Wood-Framed Two Story Building With Tall Shear Walls and Many Windows

[mfstructural]

I'm reviewing a set of drawings provided by an architect and recently had a meeting with the architect, owner, and GC. The architect would like to design the building with steel framing, but the GC is claiming that it is too expensive and that wood is a better option. The building is two stories and 60x100', with a column line down the long direction supporting trusses spanning 30'. The building will be open at the interior and service as a bar/restaurant. There are a lot of windows around two of the elevations. The distance between floors is 14'-8". I would prefer steel framing for a building like this as floor loading is 100 psf and there will probably be large at the ends of the windows that carry the floor loads. Furthermore, 2x6 studs don't work for slenderness, and we are at 2x8s. Even if 2x8s are used for the walls, I feel like there will be an issue with shear wall design due to the windows and floor to ceiling heights. I wanted to see if anyone has come across this or had any input. I included some drawings.

 

[jayrod12]

I've had a few projects lately get priced for both steel and heavy timber (i.e. glulam beams and columns and CLT floors/roof) and they are coming out relatively competitive. So I can understand the contractor's point.

However, that elevation view with the windows and doors looks like you'd essentially have one segment of wall that can act as a shearwall on the upper floor. Will this one wall have enough capacity for the demand? If so, I think you could probably make a go of it.

I'm also surprised that you can't get 2x6 to work for some areas at 14'-8" tall. The slenderness aspect of walls is a real thing, but I've seen walls that tall in 2x6. Some even taller.

 

[XR250]

That bottom elevation looks pretty messy in wood. How about making that elevation reinforced masonry and the rest in wood?
Or maybe a steel frame for that elevation?
I'm also surprised with slenderness issues using 2x6's

 

[mfstructural]

I'll double check the 2x6 slenderness...I suggested a steel frame around the perimeter with stud infill. They are not open to reinforced masonry as the GC claims it is too expensive.

 

[KootK]

I feel as though this could be done in cold formed studwork pretty easily. An additional benefit of that is that it's pretty easy to integrate the odd piece of hot rolled steel into a CFM setup.

 

[phamENG]

While 5 story apartment buildings are a thing in wood, wide open spaces with tall ceilings aren't exactly wood's strong suit. This one will test the limits on some of it.

As it is, you won't be able to make those spans work. I did a house a while back with 30' floor truss spans (long story) and we had to go to 24" trusses just for residential loads. To hit assembly loading with adequate vibration performance in wood you'll be flirting with 36" deep floor trusses or maybe LVL/Glulam joists if you can't add another column line.

Lateral will be tough for the side elevation as XR pointed out. Probably warn them that they'll likely need steel there anyway. Upper floor and the front elevation may be doable with FTAO analysis and detailing if your lateral loads aren't too bad.

What about the elevator? Will they be expecting you to do that as a wood shaft wall?

Can it be done in wood? Sure, with some changes. Should it be? Probably not. Wood is cheaper than steel. But steel is disproportionately stronger than wood is cheap.

For design, I'd probably charge at least a 15% premium for the headache of designing this thing in wood. Not to mention the fact that part of the reason its so much cheaper is the contractor can grab the residential guys who will absolutely screw up all of the more detailed stuff required for doing this thing right.

 

[mfstructural]

phamENG: all good points, when I sent my initial proposal over it was for steel as that was my understand but then when I came into the meeting I was caught off guard with all the wood talk. I resent a revised proposal for a significantly larger amount (just as you said) as I know this will be a headache. Even if it does work, just like you also said, I'm worried he's going to pull a residential crew that might not understand the more intricate wood details this would require. I can't force them to have me out there inspecting everything although I did include 3 site visits during construction. The GC says that his "truss guy" says he can make 24" deep trusses work for 30' spans....but I am concerned about vibration as well. The elevator shaft will be CMU. I think from liability standpoint as well, I'd prefer steel as wood may present some issues in the future that may have to deal with.

 

[XR250]

Not to mention the fact that part of the reason its so much cheaper is the contractor can grab the residential guys who will absolutely screw up all of the more detailed stuff required for doing this thing right.

Right On!

 

[phamENG]

I can't force them to have me out there inspecting everything although I did include 3 site visits during construction.

Does your jurisdiction have special inspections? If so, make sure you clearly define a list of inspections. The code has a list of things that are required, but there's a line in there that says, effectively, "and anything else the engineer says."

No inspection, no report, no CO.

 

[phamENG]

The GC says that his "truss guy" says he can make 24" deep trusses work for 30' spans

Set your deflection limits to L/480 for live load and/or define a minimum effective EI value for the truss to keep the natural frequency of the floor system in the desired band and see what they come up with.

Strong backs will help, but I've been on a bouncy nightclub floor....yikes...