ASCE 7-10 - Cantilevered Column Systems

[pluralitus]

I'm hoping to simplify a high seismic design that I'm working on. It's a 2-story office building with low roofs on the east and west side. Each of these roofs are ~30'-0 tall and supported by BRB's in all 4 directions and have a 2nd floor connecting the 2 portions at ~15'-0. At the center of these 2 sections is a "pop-up" high roof that is ~10'-0 above the lower roofs (~40'-0 total height) on either side.

Could I call this higher roof area to be supported by a Cantilevered Column System (height 10'-0) in each direction and transfer that load to the low roof diaphragms? The code mentions how cantilevered columns extend up from their "base", could the base be the lower roofs on either side?

 

[KootK]

An interesting condition. Some thoughts:

I'm hoping to simplify a high seismic design that I'm working on.

Is it not possible to concentrically brace the perimeter of the pop up? If so, I'd consider that simpler than cantilevered column, particularly if the cantilevered columns would be stacked upon the BRBF columns. In what way are you considering the cantilevered column approach to be a simplification?

Do your popup roof columns carry through the structure below continuously? I assume so.

At the center of these 2 sections is a "pop-up" high roof that is ~10'-0 above the lower roofs (~40'-0 total height) on either side.

Any chance that pop up is small enough in scale that you could lay claim to the ASCE7 provision for small, rooftop structures (penthouse provision)?

You might consider making use of ASCE's two stage analysis procedure if your structure would qualify for that.

There are a couple of things that I don't love about treating the popup as cantilevered columns:

a) I feel that your definition of the seismic base becomes a bit complex. On the one hand, column hinging probably occurs at the low roof level. On the other hand, for drift etc, I think that one could make the argument that the base is really the elevation at which the extension of the popup roof columns below the low roofs has a rotation of zero.

b) If you have to treat this as a combined lateral system vertically, you may have to use the R-Value for the popup for the entire building. This sounds pretty punitive to me given the much better R-Values that BRBF have compared to cantilevered columns.

A version of this that might be simpler to execute could be treating (and detailing) the popup as pin based, special moment frames. Possible advantages of that approach:

c) SMF have R-Values closer to those of BRBF if you're forced to take the worst of the two for the whole building.

d) If the two low roofs wind up moving independently, you've got a fair bit of deformation capacity at the base of the popup columns to accommodate that. It would be unfortunate if the popup roof diaphragm wound up being used as a drag strut to connect the two low roofs to some extent.

 

[pluralitus]

Thanks for the reply!

You're correct, the columns extending to the high roof do go all the way down to the base. On 2-sides, the BRB's extend up to through the high roof, so we are good in that direction. My concern is in the other direction, where we can't have full height bracing. See below (hopefully, this is my first thread!).

Currently, I have short chevrons (BRBs?) extending from the low to high roof. They can't go all the way down due to architectural features. I was hoping we could just take everything out through bending of those columns, but it sounds like that would really complicate the main seismic system and detailing...

 

[KootK]

The popup is pretty small relative to the surrounding structure. I suspect that you probably can use the columns in bending via some of the strategies that I mentioned previously

 

[pluralitus]

I think you're right - It only makes up ~10% of the total roof area and is less than 18'-0 above the low roof DBE. If that popup is a "penthouse", then we should be able to use the lower roofs for the structural height. That would put us below 35'-0 and we'd be able to use OCBFs, which would really simplify things.