Towers: Aluminum vs Steel Construction
Nick Anderson
towers? Aluminum is light, but not as strong.
Could a 200 lb person stand on the aluminum cross pieces without bending
or breaking them? Or should a tower that will be climbed (safely of
course) be steel? The tower under consideration will be either guyed or
supported at the end of a barn, so tip over and/or buckling are not the
issue here, just human body support on the cross piece.
Any brands (steel or aluminum) better than others? I'm looking at
Universal Tower.
Just looking for opinions and experience.
Nick Anderson
WA8NTW
Gary Coffman
>What are the prevailing opinions on aluminum vs steel construction for
>towers? Aluminum is light, but not as strong.
>
>Could a 200 lb person stand on the aluminum cross pieces without bending
>or breaking them? Or should a tower that will be climbed (safely of
>course) be steel? The tower under consideration will be either guyed or
>supported at the end of a barn, so tip over and/or buckling are not the
>issue here, just human body support on the cross piece.
See my posts under the antenna mast thread. Aluminum has no safe
fatigue limit. It will crack and fail after some amount of flexure loading.
Wind, and climbing, impose those sorts of flexure loadings. OTOH
mild steel does have a safe fatigue limit. If a steel tower is correctly
designed, installed, and kept within that safe limit, it will not suffer
fatigue failures, ever.
I don't know of any commerial radio site that would consider the use
of an aluminum tower today. The question isn't if it will fail, it is, when
is it going to fail? You should be no less prudent.
As to which tower to consider, no one ever went wrong buying Rohn.
Depending on the loading you expect, 25G or 45G would do nicely.
Gary
Gary Coffman KE4ZV | You make it |mail to ke...@bellsouth.net
534 Shannon Way | We break it |
Lawrenceville, GA | Guaranteed |
Brian Kelly
wrote:
>What are the prevailing opinions on aluminum vs steel construction for
>towers? Aluminum is light, but not as strong.
>
>Could a 200 lb person stand on the aluminum cross pieces without bending
>or breaking them? Or should a tower that will be climbed (safely of
>course) be steel? The tower under consideration will be either guyed or
>supported at the end of a barn, so tip over and/or buckling are not the
>issue here, just human body support on the cross piece.
>
>Any brands (steel or aluminum) better than others? I'm looking at
>Universal Tower.
>
>Just looking for opinions and experience.
>
>Nick Anderson
>WA8NTW
>
Aluminum is an excellent structural material *IF* its done right.
Rumors to the contrary the aircraft-grade aluminum alloys are just as
strong or stronger than the low-carbon steels used for tower
construction. Alloy 6061 is the most commonly used. Aluminum has
three characteistics you have to watch when used for towers: It's
three times less stiff than steel - which has nothing to do with it's
strength. An aluminum tower will sway three times as far during a
given gust. Aluminum does not have nearly as high a resistance to
abrasion as steel has. Thus an aluminum sleeve bearing for a mast is
not a good idea. An aluminum mast spinning in an aluminum sleeve is a
nightmare scenario. Last, welding aluminum is more difficult than
welding steel so you have to have a welder who really knows his
"stuff" to do it right.
>
Your question with respect to the bending of the cross-members during
climbing can only be answered via a stress analysis which cannot be
done without the detailed specifics of the cross-member; diameter,
material, length, load, etc. Toss the question at the manufacturer.
>
I've had only hands-on experience with one aluminum tower. It was an
early Universal which was considerably overloaded and the tower
screwed itself into a pile of scrap when a high wind struck the huge
tribander.
>
I examined the carcass as did a metalurgist friend and fellow ham and
we both came to the conclusion that a contributing factor to the
accident was insufficient weld penetration. Samples of the failed
welds were shipped to Universal and they quickly agreed with our
failure analysis and couldn't ship the tower owner a complete
repacement tower fast enough, no questions asked, they even paid the
freight bill. The new tower had much better welds and stayed up for
decades despite the overloading.
>
When that tower was finally taken down it was as good as new. The Rohn
45 steel tower I had was a rusted pile of junk when I took it down.
Both towers had been up for about 30 years.
>
I'd do a Universal in a wink if I was shopping for a self-supporting
tower.
>
Brian Kelly w3rv
Brian Kelly
>>Any brands (steel or aluminum) better than others? I'm looking at
>>Universal Tower.
>
I'm losing it. I'll swear that old aluminum tower was made by
Universal. There is a Universal out there which builds custom
commercial steel towers but that company is only a few years old.
There is a Heights tower which has been around forever and they do
build aluminum ham towers. Maybe that aluminum tower I described was a
Heights.
>
Incidentally, I disagree with Gary's comments with respect to
unpredictable aluminum fatigue failures. The next time you get a
window seat in any airliner watch how much the wings flex in even mild
turbulence, they bounce all over the sky to a scary extent. I haven't
heard of any of the aluminum wings falling off any of the large fleet
of 66 year old DC-3s which are still in daily commercial service, many
of those airframes have well over 100,000 hours of flying time logged.
If aluminum is good enough for Douglas in 1934 and Boeing today I
reckon it oughta be good enough for ham towers.
>
A little closer to home one might also note the number of old aluminum
beam antennas which have been bent and stressed every which way by the
wind over the years without fatigue failures.
>
w3rv
Brian
> On Sun, 09 Apr 2000 19:41:21 GMT, ke...@dvol.com (Brian Kelly) wrote:
>
> >>Any brands (steel or aluminum) better than others? I'm looking at
> >>Universal Tower.
> >
> I'm losing it. I'll swear that old aluminum tower was made by
> Universal.
See the AES catqalog. Lotsa "Universal" aluminum towers there.
> There is a Universal out there which builds custom
> commercial steel towers but that company is only a few years old.
> There is a Heights tower which has been around forever and they do
> build aluminum ham towers. Maybe that aluminum tower I described was a
> Heights.
> >
> Incidentally, I disagree with Gary's comments with respect to
> unpredictable aluminum fatigue failures. The next time you get a
> window seat in any airliner watch how much the wings flex in even mild
> turbulence, they bounce all over the sky to a scary extent.
B-52's actually need to flap their wings to get off the ground.
Especially with a big payload. :^)
Bob Lewis
that must have been 30 years old at the time we were working with it
almost 10 years ago. It was still in good shape. It had a motorized
tilt-over assembly. On the other hand, I've seen some steel towers
that were covered with rust. I don't know that you can clearly prove
one better than the other - I think cost is a big issue as well.
Remember to compare apples to apples - free standing aluminum to free
standing steel. A Rohn 25 is *not* rated for free standing unless you
only need 30 feet. Also be sure to check wind load specs. Many of the
free standing crank up towers I looked at several years ago only
specified wind loads when fully retracted. Also consider how you're
going to handle delivery. If a truck shows up with a 500 lb steel
tower *you* are responsible for unloading it.
Mark Keith
>
> On Sun, 09 Apr 2000 19:41:21 GMT, ke...@dvol.com (Brian Kelly) wrote:
>
> >>Any brands (steel or aluminum) better than others? I'm looking at
> >>Universal Tower.
> >
> I'm losing it. I'll swear that old aluminum tower was made by
> commercial steel towers but that company is only a few years old.
> There is a Heights tower which has been around forever and they do
> build aluminum ham towers. Maybe that aluminum tower I described was a
> Heights.
> >
> Incidentally, I disagree with Gary's comments with respect to
> unpredictable aluminum fatigue failures. The next time you get a
> window seat in any airliner watch how much the wings flex in even mild
> heard of any of the aluminum wings falling off any of the large fleet
> of 66 year old DC-3s which are still in daily commercial service, many
> of those airframes have well over 100,000 hours of flying time logged.
> If aluminum is good enough for Douglas in 1934 and Boeing today I
> reckon it oughta be good enough for ham towers.
> >
> A little closer to home one might also note the number of old aluminum
> beam antennas which have been bent and stressed every which way by the
> wind over the years without fatigue failures.
> >
> w3rv
I've got video of a program about the 777 where they showed the stress
tests on the wings to the point of breakage. It was quite predictable in
that case, and they knew beforehand, the amount of stress needed to
break, and they also knew which wing spar the breakage would occur.
Quite loud. I can only imagine the terror of being in a jet that the
wings broke off due to overspeed etc. It's louder than a shotgun in your
ear, and there is no doubt you are in a world of trouble. But I have no
real opinion on the tower issue. I've never compared. I do hear alot of
aircraft metal fatique stories though. So even if predictable when new,
it can become a problem after time, if flexing is involved. In jets
every cycle of takeoff/landing puts a bit of fatique in the metal. But
normally takes quite a while to really become a problem. Say the Hawaii
air 737 metal rip over the pacific. MK
--
http://web.wt.net/~nm5k
W5LZ
Nick,
I haven't read all of the replies yet, but think that
the majority will peobably recommend steel, and your
choice will depend on co$t versus replacement time.
Rather than go through all the 'same' reasons everyone
else is going to offer let me say this; It's always
better to 'over-do', than to 're-do'.
