SFRS for braced frame decks

[McEngr]

I'd be curious what system you guys use for SFRS in accordance with Table 12.2-1 of ASCE 7-10.  I tell engineers to design with a cantilevered column system, let the braces stiffen up the system if desired to do so or if needed, but use an R=1.5 per G.6.  

Using an R=6.5 is incorrect, of course for light-frame construction.

 

[tj_PE]

I would agree. Definitely wouldn't use R=6.5, because it's for light framed bearing/shear wall construction

 

[cal91]

Yes, R = 1.5 for timber frames, whether it is a cantilevered post or posts with kicker braces.

 

[David Connor SE]

I would use ASCE 7-10 - Chapter 15 - Seismic Design Requirements for Non-Building Structures, Table 15.4-2 since a deck is not a building or similar to one. 

There isn't anything in that table matches wood decks except "All other self-supporting structures, tanks, or vessels not covered above or by reference standards that are similar to buildings."  That is listed as R=1.25.  Might be conservative, but probably won't break the bank if you are already using R=1.5. 

 

[TehMightyEngineer]

Interesting topic. I have a building structure supported on steel screw piles in a low-seismic region I'm working on right now. Screw piles extend out of the ground and I have cable bracing between posts.

I'm designing it for an R = 3 but I'm kind of blending my lateral resistance from the fixed steel posts and the cable bracing. R = 1.5 seemed too low for this kind of system, and wind controls over seismic at R = 3 . Does it make sense to keep using R = 3 or would you folks run this at R =1.5?

 

[David Connor SE]

Interesting topic. I have a building structure supported on steel screw piles in a low-seismic region I'm working on right now. Screw piles extend out of the ground and I have cable bracing between posts.

I'm designing it for an R = 3 but I'm kind of blending my lateral resistance from the fixed steel posts and the cable bracing. R = 1.5 seemed too low for this kind of system, and wind controls over seismic at R = 3 . Does it make sense to keep using R = 3 or would you folks run this at R =1.5?

I would say R = 3.0, steel system not detailed for seismic resistance since you have cable bracing. However, you do say something about "blending the lateral resistance" which might require the R=1.5 value.  In some buildings I do, we have interior steel braced frames, and intermediate precast wall panels at the perimeter. We use R=3 since the interior steel frames have the lower value. 

The foundation system used doesn't dictate your R value from everything I have come across.  

 

[TehMightyEngineer]

I would say R = 3.0, steel system not detailed for seismic resistance since you have cable bracing. However, you do say something about "blending the lateral resistance" which might require the R=1.5 value.  In some buildings I do, we have interior steel braced frames, and intermediate precast wall panels at the perimeter. We use R=3 since the interior steel frames have the lower value. 

The foundation system used doesn't dictate your R value from everything I have come across.  

Kind of my thought; essentially I'm modeling the steel piles as fixed 5 feet below grade. Thus, they do have some cantilevered capacity with works with the cable bracing in my structural model. If I ignore the piles the structure turns into a three-sided structure though, so I am relying on the cantilevered steel piles to some degree. Might be worth running a combined horizontal system analysis with R = 1.5 for the side that doesn't have cable bracing; fairly confident it won't control over wind even with that though.

 

[tj_PE]

Semi related - check with building official ahead of time if it's getting permitted. We had a runaround on a temporary structure supported on scaffolding with cable braces for lateral and the scaffolding engineer was only allowed to use R of 1.5, but if time wasn't of the essence, the build g official said we could have argued the case and maybe gotten an exception. 

 

[tj_PE]

That was in response to @TehMightyEngineer

 

[TehMightyEngineer]

Semi related - check with building official ahead of time if it's getting permitted. We had a runaround on a temporary structure supported on scaffolding with cable braces for lateral and the scaffolding engineer was only allowed to use R of 1.5, but if time wasn't of the essence, the build g official said we could have argued the case and maybe gotten an exception. 

Good advice tj. We're also in a rush to get it permitted; I'll see what our code enforcement official says when we submit for permit. I should have enough time to try running the entire bottom area as R = 1.5 and I'll see if seismic controls over wind; should at least give me a better idea of whether this is an issue or not.