Span for 4X4 (!) joists
mgl...@lamar.colostate.edu
an old house and bizarrely framed. To make a long story short, there's
only 3.5" of room for joists. My plan is to go with doubled 2X4s for
joists, 16" o.c., across a 70" span. Anybody out there with a span
table? Does this sound reasonable?
Thanks.
=-=-=-=-=-=-=-=-=-==-=-=-=
Mike Lacy, Sociology Dept. voice (303) 491-6721
Colo. State Univ.
Fort Collins CO 80523 fax (303) 491-2191
patterson,george r
<mgl...@lamar.ColoState.EDU> wrote:
>I'm replacing the rotted joists and floor underneath my bathroom. It's
>an old house and bizarrely framed. To make a long story short, there's
>only 3.5" of room for joists. My plan is to go with doubled 2X4s for
>joists, 16" o.c., across a 70" span. Anybody out there with a span
>table? Does this sound reasonable?
My span tables and beam calculation tables all start at 6" wide material.
Since 2x6 material is fine for a 6' span, you should be ok with what you
have in mind. If you want to play it safe, rip some 3.5" x 70" strips of
good plywood and sandwich them between the 2x4s for stiffening.
-----------------------------------------------------------------------
| - The 90/90 Rule -
George Patterson - | The first 90% of the work takes 10% of the time.
| The last 10% of the work takes the remaining 90%
| of the time.
-----------------------------------------------------------------------
Gary Heston
>I'm replacing the rotted joists and floor underneath my bathroom. It's
>an old house and bizarrely framed. To make a long story short, there's
>only 3.5" of room for joists. My plan is to go with doubled 2X4s for
>joists, 16" o.c., across a 70" span. Anybody out there with a span
>table? Does this sound reasonable?
My pocket reference doesn't show anything narrower than a 2x6, but those
would have slightly less cross-section than doubled 2x4s (8.25 in^2 as
opposed to 10.5). Assuming your local codes consider them to be adequate
substitutes, these are the spans for 2x6 joists on 16" centers adequate
for a 40lb/f^2 load:
S. Pine/ D. Fir 10'
Western Hemlock 9.5'
Spruce 9.08'
So, I'd think you'd be quite safe with only a 70" span.
--
Gary Heston, at home..... ...which is now running Unix... :-)
ga...@cdthq.uucp or ...uunet!sci34hub!cdthq!gary (for clueless MTAs)
The brain is a mass of cranial nerve tissue,
most of it in mint condition. Robert Half
stan e black
Gary,
My tables indicate 2 x4's will work also, however, be careful with your
assumptions about cross sectional areas as indicators of strength. At 6'
spans and 1000 psi, Fb, a 2 x6 will carry 840 lbs while 2 @ 2 x4 will
only carry 680 lbs even though the area is greater. As in real estate,
location ( of the material in this case) is very important.
Stan Black, Architect, Boston
mgl...@lamar.colostate.edu
------ about using doubled 2 X 4 joists across a 6 ft span.
Thanks to all of you for your help. It is interesting that a single
2 X 6 will carry a heavier load than a double 2X4.
This suggests a different, and possibly better solution to my problem, to
which I'd appreciate your reactions. I *could* notch the end of a single
2 X 6 and get it to fit my situation:
[ side view of the notched 2 X 6 joist]
<----------6ft-------------------->
----------------------------------|
| ^ |
| | 3.5" |
|_______V |
| |
<--3"-->|------------------------|
=========
||
|| (foundation wall)
||
Might this actually be stronger than a doubled 2 X 4, without any
notching of the end?
patterson,george r
<mgl...@lamar.ColoState.EDU> wrote:
>This suggests a different, and possibly better solution to my problem, to
>which I'd appreciate your reactions. I *could* notch the end of a single
>2 X 6 and get it to fit my situation:
>----------------------------------|
>| ^ |
>| | 3.5" |
>|_______V |
> | |
> <--3"-->|------------------------|
There's a tendency for the joist to split starting at the "V" in this
drawing. There are a number of ways to prevent this, the easiest of which
is to nail and glue a layer of thin plywood on both sides. You'd have
something like this -
----------------------------------|
| | |
| 1/4" plywood | |
|________ | |
| | |
|------------------------|
X
The method used in timber frame construction (where appearance of the
framing counts heavily) would be to drill a 3/4" hole vertically about
where the "X" is in my sketch and glue in a 3/4" dowel. The hole and dowel
would run completely through the joist. Some tradesmen would drill such
a hole at an angle (up towards the center of the joist).
-----------------------------------------------------------------------
|
George Patterson - | When you're working on a project and drop
| something, it will never reach the ground.
|
-----------------------------------------------------------------------
Peter Amick
doubled 2x4s are not as strong as a single 2x6. It's the depth of
material that counts most here, not the width. There is another way to
make up for some of the lost strength, though, and that is to increase
the frequency of supporting members, e.g. 12" centers instead of 16".
This will tend to spread the load and to some extent compensate for the
shallowness of the members.
Bruce R. Miller
In article <Dec28.171...@yuma.ACNS.ColoState.EDU>, mglacy writes:
> ...
> ----------------------------------|
> | ^ |
> | | 3.5" |
> |_______V |
> | |
> <--3"-->|------------------------|
>
> =========
> ||
> || (foundation wall)
> Might this actually be stronger than a doubled 2 X 4, without any
> notching of the end?
Seems like it would be stronger at the middle, but only slightly so at
the ends. As another poster said, I'd expect it to split at the corner
of the cutout.
But here's another idea how to relieve that: use joist hangers. That
would remove the load from the trimmed ends of your joists.
Only problem is that the vertical ends of the hangers have only got ~2"
to nail to. Probably not enough. Course you could use 2x8s ...
