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?

 

[mike20793]

No, I don't consider composite action. If you do the shear flow calculation, it requires quite a few nails for composite action since the connection is at the maximum shear stress of the combined section. I use twice the flatwise bending properties. Most of the time it works, but there are some cases where I've added blocking below the top plate to distribute the load to the studs accordingly. I've also switched studs to 24" spacing to match the spacing of floor trusses (to avoid the top plate transferring the load).

 

[msquared48]

Or hang the joists off a rim joist so the plate drops out of the picture. Well, almost...

Mike McCann, PE, SE (WA)

 

[KootK]

If your splices are randomly placed, are you not down to the bending capacity of a single 2x?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[woodman88]

Typically I use 1.5 times a single plate to account for a plate slice plus the cantilever action of the sliced plate. But depending on the detail that occurs at that location one can come up with additional ways to stiffen the plates.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[KootK]

I'm going to adopt that approach Woodman88. Thanks for sharing it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

I usually do not worry about the splices. I consider the cantilever action and the nominal amount of composite action sufficient to pick up the slack. Don't forget about the small contribution from the wall sheathing. I imagine the composite action, in reality, is significant, even if the numbers do not tell us that.

 

[msquared48]

If the splices are specified to occur over a stud, there should be no problem...assuming the contractor follows the plans. Oh well, so much for that idea.


Mike McCann, PE, SE (WA)

 

[XR250]

Yea, seriously- like that will happen. Same reason I try to avoid designing houses using "advanced framing techniques" Too much to expect out of most framers.
Just had a contractor rip the 16 ft. flat roof off a house and install 60 ft. clear span, (yes 60 ft.) 24" o.c. roof trusses on 16" o.c. walls. I think he will have a top plate problem.

 

[DaveAtkins]

I design the double top plate assuming no composite action. I did an analysis on RISA once, with supports at 16" oc and point loads at 24" oc, and determined the maximum bending moment to be PL/5 (therefore, it is between PL/4--simple span, and PL/8--fixed ends).

DaveAtkins

 

[manstrom]

Non composite. It is somewhere between fixed-fixed and fixed-free. Moment or shear controls depending on the loading type. I always use southern pine. The point load limit is something on the order of 1400# (I'm going from memory, maybe I'm a little off)

If you think too hard about this your head will explode. Specifically - load transfer from floor to floor, and point loads under trusses / joists.

I use southern pine #1 top plates and limit stud spacing to 16" o.c. I specify cripples over studs so that the wall loads from upper floors transfer through to the studs, not the top plates. Using this you should be good for most cases unless you have a wall picking up long spans on each side.

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

 

[DaveAtkins]

In multistory buildings, I always show a rim board. This transfers the stud loads from the floors above into the studs below without causing bending of the top plate.

DaveAtkins

 

[medeek]

I've never given floor joists on top plates too much thought since they are typically connected to a rim joist and my exterior walls are always 2x6 with studs 16" o/c. I always require a min. 48" splice with the splice joints occurring over a stud (at least that is the way they are supposed to build it). If I have any significant point loads I ensure that there is post of some sort underneath it.

I could see this becoming an issue though if there is an interior load bearing partition with long spans on each side and the floor joist is in a multi span situation versus simple span.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

I'm starting to worry about floor truss applications now. They'll often have a 2x6 ribbon that's kind of like a rim board. Trouble is, it's flush with the top of the truss, not the bottom.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

@manstrom;

We can't even get SYP 2x4's here unless they are pressure treated. (central NC)

 

[BLSPAM]

This is a huge issue with multi story wood buildings with long floor trusses. I've found that the industry standard of just a ribbon at the top of the truss under the bottom plate is not adequate and will specify either 2 ribbons (one at the top and one bottom of the truss) or a full height rim. Unfortunately I get a lot of push back from truss suppliers, builders and architects who have never seen this before. It makes me wonder "has anyone ever followed the load path through the structure or am I missing something" I would love to be enlightened on this topic!

Thank you for your time
Bill Lindau

 

[XR250]

I usually specify 2x10 ribbons or just use an engineered band board. I have seen too many ribbons deflect under point loads.

 

[KootK]

Something here mystifies me. If the ribbon or band board is supported by the floor trusses, as is generally the case, the point load on the sill plate issue isn't really helped at all. The point load just steps sideways a bit and gets delivered to the sill plate through the truss right? It seems to me that a full height rim board or a low ribbon would help, but a high ribbon would not.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msquared48]

The top of the ribbon board and the top of the joists are usually at the same level. If hangers are used joist to ribbon, then the ribbon board becomes a beam that is stiffer than the top plate, so the top plate becomes a non issue.

Mike McCann, PE, SE (WA)

 

[KootK]

The top of the ribbon board and the top of the joists are usually at the same level. If hangers are used joist to ribbon, then the ribbon board becomes a beam that is stiffer than the top plate, so the top plate becomes a non issue.

But this is only true if the bottom of the ribbon board is in contact with the top of the double sill plate, correct? In which case, it's usually just full depth rim board.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.