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Re: Analyzing Existing Joists for Snow Drifts....

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I tried to reply to this the other day but I don't think it made it, I had thought about the same things regarding the existing joist as Paul has stated.

This is a good point, the buiding may not be working under current Code or the Code it was built under prior to the addition of the snow on the roof. I believe the IBC (if that is your Code) provision is that if a modification to an existing building causes an increase in force to an element of 5% or greater, you must bring the element up to current Code. This may include the frames and the footings as well.

But without information on the existing system it is going to be a challenge to determine what effect the new snow is going to have on the stability of the entire building. This is the kind of project I have been stuck with before. An assumed small analysis that was given to me with a small engineering fee turns into a much bigger issue. Then I have to explain that why they won't have their answer tomorrow.

I knew a guy who would take this kind of project and seal "only the existing joist and it's modifications" part of the existing building and place this disclaimer on his drawings, then show joist reinforcing required and not look into the rest of the system. To me this is unethical because you have only done part of your duty to protect the public.



Will Haynes, P.E.




From: ad026(--nospam--at)hwcn.org (Paul Ransom)
Reply-To: <seaint(--nospam--at)seaint.org>
To: seaint(--nospam--at)seaint.org
Subject: Re: Analyzing Existing Joists for Snow Drifts....
Date: Sun, 9 Jan 2005 12:32:00 -0500

> From: "Daniel Boltz" <dboltz(--nospam--at)1st.net>
> To: <seaint(--nospam--at)seaint.org>
> Subject: Analyzing Existing Joists for Snow Drifts....
>
> An architect is adding an addition to a school that will cause snow drifts

> These are both less than the allowable loads. My problem is that I cannot
> determine if the snow drift is causing a shear reversal due to the point

> insignificant.  My question is, how much distanc! e off of the original
> zero shear location is considered "insignificant"?

I assume that you are concerned about a force reversal to compression in
the OWSJ web member. This is not necessarily bad. Magnitude is the real
issue.

Your first ballpark estimate would be to consider how far from centre
the zero shear has moved in relative terms. That will give you a good
feel for the possiblility of detrimental web load reversal effects (e.g.
6"/(40'/2) = 0.025, 2.5%). Looks trivial to me.

Secondly, how far is that with respect to the chord panel points?
Typically, there will not be any gravity load related difference in web
load direction if the new zero shear is within the same "panel".

> utilized. I'm kind of at a loss for the Metal Building as 1, they usually
> have terrible lateral drift limitations which the addition's expansion
> joint is not currently accommodating, 2, the foundation construction is
> unknown, and 3, the original manufacturer is out of business and the
> school has no existing drawings for the frame.  Short of field measuring
> the frames and purlins and reanalyzing the structure, I don't know of a
> good, safe route to reinforce the existing frames.

Without good background data on the structure and foundations, there is
little about such a renovation to which I would put my seal. You should
also consider some material tests.

--
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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