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Re: earthquake in STAAD-III

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When you use SRSS load combination of STAAD-III, all the compression  members are designed as tesile members in design mode.
For the above problem, I e-mailed receiving mail that " above design error happen since Srss combination always give positive force values". It seems like they did not fixed or deleted Srss command yet.   
           young ki Kim (P.E) 

>>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)        | this distance"   (last words of Gen.
>___________________________| John Sedgwick, Spotsylvania 1864)