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.

 

[molibden]

What happens if you divide load by half and analyse a single top plate?
In reality there is also sheathing on at least one side which surely helps.

 

[dengebre]

Molibden, I have considered that approach but then the single top plate fails in flexure.

 

[dold]

Are you analysing it as a continuous beam or simple support with a 16" span?

 

[XR250]

Usually does not work. Consider using a 1 3/4" LVL for a band. That will distribute the loads above evenly to the studs. Then the top plate is only managing loads from one story of floor or roof.
Also gets rid of the pesky 2x4 truss ribbon which has a hard time with point loads.

 

[dik]

The use of double top plates is to provide continuity.

Dik

 

[BridgeSmith]

I had always assumed that the double top plate was to allow more places for nails or screws from the sheathing into the top plates, so that the diaphragm action of the sheathing would better support the top plates. Of course, I'm a bridge guy, so I know little about wood framing design.

Although, for composite action in continuous bridge girders, the plies don't necessarily have to have the full shear capacity right at the max moment location. I suspect as long as there are enough nails to provide shear capacity within a few feet of the span where the truss load is, it will still act as a composite section.

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

 

[phamENG]

It also depends on the span direction. If you're spanning from a central corridor to exterior wall, it probably won't work. If demising walls act as bearing walls, possibly with a bearing wall within the units, then you have a better chance. Blocking of some sort is typically required anyway to restrain the ends of the trusses and will prevent your point loads from accumulating too badly.

 

[dengebre]

Continuous beam over multiple supports. Here is an example for discussion purposes:

20’-0” long floor trusses @ 24” o.c. bearing on a 2x6 demising wall studs at 16" o.c. Truss Reaction = 75 psf x (2’ x 20’) = 3,000 lbs.

Forces in top plates (modeled as continuous beam over multiple supports at 16” o.c.):
M = 8,270 lb-in, V = 1534 lbs.

Analyzed as single top plate (A = 8.25 in^2, Sy = 2.063 in^3),
SYP No. 2: Fb = 1,250 psi; Cfu = 1.15
fb = 0.5(8270)/ 2.063 = 2,005 psi > 1.15(1250) = 1,438 psi NG

Analyzed as built-up double top plate (A = 16.50 in^2, Sy = 8.25 in^3, Iy = 12.375 in^4),
fb = 8270/8.25 = 1,002 psi < 1,438 OK
fv = 1.5(1534)/16.50 = 140 psi < 175 OK
VQ/I = 1534(6.188)/12.375 = 767 lbs/in
Per NDS, Va for 10d nail = 128 lbs
128 x 3 rows = 384 = 767s  s = 1/2" o.c. nail spacing!

Of course, a solution is to align the studs and trusses at 24" o.c., but that is not what I've seen as common practice.

 

[phamENG]

I don't see any issues with your analysis. I agree that, in this case, it doesn't work by the numbers. I'd say a lot engineers don't take the time to really check that. I think they get away with it for the following reasons:

1) Every other truss can line up with a stud in this arrangement. So if the double top plate does start to deflect under one at mid-span between studs, the load will redistribute some to its neighbors.

2) The top plate will have sheathing connected to it. That sheathing is capable of resisting horizontal shear, so it's capable of vertical shear, too. For the top plate to deflect, you'd have to fail the connection between the sheathing and the plate, or the sheathing would have to buckle.

3) Wood reference design values are set at the 5th percentile. The wood in the building has a 95%+ chance of being stronger than what you think they are.

I don't advocate using this reasoning. These things are either impossible or impractical to quantify and use with any sort of precision (even wood precision, which isn't very precise). But they are there and we can at least give them a qualitative consideration, particularly when looking at a building that's been there for 150 years but "fails" on paper. For new construction, stick with what you're doing and prove stress levels are within the allowable range based on discreet and predictable load paths.

 

[dengebre]

Thank you phamENG,

I have reviewed the drawings of three other projects (3-5 stories) by other engineers, and they all used a double top plate with trusses at 24" o.c. and studs at 16" o.c. I just wanted to make sure that I am not missing something.

 

[BridgeSmith]

We only use VQ/I for fatigue checks on composite bridge girders. For strength, we calculate the smallest axial force capacity on either side of the shear plane (in your case the axial capacity of one of the 2x plates) and provide shear capacity equal to that force between the max moment location and the end of the span. In your case, where the span is short relative to the cross section, the 'effective length' over which the shear capacity between the plates needs to be provided will likely be significantly more.

More likely, it's one of those empirical 'there haven't been any problems doing it this way, so that's how we do it' kind of things.

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

 

[JAE]

I guess for trusses I've seen 24" as a more common spacing. 2x10's, etc. are 16" o.c. but not trusses.

 

[XR250]

The use of double top plates is to provide continuity.

I assume this was related to my post about using the LVL band. You are correct and the band is in addition to the dbl. top plate. 14" trusses would use a 1 3/4x14 LVL band. Removes all the stated issues and gives a nice beefy place to break your sheathing or tie-down straps.
Regardless of what other engineers are doing, if you are not dealing with the top plate bending in a 3 story or greater building, you are taking a risk. Does the sheathing help? Assuming 6" O.C. nailing, you have, what, 3 nails? - so add 300 lbs to the capacity of the top plate.

 

[BAretired]

I remember running into that situation several years ago involving roof trusses at 24" c/c. The architect did not want to space studs at 24", so I specified a triple top plate. The architect did not like that either, so he went to another engineer who said the double top plates were okay. Case closed.

BA

 

[dhengr]

Dengebre:
You are asking the question the wrong way. It should be, ‘when do they work and what are the upper limiting loads and conditions?’ They are there to take the floor and roof framing loads into the stud walls, and to provide a continuity tie along the top of the wall, particularly at corners and intersections of walls. They almost always worked, whatever the relative spacing of the two systems, except in a few extreme situations. Then, along came manufactured roof trusses, and at 24” o/c no less. So then the question became what truss spans and roof loads where the upper limit. Now we have floor trusses too, and more than two floors and a roof. So, each time one of these changes happen, given the lumber grades you commonly use, you kinda have to go through the exercise (your calcs. above) again to determine the upper limits where a detail change becomes necessary. And, you tuck those calcs. and info away in your std. framing info file untill the next change, it becomes second nature, like knowing that a 2x6 won’t span more than 30’.

 

[bones206]

Some historical threads to cross reference:

thread507-381140
thread507-18966
thread507-240803
thread507-358022
thread337-150634
thread337-241949
thread507-425575

 

[MOJOJOHN]

I have always found that for small reasonable spans for a single family home, usually close. For large spans, the short answer is no unless you include work done by the sheathing.

Fought this battle with a large wood church 40 years ago. Put in a triple 2 x 6 top plate on 2 x 6 walls. They built it but I lost the client over it. No big loss.

Regards,

 

[phamENG]

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?

 

[BAretired]

It's possible that it's not so much the cost that worries them as their need to validate previous projects where double top plates were used. Just a guess.

BA