horizontal distribution of seismic forces

[nrguades]

I usually compute seismic forces manually, and apply it to my model. If I have a rigid diaphragm, I distribute the seismic force at each level based on a rigid diaphragm distribution, where the rigidity of the column or shearwall greatly matters.

In staad, where the program generated seismic forces is used, I observed that the horizontal distribution of the seismic forces, seems like, based on the defined weight on each node. The greater the weight covered by the node the greater the direct shear and torsion at that particular node. Any comments? thanks

 

[doejohn]

Well, I'm not a Staad user, but it's approach would make the inertial load distribution proportional to the mass (weight) distribution--sounds like F=ma to me, just like when we consider the lateral load to act at the floor CG. How much of that force ends up being distributed to the vertical components of the lateral force-resisting system depends on the stiffness distribution, which Staad is, or should be, handling for you. I don't know how Staad models your floor, but if it's stiffness is much greater than the vertical elements, you should get the same horizontal distribution of forces in the vertical elements as a rigid diaphragm manual calc.

 

[lippie]

Are u assuming that the rigid diaphragm is like a slab and should attract all the mass? If so, where should they be lumped? At the master node? What if the rigid diaphragm covered only part of the floor?

 

[nrguades]

Thanks for the reply, in situation where rigid diaphragm covered only part of the floor, I usually compute the equivalent force corresponding to the the rigid diaphram portion.Compute the center of rigidity and center of weight and do the rigid diaphragm distribution analysis.On the other part, I distribute the seismic forces based on the weight of its node if it is a pure frame, but if it is connected by flexible diagphragm,the forces will be distributed per tributary area...