Vertical Distribution of Seismic Forces 2

[ronstruc]

By hand I am designing a 2-story light framed (wood) building that rests on a rigid concrete suspended slab, a 1 story parking structure below. I am inclined to design the wood building all by itself with the simplified base shear method and then design the concrete diaphragm with its own base shear added with the shear from the above wood building. In essence the design would be like a buiding on top of another building. The alternative is to design the building as a whole. If I design the building as a whole using the vertical distribution of seismic forces in asce7-02 the wood building recieves forces at the roof level of 243kips where its actual force should be around 16k. I can't see how this could possibly be reasonable. Anyone have any thoughts on my inclination? Thank you

 

[AUCE98]

I would agree with you in your analysis approach of desinging the two structures seperatly. Reason being, the concrete structure would most likely have the majority of seismic mass as well as stiffness. Therefore the wood protion of the building will "go for the ride" along with the supporting concrete structure.

Regards,

AUCE98

 

[JAE]

Look through your code, in the UBC (and, I think, the IBC) there's some provision I seem to remember that allows this seperate analysis if a lower section of building is much stiffer than an upper section. I am away from the office/codes/etc. - but check it out - I think its in there.

 

[jimbo]

Hey, guys !!! It is separate, not seperate. (There is arat in separate)

The spelling police.


Buy a dictionary, keep it nearby and USE it. Webster's New World Dictionary of American English is recommended, and Webster's Collegiate Dictionary.

 

[jjeng2]

Ive been thinking about this one off the top of my head. I havent looked up anything so Im just speculating. The large forces you are finding are from the mass of the concrete structure. I dont see those forces "making their way" to the top of the wood structure. But the structures will be attatched so movement/drift on the top of the concrete structure will be imparted to the base of the wood structure. I would think you can design them seperately but the concrete structure is not rigid like grade level would be rigid. I guess if you wanted to do a real analysis you would analyze the concrete structure dynamically and the apply the shaking at the top as a ground level earthquake force to the bottom of the wood structure. This makes sense. Input the seismic ground forces to the concrete structure. Output the frequency of the concrete structure, making sure to include the mass of the structure above(as lumped into the mass of the roof of the parking). That output becomes your ground movement for the wood structure. How one would translate that to a static analysis by hand? I have no idea.

 

[JAE]

jimbo - sorry, that's always been one of those words that just don't stick in my brain. But I do know the difference between ensure and insure.

 

[JAE]

OK,
In the IBC 2000, Section 1617.6.3.1, Exception 3 gives the two stage method that you can use.

In the UBC 1997, a similar two-stage analysis can be found in section 1630.4.2. (See also 1629.8.3, No. 4)

 

[ronstruc]

Thank you all for your response. jjeng2 I agree with your thought process, however I don't feel that taking that approach is necessary to be that exact for this situation. Thank you JAE for the code reference, I found it in the 2003 IBC under 1617.6.2.3.1 exception 3.