'Doc
Gary Coffman
>Incidentally, I disagree with Gary's comments with respect to
>unpredictable aluminum fatigue failures. The next time you get a
>window seat in any airliner watch how much the wings flex in even mild
>turbulence, they bounce all over the sky to a scary extent. I haven't
>heard of any of the aluminum wings falling off any of the large fleet
>of 66 year old DC-3s which are still in daily commercial service, many
>of those airframes have well over 100,000 hours of flying time logged.
>If aluminum is good enough for Douglas in 1934 and Boeing today I
>reckon it oughta be good enough for ham towers.
Well, if you pulled your tower down on a regular schedule, checked it
for signs of fatigue, and replaced every part showing such signs, as
the airlines must do, then an aluminum tower might be Ok. But no one
does that, so aluminum towers fall when they fatigue, crack, and fail.
That's why commercial sites will only use steel towers.
W5LZ
Brian,
The "BUFF's" don't have to flap their wings, they
just want to!
'Doc
Reg Edwards
> aluminum towers fall when they fatigue, crack, and fail.
Exactly the same with steel towers..
==========================
> That's why commercial sites will only use steel towers.
There can be only one reason for using steel - for the same strength
and reliability, steel towers must cost less. American engineers and
accountants are not stupid.
---
Reg.
Brian Kelly
>
>I've got video of a program about the 777 where they showed the stress
>tests on the wings to the point of breakage. It was quite predictable in
>that case, and they knew beforehand, the amount of stress needed to
>break, and they also knew which wing spar the breakage would occur.
>Quite loud.
>
you're talking about is static destructive testing which is very
pedictable via finite element analyses given the well-known published
yield and ultimate stresses of the materials used to build the wing
structure. Very damatic but a total no-brainer used mostly to confirm
the fact that the software model of the wing is OK.
>
Fatigue failure analyses and testing on the other hand involves cyclic
loading, as in how many times can one push the wing four feet up then
four feet down before the wing breaks off? Not easy. Involves very
long term lab material testing which you ain't sit thru on TV.
>
The fatigue failure characteristics of the common steels and aluminums
are equally well known so I don't understand what the big deal is as
far as the "unpredictable" behavior of aluminum vs. steel is
concerned. The only difference is some well known numbers and they're
numerically very similar.
>
The bottom line here is that if one designs a structure which is never
cyclically loaded above some specific stress level the structure will
theoretically never fail in fatigue. That stress level is called the
"endurance limit" of the material and it's heat treated state, etc.
Endurance limits are not widely published, in fact the only really
good source for those numbers is MIL-HDBK-5 which only the aeropspace
guys bother with. This whole topic area also involves the implications
of cyclic stresses superimposed on static stresses, it gets really
ugly and is 'way outside any discussions involving ham radio. Towers
don't have to fly and are therefore designed accordingly, end of.
>
Most designers of non-critical static structures like towers and beams
design to a maximum allowable stress of 1/3 the published yield stress
to make sure that the fatigue issues are covered. For instance the
published yield point stress of 6061T6 aluminum is 60,000 psi give or
take. The national model building code allows 6061 aluminum towers to
be stressed to 24,000 psi but no thankew, I use 1/3 of 60,000 psi and
design to 20,000 psi. Nothin' I ever put up has come down yet unless I
put the wrenches to it.
>I can only imagine the terror of being in a jet that the
>wings broke off due to overspeed etc. It's louder than a shotgun in your
>ear, and there is no doubt you are in a world of trouble. But I have no
>real opinion on the tower issue. I've never compared. I do hear alot of
>aircraft metal fatique stories though. So even if predictable when new,
>it can become a problem after time, if flexing is involved.
>
designers are required to push the limits of material properties in
order to get the things flying and making money. It's a very dicey
statistics game. No such thing as absolute values for material
properties. If the material has an invisible internal flaw or some
other step in the machining or other processing step creates
unexpected stress risers all bets are off those defects can lead to
premature failure of the component and the aircraft.
> In jets
>every cycle of takeoff/landing puts a bit of fatique in the metal.
>
slightly unbalanced 60,000 rpm turbine we're talking 216,000,000
stress cycles per HOUR. Yeah, they come apart . .
>But
>normally takes quite a while to really become a problem. Say the Hawaii
>air 737 metal rip over the pacific. MK
>
>--
>http://web.wt.net/~nm5k
>
w3rv
Brian Kelly
>Brian Kelly wrote:
>
>
>B-52's actually need to flap their wings to get off the ground.
>Especially with a big payload. :^)
>
. . . you AGAIN? . . ! . . never mind! B-52 wings droop because
they're supporting four heavy old tubines per wing when they're on the
ramp. Once the jock in the left seat gets the beast up around flying
speed 'way down the runway the wings have develped enough lift force
to bend the tips of the wings up around eight feet vs. the parked
position. Once the wings are satisified that the engines are lifted
they then work on the getting the fuselage and it's weapons load
lifted. The wings doan "flap" Burke, get a life . .
>
rv
Garry
Garry
David VanHorn
Hash: SHA1
>The wings doan "flap" Burke, get a life . .
Never flown through much turbulence, have you?
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Brian Kelly
>Well, if you pulled your tower down on a regular schedule, checked it
>for signs of fatigue, and replaced every part showing such signs, as
>the airlines must do, then an aluminum tower might be Ok.
>
inspections for tower fatigue cracks.
>
> But no one
>does that, so aluminum towers fall when they fatigue, crack, and fail.
>That's why commercial sites will only use steel towers.
>
fatigue crack? Since when was steel declared "fatigue proof" but
aluminum is not fatigue proof?
>
Did you ever see a steel tower fall due to corrosion? I have. Have you
ever seen an aluminum tower corrode and fall? I sure have not.
>
>Gary
>
Aluminum is around three times more expensive per pound than steel
plus any idiot can weld steel (idiots cannot weld aluminum) so
obviously we'll find more steel towers than aluminum towers. It's all
in the production costs, not in technical issues.
>
w3rv
Brian Kelly
<G4fgq...@btinternet.com> wrote:
>Gary Coffman wrote
>> aluminum towers fall when they fatigue, crack, and fail.
>==========================
>Exactly the same with steel towers..
>==========================
>> That's why commercial sites will only use steel towers.
>==========================
>There can be only one reason for using steel - for the same strength
>and reliability, steel towers must cost less.
>
For the same strength yes. For the same reliability, no. Very few
buyers put any value on long-term reliability, what matters is getting
it up at minimum cost. The buyer will be long gone when the thing
becomes a maintenance headache.
>
>American engineers and
>accountants are not stupid.
>
Is this an invitation to open that bag of worms?
>---
>Reg.
>
rv
Brian Kelly
wrote:
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>Hash: SHA1
>
>>The wings doan "flap" Burke, get a life . .
>
>Never flown through much turbulence, have you?
>
I'm a Part 105 ultralight pilot.
David VanHorn
Hash: SHA1
> >>The wings doan "flap" Burke, get a life . .
> >
> >Never flown through much turbulence, have you?
> >
> I'm a Part 105 ultralight pilot.
I've seen them flexing very significantly on 747s. (of course that's
not an ultralight)
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Brian Kelly
wrote:
>
>
>Well, if you pulled your tower down on a regular schedule, checked it
>for signs of fatigue, and replaced every part showing such signs, as
>the airlines must do, then an aluminum tower might be Ok.
>
are those in turbine engines. Those parts are almost always steel or
one of the high-temperarature alloys like the Hastelloys and Inconels
which are used for the blades. That's why engines get torn down for
inspections much more frequently than do the aluminum airframes.
>
w3rv
Brian Kelly
wrote:
>
>> >Never flown through much turbulence, have you?
>> >
>> I'm a Part 105 ultralight pilot.
>
>I've seen them flexing very significantly on 747s.
>
of the design tricks Boeing uses to reduce the lumps & bumps for the
fares and the rest of the airframe.
>
>(of course that's
>not an ultralight)
>
all over in the things under conditions you never even know are out
there when riding in an airliner.
>
w3rv
Richard Harrison
"Ok. But no one does that, so aluminum towers fall when they fatigue,
crack and fail."
Another hazard is the low melting temperature of aluminum. A frequent
poster in this newsgroup, Gary Deutschman, tells the story of a
lightning stroke melting a leg to bring down his tower.
Best regards, Richard Harrison, KB5WZI
Mark Keith
Yep, but that would surely be a case of improper grounding of the tower
wouldn't it? If a 10 gauge wire can take a strike with little heating ,
if properly grounded, an aluminum tower should have no problem at all.
If it did, it wasn't grounded properly. Or would seem to me anyway. MK
Richard Harrison
"Yup, but that would surely be a case of improper grounging of the tower
wouldn`t it?
I think you nailed it, Mark. Gary seems to be absent, but if I recall,
he found evidence of poor contact between tower sections despite the
mechanical load.
Whatever happens with a steel tower it takes a very high temperature to
melt. The melting point for iron is about 1535 degrees C, but for
aluminum the temperature is about 660 degrees C. The alloys of these
metals can be considerably different, but are generally in the
neighborhoods of the base metals.
Gary V. Deutschmann, Sr.
