Seismic Weight and Lateral Loads

[Nevile_Bartos]

Hi All,

I have a basic question for you (Disclaimer - I am a student, please go easy on me)

I am trying to understand seismic weight and lateral loading in SAP.

As a test, I have a 2D steel braced frame, with self-weight dead load only on it. I apply a x-direction quake load, with a user coefficient ,C, of 1. I think I should expect the x direction base shear to equal the structure mass (i.e, the dead load reactions), but in my test they are not equal. The X direction base shear is about 60% of the frame self weight.

I have searched and come up with a few posts on the mass source. Mine is set to Element self mass and additional mass.

Why does the weight used for the lateral loading not equal the frame weight, when C is equal to 1?

 

[driftLimiter]

How is your hand calculation for the seismic mass of this system setup? Seems to me a hand calculation would give around the same result as sap2000.

Generally some % of the total mass is assigned as seismic mass.
For many cases this is the portion of mass tributary to the lateral degree of freedom for each story.
For a 1 story building frame like this you have the sum of (the weight of the beam + 1/2 weight of columns + 1/2 weight of braces).