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Bearing stress in truss bottom chord

Bearing stress in truss bottom chord

(OP)
What I have is ordinary light metal plate prefab wood trusses (mostly 2x4 members) spaced at 2' centers with 3 bearing points, and the interior bearing point is a 2x4 stud wall, and of course I can't change the wall to 2x6 because of the architect.

The truss calculations say the bearing length needs to be about 5", much more than the 3.5" actual. I calculate the bearing length required to be about 3.8".

Am I missing something? Is the bearing stress combined with the tension stress in some ratio?

I calculate reaction divided by allowable bearing stress to get required bearing area on the bottom edge of the truss bottom chord, and Load Duration Factor is not applicable. All working stress design.

RE: Bearing stress in truss bottom chord

can you add 2x on each side of the truss?

RE: Bearing stress in truss bottom chord

(OP)
I am not the truss detailer or manufacturer, I just need to get a second opinion on the calculations submitted to me. (I am the project EOR).

A doubler is a possibility, but is not up to me to make that choice if necessary. Maybe they (truss mfr) can use a different species of wood for the bottom chord.

RE: Bearing stress in truss bottom chord

Surely there must be some connection of the truss to the plate. You can incorporate a bearing plate or similar into that connection to extend the width of bearing to make up for the lack of length.

RE: Bearing stress in truss bottom chord

(OP)
hokie,

The truss sits on the wall top plate with only a Simpson H2.5T

I didn't see anything in the Simpson catalog that would help, and if the bearing stress is simply reaction divided by unfactored allowable bearing stress, then the truss software must be in error.

RE: Bearing stress in truss bottom chord

You might throw some numbers on here so we can see the approach. Might help us help you.

RE: Bearing stress in truss bottom chord

AELLC:
Is that 2x4 bearing wall strong enough to take these reactions? Is there a stud or two immediately under each truss? Is the truss really continuous over this wall, or is it more like two half trusses, thus simple beam reactions, or could it be? The Arch./owner dictated this abortion, so surely they can take 1.5" out of the kitch./dinning/great rm. so you can have the 2x6 wall their design dictates. Put a 2x6 top pl. on top of a 2x4 pl. on the wall, and hide it with the cove molding this complex roofed house deserves. Take some .125" or .1875" bar stock, 3.0" wide x 6" long and put this atop the wall, under each truss, they are trying to pick up a little more bearing length under their bot. chord. Better yet, use a 6" long bar sized channel 2.5 x .625 x .1875", with the legs up under each truss. You must check the compression perpendicular to the grain of your wall top pls.

RE: Bearing stress in truss bottom chord

Like JAE stated without numbers you can not get an answer on this. One option (and I do not know why the truss manufactor did not do this) is to have a vertical web at the bearing and run it through the bottom chord so that the parrallel compression of the web can be used to reduce the truss bearing requirements down to 3.5". It will not make the wall work but the bearing to the truss will. They may be trying to be nice and show you that you have a problem? but a phone call from them would of been better.

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.

RE: Bearing stress in truss bottom chord

(OP)
I calc'ed this again - the truss software was correct, it was my mistake. I had the studs at 12" o/c but I am also going to specify that a stud occurs directly under each truss.

The 5" bearing length was correct, so the truss mfr is going to have to come up with a solution.

RE: Bearing stress in truss bottom chord

(OP)
dhengr -

This is a custom revision of a standard plan, so the Homebuilder can't change the wall to 2x6 - it has been 2x4 for at least 10 years.

It has always been studs at 12" o/c but they weren't specified as being aligned at the each truss.

RE: Bearing stress in truss bottom chord

(OP)
Here are some calcs - the brg stress is 524 psi which is too much for SPF but would be OK if they used DF-L.

The double 2x4 plate is overstressed in bending, but fortunately not very much with dead load only. We are near Phoenix, and it is very unlikely to get any kind of live load on a 5:12 roof with concrete tile, unless a momentary downdraft with one of our local microburst weather events.

RE: Bearing stress in truss bottom chord

I would have thought wind uplift would be the controlling factor in the top plate bending.