I is here!
I did not receive the messages that were the precurser to this thread,
but it looks like your talking about my tower that blew up 2 years ago
after the lightning ate it.
The final determination was that the joint between the base section
and the upper section of one leg was corroded and not making ground
contact. However, ground should have still been direct through the
other 2 legs.
In any case, the other thing that was found was that the leg was full
of water. So apparently what happened was the lightning strike boiled
the water inside the leg and blew the leg apart rather than it
melting.
It's one of those incidents I have been trying to forget, if ya know
what I mean, hi hi.....
TTUL - 73+ de Gary - KGØZP
Brian Kelly
(Richard Harrison) wrote:
>Gary Coffman wrote:
>"Ok. But no one does that, so aluminum towers fall when they fatigue,
>crack and fail."
>
>Another hazard is the low melting temperature of aluminum. A frequent
>poster in this newsgroup, Gary Deutschman, tells the story of a
>lightning stroke melting a leg to bring down his tower.
>
The melting point of aluminum vs.steel is only a few hundred degrees
and aluminum is a much better conductor than steel so, without
actually doing the numbers, it's probably close to a tossup wrt to
whether an aluminum tower is more suseptible to getting fried during a
direct hit than is a steel tower. There was something else in play in
that instance.
>
One being that mongo direct strikes can generate more Joules in the
structure than any tower can dissapate without frying.
>
>Best regards, Richard Harrison, KB5WZI
>
w3rv
Richard Harrison
"I is here!"
Gary Deutschmann is now heard from and knows the sad story of his
disaster. Sorry I got it wrong. Sounds as if he had a blast and not a
melt down.
It is a fact , none the less, that lightning melts and splatters metals.
Lower melting point metals are affected more than higher melting point
metals in spite of thermal conductivities.
Mark Keith
I still think a poor ground is usually the root of the problem with a
metal meltdown. In his case , for the water to boil and expand it had to
get hot first. Mainly from the ungrounded leg. If it had been well
grounded, this shouldn't have happened. When I took a hit on my mast, I
didn't see any of this at all. I am using a rat shack pushup, with an
extension at the top using conduant which is only 1/2 to 3/4 inch, and
fairly thin wall.
I had the top of this taped up with electical tape to keep water out. I
took a direct hit on this mast, and then later rolled it down to check
it out. All it did was blow a nice neat hole in the tape, just like a
kindergarten pencil had been jammed thru it. The metal had absolutaly no
damage at all. Not even an arc mark or anything. With the exception of
the blown out tape, I could not find a single bit of damage. Also I
decided to quit using the tape on the top. I think it helped the top of
the mast to stream easier , so I quit doing that. I think the tape
helped attract the strike, but may be wrong on that. It did have kinda
pointy places where I had wrapped the tape around the end. Also seems to
me it would be the electrical conductivity that would help avoid metal
damage rather than thermal. Dunno, but I'm still of the mind that metal
damage from lightning is a sure sign of an inadequate ground connection.
To handle a monster strike, you need monster grounding. ufer, radials,
etc. Note lightning rod systems that take strike after strike, and don't
fail. I'd be curious to ride up on one of our TV towers to look at
possible strike effects. They get zapped almost weekly it seems. But
they don't let very many people go up on those anymore. MK
Gary Coffman
>The bottom line here is that if one designs a structure which is never
>cyclically loaded above some specific stress level the structure will
>theoretically never fail in fatigue. That stress level is called the
>"endurance limit" of the material and it's heat treated state, etc.
Quoting from Machinery's Handbook, "Endurance limit is the stress
value below which the number of cycles to failure is infinite. Steels have
endurance limits that vary according to hardness, composition, and
quality, but many non-ferrous materials do not." Aluminum is one
of those non-ferrous materials which does not have an endurance
limit.
You can design a steel structure so that the stress is below
the endurance limit for that steel and it will never suffer fatigue
failure. You cannot do that for an aluminum structure. It has no
endurance limit. *Any* stress contributes to fatigue failure of
aluminum structures, and the effects are cumulative according
to Miner's Law. Thus every aluminum structure has a finite life.
That life depends on the nature and number of stress cycles the
structure must endure. There are two types of cyclic stress of
interest to us here, high cyclic and low cyclic. Each type follows
Miner's Law, but their locations on the S-N curves are different.
High cyclic stress (loadings of 10% or greater of the yield stress
of the material) leads to rapid failure, low cyclic stress (much less
than 10% of the yield stress of the material) leads to a longer time
before failure.
For antenna structures, high cyclic stress would be equated
to gust loadings in a storm. Low cyclic stress would be the
response to ordinary wind loadings. Note that both stresses
are cumulative for aluminum because it has no endurance
limit.
For steel, only loadings above the endurance limit are cumulative.
If we design so that the endurance limit is above the highest credible
gust loading, we can be assured that a steel structure will not suffer
fatigue failure. We can't do that for an aluminum structure, because
it has no endurance limit and all stress is cumulative.
Since the frequency and severity of storms is to a large extent
unpredictable for a particular site, we can't set a hard end of life
figure for an aluminum structure. But we know that it will have one.
Sooner or later, fatigue will get it. It might be 6 months, a year, or
even 10 or 20 years, depending on the particular set of loadings it
will suffer at the particular site, but it will fail (and not necessarily
during a storm, storms may only accelerate fatigue, and the tower
can later fail on a relatively calm day when the final straw is added
by ordinary low cyclic stress).
With steel, we can design so that the highest credible loading will be
below the endurance limit, and can then be assured that it will never
suffer fatigue failure. That's not what tower designers typically do,
however, because it would result in a more expensive and massive
structure than the customer would tolerate or could afford.
What they typically do is design so that low cyclic stress falls below
the endurance limit, and allow high cyclic stresses to contribute toward
fatigue failure. That's reasonable, since low cyclic stress is orders of
magnitude more frequent than high cyclic stress, so the tower lifetime
will still be quite long on average (good enough that only infrequent
inspections are required to catch fatigue cracking before it results in
a catastrophic failure).
Aircraft have to use aluminum to keep their unloaded weight down,
and must have very frequent inspections to monitor the cumulative
fatigue damage they've suffered in order to catch problems before
they become catastrophic failures. Most tower owners are unwilling
to maintain that sort of inspection and replacement schedule, so
aluminum is a very dangerous material to use for load bearing towers
and masts. (We can tolerate aluminum antenna elements because
they are generally light enough that failure would only extremely rarely
have catastrophic consequences, and their high RF conductivity is a
major advantage over steel.)
Gary Coffman
>Gary, KG0ZP wrote:
>"I is here!"
>
>Gary Deutschmann is now heard from and knows the sad story of his
>disaster. Sorry I got it wrong. Sounds as if he had a blast and not a
>melt down.
>
>It is a fact , none the less, that lightning melts and splatters metals.
>Lower melting point metals are affected more than higher melting point
>metals in spite of thermal conductivities.
That's true, Richard, but steel also has about 17 times the skin resistance
of aluminum, so it will suffer about 17 times as large an energy dump as would
an aluminum conductor in a strike. That more than offsets the advantage of
steel's higher melting point. Even at DC, an iron wire will melt while carrying
less current than than is required to melt an aluminum wire of the same mass.
A good conductor generally suffers no damage in a lightning strike. It is the
higher resistance points that get the majority of the energy dump. If a tower
does suffer damage, it will invariably be at a poor joint where resistance
is highest.
W5LZ
In applications where weight is a factor (aircraft),
aluminum, oe other more exotic alloys are used. Where
weight is not a factor (bridges), steel is usually used.
Weight for a tower is usually not a factor. I would
use a steel tower.
Not very scientific, but works for me.
'Doc
south...@my-deja.com
Gary Coffman <ke...@bellsouth.net> wrote:
>
> Quoting from Machinery's Handbook, "Endurance limit is the stress
> value below which the number of cycles to failure is infinite. Steels
have
> endurance limits that vary according to hardness, composition, and
> quality, but many non-ferrous materials do not." Aluminum is one
> of those non-ferrous materials which does not have an endurance
> limit.
Pardons for bumping in...
My copy of MIL-HDBK-5 (RevE) has several S/N diagrams for aluminum
alloys, various tempers. Example: 6061-T6 alloy, longitudinal
direction, 10KSI, 1x10E8 cycles.(Page 3-242). Example:7475-T7351 plate
alloy, logitudinal and long-transverse direction, 18 KSI, 1x10E8 cycles
(page 3-388).
>
> You can design a steel structure so that the stress is below
> the endurance limit for that steel and it will never suffer fatigue
> failure. You cannot do that for an aluminum structure. It has no
> endurance limit. *Any* stress contributes to fatigue failure of
> aluminum structures, and the effects are cumulative according
> to Miner's Law. Thus every aluminum structure has a finite life.
In the real world (which is the whole point of this discussion) you are
absolutely correct and I agree that aluminum structures will fail. But
that is not due to the lack of an endurance limit for aluminum.
Aluminum fails in structural tower applications because it is highly
notch-sensitive and therefore more prone to develop cracking at zones of
stress concentrations. You can't build a structure without stress
concentrations somewhere, which drastically reduce the already low
endurance limits for aluminum.
