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Re: Seismic Bracing for Pipe Risers -- 1965

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Robert,

That was my first thought too, but if those connections (at the web of beam
and riser sleeve) behave as pins isn't the little "box" we're creating
unstable?  Seems to me that there must be a shear component (for big risers
near the top of the building it's four or five thousand pounds in each of
the two plates), and the only way those shears can be transferred from riser
to web is through weak axis flexure in the plate -- e.g., end moments at the
welds.

         =================   Web of Beam

             <---         <---            Shear = P / 2
             --->         --->
              (   )         (   )              Moment = ( P / 2 ) x ( e /
2 )
                              ^
                |              |        (Generated Force Couple)
                |              |
               o             o        Points of Inflection (Near Mid-length)
                |              |
                |              |         (Generated Force Couple)
               V
              (   )         (   )              Moment = ( P / 2 ) x ( e /
2 )

             <---         <---            Shear = P / 2
             --->         --->

               -------->O           Seismic Lateral Force at Riser = P
                                           (At Distance "e" from Web of
Beam)

I'm therefore thinking its a simple statics (stability) problem and that
assuming one doesn't want to assign rotational resistance to the one-sided
fillet welds there is no need for an FEM model or any other analysis -- the
thing just doesn't work.

Drew

----- Original Message -----
From: "Rogers, Robert" <Robert.Rogers(--nospam--at)Woolpert.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Monday, December 18, 2000 1:59 PM
Subject: RE: Seismic Bracing for Pipe Risers -- 1965


> 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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