8 Story EBF with OCBF penthouse - tips for modelling the penthouse story?

[jittles]

I have an 8 story steel frame building in schematic design. I am running a comparison of an EBF system vs a BRBF system to hand off to the contractor to price out. Both of the systems are being designed under ASCE 7-05 using the fixed beam-column connections for R=8.

We are permitted to have the (mechanical) penthouse story be OCBF framing and I am looking for modeling tips or techniques.

I have the model built in RAM, and I can analyze it under R=8, which gives me story forces under a system R of 8. My proposal is to take the penthouse level shear as analyzed under R=8, multiply it by 8/3.25, and apply the difference to the frames at the penthouse level. I would then apply equal and opposite shear loads at the roof level just below the penthouse to cancel out the added shear. The shear would be gone, but the overturning due to the increased shear would still be present and track through the model.

Does this sound like a reasonable approach? Is it necessary? The alternative is to just grab loads and design the OCBF completely outside the main building model, but I want to make sure I don't miss any added forces on the rest of the structure below.

 

[SAIL3]

don't understand why you want to get rid of that increased shear?...anyway, you may have other issues like soft stories supporting a more rigid one.....not a good idea for seismic

 

[jittles]

The story shears are generated assuming an R=8, but locally I want to design the OCBF as an R=3.25 system. I don't want to penalize the R=8 system below by adding additional loads to the whole building, I just want to design the single story frame for higher loads, or at least that's what I think I should do.

 

[WinelandV]

jittles,

I think you need to look at ASCE 7 Section 12.2.3.1. For the general condition of combinations of framing systems in the same direction, the R-value at a single story can't exceed the lowest value of R that is used above that story. There are exceptions, however. Specifically, exception #2 may apply in your situation.

 

[UcfSE]

Could you have a separate model for the penthouse?

 

[jittles]

UcfSE:

Yes, that is always possible. For peace of mind regarding keeping all model information up to date it is convenient to have as much as possible in one model. This is mostly due to the design being at the schematic level still, with things changing often.

winelandv:

Exception (1) applies in this case.

 

[SAIL3]

follow the yellow brick road in the codes(IBC/ASCE) regarding "weak" stories...using your numbers the stiffeness ratio 8/3.25=2.46...this is the overall issue to be addressed...

 

[TDIengineer]

For a penthouse structure, that is considerably smaller in footprint than the main structure, I dump the mass in the roof diaphragm of the main structure.

Analyze it separately. Confirm that the footprint of the penthouse does not exceed the code maximum for a penthouse.

I normally would not go with a 2-step analysis with different R's.. especially if the penthouse mass is not large enough and tall enough that you feel it is affecting the building period and mode shapes. Do make sure you have the added mass though, and in the right place on the diaphragm.