The endurance limits in MIL-HDBK-5 were only for smoothly machined test
specimens that had no stress concentrations and were uniformly cycled --
that's how they got the limits. But build a structure out of the
material and cycle it, and it'll fail at far less number of cycles than
are even predicted by the alloys S/N curve. Quoting again from
MIL-HDBK-5:
"Fabricated parts in test have been found to fail at less than 50,000
repetitions of load when the nominal stress was far below that which
could be repeated many millions of times on a smooth machined specimen."
(page 1-16)
So, while aluminum does indeed have an endurance limit, it's useless for
many design purposes.
Build the tower out of steel.
I enjoy your posts, Gary - thanks for sharing your vast knowledge.
Kevin Koskela
Sent via Deja.com http://www.deja.com/
Before you buy.
Gary Coffman
>In article <io49fsk5fohvunjtl...@4ax.com>,
> Gary Coffman <ke...@bellsouth.net> wrote:
>>
>> Quoting from Machinery's Handbook, "Endurance limit is the stress
>> value below which the number of cycles to failure is infinite. Steels have
>> endurance limits that vary according to hardness, composition, and
>> quality, but many non-ferrous materials do not." Aluminum is one
>> of those non-ferrous materials which does not have an endurance
>> limit.
>
>Pardons for bumping in...
>
>My copy of MIL-HDBK-5 (RevE) has several S/N diagrams for aluminum
>alloys, various tempers. Example: 6061-T6 alloy, longitudinal
>direction, 10KSI, 1x10E8 cycles.(Page 3-242). Example:7475-T7351 plate
>alloy, logitudinal and long-transverse direction, 18 KSI, 1x10E8 cycles
>(page 3-388).
Those diagrams clearly show that aluminum does not have an endurance
limit as defined by the ASTM. Endurance limit is defined as the stress level
below which a material will not suffer fatigue failure even if cycled an infinite
number of times. This is shown on the S-N diagram as a floor on the S-N
curve such that the curve never reaches the X axis.
In the examples you give, the S-N curve *does* reach the X axis. That shows
that aluminum does not have an endurance limit. If you look at a S-N diagram
for steel, you'll see that the curve flattens out until it is parallel with the X axis
at some stress value greater than zero. That value is its endurance limit. It can
withstand an infinite number of cycles with stresses below that limit without
fatigue failure.
In case it isn't obvious, if you cross to the right of the S-N curve, the material
will fail due to fatigue. As long as you stay to the left of the curve, it will not
suffer fatigue failure. Because the curve for aluminum touches the X axis,
there are only a finite number of cyclic stresses it can survive, no matter how
small the stresses are. Because the curve for steel has a floor for the S-N curve,
it is possible for it to survive an infinite number of cycles without fatigue failure
as long as the stresses are below the endurance limit value.
Endurance limit is an important property of ferrous metals (and a few non-ferrous
metals). It is what makes springs practical. You cannot make an aluminum spring
which will not suffer fatigue failure, but you can make a steel spring that won't fail
due to fatigue, as long as your design keeps the stresses below the endurance limit
for that steel.
As you mention, fabricated structures have stress risers. Those will be the
first points of failure if the endurance limit is exceeded (or if, like aluminum,
the material has no endurance limit). This means that real structures have
to be designed for stress loadings below that given for the bulk material in
order to allow for the presence of these stress risers. (MOM programs can
give us a good idea of the location and magnitude of these stress risers, so
we can more closely approach the endurance limit while maintaining a
margin of safety.)
south...@my-deja.com
Ah, I see now. The MIL-spec defines that "infinity" for their purposes
equals 1x10E08 cycles (that is, 100 million cycles) of a given stress
value. If you look at the diagrams and on the pages I cited, the
figures actually don't touch the x-axis at any stress value --
because the x-axis only extends to 100 million cycles and then stops
there. What the spec does is provide data to show that aluminum tested
on their test specimens will withstand 100 million cycles if the stress
is below a certain limit. Although 100 million cycles is a lot, it
certainly isn't "infinity" (the ASTM definition) and it may be that the
S/N curve will eventually touch the x-axis, but it isn't depicted in the
figures because they don't show test results for that high of a number
of cycles.
Just curious - if you look at your reference S/N curve for 6061-T6,
at 1000 psi (ie, 1 KSI), how many cycles are predicted before failure
occurs?
How many cycles for 500 psi? The MIL-Hdbk doesn't show sufficient data
to answer when failure at this loading will occur.
Thanks for the info...
Kevin
Arthur Unwin
flexing,ferrous versus non ferrous,buckling, welding e.t.c.are justn the starting point of
criteria.
In the real world quality control, interpretation of test results i.e.similarity of test
material surface
and makeup compared to actual materials and construction methods used are much more
important
as well as the prevailing temperatures when welding, such is the reason for safety
factors.
Of course many tower manufacturers are not high on the technology front relying on
basically on how things
are usually done in the industry and correct when and if problems occur later.
Relating a personal experience, I had a Rohn 45 foldover tower that broke under a normal
load condition.
On examination I determined that one leg of the tower failed in tension with clear
'necking' prior to failure.
( for those who are not into it, necking is a term used when a piece of metal thins out or
waists like chewing gum
prior to failure due to tension and resulting compression forces when it is pulled apart)
When performing a stress analysis I was shocked to find that the recommended loads matched
my analysis
BUT WITHOUT A SAFETY FACTOR BUILT ADDED IN)
I took the failed part and my analysis to my dealer and he was quite excited because the
manufacturer had just
changed the design in that particular area without explanation and of course now all was
explained!
So to reitterate, academic basic analysis of mechanical engineering is fine but if a
manufacturer fails to insert a safety factor and relies on a maximum load message in the
instructions or has no x ray facilities for weld checking,Q.C.on materialls used e.t.c.
discussion of metallurgic comparisons are really meaningless ( nothing personal intended
ofcourse)
tho I do find the thread educationaly interesting.
Regards
Art Unwin KB9MZ
Gary Coffman wrote:
> On Thu, 13 Apr 2000 03:01:51 GMT, south...@my-deja.com wrote:
> >In article <io49fsk5fohvunjtl...@4ax.com>,
> > Gary Coffman <ke...@bellsouth.net> wrote:
> >>
> >> Quoting from Machinery's Handbook, "Endurance limit is the stress
> >> value below which the number of cycles to failure is infinite. Steels have
> >> endurance limits that vary according to hardness, composition, and
> >> quality, but many non-ferrous materials do not." Aluminum is one
> >> of those non-ferrous materials which does not have an endurance
> >> limit.
> >
> >Pardons for bumping in...
> >
> >My copy of MIL-HDBK-5 (RevE) has several S/N diagrams for aluminum
> >alloys, various tempers. Example: 6061-T6 alloy, longitudinal
> >direction, 10KSI, 1x10E8 cycles.(Page 3-242). Example:7475-T7351 plate
> >alloy, logitudinal and long-transverse direction, 18 KSI, 1x10E8 cycles
> >(page 3-388).
>
> Those diagrams clearly show that aluminum does not have an endurance
> limit as defined by the ASTM. Endurance limit is defined as the stress level
> below which a material will not suffer fatigue failure even if cycled an infinite
> number of times. This is shown on the S-N diagram as a floor on the S-N
> curve such that the curve never reaches the X axis.
> Massive delete
Brian Kelly
wrote:
>
>Quoting from Machinery's Handbook, "Endurance limit is the stress
>value below which the number of cycles to failure is infinite. Steels have
>endurance limits that vary according to hardness, composition, and
>quality, but many non-ferrous materials do not."
>
Hartog to the Machinery's Handbook when applied mechanics becomes the
discussion.
> Aluminum is one
>of those non-ferrous materials which does not have an endurance
>limit.
>
usually defined by the stress level where the s-n curve hits the
100,000,000 cycle line as Machinery's Handbook states in a paragraph
following the one you cited but sorta missed I guess.
>
Yeah, might be that an aluminum tower might come down via fatigue when
the 100,000,001st breeze hits it. In the meanwhile some uncountable
number steel towers have rusted into oblivion.
>
If you feel so strongly that aluminum towers are crap due to fatigue
issues then kindly provide the info on one documented example of an
aluminum ham antenna tower which has bored in due to a fatigue
failure. Or one example of one of the big long-haul aluminum power
transmission towers having likewise having dropped on the farm due to
a fatigue failure.
>You can design a steel structure so that the stress is below
>the endurance limit for that steel and it will never suffer fatigue
>failure.
>
phenomenon, all fatigue failures are random events which are scattered
all over the statistical distribution plots. Methinks you've gotten
yourself into some kinda horizontal asymptote worship, above the
asymptote is bad and everything below the asymptote is good,
*everything* is a one or a zero. As if.
