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# RE: Boggled Out By Load Combinations

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Boggled Out By Load Combinations
• From: "Bill Allen" <T.W.Allen(--nospam--at)cox.net>
• Date: Fri, 16 Apr 2004 09:57:37 -0700

```Michael -

I wasn't being intentionally mysterious. It's just that, if I presented a
lot of detail about the project, responses could have been more oblique
(offers of other solutions rather than addressing the original question -
common occurrence on this list). I just tried to simplify the issue as much
as possible so that I could get the most direct response. That's all.

The project:

I've got two buildings (pretty much identical) separated by 12 feet. The
architect wants to put a clerestory roof element (with very little glazing)
over the 12 foot separation which pops up about 8 feet. This element will
form a high volume corridor. I intend to put steel tube frames (upside down
"U"s) at 12 feet on center with each leg resting on the "exterior" walls of
the two buildings. If the tubes are small (4x4x1/4), they will be pretty
flexible and not transmit much force to the existing building. I don't want
it to act as a tie, but I want to be able to accommodate the loads.

The maximum Delta_sub_m is 7.87 inches. If I include a load case of a
support displacement of 7.87/1.4 = 5.62 inches and allow a 1/3 increase in
allowable stresses, the TS4x4x1/4 frame is O.K. In addition, the shear at
the base of the frame for this load case is about 1,700 lbs, which should be
a reasonable load to detail, (although I would rather detail for Delta_sub_s
divided by 1.4). This issue is my original question, just to stay on point.

With regards to performance, there is a cant strip where the base of the
tubes sit and I've warned the architect that there's going to be some
movement. Hopefully, he'll provide some high quality "elephant snot" to
accommodate the movement. Also, at the end of the building, we are providing
a seismic separation where there is an end wall condition at the corridor.

Hope this paints a clearer picture.

T. William (Bill) Allen, S.E. (CA #2607)
ALLEN DESIGNS (http://www.AllenDesigns.com)
San Juan Capistrano, CA

:-----Original Message-----
:Sent: Tuesday, April 13, 2004 12:32 PM
:To: seaint(--nospam--at)seaint.org
:Subject: Boggled Out By Load Combinations
:
:Bill Allen wrote:
:
:"Michael -
:
:I don't believe I ever said steel, but, even if it was steel, couldn't =
:it be a steel spring?
:
:Regardless, that's off the point. Are you saying that I don't have to =
:design for a any force due to the relative movement ("multiplier is
:zero")?
:
:I was hoping I could design for Delta_sub_s divided by 1.4, but as it =
:turns out, designing for Delta_sub_m divided by 1.4 is possible. I just
:don't = feel this is the right value to design to. After all, I don't
:have to design = any other member in the structure for a strength to
:withstand a deflection = of Delta_sub_m, just provide enough space
:between the structures to = accommodate this deflection."
:
:
:Bill,
:You didn't say steel, but you talked about ASD and about a member thin
:enough to act like a rubber band.  That sounds like steel to me, unless
:it's some kind of rope or cable.  Or, I suppose, a steel spring.  Geez,
:you're being mysterious.
:
:I'm not saying you don't have to design for any force.  I'm saying I
:can't imagine how you can, so it seems like you have to detail the
:member to slide or otherwise get out of the way of the movement.  To me,
:this is what "provide enough space between the structures to =
:accommodate this deflection" leads to.
:
:So, what is it?
:
:Mike Hemstad, P.E., S.E.
:TKDA
:St. Paul, Minnesota
:
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```