S_acceleration directions

Hi,

I am doing static equivalent analysis of siesmic analysis for a 150M long header which is anchored at one end and free on the other.

I have made 6 loadcases considerng Seismic Accelerations in +X, -X, +Z, -Z, +Y and -Y directions and very recently my counterpart told me not to consider seismic accelerations in negative directions also.

If my counterpart's comment is correct, the only reason which i find behind that comment is "We are adding double acting supports everywhere, so it will take care of the accelerations in the opposite directions"

The reason for which i am stressing my team to consider Accelerations in negative directions also is that the limit stop at the center of header is taking more load in negative acceleration case( almost twice the values of positive acceleration case).

Regards,
Vikas

If you're performing a true "dynamic seismic" analysis, then the computations involve a number of SRSS operations. Therefore, your counterpart is correct, you don't need to evaluate both the "+" and "-" directions. Also note that in a "dynamic" analysis any non-linear boundary conditions are linearized (that is why you set the 1st option on the Control Parameters dialog - "Static Load Case for non-linear Restraints").

If you're performing "static seismic", then you're simulating the dynamic event with a uniform "g" load. In this situation, the "+" and "-" directions could give you different results. However, this is an approximation, and really only valid if the response of the system is dominated by a single degree of freedom. (Of course you won't know that unless you perform both analysis and compare the results.)

Thank you very much Mr Richard for your quick and detailed reply.

I am performing "static siesmic" analysis. Can you please put some more light on last sentence.

regards,
Vik

Dear vikas,
pls explain me whenever you get time.

If your model is linear, then there is no reason to explicitly run both positive and negative g loads.
BUT, if you are looking for restraint loads, it might be simpler to run both to get the range of loads; for example, 1) operating plus g in X and operating minus g in X.
Or:
L1:W+T1+P1 (OPE)
L2:W+P1 (SUS)
L3:U1 (OCC)
L4:W+T1+P1+U1 (OPE)
L5:W+T1+P1-U1 (OPE)
L6:L1-L2 (EXP) [ALGEBRAIC]
L7:L2+L3 (OCC) [SCALAR]
Where:
use L1,L2,L4&L5 for strucutal evaluation (displacements & restraint loads)
use L2,L6&L7 for stress evaluation

If you have nonlinear restraints in your model, then more work is required.

Thank you Mr Dave.