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Wind uplift forces must be determined by the design professional and shown on the contract drawings as NET UPLIFT. (The net uplift force on the roof joist is the gross uplift minus the dead load including the joist weight.) This temporary reversal of loading creates compression forces in the bottom chord which, as a result, may require lateral bracing. The Steel Joist Institute (SJI) recognizes this by specifying a single line of bridging near the first bottom chord panel point to brace the bottom chord. The remainder of the bottom chord must be checked by the joist company (NCJ) to see if the SJI standard bridging is sufficient to brace the members in compression. The webs (diagonal members) of the joist can also be subject to stress reversal and this may require a reduction in the end panel space to accommodate the resulting compression in the end web. Thus the web layout may change from the standard dimensions published in the New Columbia Joist Company catalog. The modified joist model
is checked for the normal downward loading of the dead plus live loads and the worst case is used to determine the joist components.
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