Learning Forum

[SriVin]

Hi all. NOOB alert.

I am trying to perform a time history analysis building frame. When I apply the time history function

1) not as base excitation, force being applied at centroid (correct me if wrong) of the building. I get a response as it should be.

2) If the same function is applied as base excitation, ends of all four columns are fixed. The frame undergoes rigid body motion.

To stimulate an earthquake in reality, shouldn't the function be applied as a base excitation function ?

What might be the reason ? How to solve it ?

Thank you !!

[peteroznewman]

Read this post and see if that helps explain things.

[SriVin]

So, as I understand it

Absolute = Yes => base is set into motion as per the function

Absolute = No => Base is fixed and mass is set into motion as per function ?

Earthquake load on frame, is essentially response of mass of building (lumped at floor level) due to fixity of base.

So, absolute = No, is a better approximation ?

Also I got same response when I performed, No base excitation and Base excitation with absolute no.

Thank you

[peteroznewman]

Absolute = Yes means the frame of reference for displacements is on the ground while the fixed support moves according to the acceleration input.

Absolute = No means the frame of reference for displacements is on the fixed support while the fixed support moves according to the acceleration input.

The stress and strain results are identical in both cases.  It is just where the displacement is measured from that changes, not the dynamics of the problem.

Using Base Acceleration is equivalent to applying an Acceleration load to the mass in the model and having a fixed support.

 

[SriVin]

Thank you, Mr.Peter !!