Seismic Analysis with a roof pop-up

[Greenaqua]

I have a one story building that has a roof pop-up about 4' higher than the main roof centered in the building with clear-story windows all around the pop-up. I would like to brace the popped up roof with steel columns that would be treated as pinned at the foundation and pinned at the main roof level then cantilever up to the pop-up roof. The main roof has plywood shear walls bracing it around much of the perimiter. This is in seismic design category D. The seismic force from the mass of the pop-up would travel into the steel columns and then put a reaction on the main roof and then travel through that diaphragm to the plywood shearwalls.
I am a bit confused how to analyze this structure. Using cantilever columns with an R=2.5 (special) seems to be overkill since the popup is only about 15% of the roof. The plywood shearwalls which is what transmits the lateral forces from the main roof to foundation gives an R=6.5! I am only using the steel columns to get the load to the main roof.

How do you guys on the west coast handle situations like this?

Thanks

 

[woodman88]

ASCE 7-05 Section 12.2.3.1
"EXECEPTIONS:
1. Rooftop structures not exceeding two stories in height and 10 percent of the total structure weight.
2. Other supported structural systems with a weight equal to or less than 10 percent of the weight of the structure..."

Is one possible way

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Greenaqua]

That is one way but the pop-up doesn't meet that provision because it is 15% of the weight....

 

[woodman88]

15% of the weight??? Of the roof (per your first post) or of the total structure weight (including the popup weight)?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Greenaqua]

The pop up roof is both 15% of the area of the total roof and 15% of the weight of the total...
For ease of numbers let's say that:
The pop-up roof weight =15kips
The main roof weight = 85kips
Total roof weight =100kips
15/100 = 15%

I think strictly speaking from the code I will need to use an R = 2.5 as a cantilever column / special moment frame which seems to me like complete overkill for this....If only I were allowed engineering judgement...
I also will have horizontal irregularity type 4 (out of plane offsets)

 

[OHIOMatt]

My reading of the expection, is that it be less than 10% of the total structure weight, not just the roof.

With that being said, I would reccomend you reconsider the idea of cant. columns. I am guessing that you have the perimeter both low and high roof framed with beams of some sort. Why not rigidly connect them and use the provisions of some type of moment frame.

Since it is framed with windows, deflection would be greatly reduced.

 

[msquared48]

Don't know where this is, but you should be looking at 5 to 15% of the weight of the structure for seismic. Any chance of using four WF steel ordinary moment frames to do this?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[DST148]

Greenaqua: Ref ASCE 7-05, Cantilevered column system - A seismic force resisting system in which lateral forces are resisted entirely by columns acting as cantilevers from the base. The pop up roof in your case would not qualify as a cantilevered system.