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

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Analyzing Existing Joists for Snow Drifts....
• From: "John Riley" <jriley(--nospam--at)cbbel.com>
• Date: Tue, 11 Jan 2005 14:51:45 -0600

Possibly I've interpreted something incorrectly, but my rough calculations
indicate the point of zero shear shifts only 3.44".  Making no attempt to
answer your question of "how far is OK", I submit that this shift is less
than the example you're following.

John Riley
------------

An architect is adding an addition to a school that will cause snow drifts
on existing roof joists and an existing metal building's roof.  At Roof 1, I
have 22J8 @ 4'-0" on center spanning 40'-0".  The 60-year SJI Manual lists a
capacity of 225 plf, maximum end reaction = 5000 lb, and maximum moment =
540,000 in*lb.  The addition is causing a snow drift of 40 psf for a length
of 13.6' from the support.  With an existing DL=22 psf, balanced SL=23.1
psf, I'm calculating M = 426,960 in*lb and V = 4300 lb.  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 of zero shear moving from
the center of the joist.  I'm using a 1996 National Steel Construction
Conference paper written by D. Ed Ryan of Vulcraft.  In this example, he
states that since the location of zero shear is only 6" off center in his
example, the possible shear reversal is insignificant.  My question is, how
much distanc!
e off of the original zero shear location is considered "insignificant"?

I've been told by another engineer that he usually field measures the joist
dimensions, chords, and web members, analyzes the truss on RAM for the
original loads, then adds the snow drift to determine how the forces have
changed throughout the truss.  If reinforcing is required, he adds a
continuous plate to the bottom chord, welds round bar to the top chord
angles, and reinforces the web members with angles as required.  This seems
like a lot of work if there is another method that could be 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.

Any help is greatly appreciated.
Dan

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