RE: Bearing stress in truss bottom chord

Not really a wood guy but I will give my opinion anyway. ha

I thought Simpson had some sort of bearing enhancer inside of the catalog. I'm not sure if they would help or not:

http://www.strongtie.com/products/connectors/tbe_p...#

If that doesn't work and the mfr is not helpful with your problem, couldn't you detail the top plate of your bearing wall down a little bit an put a steel shim large enough and thick enough to get proper bearing on the truss and the wall (a 5-1/2"x5-1/2" plate)? I would think the ceiling would be strapped so the added thickness wouldn't be a problem.

Just a thought.

RE: Bearing stress in truss bottom chord

(OP)
Hokie,
The uplift runs about 540#. The roof is relatively heavy (22.5 psf) and wind is not to bad in this region.

SteelPE,
Those TBE's do look very practical. I am just going to see if the truss mfr is willing to change the species of wood of the bottom chord.

RE: Bearing stress in truss bottom chord

A couple of points to consider,
1) The double top plates are acting like continuous beams over supports. Now one of them maybe spliced within the span but would be two cantilever beams for support or splice over a stud so it is free one end and continuous the other. Did you try analysising it this way?
2) For the bearing requirement, did you use the bearing factor (per the NDS) of 1.25 for 1.5' bearing and 1.1" for a 3.5" bearing?
3) SPF(S) was not to common of a grade of lumber in the Phoenix area ten years ago. They were using SPF which has a 425? psf bearing value. Unless they are showing the (S) at the end the values will be for the plain SPF which are higher.

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.

RE: Bearing stress in truss bottom chord

(OP)
Woodman

1) This point is worrisome. If it were true, then even common situations such as 2x6 double plate with a 40 foot long truss would make the plates fail. I hope the splices occur at a stud - and I certainly will specify so.

2) That looks the allowed increase for the top plate, but the bearing length problem is more for the truss, and I don't think the increase is allowed for the truss. Originally, my calcs were for the top plate, and I did use that factor, and the specified wood for top plate is Hem-Fir (405 psi) - it was only slightly overstressed.

3) When I went to the WWPA website they listed commonly used species for the Western USA. They show SPF(S) (335 psi) but not SPF. The truss calcs show SPF, but I don't know what the allowable is for that.

It turns out most of the trusses have flat bottom chords, so I think the Simpson TBE4 is the logical solution. There were a couple of trusses where the bottom chord steps down, which makes the TBE4 impossible to install, so I am going to ask the truss mfr to make the vertical web there (actually it is about 80 degrees) go thru down to the plate elevation as suggested by you in your previous post.

RE: Bearing stress in truss bottom chord

If you are going to design with wood you should have a copy of the NDS and Supplement to read. One, it will give the values for SPF lumber and Two, it will tell you when and how to use the bearing factor. Such as, all bearings less than 6" place more than 3" from the end of the members. So at your bottom plate splice at a stud it can not be used.

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.

RE: Bearing stress in truss bottom chord

If it proves to be that much of a problem driver, then just go to a triple top plate so that there are always two plates continuous over any stud...

Mike McCann
MMC Engineering

RE: Bearing stress in truss bottom chord

AELLC:
Read my 21OCT13 post, the truss people need a little more bearing length on their bottom chord over this 2x4 wall. “Take some .125" or .1875" steel bar stock, 3.0" wide x 5.5 or 6" long and put this atop the wall, under each truss, they are trying to pick up a little more bearing length under their bot. chord. Better yet, use a 6" long bar sized channel 2.5 x .625 x .1875", with the legs up under each truss.” The steel is wider than 1.5" which helps your top pl. problem; and its longer than 3.5" which solves their problem. The steel acts much like a column base pl. to spread the bearing load out along/on each member.

RE: Bearing stress in truss bottom chord

(OP)
Mike-

Agreed. It is not a problem with full custom houses because I can specify anything necessary, but this case is a 10-year old design for a standard tract home which is being semi-customized for an owner, and I am working for the construction company. They don't tolerate changes to a design they assumed was good for 10 years. The original design was done by someone else and I didn't detect this wall problem at the time - and the truss calculations were not submitted to me until the plans checker noticed bearing overstress of the truss chords.

RE: Bearing stress in truss bottom chord

(OP)
dhengr:
No that would be too expensive - I am replacing the H2.5's with TBE4.

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