>
The most highly developed fatigue data and failure analysis
methodologies have been developed by the rolling-element bearing
industry in profusion. There is no such thing as "steel will *never*
fail in fatigue" because there is no such thing as a perfect material
as the bearing industry well knows and has
demonstrated. "Never fail" is an oxymoron in this game and that
factoid not covered in your Machinery's Handbook. Brand new
out-of-the-box steel bearings fail regularly via fatigue at under 10%
of their rated service life. In applications where the stresses in the
bearing material aren't anywhere near the published endurance limit
stress of the usual 52100 bearing steel.
>
> *Any* stress contributes to fatigue failure of
>aluminum structures, and the effects are cumulative according
>to Miner's Law. Thus every aluminum structure has a finite life.
>
we're still waiting. I've never heard of an A&P coming up with a
fatigue-cracked DC-3 main wing spar yet on any DC-3's annual, or at
least I can't find any ADs on DC-3 main spars. How many load cycles do
you suppose those have been thru after 2/3 of a century and 100,000+
hours in the air? Why wouldn't one expect a ham tower designed to the
far more conservative national building code structural standards to
also survive those same 66 years? Considering that every wind which
comes along which loads ham towers also loads parked DC-3s. To which
we also might ponder the fact that the aluminums used to build the
DC-3 are metalurgically primitive by today's standards. Don Douglas
would love to have had the cheap aluminum merchant alloys we use today
to build antennas and towers when he got his 3 up and running.
>
>
>Sooner or later, fatigue will get it. It might be 6 months, a year, or
>even 10 or 20 years, depending on the particular set of loadings it
>will suffer at the particular site, but it will fail (and not necessarily
>during a storm, storms may only accelerate fatigue, and the tower
>can later fail on a relatively calm day when the final straw is added
>by ordinary low cyclic stress).
>
per previous.
>
How long is long enough Gary?
>
The nut of the matter is that there are a number of tradeoffs to be
made when considering any tower acquisition. This fatigue thing is
only one of those tradeoffs which you sir have tried diligently to
blow completely out of proportion relative to it's actual importance
for some inexplicable reason.
>
The rather minor downside of aluminum towers is the cost per foot of
height per pound of load capacity vs. steel towers, the rest is all
uphill. Ya pays yer money and takes yer choice as usual.
>
>
>Gary
>
w3rv
Brian Kelly
>
>an infinite
>> number of times. This is shown on the S-N diagram as a floor on the
>S-N
>> curve such that the curve never reaches the X axis.
>
>
>Just curious - if you look at your reference S/N curve for 6061-T6,
>at 1000 psi (ie, 1 KSI), how many cycles are predicted before failure
>occurs?
>
>How many cycles for 500 psi? The MIL-Hdbk doesn't show sufficient data
>to answer when failure at this loading will occur.
>
>Thanks for the info...
>
>
>Kevin
>
w3rv
south...@my-deja.com
ke...@dvol.com (Brian Kelly) wrote:
> >
> Awshit . . no mindless asymptote ya can really wrap around eh?
> >
Actually, you got it exactly right.
> >
> >Just curious - if you look at your reference S/N curve for 6061-T6,
> >at 1000 psi (ie, 1 KSI), how many cycles are predicted before failure
> >occurs?
> >
> >How many cycles for 500 psi? The MIL-Hdbk doesn't show sufficient
data
> >to answer when failure at this loading will occur.
> >
> >Thanks for the info...
> >
> I just cain't wait for the "answers" . . .
> >
As you reduce the stress value for aluminum closer to the 0-stress line
(which is the x-axis of the S/N curve), the number of cycles to failure
increases by orders of magnitude.
Actually, I agree with Gary on the definition of endurance limit and its
requirement for "infinite" number of cycles - ferrous metals have a
clearly defined shape to their S/N curves which flattens out to a very
nicely defined stress value that is referred to as "endurance limit" or
"fatigue limit."
The materials texts provide tables of fatigue limits for a variety of
metals - different values for different alloys of steel and so on.
These same tables also provide "fatigue limit" values for aluminum - but
they always include a footnote which defines what number of cycles
applies to that limit. That's because steel can be cycled below it's
fatigue limit indefinitely, but aluminum has a specific number of
cycles (from the S/N curve) which applies to that specific stress value.
And, most texts also follow up with the statement that "Aluminum has no
endurance limit" -- this statement oversimplifies the issue and leads to
incorrect assertions.
A case in point:
Gary said: "In the examples you give, the S-N curve *does* reach the X
axis. That shows that aluminum does not have an endurance limit. "
This is where I disagree -- first of all because the figures I cited
do *not* show the S/N curve touching the x-axis, and the S/N curve for
any material (that has an S/N curve) cannot *ever* actually touch the
x-axis. In the case of aluminum, it is asyptotic to the x-axis, but it
does not touch it. Here's why:
At the x-axis of any S/N curve, what is the stress value? The stress
value along the x-axis is 0 (zero) psi. If the test article is
receiving 0 psi, it cannot be under stress cycling and therefore will
never fail due to fatigue. If the S/N curve actually touched the
x-axis, it would mean that the test article failed without being
subjected to any cyclic stress at all.
As has been noted, the S/N curve for aluminum actually never levels off
to a clean fatigue limit that will apply for an infinite number of
cycles. The S/N curve just gradually decreases until it becomes
asyptotic with the x-axis. That's what leads to the textbook statement
that "Aluminum doesn't have an endurance limit." That's true, if you
restrict the definition of endurance limit to only apply to infinite
cycles.
But designers still need to design stuff with aluminum, and still need
to account for fatigue somehow. That's why they just use some high
number of cycles (100 Million or 500 Million) and call it close enough
to infinity for practical purposes, and use the fatigue limit from the
S/N curve that works in their design. At least, that's how I do it.
Getting back on-topic, designing a structure for an antenna tower could
certainly be done out of aluminum, but it would be big, heavy,
butt-ugly, expensive, and overly complex for the circumstances. Steel
is simply the correct material for the job.
Aluminum sux, steel rulz for towers. (g)
Yuri Blanarovich
Why in the world anyone would want to subject, make prone, any component in the
towers/antenna installation to fatigue stresses? You allow that, you are a fool
waiting for disaster. There are just so many cycles that lead to failure.
There are very well known measures to avoid, prevent, break the conditions
leading to oscilations and fatigues failures. Like putting ropes in the
elements (if they are not design properly), guying the towers (even with rope
or Inverted Vees) or putting damping mass at critical points. You see something
vibrating/singing in the wind, stop it immediately, don't wait for elements to
break and drop to the ground. Guy wires on the crankup self supporting towers
don't have to be tightened, just have them there, when wind comes and tries to
bend the tower, "lose" guys will take the force and help the tower to stand.
Two strongly made Inverted Vees would do it.
On the subject of steel vs. aluminum (and use of rivets). I have seen arguments
on TowerTalk reflector pro and con and comparing/justifying one over the other.
Steel is more common and stuff fabricated from it more uniform (safer?) but
more corrosion prone. Aluminum is more tricky to weld and there were problems
with some towers with welds being crystalized and breaking off. Al resists
corrosion and is light.You need to know the properties/advantages and use it
accordingly.
Use of rivets alone in joining the tubing is mickey-mouse. You have two
circular surfaces (not flat with lotsa rivets like on the airplanes) joined
with few rivets taking the stress from vibrations and movements. They will
eventually work themselves lose. I still use slit tubing with "heavy" hose
clamps. Provides good mechanical and electrical connection and it can be taken
appart, adjusted and serviced. (You want to be double safe, put the rivet
through that.)
Yuri, K3BU, VE3BMV
south...@my-deja.com
Heh heh. Yeah, but it's my life! I live for it...
You said:
> Why in the world anyone would want to subject, make prone, any
component in the
> towers/antenna installation to fatigue stresses? You allow that, you
are a fool
> waiting for disaster. There are just so many cycles that lead to
failure.
> There are very well known measures to avoid, prevent, break the
conditions
> leading to oscilations and fatigues failures.
Cyclic and fatigue loading should always be examined (even if only
briefly) for structures that are subjected to real world stresses such
as wind and gusts, temperature expansion/contractions, and so on. With
a good design, fatigue can be managed nicely (and safely), even for
aluminum. Aluminum is a fairly well-understood material, including its
fatigue properties, and the aerospace industry has lots of guides and
rules of thumb that provide a high degree of confidence. No problems
most of the time.
You see something > vibrating/singing in the wind, stop it
immediately, don't wait for
elements to
> break and drop to the ground.
If something is vibrating/singing in the wind, that's another issue --
if this is happening, then chances are the structure (or a piece of the
structure) is diplaying its natural frequency. Good structural
designers account for natural frequency in the design - the materials
are sized to accept resonance without exceeding the fatigue limits. In
fact, that's exact how I analyze a structure - find the natural
frequency, determine the response at the natural frequency, and then
size the material to withstand loads at the natural frequency for
whatever lifetime is desired for the structure. No problems.
In the case of home / ham antenna structures, it's a crapshoot as to
whether the natural frequency has been considered. When in doubt, make
it bigger and stronger, use more guy wires, etc. A complex analysis
isn't necessary, and one could way over-analyze something. For a home
structure (not a commercial one) just buy some big ol' honkin' tubing
and bolt it or rivet it together. The methods you described sound fine
to me. Just err on the side of safety, as always.
