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Re: URM

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At 15:39 1/8/98 -0800, Mark D. Baker wrote:
>Thanks to Dennis for responding to my earlier post. I am suprised no one
>else has an opinion regarding collectors for crosswalls.
>
>By definition (see UCBC commentary) "A crosswall is not considered as a
>shear wall in that it need not be designed for the tributary loads of
>the flexible diaphragm or diaphragms. The crosswall decreases the
>displacement of the center of the diaphragm relative to the shear walls
>and wil provide damping of the response of the diaphragm to earthquake
>shaking."
>
>My take on this is that installation of collector elements would not be
>appropriate in that the crosswall is not carrying (collecting) trubutary
>load of the diaphragm. The code requirement for the connection of these
>walls to the diaphragm is only that they be connected for the crosswall
>capacity. In most cases this can be achieved within the length of the
>crosswall.
>
>Have I sufficiently baited anyone else to respond ? :)
>
>Regards,
>
>Mark D. Baker
>Baker Engineering


[Bill Cain]  The UCBC Commentary cited ["Commentary on Appendix Chapter 1
of the 'Uniform Code for Building Conservation[tm]', Seismic Strengthening
Provisions for Unreinforced Masonry Bearing Wall Buildings" prepared by
SEAOC, June 20, 1992, which applies to the 1991 edition of the UCBC] goes
on to say:

Re: Existing Crosswalls:
" The capacities of the connections of the existing crosswall to the
diapharagms above and below need not be investigated if the crosswall
extends to the diaphragm level above.  If the crosswall only extends to
ceiling joists that are separate from the floor or roof framing above, then
continuity of the cross wall must be provided.  If the cross wall extends
to a ceiling that is applied directly to the roof or floor framing, the
capacity of the connection need not be investigated.  Footnote 2 of Table
No. A-1-C limits the total capacity to 300 lbs per foot regardless of the
combined capacity of the existing materials on the cross wall.  This
limitation was provided in lieu of an investigation of the capacity of the
connection of the crosswall to the diaphragm."

and Re: New Crosswalls:
"Connections of new cross walls to diaphragms shall be designed by the
applicable sections of the Building Code.  This includes the design of a
vertical connection of the end of the crosswall to the foundation, or the
crosswall or support below the level of the crosswall.  This connection is
required to protect the floor framing from wall overturning effects.  The
dead load of the floor or roof framing supported by the crosswall cannot be
used to resist the calculated overturning due to lateral forces.  However,
the calculated force at the end of the crosswall need not use an
overturning moment accumulated from more than two stories including the
level of calculation."

[My comments]  Although the intention of the UCBC process is not to require
investigation for connection existing crosswalls to the diaphragm, we must
remember that the purpose of the cross wall is to reduce the diaphragm
deflections and provide damping through hysteretic behavior.  To do so,
there has to be a connection that can take some level of load.  Although
not required by UCBC,  I personally have always tried to be sure a
connection does exist capable of the 300 lbs per foot allowable crosswall
capacity.  The details (framing clips, etc.) usually are not difficult or
expensive to achieve.   Most of the hysteretic behavior comes from
deformation and progressive failure of the crosswall materials, not the
connection.  Also, providing a reasonable and defineable load path, even if
it is not formally designed to a particular force level, has always made me
more comfortable.  If you look at the typical construction practices for
the floor system of URM buildings, some method of forces transferring
throughout the width of the diaphragm is usually possible to reason out so
that although a formal collector may not exist, one can ususally find a
reasonable force path that accomplishes the same effect if the diaphragm
spans are not too great (hence the spacing limitation on cross walls).

The one guiding principle I use in URM's as with any building is:  TIE THE
STRUCTURE TOGETHER!   

    
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BILL CAIN, SE
OAKLAND, CA
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