Interior Bearing Walls

[jike]

For a commercial grade building, one engineer in our office likes to use 2x8 walls to have sufficient bearing length for wood roof trusses (DL = 20 psf and LL = 55 psf). The architect and prefer 2 x 6's. The problem is making them work. If I allow 1/2" between truss ends, that only provides 2.5" of bearing. For single ply trusses, that allows a maximum load of 425 psi(SPF) x 1.5" x 2.5" = 1594#. I have truss loads of 2044#.

Can I have the truss supplier provide a scab for additional bearing or a Simpson truss enhancer or should I go to a 2x8 wall?

 

[UcfSE]

Can you use a stronger species of wood? Can you offset the trusses so you can have the full width of the 2x6 for bearing?

 

[jike]

Offsetting the trusses is a thought although it may require cutting some sheathing. A stronger specie of wood often gets ignored by both contractrs and inspectors.

 

[jheidt2543]

What about looking at 2x6 studs with a double 2x8 top plate? Another possibilty along the line of UcfSE's comment, use an engineered wood member at the plate, like a paralam or microlam?

 

[CJJS]

You could try to use the bearing area factor Cb. This may get you closer to the allowable that you need.

 

[FSS]

Are you sure SPF is the top plate material. In many areas, it is more common to see SYP top plates. Then you are looking at 565 psi allowable.

 

[jike]

SPF #2 is normally what I see used in this area. SYP is a good option but we will have to make sure that our inspectors enforce it.

 

[RARWOOD]

When I was a consulting engineer I would specify a maximum bearing stress that the truss supplier could not exceed at their bearing locations. I was probably not popular with truss suppliers for doing that.

Most of the buildings I designed were in the Minnesota area, and where at that time it was very common to use SPF top plates. When I first started checking truss shop drawings I found that the truss suppliers would use high quality wood for their chord members and base the bearing requirements on the allowable bearing stress for the chord. If the truss supplier was using Douglas Fir and the top plate was SPF there was a conflict.

So after my first few buildings I started specify a maximum bearing stress so that I knew the truss reactions would not exceed the top plate bearing stress.

Either approach you take is going to increase some ones cost. You might talk to a local truss supplier to obtain an estimate of the increased cost to them for meeting the bearing stress if 2x6 SPF top plates are used.

I would guess though that the savings arising from the use of 2x6 walls instead of 2x8 walls would exceed the extra cost to the truss supplier.

 

[woodengineer]

NDS does allow for a bearing factor increase when the bearing length is less than 6" and you are not closer than 3" from the end of the beam (wall in this case).

Fc'(Perp)=Fc(perp) X Cm X Ct * Cb

where, Cb = (L+.375)/L therefore you would be able to increase your allowable bearing from 1594# to 1833#. In your case this didn't help but might on future jobs.

In your case, have the builder price out bearing enhancers and 2x8 studs. They may also opt for LSL (Timberstrand) top plate.

Jordan

 

[onteng]

Offsetting the trusses is still the cheapest solution. Alternate measures to review may be metal truss enhancers. I know Simpson Strong-Tie has this type of connector which provides uplift resistance and also increases bearing capacity.

You could also indicate truss scabs at the support points to increase bearing. This is usually a small section of lumber connected to bottom chord at the wall. We have specified this in the past and became part of truss design