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Re: earthquake in STAAD-III
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- Subject: Re: earthquake in STAAD-III
- From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
- Date: Sat, 12 Dec 98 17:51:13 -0600
>The method of combining modal results in >all of the response spectrum values being reported as positive values thus >making it meaningless to draw a moment diagram. Not entirely meaningless. Although reported positive the combination actually has no sign since it changes throughout the event. You can't calculate an external load at a node by summing the internal loads of the attached elements, but there are a couple of things you can do, if you really need the information. You can get external loads for the member in question for each mode, then combine the modal loads with the same methodology used for the stress combination (CQC, SRSS, whatever). These loads are equivalent nodal forces to the distributed dynamic loads, so your moment diagram will be a piecewise linear approximation to the actual moment diagram. Another method useful for connection loading is to write off the combined modal displacements and impose them as displacement constraints in a static run along with the original displacement constraints. The resulting load set is again equivalent to the structure dynamic loading (since it produces the same displacement), only with the member internal load signs preserved. If you use any sort of reduced analysis you don't need to use every calculated displacement, only the master DOF displacements from the expanded modal solution. You can add gravity and live loading to the problem however you like. The pitfall here is that the combined modal solution is always an envelope--any single stress, load or displacement response is the maximum expected during the event, but the individual maximums aren't simultaneous. The structure will probably never assume the shape determined by the combined modal displacements, so a moment diagram corresponding to the displaced structure will be an envelope of all the maximum internal moments, not a moment diagram at some instant in time. Christopher Wright P.E. |"They couldn't hit an elephant from chrisw(--nospam--at)skypoint.com | this distance" (last words of Gen. ___________________________| John Sedgwick, Spotsylvania 1864) http://www.skypoint.com/~chrisw
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