Truss Question (calling RTR) 2

[XR250]

Ok, second post in a day. I would like to get rid of these flush beams and run these attic room trusses out to the dormer wall . The beams do not stack below so I need a bunch of transfer beams. I assume the dormer roof profile would get built into the trusses? The truss span is about 32 ft. I am assuming a 16 ft. wide room @ 30 psf LL.. Roof LL = 20psf
The shaded area is the proposed room footprint. Is theis practical or should I keep the beams?
Thanks

 

[lexpatrie]

Practical is a whole different question, is it possible? Sure. Check with the truss supplier and design the wall and header for the appropriate load. Either the do the trusses so there's a sloped part and a flat part, or they run the trusses (as drawn) to a common support point, you could potentially put the header upset into the truss space and support them there, then overframe the majority of the dormer, and a gable end truss at the front wall which is relatively non-bearing.

I think the key is having enough depth for the truss at the flat part to work, so the one at the dormer ridge is the "easiest", the ones at the valleys are the most difficult in terms of material and stresses.

Or you put a girder truss at the primary wall, that's in the truss space and the truss designer does the hangers. Hand frame or scab or whatever to get the dormer.

Truss supplier may have an idea which is "better".

 

[bones206]

Could you bracket the dormer with girder trusses, then put the header beam from GT to GT? Similar to this:

 

[XR250]

Or you put a girder truss at the primary wall, that's in the truss space and the truss designer does the hangers. Hand frame or scab or whatever to get the dormer.

Are you saying replace the beams I have shown with a girder truss? If so, I am trying to avoid that as it doesn't stack on the first story.

"I think the key is having enough depth for the truss at the flat part to work, so the one at the dormer ridge is the "easiest", the ones at the valleys are the most difficult in terms of material and stresses."

Yup, that is my concern

Could you bracket the dormer with girder trusses, then put the header beam from GT to GT? Similar to this:

Potentially. Unfortunately nothing below the primary wall stacks on the first story so I would have to deal with the point loads from the side girders down there.

 

[phamENG]

I don't share that concern at all. All the other trusses have, what, a 4.5" depth at the exterior wall? Each of these trusses perpendicular to the gable end will have gradually deeper heels. I've designed similar roof truss layouts before, though they are usually interior to the building at some roof profile change. Similar concept, though.

 

[RontheRedneck]

I have several thoughts.

Yes, it should be possible to run the attic trusses out to the front dormer walls. But as Lexpatrie noted, it could be a problem a you get closer to the eaves at the dormer. You need height in order to get the attic truss to work. If the height above the wall (or heel height) and pitch are low, some of them may not work.

Cost could be an issue. As you likely know, it costs a lot more to make one of anything that it does to make several identical ones. If you step the TC up, you'd have maybe 10 or 12 unique trusses. Some pieces would match between setups, but not all. That would slow down the plant, and I'm sure they would charge accordingly.

It might be possible to extended the B.C. out to the bearing. It depends on what species and grades of lumber are available to the truss manufacturer.

I saw that you mentioned a 30# live load on the attic floor. I never design with less than 40# on any floor. I don't care what the code says. I think it allows for floor embers to be designed in ways that will make them under-perform.


Running attic trusses across a stair opening is a problem. You can't just cut the BC out anywhere you want to. The B.C. is in tension, and that has to be resolved.

Opinions on that vary from one manufacturer to another. Yours might refuse to do it, or they might be more accommodating.

I noticed your layout calls for the attic trusses to be 16" O.C. There's really no reason for that. It just adds a lot of trusses, and will likely make the job cost 40% more. (A rough number)

Is there a reason why the attic room is not as wide to the left of the stairs?

One final thought - I usually don't make rooms 16' 0" on attic trusses. (Unless the customer insists) The face of T&G plywood is something like 47 3/8". So I make the room 15' 10". That way they don't have a narrow strip to fill in.


Hope some of that helps.

 

[XR250]

Thanks for the info, Ron.
at the contractor's direction, I ended up substituting girder trusses for the flush beams and taking care of the reactions below with a transfer beam.
Good call on the 16" O.C. - That was a mistake.
Will change it to 40 PSF.
At the stairs, I was thinking it would be two mono trusses or I could stick frame that area. It is only about a 16 ft. max. span so 2x10 joists and 2x8 rafters will take care of it (16" O.C.)