2x4 Double Top Plate Capacity 1

[XR250]

How are you guys calculating for the point load capacity of a double 2x4 top plate? Are you taking into account any composite action between the two members with the limited amount of code nailing or just basing it on twice the flatwise bending properties of a 2x4?

 

[msquared48]

Yes.

Mike McCann, PE, SE (WA)

 

[BLSPAM]

Usually the plates can take the load of the truss but not the load from the walls above. Once the loads accumulate the plates are over-stressed. If the plate cannot take the load of the truss a full height rim with joist hangers or nails/screws to the truss may work. I have not been able to get a wall panel/truss supplier to do this calculation because its a system and not a component.

And now its my fault?
Bill

 

[msquared48]

But full depth blocking will do much the same thing, just not as effective as a rim joist or truss.

Mike McCann, PE, SE (WA)

 

[BLSPAM]

Any engineer who looks into this issue usually comes up with the same answers. Why are there so many standard details and CD's with just the upper ribbon? It is very frustrating when the calculation ends with "then a miracle happens" and everyone buries their head in the sand.

Bill

 

[XR250]

yup, which is why I dislike floor trusses and I-joists

 

[manstrom]

The plates take the load from the individual truss of the single floor above. It is usually not an issue unless you have big tributary on each side of a wall or heavy loading. Cripples / blocking over the studs transfers load directly to the studs.

The only issue that I have ever seen related to this was with 30' trusses in a clubhouse supporting heavy DL and 100 psf LL. The plates crushed slightly, but did not fail.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[BLSPAM]

So we have a consensus?

A single upper ribbon results in the accumulative load being placed on the top plate of the wall below, potentially at locations where there is no stud. Thus resulting in the bending of the upper plate.

If the ribbon is stiff enough it may transfer the accumulative load to the trusses with studs directly below them. this may overload those studs

 

[mike20793]

This issue is exactly why I specify stud and trusses at the same spacing, minus roof trusses. I've had many contractors rather go to 24" stud spacing or 16" truss spacing than have to install blocking, hangers, etc. We have a detail showing our maximum out of alignment tolerances. The contractors never have an issue with this, especially when multiple studs are used.

 

[dcarr82775]

I did a RISA analysis as Dave noted way up in the thread taking a 10-ft length of wall and staggering some splices accounting for continuity and such. I found the double plates are ok until you get into long span floor or roof trusses. On the 4-5 story wood jobs since the trusses are at 24" I tend to spec stud bundles under each truss on the interior walls. Exterior walls I do a rim board with studs at 16". As described above the ribbon is a steaming turd of a detail, and I choose not to associate myself with it. When the owner or architect has balked in the past I explain many of the things others already mentioned and they seem to understand and accept the rim instead of the ribbon without too much of a fight.

 

[manstrom]

BLSPAM, you are correct. Add blocking / cripples over the studs and your problem goes away.

I have been privy to 2 schools of thought at 2 different companies:

*Align the trusses with the studs. Studs at 12", trusses at 24". Add blocking between trusses (over studs)

or

*Trusses at 19.2" studs at 16" o.c. Add blocking over studs. SP plates






When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller