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RE: Seismic Bracing for Pipe Risers -- 1965

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Drew,

Doubt it a "design guide" exist for this contraption.  Sounds like a solid
mechanics problem where one really has to take a close look at local
stresses (plate type FEM modeling).  Trying a simplified approach one may
consider that the lateral force component at the center of mass of the riser
(i.e., centroid of the pipe) would generate a shear and tension force on on
the welds (one side) and would put the other plate into bearing against the
web.  This would happen like so:


        =================   web of beam
                   ^
              |    |
              |    |         Plates for Attachment (Generated Force Couple)
              |    |
             V

    -------->0           lateral force at center of riser

This considers the two plates acting as a composite unit.  Interesting
problem.... my suggestion is just an off-the-cuff thought.....

Robert C. Rogers, PE
robert.rogers(--nospam--at)woolpert.com

-----Original Message-----
From: Drew A. Norman, SE [ mailto:DNorman(--nospam--at)dnormanse.com
<mailto:DNorman(--nospam--at)dnormanse.com> ]
Sent: Sunday, December 17, 2000 7:23 PM
To: SEAINT List Service
Subject: Seismic Bracing for Pipe Risers -- 1965


Calling old high-rise hands:

My firm has been engaged by the owner of an existing (35 year-old) high-rise
building in seismic zone 4 for the purpose of evaluating and improving the
earthquake survivability of its HVAC system.  One concern is lateral support
for pipe risers (up to 12" diameter) which carry both heated and chilled
water up and down the tower.  Reference documentation locates but does not
detail "guides" which the original designers provided to laterally support
the pipe.

Field observations at a few (we believe typical) locations reveal that these
consist of a short (6"-12") pipe sleeve, several inches larger in diameter
than the OD of the pipe they "guide" (internal stand-off bars restrain the
riser).  The assembly is supported by a pair of vertical steel plate
outriggers 3" to 6" apart and 4" to 12" deep.  These are welded to the web
of a wide flange beam that "trims" the shaft opening in the floor and to the
outside of the sleeve (typically 3" to 6" clear of the edge of the shaft).
They are relatively light (say 1/4") and have fillet welds on one side only
at both the beam and the sleeve.

I have seen this kind of support before but have not had occasion to analyze
one, and I don't have any documentation for capacity.  Does someone out
there recognize what I am describing and (I should be so lucky) have an old
design guide or something that might help us decide how much load one of
these things can safely take?  My principle concern is for restraint of
motion parallel to the side of the shaft.  Since it seems to me that those
single-sided fillet welds must be treated as "pinned" connections, the
assembly doesn't seem to have much ability to resist transverse load.

Any comments or suggestions would be appreciated.

Drew A. Norman, S.E.
Drew Norman and Associates
Pasadena, California


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