Light gauge straps with wood framing

[UPJEng04]

I am working on a 4 story wood building where the owner has requested the wood shear walls be designed without OSB sheathing using light gauge x straps. This seems like feasible option but I have a few concerns.

1. The connection of the strap to the wood wall would be the hardest thing to work out. I can only get about 150 lbs in a nail and I need to develop 2.5 kips. Not to mention how do I transfer the lateral force back into the bottom plate of the wall.

2. I am in seismic design category C. What R value do I use?

If anyone has any experience using this type of system I would like to hear from you about your design approach and any field issues you encountered during construction.

Thanks

 

[RARWOOD]

Usually the problem with using steel straps with wood walls is the connections, as you have already found out. Unless Simpson has come up with a new product, I don't think any of the steel hanger people sell a product intended for this use. If there was an easy way to use the straps with wood walls people, like Simpson would be selling it.

I think if you use thick enough steel you can make the connections. The problem then becomes applying the finish material over the straps.

If he wants to use the straps, you might suggest using steel studs at the shear wall. Another option might be to use 4x4 posts at your shear wall boundaries, then x-brace the walls with steel rod centered on the wall.

All in all I think experience has shown that the most cost effective system is to use wood sheathing.

 

[hokie66]

It is difficult enough to make light steel strapping work for a single storey building, so a four storey building using this type bracing is ridiculous. Sometimes you just have to educate owners that their ideas don't have a basis in reality.

 

[JAE]

I think you'd have problems getting a strap system to work - I'd agree with hokie66 here. Very difficult to connect the ends of the X-straps to the wood and still ensure an adequate load path.

 

[MiketheEngineer]

Go to Simpson or USP.

They make a number of hold downs that can carry that load - might need to be three inches wide with double studs and a lot of nails - but doable.

There are also threaded connectors that can work

http://www.strongtie.com

http://www.uspconnectors.com

 

[UPJEng04]

Thanks to all for your comments, and I do agree with hokie66. My problem is this owner has already built a building like this with another engineer and now this is the system he prefers. Cost is not the deciding factor here as removal of the OSB sheathing eases the construction process for the MEP trades, and can shorten the shear walls.

I still am confused as to what R value I should be using for seismic.

 

[msquared48]

I hjave seen "ewngineered" solutions that I did not agree with - abortions of the code too. Contrary to the owner's comments, just because it was approved and done in the past dopes not mean that it should have been. Moreover, codes change.

To make this work, a special holddown using end bearing on the shearwall end columns would have to be developed, with special side tabs to install the metal straps. Maybe Simpson will read this post and develop it. Maybe I should have developed it myself and made some money.

Maybe...maybe...maybe never comes.

Mike McCann
MMC Engineering

 

[JS1520]

Mike,

This connection has been developed by The Steel Network.

 

[UPJEng04]

I am feeling some resistance here, and I value you options. Take a look at this sketch. I have not addressed the floor to floor hold down but that should be easy say a Simpson HD hold-down between floors. How do you feel about load paths? Would this comply with all the applicable standards referenced from the IBC?

 

[JAE]

There's no stiffness in the vertical stud that you are connecting to (in-plane) so you'd bend and break the stud that the gusset connects to. Same thing for the top 2x of the bottom double plate.

 

[msquared48]

I agree with JAE... To make this work though, you might want to consider the use of a solid wood infill piece at the corner of the wall, thru-bolted between the gusset plates. This should help reinforce the sill plate and endwall studs against weak-axis bending.

There's also a lot of lateral movement with the nailed connections, and a double thickness of 18 gage plates that the sheetrockers will have to contend with. If this a "X" pattern strapping connection, the straps will tend to pop out the sheetrock when the straps see compression as they will bend out and in normal to the plane of the wall.

Mike McCann
MMC Engineering

 

[faromic80]

JAE and msquared48:

You said the end stud and sill plate will see weak axis bending. It will be minimal won't it??....The vertical component of the brace force will induce shear on the vertical screws and the horizontal component of the brace force will induce shear in the (horizontal) screws in the sill plate. Correct me if I'm wrong but wouldn't the only weak axis bending be induced by the eccentricity of the vertical and horizontal components with respect to the working point?

 

[msquared48]

By the geometry of the gusset plate, the nails are inducing cross-grain bending into the sill plate and the end studs which is not allowed by code. You could eliminate this by changing the design of the gusset plate

Mike McCann
MMC Engineering

 

[JAE]

Well, yes - but also, no. The assumption in designing the gusset plate assumes that the brace force is resisted by two forces resulting in a concentric resistance force. This means that each line of fasteners is assumed to take similar, balanced loads.

So as the load is applied, force will follow stiffness and the as the weaker stud flexes to avoid the load, the stronger stud (loaded in shear) would take more load, resulting in a non-concentric resisting force, causing bending/eccentricity in the gusset.

The attached sketch shows what I'm talking about.

 

[DaveAtkins]

I don't think there is any bending in the gusset plate. The two moments on the gusset plate cancel each other out.

DaveAtkins

 

[msquared48]

Dave:

Of course there is bending in the gusset plate - adding the gusset strengthens the connection by inducing a moment resisting capability into the connection, and the gusset. As I suggested in my previous post, the gusset's moment resisting capacity would be weakened by cutting out the gusset to either side of the diagonal, clear to the other studs and sill plate, but it would still resist some moment, just a lot less.



Mike McCann
MMC Engineering

 

[DaveAtkins]

I still disagree. If you created a gusset as you suggest (with three "fingers," like I have seen on heavy timber trusses), wouldn't the diagonal load pull on one of the fingers, while the resisting loads push on the other two fingers?

DaveAtkins

 

[MiketheEngineer]

By the time you get this all figured out and build the bracing - OSB looks cheaper to me. Easy to design and meets codes. Each brace would have to be custom designed and built. Carpenters know how to install OSB. Also, building code guys love to see completely sheathed walls.

Like I and others have said - hold downs are cheap and easy to install.

Good Luck

 

[JAE]

Another issue I see here is that the strap is on one face of the stud assembly.

This means that the brace loading is not concentric with the studs themselves and will produce some amount of twist in the wall - or a resultant out-of-plane force.