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Re: earthquake in STAAD-III

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>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