I agree pretty much across the board with this. Aluminum vs steel is
fun to debate (sort of like the incessant 7.62MM Nato vs 30-.06 debates
on rec.guns...) but when it comes right down to it, both materials can
be made to work just fine. Just account for the limitations of whatever
material you use and all will be well.
Regards, Kevin
Richard Harrison
"Even at DC, an iron wire will melt while carring less current than is
requires to melt an aluminum wire of the same mass."
Agreed. We are assuming different heat sources. Gary is describing heat
produced by resistance in the material. I was referring to heat produced
by an electric arc as in welding, burning, and carbon arc lamps. The arc
in a lamp vaporizes the carbon electrodes. The heat produced has little
to do with the material in the electrode. The rate of consumption does.
Brian Kelly
>I am getting fatigued from all the fatigue analyses :-)
>
>Why in the world anyone would want to subject, make prone, any component in the
>towers/antenna installation to fatigue stresses? You allow that, you are a fool
>waiting for disaster.
>
Any non-static varying load on any component of any tower constitues a
"fatigue stress cycle". Climbing a tower fatigue cycles the horizontal
cross braces.
>
>There are just so many cycles that lead to failure.
>There are very well known measures to avoid, prevent, break the conditions
>leading to oscilations and fatigues failures. Like putting ropes in the
>elements (if they are not design properly), guying the towers (even with rope
>or Inverted Vees) or putting damping mass at critical points. You see something
>vibrating/singing in the wind, stop it immediately, don't wait for elements to
>break and drop to the ground.
>
Don't assume that high stresses/fatigue loads are necessarily related
to audible noise or "singing" (unless it's Rap). In fact just the
opposite is more likely, Yagi elements which "sing" are actually
moving very little based on inertia/time considerations thus element
"music" induces hardly any stresses on the elements. This silly old
wive's tale has been floating around ham circles since the first 20m
Yagi went up and started "talking" in the wind. The real enemy is the
sub-audible element ocillations you can see from the ground but can't
hear.
>
>
>
>clamps. Provides good mechanical and electrical connection and it can be taken
>appart, adjusted and serviced. (You want to be double safe, put the rivet
>through that.)
>
To hell with rivets, they have to be drilled out. Split outer tubes
and sets of 304 stainless 10-32 screws, inside-toothed lockwashers and
heavily torqued nuts has always worked for me.
>
>Yuri, K3BU, VE3BMV
>
w3rv
Richard Harrison
"I`ve had it with the corrosion hassles and weight. I`m moving to
aluminum."
The solution to rust is galvanizing. The solution to weight is adequate
footing and hoisting the load up and down, not the tower.
We used winches to hoist and lower our s-w broadcast curtain arrays for
maintenance. Steel towers were never a problem.
Richard Harrison
"Would a cast iron tower in Florida last as long?"
The Egyptian pyramids only endured because they were too massive for
thieves to remove. Aluminum in England or cast iron in Florida may be
durable but would likely be stolen for salvage value.
Brian Kelly
>In article <38f67793...@news.dvol.com>,
> ke...@dvol.com (Brian Kelly) wrote:
>> >
>> Awshit . . no mindless asymptote ya can really wrap around eh?
>
>Actually, you got it exactly right.
>
I better have it right.
>
>
>But designers still need to design stuff with aluminum, and still need
>to account for fatigue somehow. That's why they just use some high
>number of cycles (100 Million or 500 Million) and call it close enough
>to infinity for practical purposes, and use the fatigue limit from the
>S/N curve that works in their design. At least, that's how I do it.
>
That's how every professional mech designer deals with all of the
non-ferrous recipes.
>
>Getting back on-topic, designing a structure for an antenna tower could
>certainly be done out of aluminum, but it would be big, heavy,
>butt-ugly, expensive, and overly complex for the circumstances. Steel
>is simply the correct material for the job.
>
>Aluminum sux, steel rulz for towers. (g)
>
Yeah, yeah, having been down the ham steel tower road for the best
part of a half century, too many bad trips, I've had it with the
corrosion hassles and weight. I'm moving to aluminum. I'll let you
know if it's still standing after a half-billion breezes hit it . . .
endurance limits, science, engineering, hams advancing the state of
the radio arts and all that . . .
>
Somebody gotta plant comparable unguyed Rohn and Heights crankups with
equal big beams on a hilltop at some ugly salty seaside qth and settle
this "discussion" for once and for all.
>>
>Kevin
>
Brian Kelly w3rv/M.E.
Reg Edwards
top, 3 feet diameter at the base, do you think it would last as long
as the pyramids of Egypt ? Would a cast iron tower in Florida last as
long ?
--
Reg.
Richard Harrison
"The buyer will be long gone when the thing becomes a maintenance
headache."
The b-c station where I worked 51 years ago is still using the same two
towers today and they weren`t new in 1949. The station is
non-directional daytime and directional at night. These painted,
galvanized, towers still look good and likely will still look good in
the year 2049. I only wish the programming had held up as well as the
towers.
Brian Kelly
I suspect that those towers are being well-maintained. It's getting
down to aluminum being much cheaper than paint.
>
>Best regards, Richard Harrison, KB5WZI
>
w3rv
Brian Kelly
(Richard Harrison) wrote:
>Brian Kelly, W3RV / M.E. wrote:
>"I`ve had it with the corrosion hassles and weight. I`m moving to
>aluminum."
>
>The solution to rust is galvanizing.
>
I've seen more rusted-out hot-dipped galvanized Rohn steel tower
sections than I can count. In the case of tubular construction there
is no way to maintain the interior surfaces, most of the failures I've
been involved with have been sections which corroded from the inside
out. Get water inside the leg and it's all over.
>
>The solution to weight is adequate
>footing and hoisting the load up and down, not the tower.
>
In the case of small crankups, which is what I'm considering, weight
is a very big issue.
>
>We used winches to hoist and lower our s-w broadcast curtain arrays for
>maintenance. Steel towers were never a problem.
>
If you're in an arid part of the country like the southwest a steel
tower will last far longer than one which is here in the eternally
damp salt and chemical laden air in the northeast corridor. That's
why fifty year old cars in Phoenix still aren't rusted yet but many
fifteen year old cars here land in the compactors due to rust.
Bob Lewis
I think water in the leg (and eventual rust) is a given - from
condensation. That's why Rohn has you set the base section in a couple
of inches of gravel - so it can drain out.
Brian Kelly
wrote:
They put vents in the top sections for that reason. I had the water
problem with my own Rohn 45 tower. The location is swampy so I ran the
legs down thru the concrete into a foot of gravel plus I drilled 1/8"
holes in all three legs right at the upper surface of the concrete. At
some point one of the holes got plugged, probably with "rust chips".
When I took the Sawzall to the bottom section to cut it off the
concrete the leg had a couple feet of water stored above the hole.
Kept the Sawall blade nice and cool . .
>
w3rv
Arthur Unwin
I can't think of a more dangerous "solution"
Regards
Art Unwin KB9MZ
Brian Kelly wrote:
> On Sat, 15 Apr 2000 12:55:09 -0400, "Bob Lewis" <aa...@erols.com>
> wrote:
>
> snip
> I drilled 1/8"
> holes in all three legs right at the upper surface of the concrete. At
> some point one of the holes got plugged, probably with "rust chips".
> snip . .
> >
> w3rv
N2EY
writes:
>>The b-c station where I worked 51 years ago is still using the same two
>>towers today and they weren`t new in 1949. The station is
>>non-directional daytime and directional at night. These painted,
>>galvanized, towers still look good and likely will still look good in
>>the year 2049. I only wish the programming had held up as well as the
>>towers.
>>
>I suspect that those towers are being well-maintained. It's getting
>down to aluminum being much cheaper than paint.
>>
There is a VLF station in Sweden (17.2 kHz?) that uses the last operational
Alexanderson alternator in the world. It has a pair of 800+ foot tall towers
that hold a truly enormous multiwire flat-top. The towers are triangular in
cross section and self-supporting by virtue of being similar to windmill
towers. This installation has existed since 1917 or so - because they kept
painting the towers. Of course they are steel, because that was about the only
option then.
A few years ago they faced the daunting porspect of removing all the many
layers of LEAD BASED paint that had been put on over the years, and repainting
with epoxy. It was not an inexpensive or easy job.
In this area (EPA) most of the traffic signal poles and cantilevers are steel.
All of the newer ones are galvanized steel - but many are rusting, even after
less than ten years. Some of the newest installations are being done in
aluminum. It will be interesting to see which lasts longer.
Local street lighting poles have long been aluminum. Local power folks still
use wood. In Florida, precast concrete power poles have become common. (Anyone
for a concrete tower?)
I know of at least one local steel tower (amateur) that was properly installed
and maintained. It was taken down and moved to a new location, but failed
catastrophically while being raised because the hollow tubing had rusted out
from the inside. The outside had been kept painted, and the whole thing
appeared to be in good shape. Yes, there were drainage holes in the tower legs.
