Wood trusses on wood studs

[WWTEng]

I have a 50'x80' single story structure. Roof framing is pre-fab gable wood trusses spanning the 50' direction and I would like to space the trusses @ 24" oc. The question I have is about stud spacing for the load bearing walls. Ideally I would like to space the studs @ 12" oc topped with a double 2x bearing plate. Each truss is to be lined up with a stud in the wall.

What this means that every other stud of the wall would be support a truss. Is a single stud a good/acceptable practice for supporting roof trusses? Should the studs be double at bearing locations?

Studs are 14' feet tall and the peak of the truss is 14' (28' building height).

 

[rijaltd]

Your exterior wall studs will need to be designed not only for the roof load, but also for the wind load on the exterior walls. It is quite possible that you will need to use double or even triple studs at the locations of the trusses. You should hire an engineer to design your wall studs as there are many factors to consider due to code mandated strength and serviceability requirements.

 

[WWTEng]

rij, I know that I have to design for wind and I have accounted for that. My question is more of a approach inquiry, if single stud works for wind & axial, is it acceptable practice or should the studs @ bearing be doubled just as a thumb rule.

 

[structSU10]

To get to your general point, it is whatever the design dicatates. You should have some load sharing due to the double plate at the top, but likely not much, as well as load transfer through sheathing, but that is hard to quantify. I would just design the stud for the axial + bending load that is appropriate and see what works - species, grade, and size wise, and adjust framing to see what is potentially most economical, depending on the time I have to refine the framing.

 

[MiketheEngineer]

Usually one is enough but based on your height StructSU10 has it right. Check it out.

 

[msquared48]

At 14 feet with wind and axial loads to the studs, you are very likely looking at a 2X8 wall to control lateral deflection of the wall.

If you canb get the vertical and lateral to work with double 2X8 studs considering live load deflection at L/360 for any sheetrock, the contractor will love you, but probably not the Architect. Then you nay have to go to three or four 2X6's under each truss.

Some plywood shear wall options in the IBC can be applied over studs at 24" on center.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[WWTEng]

M^, I ran more detailed numbers this afternoon and looks like I am going to need (2)2x8s @ 24" oc and at jambs I need 3 or 4 2x8s (may even use 6x8s).

But now I have a question about IBC table 2306.4.1. Foot note f talks about 3" nominal member and it is not clear to me if it could be double 2x8s?

 

[msquared48]

I realize this is a different application, but the 3" nominal, double sill 2X plate configuiration with bookoo nails (very professional structural engineering term here, but it works) I have used with success, but it depends on the jurisdiction. So would be the case here. Double 2X vertical studs at the vertical splice between plywood sheets also works - it's all about splitting the studs with the size and spacing of the nails, and double studs does provide extra area - more than a 3X..

I would consider using 3.125 X 7.5" glulams, or even 5.125 X 7.5 if needed here. Some Architects like the wood look if it is at a window wall where you would not have to sheetrock over the multiple 2X8 or 3X minimum member. Otherwise use 4X members, as 3X may be hard to get in some locations. Then there is no question with the plans checker.

Interestiong to note that you must be usong IBC 2006, because that table is 2306.3 in the 2009 IBC. Note f is unchanged.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[shobroco]

I would be astonished if you need that kind of wall framing. I don`t know what your loading is or your occupancy but a 50 X 80 building is not very large and they are commonly built around here to larger sizes, 16' high, with lumber studs. I've done ag buildings (yes, lower factors of safety & higher deflections allowed) of 80' span & 16' walls with single 2x8 studs at 16``.

 

[BenAustralia]

I do a lot of houses in Australia. Standard is (in mm, you can convert):

2/ 90x45mm as the Top plates.

90x35 mm studs @ 450mm centres. This is fine for gravity loads, wind uplift loads, and wind pressures inward/outward.

Nogging between studs at mid height for a standard 2700-3000mm high wall.

Trusses can sit ANYWHERE on the top plates. They are designed to span the 450mm for gravity loads, and the tie down spacing for wind uplift loads.

We only ever line up studs/jamb studs (packed) under bigger Girder trusses that carry a lot of load.

Hope this helps. This is Standard and is used on the majority of houses. Many also just use 70mm wide studs, if they want to cheap out.

 

[hokie66]

Ben, the building described above has 14' (4270) high stud walls.

 

[BenAustralia]

In that case, I would specify 2/90x35mm MGP12 studs @ 300mm centres. But this is obviously highly dependant on wind pressures.

For high wall homes, or those with stair voids to an external wall, we often use closer spaced packed high studs.

 

[WWTEng]

schobroco, Ben

Actually 2x8 studs @ 16" oc do work but I wanted to line up the trusses (spaced @24") with a stud. I guess what you are suggesting is that the trusses don't necessarily need to line up with studs @ each and every location and that the top plate could be used to span between studs as required. My truss reaction is about 2.6 k (25psf D, 25 psf L plus a 350# point load at some locations) and based on that (2)2x8 top plates would fail for worst case ie truss right between two studs. So I feel that I do have to line up my trusses with studs.

Typical wind load is 20 psf

FYI-I have done 3 project with wood trusses but they all used masonry bearing walls. This is first one with stud bearing wall, thus the curiosity. Thanks for all the info.

 

[MiketheEngineer]

2x8 walls are rather expensive regardless of the 2x8 cost when one adds in the cost of windows, doors, jambs, etc. Multiple 2x6's would be better. Also - if sheathed - you do get some bracing help in the "weak" direction!!

 

[RFreund]

I have often heard of potential shrinkage problems using 2x8 studs (as well as price concerns as previously mentioned). I will try to find exactly what there problems are, but I know we have tried to avoid them in past. Alternates are closely spaced 2x6 or composite lumber. I have seen more than a few instances where the stud spacing does not match the truss spacing. Not ideal, but I believe it can work.

EIT

http://www.HowToEngineer.com

 

[BAretired]

With 16" stud spacing and 24" truss spacing, every second truss can be made to align with a stud. The remainder bear halfway between two studs. If a double stud is used where stud and truss align, then every stud carries a compression of one half truss reaction.

With 12" stud spacing, there is essentially no load sharing among studs. The aligned stud carries nearly all of the truss reaction while the other carries essentially no vertical load at all.

Double studs at 24" o/c is another option but may be problematic for drywall.


BA

 

[BenAustralia]

Add another top plate?

I guess its hard for me to give advice given the difference in standard practice and costs of components etc.

Here our tradesmen are quite expensive and lazy, getting them to line the trusses up with studs is not economical, and they'd not take kindly to the idea of it either! (at least for residential).