Light gauge steel delegated design 1

[Gopher13]

A building addition I am working on includes a new mechanical penthouse on the existing roof. Light gauge steel studs will be used for the walls. I do not have any experience in light gauge design and was given a short schedule to design the entire addition so I am thinking about delegating the design of these. I would like to use these walls as shear walls for the penthouse. My question is where should the delegated design stop? Do I design the plywood sheathing and associated fasteners and fastener spacing to complete the shear wall or would they?

 

[spieng89]

You can put the shear load the wall will take on the plans and have them submit the design for review. Only issue is when delegating a design, you never know what you'll get back and if it's 'review-able'. The more info you give upfront, the easier the review.

 

[BUGGAR]

Is there a reason you want to make these wall shear walls, or is laterally braced OK? Some light gage people like to use strapping for shear so they avoid relying on wood framing trades. This will give you more flexibility in the type of siding you use as well.

 

[milkshakelake]

I do delegated cold formed steel design all the time. Rarely, the delegating engineer/architect is good enough to give all criteria, like wind pressure, deflection criteria, and loading. One time, I got all the wind/seismic shear loads, and they specified certain things like minimum strap and post size so it matches their lateral design, but that was a huge project and not a penthouse. But usually they give almost no criteria, I end designing the whole thing, and once in a blue moon the EOR comes back months later and disagrees with my criteria. Then I have to revise and decide whether to issue a change order. Just be clear about what you need them to do exactly; it saves a lot of headache. It's up to you how much of the lateral design they do.

Also, specifying plywood has architectural implications with combustible/non-combustible class, energy calculations, and wall type details.

 

[txeng91]

Is the primary structure hot rolled steel and the light gauge studs are just infill? That configuration is pretty typical, in which case you would just use steel bracing for lateral resistance and the studs would only support c&c wind loads. I’ll say that designing light gauge infill framing is relatively easy and wouldn’t take much effort to learn. If the studs are load bearing and used for lateral resistance it then it becomes more complex, but not that much different than conventional wood framing. I’ll say that I once did all the sizing and detailing for the infill framing on one of my larger projects and the contractor still hired a sub to engineer the light gauge because it was in the specs. I received a submittal with hundreds of pages of calculations only for the studs and connections to be undersized. Resulted in a lot of unnecessary back and forth ending with them conforming to my original design.

If you do end up doing the design I would of highly recommend Simpson’s CFS program. Very cheap and user friendly.

 

[XR250]

You need to give them a base to build upon - such as a tube steel frame so they have something solid to attach their framing. It is not their job to figure out what is there and its capacity - that is your job
Design it so it can take any possible load they put on it from gravity to wind to shearwall overturning. They can handle everything else.
FWIW, if EOR's were being this abundant, I might still be doing metal stud design!

 

[Gopher13]

Thanks for the replies!

Attached is a little drawing of the mechanical penthouse. As you can see, there is not much left of the wall on the right side.

 

[Stenbrook]

I have been on both sides of this. In my experience as a light gauge delegated designer, most of the projects we receive, the engineer of record has not even given the light gauge framing a second thought. We end up doing all of the coordination between the architect and structural engineer and submit a shop drawing for review with anywhere from 1-30 questions that have to be verified because the information isn't in the drawings. In the small cases where the EOR has considered the light gauge, most of the time they have been overly conservative and just plain wrong with how they are framing it. For instance, no one apparently understands that you need a slip connection for your stud to exterior steel framing. You can't just shoot the flange into the edge angle with a couple PAF and call it a day.

With that being said, we have done the light gauge for non-load bearing, load-bearing and everything in between. As stated above by milkshakelake, if you specify the loading you want the studs to be designed to, any light gauge designer should adhere to that loading. If you don't give them anything though, then they will have to make assumptions that you may not necessarily agree with and it could become a huge hassle.