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Rigid wood diaphragms, more to know[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "seaint(--nospam--at)seaint.org" <seaint(--nospam--at)seaint.org>
- Subject: Rigid wood diaphragms, more to know
- From: merrick group <merrickgroup(--nospam--at)compuserve.com>
- Date: Tue, 23 Mar 1999 21:31:52 -0500
UBC earthquake rigid wood diaphragm A plywood official reported that to analytically test for a rigid diaphragm (94UBC, 1628.5 third paragraph) there needs to be an equation for unblocked diaphragms. Therer is no equation for such. He believes it is not then intended for unblocked diaphragms to be considered rigid. He then hints of existing fudge factors for blocked diaphragm equations to represent the unblocked. Possible factors are. ******Increase the deflection six times for loads parallel to unblocked edges. ***Increase the deflection three times for loads parallel to the joists. Most wood floors are glued and are most likely rigid. No fudge factors For this one. The method of tributary area reduces the likely result of highly eccentric shearwall systems. Tributary area method (most of the time) will control the seismic design. But does allow larger cantilevered diaphragms to be justified. The code only suggests a rigid diaphragm system for seismic and not wind forces. But it sure creates a great back up to justify reduced deflections to extend cantivered diaphragms! (do not assume rigid please) This might free the need for the cantilevered columns. Wood diaphragm force distributions by tributary area, are modeled as being flexible in shear and rigid in flexure. Following is a summary of models that I have heard of. 1. Flexible shear, rigid flexure: This is the tributary area method 2. Rigid shear, flexible flexure: This is where interior shear walls are increased per a continuous beam model 3. Rigid shear, rigid flexure: By demand of the code section of 1628.5. This is the method used for simple concrete structures, and seems to be the recommended approach per code. 4. All of the above enveloped has yet to be recommended as enveloped. 5. The enveloping of case 1 and case 2: is found in ATC-4 I have yet to find a program intended for 1994 UBC wood shear walls, with the capability of rigid diaphragm checks. I know of one programmer who is now trying to implement the issue as a secondary check. Evidently he got enough calls. The above is mearly a group of expressed ideas and not my opinions or recommendations as to how to design. I hope that the concepts can be intertained and responed to. Of course with the same limit of liability.
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