Do Double Wood Top Plates Work? 7

[dengebre]

For multi-story wood buildings, I have very often seen the wall studs spaced at 16” o.c. and the trusses spaced at 24” o.c. The trusses bear on a double top plate across the top of the wall studs. This typical configuration will result in every other truss bearing at midspan between the wall studs (the top plates behave as a beam over multiple supports). When analyzed as a built-up member, the double top plates are adequate for both flexure and shear. However, in order to behave as a built-up member, the plates must be fastened together so that they satisfy shear flow, VQ/I. Every time I perform this calculation the result is multiple 10d nails at a prohibitively tight spacing. What am I missing here? Trusses at 24” o.c. on a double top plate on studs at 16” o.c. seems to be common practice, but it does not appear to work.

 

[dhengr]

It can’t be that they care about costs, just look at the crap they design with God awful ugly, expensive details and conditions and they don’t give a ding about cost. And, then they want to save a buck on structural materials and details so they can afford a better grade of granite for a counter top. Mostly, they would have to change a few typical wall details with double top pls. which show all of their structural prowess, and they don’t have anyone on staff who knows how to do that any longer.

 

[phamENG]

Both good points.

 

[XR250]

I'm really curious why there are stories about architects getting so mad about a triple top plate. Sure, it costs a little more, but what's $0.40/lf in the grand scheme of things?

The framers are also gonna have to cut 1 1//2" off each stud to maintain standard ceiling heights - otherwise the sheathing and sheetrock will be more time consuming.

 

[BridgeSmith]

The framers are also gonna have to cut 1 1//2" off each stud to maintain standard ceiling heights...

There are studs available precut to length for double top plates? I'm just asking because I haven't seen them. Of course, I've only looked at what's available at the big box stores.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Brad805]

On multi-storey projects you order from the mill. They make what you ask for.

 

[BridgeSmith]

That makes sense, Brad805. If the triple (or double) top plate is a change, then that would be a much bigger deal than if it's part of the original plan.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[phamENG]

But yes, even the big box stores sell pre-cut 2x4s. Ever noticed the 92-5/8" 2x4s? That plus 3x1.5" gives you 97-1/8". 5/8" drywall ceiling and 1/2" floor covering puts you right at 8' ceilings.

Even so, 8' ceilings are rare in custom homes. Most are 9'-12'. And as Brad pointed out, larger projects will have mill or lumber yard orders and it won't matter (unless it's a change order, as BridgeSmith points out).

 

[Celt83]

The only way I have seen the double top plate justified is for a single floors worth of load and the analysis done considering the vertical studs as springs rather than rigid supports. We typically note continuous stud framing from truss bearing to foundation/transfer element, which would require additional full height studs injected into walls when the wall stud spacing doesn't match the truss spacing.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[XR250]

There are studs available precut to length for double top plates? I'm just asking because I haven't seen them. Of course, I've only looked at what's available at the big box stores.

Yes, 93" and 105" studs are the most common for 8 and 9 ft. ceilings. (92 5/8" and 104 5/8" in many locales)
92 5/8" + 1 1/2" bottom plate and 3" top plate = = 97 1/8" - 1/2" sheetrock ceiling = 96 5/8" - gives 5/8" clear at the base of the sheetrock that is covered by the baseboard.
Did not realize for larger projects, mill orders can be placed, but makes sense.

 

[r13]

After review a wood design course offered by WoodWorks (Wood Product Council), I deleted my previous post, and came to agree with Molibone, who said in his rensponse:

"In reality there is also sheathing on at least one side which surely helps."

The shear is essentially applied to a deep beam, that consists of flanges (chords), and web member (sheathing and studs), thus, shear flow shall be calculated over the entire beam, rather than local member (chord) only. Please correct me, if I am wrong. Thanks.

 

[bones206]

I wouldn't count on the presence of sheathing across the top plate. Say you have 9 or 10 ft walls and the sheathing is only installed up to 8'. At the same time you could have large concentrated construction loads placed on the deck such as stacks of lumber or other material.

I haven't done wood design either, so I'll let others comment on the likelihood of that occurring. But I've worked as a carpenter and I know we liked to stack as much material up on the level we were working on as we could get away with.

 

[bones206]

Here's an example of the scenario I described:

 

[r13]

Bones206,

After completion and before loading, it will be covered by what, or will not - just leave an opening?

 

[bones206]

I’m just looking at the construction loading case. Just speculating that it could be the governing load case for the top plate while also not having the benefit of the sheathing.

 

[r13]

I see. Thanks.