73 de Jim, N2EY
Dave Heil
Brian,
My 60' Universal aluminum traveling tower (22", 18", 18",14", 14", 14")
was purchased by me in 1977. It stood in two different Cincinnati
locations and has been used either complete or partially in 6 Foreign
Service assignments. It has been subjected to high wind, extreme heat
and salt air (coastal Africa); and to extreme cold and ice (Finland
during two assignments). In Botswana it carried a TA-34XL AND a 2
element Cushcraft 40m beam. When I finish in Tanzania it will be tower
number three at my West Virginia place. I'm still waiting for it to
fail.
The other two towers at my place in the states are steel--a 51 foot free
standing crankup and 70 feet of Rohn 25 guyed at two levels.
Dave Heil 5H3US, K8MN
Richard Harrison
"I`ve seen more rusted-out hot-dipped galvanized Rohn steel tower
sections than I can count. In the case of tubular construction there is
All the medium-wave broadcasters I`ve worked for had reformed-angle-leg
triangular towers. There is nothing hidden from view. They all used
galvanized towers although required by regulations to keep them smartly
painted in international orange and white. Never saw a rust problem with
any of these towers.
The international broadcaster I worked for used light-weight Wind
Turbine Corp. and Windcharger Corp. with solid rod and reformed angle
legs for rhombic antennas, and for the high stress support of curtain
arrays (When you lower one array the towers which supported the array
are still supporting its neighbors on either side without the balancing
force the lowered array provided.) This broadcaster used Stainless Corp.
tubular leg guyed towers.
We of course were curious as to how good Stainless` hermetic seal of the
leg sections really were. When a vehicle collision with a guy cable
resulted in a kinked tower section, we sliced open the legs on the
replaced tower section. The insides of the leg tubes were pristine after
a nunber of years of use. Not a speck of rust anywhere.
So, it all depends on how well the manufacturer has done his job. My
work for a conglomerate with hundreds of towers across the U.S. and
around the world almost always meant buying reformed angle leg towers
for the comfort of being able to examine all the surfaces. However, I
had an opportunity to buy a used Stainless Corp. self-supporting tower
with sealed tubular legs that had belonged to RCA and that I needed for
a microwave terminal right now, and I bought it in a heartbeat because
of my prior experience with Stainless. It was perfect, saved a ton of
money, and served like a champ for many years, until it was no longer
needed.
Brian Kelly
wrote:
>
>Brian,
>
>My 60' Universal aluminum traveling tower (22", 18", 18",14", 14", 14")
>was purchased by me in 1977.
>
Universal Dave?
>It stood in two different Cincinnati
>locations and has been used either complete or partially in 6 Foreign
>Service assignments. It has been subjected to high wind, extreme heat
>and salt air (coastal Africa); and to extreme cold and ice (Finland
>during two assignments). In Botswana it carried a TA-34XL AND a 2
>element Cushcraft 40m beam. When I finish in Tanzania it will be tower
>number three at my West Virginia place. I'm still waiting for it to
>fail.
>
is indicated . .
>The other two towers at my place in the states are steel--a 51 foot free
>standing crankup and 70 feet of Rohn 25 guyed at two levels.
>
>Dave Heil 5H3US, K8MN
>
Brian Kelly
(Richard Harrison) wrote:
>
>All the medium-wave broadcasters I`ve worked for had reformed-angle-leg
>triangular towers. There is nothing hidden from view. They all used
>galvanized towers although required by regulations to keep them smartly
>painted in international orange and white. Never saw a rust problem with
>any of these towers.
>
on a regular PM basis a steel tower should be remain solid off into
"infinity". For the most part however hams use the usual collection of
much less expensive and more compact tower designs like the Rohn stuff
which are impossible to fully maintain like the mongo windmills and
such are. Even worse most hams usually don't bother with painting the
outsides of their towers in the first place. In these parts unpainted
galvinized surfaces are history in 10-15 years unless they're painted.
>
The way I see it the primary beauty of aluminum towers for ham
applications is that aluminum towers are much better at defending
themselves from us hams than are steel towers.
>
Gotta watch drawing parallels between commercial radio and ham radio
in any number of respects.
>We of course were curious as to how good Stainless` hermetic seal of the
>leg sections really were. When a vehicle collision with a guy cable
>resulted in a kinked tower section, we sliced open the legs on the
>replaced tower section. The insides of the leg tubes were pristine after
>a nunber of years of use. Not a speck of rust anywhere.
>
the construction of those towers which, for all practical purposes, is
a bulletproof material in a bare naked unprotected state wrt to
rusting in back yards everywhere under just about all atmospheric
conditions what was the intent of the hermetic sealing??
>
>Best regards, Richard Harrison, KB5WZI
>
Dave Heil
>
> On Sat, 15 Apr 2000 21:12:59 +0000, Dave Heil <K8...@cats-net.com>
> wrote:
> >
> >Brian,
> >
> >My 60' Universal aluminum traveling tower (22", 18", 18",14", 14", 14")
> >was purchased by me in 1977.
> >
> What the heck is a "traveling tower"?? Might you have a URL for
> Universal Dave?
The tower which gets put in a 40 foot container and gets shipped from
country to country. I leave only the bases :) I don't have a URL for
Universal but they advertise in various ham mags. They're in Michigan.
Dave 5H3US, K8MN
Richard Harrison
"Assuming the use of one of the common 300 series stainless steels for
That may be what Stainless corp., a Pennsylvania tower builder would
like you to assume, at least at first encounter. Their towers are
usually constructed from the same steels as all the others for
competitive reasons. From what I`ve seen, they did a good job, and their
designs with tubular legs were exceptionally strong. We had a mechanical
failure in a curtain antenna and its tons of mass fell to the ground
abruptly. The four towers supporting the long line of arrays, at that
point, bowed like fishing poles with good hits, one pair of towers
bending towards its remaining curtain, and the other pair bending
towards its remaining curtain. It takes four towers to support the type
array we used, (4) dipoles in-phase reflected by )4 The repaired
curtain was hoisted back into place and the Stainless Corp towers
straightened on their own as if nothing had happened.
Brian Kelly
(Richard Harrison) wrote:
>Brian Kelly wrote:
>"Assuming the use of one of the common 300 series stainless steels for
>the construction of these towers---."
>
>That may be what Stainless corp., a Pennsylvania tower builder would
>like you to assume, at least at first encounter.
>
Yeah, we have a lot of bush-league scams like that around here. Heh.
>
>Their towers are
>usually constructed from the same steels as all the others for
>competitive reasons. From what I`ve seen, they did a good job, and their
>designs with tubular legs were exceptionally strong.
>
That'a a matter of how much steel you throw at a job like this which
of course translates into the depth of your pockets. It also explains
why they sealed the legs. That's pretty exotic stuff. How did they
join the sections? Couldn't use thru-bolts. Bolted flange joints?
>
>We had a mechanical
>failure in a curtain antenna and its tons of mass fell to the ground
>abruptly.
>
"Tons"? For just one element of an array? What WAS that beast?? Sounds
like an antenna I'd like to take into a dx contest on 80/160m.
>
>The four towers supporting the long line of arrays, at that
>point, bowed like fishing poles with good hits, one pair of towers
>bending towards its remaining curtain, and the other pair bending
>towards its remaining curtain.
>
There was either some good design or dumb luck involved in that one.
I'd like to have seen it. From a distance. Fact is that as long as you
don't exceed the yield stress particularly of the legs a structure
like that can safely survive huge completely elastic deflections and
come right back as if it never got bent. In the final analysis a
self-supporting tower is just a very big spring.
>
> It takes four towers to support the type
>array we used, (4) dipoles in-phase reflected by )4
>
. . . cq 'test, cq 'test . . .
>
>The repaired
>curtain was hoisted back into place and the Stainless Corp towers
>straightened on their own as if nothing had happened.
>
Now that I think about it those towers probably also had to deal with
the same nasty side loads in the process of putting the antennas up in
the first place. Somebody knew what they were doing.
>
>Best regards, Richard Harrison, KB5WZI
>
Brian Kelly w3rv
Richard Harrison
"What was that beast?"
It was four each 1/2-wave dipoles in a 4-quadrant array, with 1/2-wave
vertical spacing. An identical array right behind the driven array
served as a tuned parasitic reflector. The dipoles may have looked as if
folded, but in fact both wires were driven. Each dipole had 4 wires, a
pair in each direction which was shorted on both ends. Steel cables held
everything in place and gave the curtains most of their weight. The
whole mess was hoisted high to avoid ground losses. Elevation angle also
required antenna height. A.D.(Andy) Ring was the designer. ITT / Federal
Electric was responsible for the design and erection of the towers to
support these antennas. Everything met or exceeded expectations.
Best regards, Richard Harrison, KB5WZI rroaS.
Richard Harrison
installer. That was the wrong ITT subsidiary. It was ITT / Standard
Electric. Fedeal supplied transmitting tubes. Standard was the
engineering and contracting subsiriary.
Richard Harrison
"How did they join sections?"
Each tubular leg was sealed with a concentric disk flange on each end.
