What is this structural system's R factor?

[shemp]

I am designing a residential structure in SDC D. It is 2 stories. The 1st story is an open carport. 2 story steel HSS columns are used from the foundations to the roof. Wood shear walls are used at the 2nd story. There are NO moment connections at the 2nd story or roof. The base of the columns need to be fixed to reduce story drift.

The structure derives its lateral force resistance from the shear walls (at the 2nd story level) and through bending in the steel HSS columns. A R factor of 6.5 (per ASCE 7-10) has been used based on light frame shear walls. This would control over an R factor of 8 with special steel moment frames. Does fixing the base of the columns at the foundations require an R-factor based on cantilever columns (R=2.5)? I think it would not because the wood shear walls provide some restraint at the 2nd floor and roof diaphragms. What R factor would you consider this to be?

 

[BadgerPE]

See highlighted section in attached article for guidance. I have never performed a design like this, but recently read this article.

The highlighted section is from ASCE 7-10 12.2.3.1.1.

 

[KootK]

That's a tough one. It would depend on the story that you intend to tell.

Story 1: Inverted pendulum. Your columns act as cantilevers with a large seismic mass near the top.

Story 2: Hybrid steel moment frames where your light frame shear walls are essentially the beams of those moment frames.

Inverted pendulum would be the most conservative route, obviously.

I could get behind the lesser of the value for light frame shear walls and ordinary steel moment frames.

I wouldn't tell the moment frame story at all in a special or intermediate moment frame. In those systems, dissipation is meant to happen in the beams. With your beams being wood shear walls, that would be a pretty tough sell.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SAIL3]

if a col cantilever is not chosen , then one may have to deal with a soft story irregularity....

 

[KootK]

Correction: I should have been using the term cantilevered column instead of inverted pendulum in my post above.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[sandman21]

Your first story is not a special moment frame, only one connection has been prequalified for use with an HSS column. All special moment frames require moment connection at the beams. Your first floor is a cantilever column system your R's will be 1.25 for ordinary or 2.5 for special cantilever columns. The building will not qualify for a two stage analysis the R for the lower portion will need to be used on the walls above.