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Re: Diaphragm Chords

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Jim Kestner wrote: 
 
>To talk in practical terms, let's say we have a building with steel joists, 
metal deck and load bearing masonry walls. The joists span to the long walls 
(span parallel to the short walls).  The bond beams and the reinforcing are 
cut at each masonry control joint (so that it cannot act as a chord).  How 
does the building work to transfer the lateral forces?  I believe the deck 
works as the chord parallel to the long wall.  The first joist in from the 
short wall (parallel to the short wall) probably works as the chord in the 
other direction. < 
 
In my opinion, the concept of chord forces to resist flexure in a diaphragm
is 
partially based on an assumed greater relative stiffness at the chords.  
Otherwise, say in a uniform concrete slab, it seems it would act more as a 
deep beam than as concentrated chord forces.  (Although admittedly this 
greater stiffness may not always occur in sub-diaphragms.)  Thus I would 
always provide a definite chord element in design.   
 
I wouldn't rely on metal deck as a chord member even in the longitudinal 
direction, since it would be weak in compression and would not have enough 
stiffness to "concentrate" the tensile load at the edges.  I specify masonry 
bond beams to be continuous at roof levels, to develop this chord action,
even 
though interrupted at control joints at other locations.  Perhaps the end 
joists could be used as chord members, but the top chord of the joist must 
then be designed for the added tension/compression.  I prefer to anchor the 
deck to the CMU walls and use the CMU bond beams for chords.   
 
An improperly designed building may in fact act in the general manner you 
described, but I would not want to count on it in extreme load conditions.