These had matching bolt-hole patterns. I don`t know if these flanges
were welded on in a dry nitrogen enviroment, but the insides of a well
used section looked as if they were.
Brian Kelly
>> What the heck is a "traveling tower"?? Might you have a URL for
>> Universal Dave?
>
>The tower which gets put in a 40 foot container and gets shipped from
>country to country. I leave only the bases :)
>
>I don't have a URL for
>Universal but they advertise in various ham mags. They're in Michigan.
>
>
>Dave 5H3US, K8MN
>
w3rv
kayak
stressed *below* its yeild point; steel does not.
Michael Edelman
------------------
http://www.foldingkayaks.org (nomadics)
http://findascope.com (buying a telescope)
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Brian Kelly
<mjeNO...@spamcop.net.invalid> wrote:
>There's some confusion on fatigue. Aluminum fatigues when
>stressed *below* its yeild point; steel does not.
>
Yeah right: Sorry, no cigar on this pass.
Gary Coffman
>There's some confusion on fatigue. Aluminum fatigues when
>stressed *below* its yeild point; steel does not.
Steel fatigues when stressed above its endurance limit.
Below that limit it can be stressed indefinitely without
fatigue failure. Aluminum does not have an endurance
limit, so any stress leads to fatigue. (In other words it
can only be fatigue free if subjected to zero stress.)
Endurance limit and yield limit are very different things.
All metals have a yield limit (beyond which they are
permanently deformed), but only some metals have a
well defined endurance limit (and that's well below
the yield limit).
Gary
Gary Coffman KE4ZV | You make it |mail to ke...@bellsouth.net
534 Shannon Way | We break it |
Lawrenceville, GA | Guaranteed |
Gary V. Deutschmann, Sr.
Interesting but not very relevant to real life.
Sorta like, who cares that an air-condition doesn't make cold air, it
only removes heat!
95% of antenna's are made of aluminum. They are subjected to
continual stress 24 hours a day, if not from breezes, just from the
shere force of gravity acting against them.
>Aluminum does not have an endurance
>limit, so any stress leads to fatigue.
These are all stresses to the aluminum!
Yet I don't see too many antenna's falling down or breaking.
Albeit the same cannot be said for aluminum airplanes, hi hi.....
What would the alternative be if all antennas were constructed of
steel? Don't think I would want to stand under it!
TTUL
Gary
Richard Harrison
"Albeit the same cannot be said for aluminum airplanes."
Well there are aluminum airplanes and then there are real aluminum
airplanes. Some planes fall from the sky that are new. Many DC-3`s are
still flying high after 65 years. They`ve had more ups and downs than an
Otis elevator.
There was a common demonstration of metal fatigue that engineering
students had to make and report on. A sample metal rod is chucked
between centers for rotation. A revolution counter is attached.
A variable measured load is arranged to bear on the rod in the middle of
its span. The rod is spun with the number of turns counted at different
loadings. The rod spins until it breaks or it becomes clear that it will
just about spin forever without breaking. This happens with some metals
(steel) when the rod isn`t overloaded. With enough loading, steel will
break. The problem is to find the threshold below which steel can be
loaded and run almost forever without breaking. Above this critical
point, it is found that the number of turns at which a sample of the
same batch of steel will break at a particular loading is quite
predictable within a reasonable range, of course. The heavier the load,
the fewer the number of turns it takes to do it in.
Arthur Unwin
Richard Harrison wrote:
> Gary Deutschmann, Sr. wrote:
> "Albeit the same cannot be said for aluminum airplanes."
>
> Well there are aluminum airplanes and then there are real aluminum
> airplanes. Some planes fall from the sky that are new. Many DC-3`s are
> still flying high after 65 years. They`ve had more ups and downs than an
> Otis elevator.
> I think we can say that the British airliner Comet did go down in India
> due to fatigue in the 50s
> There was a common demonstration of metal fatigue that engineering
> students had to make and report on. A sample metal rod is chucked
> between centers for rotation. A revolution counter is attached.
> snip
I remember it well but as a reminder, this test requires a very smoth sample
with no pre built in stresses which is not representitive of the real world.
I would imagine that a similar test based on a angular section of a tower
leg
or a circular pipe leg in castre would be a whole different ball of wax, add
in a surface scratch or a metal impurity, facter in buckling, torque and
every thing
will go to hell. Sometimes knowing to much prevents progress
In the tower case I would bank on empirical results based on experience
without hanging on to hard to academics.
Regards
Art Unwin
Brian Kelly
wrote:
>every thing
>will go to hell. Sometimes knowing to much prevents progress
>In the tower case I would bank on empirical results based on experience
>without hanging on to hard to academics.
>
Ya gotta start somewhere Art, the academic pushups provide the basis
for the design safety factor.
>
Your point with respect to stress risers is well taken, however =
don't put cyclic loads on dinged parts, beam elements with large
drilled holes, etc.
>
>Regards
>Art Unwin
>
w3rv
Richard Harrison
"I think we can say that the British airliner Comet did go down in India
That was attributed to inflation of the cabin then releasing the
pressurization over and over, I believe. DC-3`s don`t have cabin
pressure.
Aerolinas Argentinas lost all 3 of its new Comets in short order and I
think it had more to do with pilot error than fatigue. Goes to show that
if you are only going to use something a few times, maybe you don`t have
to worry about metal fatigue.
I have great respect for aircraft designers. They do an awesome job.
june
Could this be why aluminum type lightposts are not common
anymore. See lots of steel ones now.
Gary wrote:
> On Wed, 19 Apr 2000 08:39:10 -0700, kayak <mjeNO...@spamcop.net.invalid> wrote:
> >There's some confusion on fatigue. Aluminum fatigues when
> >stressed *below* its yeild point; steel does not.
>
> Steel fatigues when stressed above its endurance limit.
> Below that limit it can be stressed indefinitely without
> fatigue failure. Aluminum does not have an endurance
Brian Kelly
>Great info from all on this topic !
>
>Could this be why aluminum type lightposts are not common
>anymore. See lots of steel ones now.
>
That's not the case, aluminum is overall replacing steel in structures
like that even though the gap beween the cost of steel and the cost of
aluminum has increased. We have a bunch of new signal stands, signal
bridges and crossing gates along the local RR here and the new stuff
is aluminum. Aluminum structures cost a bunch more initially but the
paybacks are lower installation and maintenance costs. Ditto ham
towers.
>
w3rv
Gary V. Deutschmann, Sr.
Hey Guys
I wasn't refuting anything Gary said, as usual, he's right on the
money!
However, there is a point in both the billfold and in your aching back
as to whether aluminum or steel is the choice material to utilize, and
don't leave out PVC either!
There are well defined benefits and drawbacks to each type of material
in use for ham equipment. More often, how it affects the hip national
bank is how the decision is rendered as to which material is utilized.
Gramps had a steel spring seated patio chair, Pop's had an aluminum
with nylon webbing.
Gramps chair lasted some 45 years, Pop's chair only about 5 years.
Gramps chair at todays prices is around $199.00, Pop's $3.99 at
WalMart.
When Gramps chair seat broke, it cost something like $2,300.00 to have
is rear end stitched back up from the deep lacerations the spring
steel sharp edges made in his buttocks.
Pops on the other hand just felt a little uncomfortable when the
webbing broke and snapped his family jewels a good one. He then opted
to replace the chair with a $5.99 special, slightly sturdier.
In the long run, I think the el-cheapo Grandpa Pidgeons Special won
out over the tried and true steel! <G>
TTUL
Gary
Brian Kelly
Deutschmann, Sr.) wrote:
>
>When Gramps chair seat broke, it cost something like $2,300.00 to have
>is rear end stitched back up from the deep lacerations the spring
>steel sharp edges made in his buttocks.
>
>
>Pops on the other hand just felt a little uncomfortable when the
>webbing broke and snapped his family jewels a good one. He then opted
>to replace the chair with a $5.99 special, slightly sturdier.
>
>In the long run, I think the el-cheapo Grandpa Pidgeons Special won
>out over the tried and true steel! <G>
>
>TTUL
>Gary
>
better from here.
>
w3rv
Bob Lewis
> >anymore.
I see aluminum lightposts all over the place. Many more than steel.
Maybe aluminum only works in Virginia.
Richard D. Reese
> >> What the heck is a "traveling tower"?? Might you have a URL for
> >> Universal Dave?
Universal is not on the web. They do have e-mail un...@voyager.net . Their
phone number is 810-463-2560 Voice and 810-463-2964 Fax. They are located
in Clinton Township Mi.
I have an 80 foot free stander from them that is a 30 inch face with 1/4
inch thick walls at the base. It is in 8 yards of concrete and has about 12
sq. feet on it. I watched it in a 60 mph wind and there was just
perceptible movement at the top. I have used steel towers since 1964 but I
am now sold on the aluminum. Photos of the old guyed steel tower and the
storm damage my be seen at http://wa8dbw.ifip.com/Tower.htm
For those interested, photos of the aluminum free standing tower are
available at http://wa8dbw.ifip.com/Towerpix.htm
73
--
Richard D. Reese WA8DBW
http://wa8dbw.ifip.com