Concrete - Wood Seismic Interaction 1

[BrianMc]

I have a building with the first level above grade a concrete suspended slab with concrete shear walls for a lateral system. The upper three floors are wood frame with wood frame shear walls as a lateral system. I have always assumed that because the periods of the two systems are so dissimilar I could treat the upper wood frame structure as independent of the lower concrete level from a lateral point of view.

A typical design scenario would be to design the wood portion as a three story wood frame building on grade. I would take base shears from the floor above into the concrete suspended slab system as a lumped force. The suspended slab would be detailed to resist the hold down forces from above.

I am looking for information on this type of system to either justify what I have been doing or to provide guidance on how one should analyze the composite system.

Any help would be appreciated.

 

[ishvaaag]

The problem with your approach is that the 3 wood stories atop the concrete story may be subject to bigger forces than to this way established.

This in akin behaviour to non structural parts attached to buildings, or antennae atop the buildings, due to some whip effect.

It looks to me as well you are unwarrantedly dismissing the seismic shear corresponding to the mass at the level of the floor just atop the shearwall (or otherway said, the forces at any and all of the stories wouldn't be being properly established).

The more simple way to tackle with this is to make a 3D (or even maybe 2D) model to perform a response spectra analysis.

If you can give a look to RISA 3D, there you have more in its tutorial more or less what you would be doing, since the process is entirely altogether independent of which are the structural materials in the building.

Yet note that the abrupt change in stiffness or the particular layout of the building may warrant by some standing code a detailed analysis that only other more sophisticated programs may deliver.

 

[Taro]

U.S. building codes (IBC, UBC) allow a 2-stage static analysis procedure for buildings with flexible upper portions and stiff lower portions. The lower portion must have a stiffness at least 10 times the upper portion and the period of the entire structure must not be greater than 1.1 times the period of the upper portion. See IBC 1617.6.3.1 or UBC 1929.8.3 and 1630.4.2 for more information.