RE: ADM Evaluation of Trac Beam
Brian Hall
I like the "Freudian Slip". :)
I would say that the concerns are based on cost comparison. If a 2x5 Trac
can do what a 2x7 Legacy can do (or better) and a 2x7 Trac can do about what
a 2x8 Legacy can do (or better - as the span charts seem to reflect) then
that can add up to some significant savings in cost in the long run. Since
this industry is so fiercely cost competitive then those dollars (however
significant or insignificant it may be) means a lot to the contractors. It
always comes down to the money.
As I am not qualified to make assumptions in this matter, I would ask the
following question; Are the spans that are given in the span charts on the
Trac Beam's website based on the members being simply supported with out any
combined loading being taken into account? If that's the case then would
the spans necessarily come down when combined loading is taken into account
(depending on what combination of members is being used)?
--
Brian
-----Original Message-----
From: Aluminum-Struct...@googlegroups.com
[mailto:Aluminum-Struct...@googlegroups.com] On Behalf Of Steve
Sincere
Sent: Sunday, June 10, 2007 9:49 AM
To: Aluminum Structure Contractors
Subject: Re: ADM Evaluation of Trac Beam
Okay, just one last post.
I compared the allowable bending moment from the Trac Beam data sheet
to what I calculate for legacy shapes (6063-T6). What I find is that
the 2x5 Trac Beam is better than what I calculate for a 2x7
legacy...according to the data. Also, the 2x7 Trac Beam is almost on
par with the legacy 2x8.
So, what's all the fuss about? I saw lots of jobs with 2x9 and larger
beams go down. Swapping out 2x7's and 2x8's with Track beams is not
going to solve the problems.
I guess the concern is that someone might be claiming they can do more
with less metal, but again, you have to look at the whole cage.
Besides, we may just get a full scale test of all the design revisions
this season.
Let the beams...I mean chips fall where they may.
Steve Sincere
On Jun 9, 2:13 am, <ssinc...@bellsouth.net> wrote:
> It dawned on me that when I was doing the spreadsheet for the last post I
was considering the unbraced length to be the entire span. (When doing
designs I let my programming work that out since unbraced lengths change
from design to design.) However, both the technical data sheet and the
product approval make no mention of unbraced length and the schmatics of the
loading indicate no lateral bracing either. Technically speaking, both of
these document could be interpreted as to be used applications without
lateral bracing.
>
> Regardless, I adjusted my evaluation spreadsheet to consider an unbraced
length in the minor axis direction (used for lateral buckling due to bending
and possibly compressive bucking...depending on which kL/r is higher). I
used a value of 8 feet since this is probably the maximum purlin spacing
anyone would use. The revised spreadsheet is attached.
>
> Note that nothing changed when considering flange buckling where one edge
is free. However, the allowable moments when both flange edges are supported
goes up, although the limiting moment is still member buckling. The numbers
match better...
>
> ...but then I noticed an erratic allowable moment profile in the data. I
decided to expand the moment calculation tables and plot the curves for each
beam on subsequent worksheets. What I saw defies logic. The curves would be
flat if the flange buckling did limt as suggested by the interpretation of
the ADM. However, the curves would also be flat if consistently spaced
lateral bracing was used. And, the curves start high at minimum span and
fairly consistently drop with increase in span if there were no lateral
bracing.
>
> So, now my question is, "How were the test specimens supported in the
lateral direction?"
>
> And one more final thing I did. I tried to find the best possible factor
of safety that could be calculated by perfect test data...no scatter. I come
up with 2.73 and included the calculations on the last worksheet. I would
appreciate it if someone could double check this for me.
>
> At this point, I'm more than a little confused.
>
> Steve Sincere
>
> TracBeam_Eval.xls
> 39KDownload
Tom Matson
Has Do Kim's engineering been subject to a peer review?
If so who reviewed it and what was the consensus?
If not, why not?
Tom Matson
(561)376-4012
Brian Smith
I went to a job that the homeowner was mad about to review the work
and some of the things they did based on the detail made me afraid to
stand under the enclosure. Attached 2x10 beams to the fascia on a 45
degree where only one side of the beam connected to the fascia and the
other side was about 1/2" away and only had a 1/2 edge distance on the
screw that held the angle on and the fascia was seperating from the
sub fascia or truss ends.