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Re: Diaphragm Chords[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Diaphragm Chords
- From: "Bill Sherman" <SHERMANWC(--nospam--at)cdm.com>
- Date: 03 Dec 98 15:21:22 -0500
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.
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