John Eyles
It thoroughly answers all the questions raised in this thread.
It doesn't have joist tables; rather, it gives you the
formulae for computing them !
It can be obtained from John Wiley and Sons, InterScience 908-469-4400.
It's hideously expensive, but tells you EVERYTHING you would ever
need or want to know (and much more) about building with wood.
For an engineer who works with wood it's a fascinating book.
patterson,george r
>I heartily recommend the Timber Construction Manual.
>need or want to know (and much more) about building with wood.
You might look for _Building the Timber Frame House_ instead. At $12.95,
it's not hideously expensive and contains at least most of the info in the
Timber Construction Manual. The beam calculation formulae in it *are*
limited to oak, Douglas fir, Southern pine, and spruce, however. If you
plan to work with another wood, perhaps the Timber Construction Manual
would be a better choice.
Published by Charles Scribner's Sons, New York. Ted Benson author.
ISBN 0-684-17286-0.
At the time I bought my copy, it cost me a couple hours' wages, but the
illustrations alone were worth every penny! There's a hammer beam truss
roof in there that's out of this world. The discussions of joints and
the strengths and weaknesses of each type are good education for cabinetry
too.
I saw a copy for sale last year at the New Jersey Woodworkers' Show, so
it should be still in print.
-----------------------------------------------------------------------
| Love, n; A form of temporary insanity curable either
| by marriage or by removal of the afflicted from the
George Patterson | conditions under which he incurred the condition. It
| is occasionally fatal, but more often to the physician
| than to the patient. - The Devil's Dictionary -
-----------------------------------------------------------------------
Chris Lewis
|In article <Dec28.171...@yuma.ACNS.ColoState.EDU>,
| <mgl...@lamar.ColoState.EDU> wrote:
|>This suggests a different, and possibly better solution to my problem, to
|>which I'd appreciate your reactions. I *could* notch the end of a single
|>2 X 6 and get it to fit my situation:
|>----------------------------------|
|>| ^ |
|>| | 3.5" |
|>|_______V |
|> | |
|> <--3"-->|------------------------|
^ ^
|There's a tendency for the joist to split starting at the "V" in this
|drawing. There are a number of ways to prevent this, the easiest of which
|is to nail and glue a layer of thin plywood on both sides. You'd have
|something like this -
We had occasion to splice 4x12 cedar beams (supporting a 2x6 T&G roof in
60lb/sq foot snow load territory) in "mid-air", as it were, and used something
a little different.
[We were extending the roof out an additional 10 feet, and the beams had
to be extended to hold it up first. They were 4x12s on 8 foot spacing.]
---------------------------+---------------------
|
+------+
|
--------------------+----------------------------
D e a A a b C
There were support points at D and C, where D-A was about 8 feet, and
A-C was about 2 feet. Ie: the D-A section was resting on A-C. (roughly
like the notched joist we're discussing, where A-C is the joist support)
In addition to putting two large carriage bolts with large washers
vertically through the tongue at both "a"s, we put another vertically
through the C-A section of one of the beams at b, purely to stop the
beam from splitting - it already had a crack. My father (the civil
engineer) wasn't entirely convinced that it was necessary, but we did it
anyways. And the inspector liked it. 20+ years later, it's still
standing - including the three beams that didn't have the anti-split
bolt. I suggested putting bolts at "b" in all beams, plus another
at "e", but my father overruled me. My father overbuilds as it is,
and I'm *much* worse... ;-)
[I believe that the 4x12 were oversized for the 10 foot span - they
already had a ~20 foot span to the right of C - back of envelope
calculation shows that the beams were designed for up to 9600 total
pounds over a 20' span]
[The T&G succumbed to carpenter ants in a few places, but that's a
different story...;-)]
This isn't entirely applicable, but I thought it'd be interesting anyways.
It seems to me that a couple of carriage bolts at the "^" in the first
drawing might serve just as well or better than George's suggestion, and
be a little easier. Especially, since incautious glue selection might be
a problem if the floor ever gets wet....
Consult an inspector on this one.
Another approach which might also be useable would be to put another
cap plate and stud wall or some other support (cleated double 2x6 or 2x8
beam plus posts) underneath the joists at the "^". Then you certainly
wouldn't have to worry about the joists splitting.
--
Look on the bright side - at least the PC's reached gender parity!
Chris Lewis; cle...@ferret.ocunix.on.ca; Phone: Canada 613 832-0541
Ferret list: ferret-...@ferret.ocunix.on.ca
Latest psroff: FTP://ftp.uunet.ca/distrib/chris_lewis/psroff3.0pl17/*
lmi...@indyvax.iupui.edu
6th edition by M.F. Spotts (iused this a textbook back in MechEng school)
The term "Moment of Inertia" is used to describe the quantative effect that the
shape and dimensions of the cross section of a beam have upon the stress, strain
and deflection behavior of that beam. In the situation of a floor beam, the
moment of inertia of the cross section about the horizontal axis in the center
of the beam is of key interest. for a rectangular beam the formula for the
moment of inertia is I = (width*depth^3)/12. Thus as the height of the beam
rises the Moment of inertia increases to the third power. In evenly loaded
beams with ends on simple supports, deflection is infersely
proportional to the moment of inertia of the beam .
for doubled 2x4's the moment of inertia (in english units) is 10.7 in^4?
for a single 2x6 the moment of inertia is 15.6
assuming nominal 1.5x3.5" 2x4's and 1.5x5" 2x6's. Cutting the 2x6's down to
2x4 at the ends should not hurt because the maximum bending moment (and tensile
stress) is in the center of the beams. This is why 2x6 walls are as strong as
2x4 walls even though the 2x6's are on 24" centers.