residential wood truss interior bearing walls 1

[eng003]

Looking for general thoughts on what spans/conditions at which one should start considering interior bearing walls to support wood roof trusses in typical residential applications.

 

[FishTheStructure]

There are no span lengths on which the interior walls must necessarily be bearing walls.

As an example, you could have a long truss which was designed for the full span and in which case all the interior walls do nothing structurally. On the other hand, you could have a much shorter "truss" that resembles a truss but is actually 2 half trusses that each bear on an interior wall. My point is that the span does not dictate if the interior walls are bearing or not.

General rules:
If a wall is bearing it must lie directly under a joint on the truss above in order to provide support to the truss.
If a wall is bearing there must be a mechanism to bring the load it picks up to the ground. If it is a single story house, the wall must be on a footing, not simply on the floor. If it is a 2 story house the wall must rest on a strong and stiff enough beam that is capable of bringing the load to the foundation.

I would never make judgements about removing interior walls in my house based on things I read on the internet. Hire a structural engineer to come for an inspection. It will not take a knowledgeable engineer very long to determine if the wall in question is load bearing.


Great spirits have always encountered violent opposition from mediocre minds - Albert Einstein

 

[eng003]

I appreciate your response and understand and agree with your statements but my question is really regarding what factors to consider in design when deciding whether to span from exterior walls or whether to provide an intermediate bearing wall. Since you can often go either way...I am looking for what other's thoughts are on pro's and cons of each. My thoughts are:

No interior bearing wall:
Pro:
-Provides flexibility for renovations in future
Con:
-If long span, truss may get more expensive

Interior Bearing wall
Pro:
-Distributes load more through several load paths
Con:
-Additional foundations required

 

[msquared48]

It also gets into the transportability of the trusses on the road. Some place the limits at 40 feet, some higher, some lower. Depends on your area.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dhengr]

Certainly, you get more movement in longer trusses, whether it be from deflection under load or from truss uplift during some seasons and in some climates . And, this must be accounted for on your interior detailing btwn. walls, ceilings and trusses.

 

[Lion06]

dhengr. How do you detail those joints at interior walls. I've often struggled with this. Even if you hold the wall down enough to not have the truss touch the wall under a full design load deflection, the gyp corner joint is still going to crack, unless you don't tape it and let the ceiling gyp slide past the wall gyp. I've never actually seen that done, though.

 

[SLTA]

Lion, we've used crown moulding at the top to accomplish that, due to temp/moisture changes seasonally - attached to the wall and not the ceiling. The Journal of Light Construction (JLC) has lots of articles and details on how to best do this.

 

[Lion06]

That's pretty slick. I'll have to get a copy of that resource.

 

[shobroco]

I think slta means the crown is attached to the ceiling, not the wall, so it can slide up the wall. The other way around simply shows a gap when the trusses lift in the winter. A more universal solution (crown is not always appropriate or desired) is to use light gauge steel corners fastened to the partitions for ceiling backing, with the ceiling drywall screwed to them & not fastened to the trusses within 2' of a partition. The metal corner provides good continous support for the joint & the drywall can deflect as the trusses lift.

 

[Lion06]

shorbroco-
How do you finish that joint? I don't imagine that it makes a nice, clean line.

 

[dhengr]

Lion:
You have to provide a slip joint btwn. the top of the wall and the trusses, so as to provide lateral support for the top of the wall, but still allow these vert. truss movements. We used to do this with a blocking means on the top plate of the wall, at each truss, but always got a fight from the builder and carpenters for this extra work. Simpson and USP now provide a piece of “L” shaped hardware which does this, with slots in the vert. leg, loose nailed to the truss bot. chord, and solid nailed to the wall top plate. I’ve actually seen walls hard nailed at the trusses, lifted right off the floor below when the trusses lifted up, there was a 1/4"+ gap btwn. the bot. pl. and the sub-fl.

Regarding the ceiling sht.rk....: in its crudest form, nail a 1x6 x 20" long flat to the top of the wall, btwn. each set of trusses, and nail/screw the clg. sht.rk. to this, or at least push the wall sht.rk. up under the clg. sht.rk. and up to this blocking. But, then do not nail the sht.rk. to the trusses until you get 16-18" away from the walls. This allows the sht.rk. to flex without popping nails and with less likelihood of cracking significantly at the walls. A small cove molding or some such nailed to the wall, but not the clg. will hide this joint. You can’t nail to the clg. because the trusses move and this joint does not. In some cases a larger cove molding can be nailed only to the clg. (to move with the clg.) and be allowed to slide vertically at the wall or on another flat piece of molding affixed to the wall. This joint may show the movement in paint or stain discoloration from behind the lower toe of the cove. The above works pretty well along the length of the wall which is perpendicular to the trusses. The most difficult detail area is at the end of the wall which goes up to the clg. where you have to let the clg. sht.rk. flex on both sides and for about 16-18" beyond the end of the wall. And, you want to avoid sht.rk joints in this immediate area too, if you can.

 

[bylar]

You let the ceiling drywall float. do not fasten it to the trusses in the area app 18" from the wall. There is a detail in one of the gypsum manufactures handbooks. I can't seem to find my copy at the moment. We just moved an I can't find anything.

 

[DLBennett]

Truss arching is a difficult issue, I've seen 2" of arch in a modest sized home (dryer vented into attic). No detailing would have stopped those cracks. The ceiling details are necessary to keep us engineers out of trouble, but I'm not sure the end product is any better because I've never seen a sheetrock crew read a drawing.
My office condo has end bearing roof trusses and ceiling/wall cracks. Some owners spend a good deal of money trying to fix them. They are not large, I accepted them long ago. But I love the ability to rearrange my interior walls as I want without much thought to the roof/attic or my downstairs neighbor.
I suggest you have a discussion with the Architect. If the building is to be inhabited by people who will hate even a small crack in the ceiling/wall, then break the trusses to reduce arching.

DBennett
Lone Star Engineering, PC

http://www.lonestarengineeringpc.com

 

[focuseng]

As mentioned, Shipping the truss is a concern and craning. Doesn't help you if you don't know the supplier or contractor though. Too tall trusses for the truck are solved with "piggybacks", generally over 13-15'+/-.

Also bearing conditions at the truss and tie downs. Longer trusses need more bearing on a wood top plate and have higher uplift. Pay attention to girder trusses especially. DON'T assume the truss designer is looking out for you on this. The fine print on the shop drawings will tell you the things they are not responsible for and they don't usually flag it for you if you don't meet bearing requirements.

Deflections as the conversation is going should also be a concern. L/240, L/360 etc... then you should consider a max limit. Keep in mind -stiffness attracts load. SO if you have partition walls under a long truss the floor joists under will start to deflect too. I once reviewed a home under construction where the floor was deflecting like crazy +2" without any occupation yet. Turns out, the roof was using a "partition wall" as a nice support structure and the floor was willing to help out.

MAP

 

[MiketheEngineer]

If it is under 40' - go clear span - although I have easily designed 60' clear spans. Obviously - the live loads you need to carry may change this a bit. The extra cost and simplicity just make it the right choice.

After 40' - your call.

Get with your local truss designer and work out the details.

My choice - clear span if at all possible.