seismic lateral

[par060]

I am about to begin modeling a simple structural steel building. I am using the 2003 IBC to determine the seismic forces. I have determined the Design Base Shear now I am trying to figure out what is going on with 1617.1.1.1

It appears that the code is coming up with another load to apply in combinations. My question is do I apply the base shear to my model...or do I multiply base shear by rho and add to it the (.2)Sds(D) and apply that to the respective storys in my building.

Don't be affraid to over simplify...I'm confused!!

 

[mrMikee]

par060,

Start with these threads and see if they help.

Thread176-98527
Thread487-121459

Regards,
-Mike

 

[MWPC]

It appears the the code wants you to add for vertical acceleration load on members where the horizontal acceleration load is additive to the gravity load. An example would be the end studs of a two story shear wall that is also a bearing wall. The effect of gravity and seismic would be additive. Therefore you need to increase the seismic force based on Eqn 16-50 for any vertical acceleration. On the other end of the shear wall the dead load should be reduced by eqn 16-51 which may increase the holddown requirements.

These are very difficult to sort out. To me the combinations need to be run member by member. Not the old level by level or wall by wall method.

It sounds like you have done your work correctly. Your earthquake shear is now the Qe term and you need to increase or decrease it by eqns 16-50,51. I hope others will chime in here to correct me if I am wrong.

 

[par060]

I think i may have it now....I can apply my base shear x rho=Qe as the horizontal force and then use DL side of the equation as additive or subtractive from the total dead load.....sound right?

 

[MWPC]

Sounds right.