Wood shear wall parallel with truss - out of plane bracingrequired?

[jeffhed]

I have a small, wood framed commercial building that has an interior shear wall (10'-0" tall) that is parallel with a roof truss and directly underneath it. I have a plan reviewer that is asking for lateral bracing on top of the wall, similar to gable end wall bracing. We usually have not provided this for short, wood framed interior walls. Our reason being that the lateral load at the top of the wall is only 5 psf*10 ft/2 = 25 plf. Very low loads. The truss manufacturer states that the truss must be braced with purlins at some spacing or directly applied ceiling in order to keep the trusses plumb. We are in a seismic design category D, so we generally do not rely on sheetrock for anything. However, if this is enough bracing for the trusses to stay plumb, we have generally considered the truss braced. Since our wall is attached to the truss it is also braced. What does everyone else do? All I know at this point is over the past 12 years, I have never seen this type of lateral bracing in construction, nor have I been asked for it on commercial buildings that we have done in the past. Perhaps this is a short coming of local construction practices. For masonry or concrete shear walls and moment frames we require bracing. My question is specifically for short wood framed shear walls. If it should be there, I have no problem with requiring it, but I don't want to back down on this if it is not required.

 

[jeffhed]

As an additional note, if I use the lowest unit shear for 5/8" sheetrock (70plf), my out of plane lateral forces are roughly 1/3 of that (25 plf). I realize the sheet rock is probably not fastened as specified for shear, but I was just using that as a reference.

 

[JAE]

I think that for the small amount of cost to laterally brace a shear wall like that I would do it.

 

[jeffhed]

JAE,
That is what I am going to do at this point, show the lateral bracing on the plans so they will issue the permit. I have already discussed it with the plan reviewer and he would not budge. However, it is the local jurisdiction, so if I do it now, I will have to do it always. Are you saying that you do it now? Or that you would just concede and do it for the permit?

 

[woodman88]

jeffhed, Per the IBC (2009) Table 2508.5 for seismic design categories D, E and F you need to use 50% the table values. As for the plan reviewer, unfortunately, you either talk them into accepting the Sheetrock (as allowed per the IBC Table), put some bracing in, or try to go over their head to the Building Official. I would go with the bracing as a plan reviewer comment change.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[jeffhed]

woodman88,
You are right. I forgot to reduce the value by 50%. The building official will back the plan reviewer, so I just added the bracing and am done with it. I see you are in Arizona. I am in Southern Utah. What do you see down there in regards to out of plane bracing of shear walls parallel to trusses in wood framed construction?

 

[woodman88]

Basically, connect it to the bottom chord of the truss and let the truss bottom chord bracing system (typically the ceiling gyp-board) control it. But, I have not had a situation where there was more than a low force condition occurring. If I had a mid to high level force I would need to consider if additional bracing may be required. Of course if I had a plan reviewer comment I would go the path you are using.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[msquared48]

Another consideration here and frequently overlooked - unless the shear wall is also a bearing wall, there must be a vertical gap between the top of the wall double top plate and the bottom of the truss. Otherwise, the truss will not be able to deflect in an equal amount with the other trusses and you will read it in the roof membrane. The coindition could produce a leak in the roof diapragm with time. I commonly use Simpson DTS clips or equal for this.

I would also directly laterally brace the top of the wall at a minimum of 8 feet on center along the wall.

Mike McCann
MMC Engineering

 

[jeffhed]

woodman88,
That is how construction is done here. Although I would consider bracing for larger loads, I would bet that there is few to none buildings that have this type of bracing installed. Around here some framers still look at you like youre crazy when you tell them they have to put in bracing for gable end walls.

msquared48,
We have discussed this before. And around here this never gets done. All walls are framed to the same plate height. The more bearing the better, right? At least that is the framers mentality around here. The wall looks like it will be just to the side of the shear wall and I specified some clips as you mentioned for shear transfer.

 

[msquared48]

Jeffhed:

That's unfortunate, but I do understand. Unless there is a separation though, with a tile roof, you will be able to read it every time.

Mike McCann
MMC Engineering