Crashing of sill plate in wood stud wall 2

[AlexWong122]

Hi there,

I am designing a header in exterior wall (2x6 stud wall) and due to the large snow load, my factored reaction force at one end of the header is about 30 kips. The head is 5 1/4" x 11 7/8" PSL beam and I would like to 5 1/2" x 7" PSL post, but I am worried about bearing capacity of the sill plate (typically it is SPF material and I will need 9" bearing length in this case). I have two questions:

1. Does it matter that if the sill plate crashes? Is there any simple way to increase the bearing area at the bottom of the PSL post?
2. If I am going to use build-up post for this header (for the purpose of increasing the bearing length), I will need 7-2x6. Is it possible? I remember I heard someone saying to limit the build-up post to 5 piles but I can't find the reference.

Thanks in advance for your inputs.

Alex

 

[jayrod12]

1. Technically crushing the sill plate is a failure. However my opinion is it's self limiting. If I was checking an existing condition I wouldn't require upgrading, for a new design, you should make it work properly.

2.Why not change the sill plate to something with a higher perp to grain strength, or provide a steel bearing plate to get the bearing area you need.

 

[pmtrevisan1]

as JR said.
if you have a bearing issue then the design doesn't meet code requirements.
you could.
1 increase bearing by adding a steel bearing plate to increase bearing area.
2 run your posts all the way to the beam in end bearing (if the plates are the issue).
you will need to provide a strap to the the plates together though.
3. see how how much crushing you get and make an engineering judgement....

p

 

[jayrod12]

Or even a 10" long chunk of Dfir or LVL or PSL as a second bottom plate that will allow for enough load spread to get 9" by the time the load gets to your spf plate. Many ways to skin a cat.

 

[Ron247]

While the failure itself of the compression perp to grain may not be catastrophic, the damage from it is generally related to why you have a header.
1. Header for an opening you walk through, not much of a problem.
2. Header for a door. King posts and liners go down, but the base of the door does not. Header presses down on top of door and binds the door from operating right. There is not much slack between a door leaf and a jamb (about 1/8")
3. Header for a window. King posts and liners go down, but cripple studs and sill do not. Header presses down on top of window but bottom of window resists moving down. Window can bind or break.

With really large loads like yours, it is hard to estimate how much it will crush. The worst I have seen was about 1/2" to 5/8" on a SPF sill. 5/8" movement will do a lot of damage to finishes. This particular case was a highly loaded steel column from above with a very small and thin base plate. Unless you cannot get it, I would at least go to a higher grade of sill plate. Southern Pine is 565 psi as compared to 425 psi for SPF. That is at least a 35% increase in allowable. Also the steel plates as suggested are a good solution.

 

[BridgeSmith]

Forgive my ignorance of wood framing, but would it be an option to cut out the sill plate under the post and have the post bear directly on the foundation?

 

[jayrod12]

Hotrod,

That is an option, and that's what I think PMT was indicating in his 2nd option. generally speaking with wood framing, they like to build the walls flat on the ground, and then stand them up. So it is nicer to have semi-continuous bottom plate to add some rigidity. I know many framers that will leave the bottom plate in for doors, and then cut it after.

 

[CANPRO]

I know many framers that will leave the bottom plate in for doors, and then cut it after.

I've built quite a few homes myself, and that is the way we also built our walls. It wouldn't be too difficult to frame the wall without the post in, stand the wall, cut the sill, and then slip the post in afterwards. That might even be preferable to changing the grade of wood for the sill in one location - that sounds like something that could be easily overlooked on site.

 

[BridgeSmith]

Yep jayrod, I see now that my idea was already proposed. I have seen the 'make the sill plate continuous and cut it later' construction, probably more often than I've seen it where it's removed before. I assumed that it was done as an easy way to keep the wall aligned until it's attached to the subfloor, but I can see how rigidity while tilting it up would be an advantage, too.

Just out of curiosity, if the post is close to the opening, can you still use a bearing plate to spread the load, or would it cause other problems if the load can't spread both ways?

 

[jayrod12]

Potentially could cause issues, however they don't need a drastic increase, they have 7" so far, need 9", shouldn't be a huge issue. You could extent the header an inch or two and provide another 2x6 Jack and you'd be at 8.5". To me that's probably close enough.

 

[Ron247]

If the sill has to be treated, where I work, the only thing we find treated is Southern Pine. I am not familiar with SPF treated lumber. Also, along the same lines, I routinely see wall studs called out on drawings as Southern Pine but the studs actually provided are SPF. The building official has never said anything about that to my knowledge. If you go to the lumber yard, most precut studs for 8' and 9' are SPF. Since the load comes in via a header that is below the top plate, you only have to worry about crushing the sole plate. If it came in from some loading above the top of your wood post, it may crush a top plate also since we do use SPF top plates here.

The max 5 laminations is in Section 15 of the 2015 NDS. Personally, I would not worry about the 5 laminations rule if I were not adding the extra lamination due to a weakness in the column itself. I am adding it for bearing reasons versus column strength reasons.

 

[BridgeSmith]

Thanks jayrod. I agree in this case getting the extra length shouldn't be an issue. Adding an extra stud gets you 1 1/2" on the opening side, and presuming the bearing plate extends the other way as well, you get a 10" bearing length without any eccentricity. My question was more of a general query.

 

[jayrod12]

My question was more of a general query.

Understood, and honestly appreciated. Sometimes I look past things like that due to the overall flexibility of wood framing to do amazing things.

My direct fix recommendation popped into my head while I thought of how to avoid your potential issue. It certainly seems signficantly easier to just enlarge the opening slightly and provide an additional member on the opening side to increase the bearing area.

 

[BridgeSmith]

I thought about suggesting the use of a bigger post, but kind of assumed that a solution that obvious was considered and rejected for space or cost reasons.

Would adding a stud on each side of the post work, or would the bearing pressure be uneven due to the differences in studs and timber post? Again, more of a general query.

 

[AlexWong122]

Thanks all for your inputs. Appreciated!

 

[jayrod12]

or would the bearing pressure be uneven due to the differences in studs and timber post?

The bearing pressure would be slightly different due to the stiffness differences, however it would be a minor amount of crushing under the stiffer post before the studs were fully engaged. To me that's an acceptable solution.

 

[dhengr]

The bigger problem when they make these posts up with 4,5, or 6 - 2x’s is that the members are often cut to length at +/- 1/8” or so in length, so the header load isn’t transmitted to the post evenly, or then on to the sill pl. evenly. The largest bearing stress will usually be on/at the inner most jack stud, due to some header deflection, and the sill pl. will crush to accommodate this. Like Jayrod12, I’d accept it if they have about the right total bearing area and the members where cut well and fastened together well.