Bottom Chord Bearing Truss w/ Raised Bearing Seat

[gte447f]

I am designing a building with flat, bottom chord bearing, metal plate connected, wood roof trusses. The truss span is approximately 45 feet. I am thinking the bottom chords will probably be 2x6 oriented in the strong direction.

The bearing walls are wood stud walls with a 2-2x6 top plate. The typical truss bearing elevation on the 2-2x6 top plate is 14'-4-1/2". At the front of the building there is a storefront glazing system with 5-1/2"x11" glulam header beams. The elevation of the top of the beams is working out to be 14'-5-1/2", just 1 inch above the typical truss bearing elevation.

Here is my question. Is a 1" raised bottom chord bearing seat OK and easy to fabricate for metal plate connected wood trusses, or is some other standard dimension better, like for example, is it better to raise the bearing seat by an amount equal to the typical bottom chord dimension (5-1/2" in this case)?

If something other than 1" is desirable, the I think easiest way to adjust the truss bearing elevation will be to add 2x6 nailer plates to the top and/or bottom of the glulam beams.

Hope this makes sense.

 

[Eng16080]

I can't speak to the ideal dimension. You would have to talk to a truss supplier. (There's a guy here, RonTheRedneck, who's an expert in this, if you could somehow summon him.)

I can say, however, that I've used this sort of detail many times without issue. It's too bad that the header can't be 10" to avoid this, but I'm sure you've already spun your wheels trying to make that work.

 

[jayrod12]

I don't see an issue with doing this. However, I would recommend trying to get the wall plate and the beam bearing to align one way or the other. Maybe it's as simple as adding another plate and pushing the beams up the 1/2" to make everything bear at 14'-6". That would allow for all trusses to be the same (which is usually where the economy lies). and then the trusses could be either 1 1/2" shallower, or the entire building ends up 1 1/2" taller.

 

[gte447f]

Thanks Eng16080 and jayrod12.

Jayrod12, good advice to make an adjustment to get everything at the same bearing elevation. Unfortunately, it is too late in the game for any wholesale changes on this particular project, so I am just going to go with the 1" raised seat condition. There are several other "issues" with this arch design, so if I end up doing anymore of these buildings with this arch, I will add this to the list of items to coordinate early in the process.

 

[jayrod12]

Been there. Got to make the best of a bad situation.

 

[lexpatrie]

I fail to see the value. Do what you can to make it uniform bearing for all the trusses.

 

[RontheRedneck]

I don't have an example to post at the moment, but we do this sort of stuff all the time. In this case it sounds like a 2X6 vertical that's held up an inch (or whatever) would do the trick.

On a note that's not exactly related - I strongly recommend having adequate depth for the trusses. The shallower they are, the more prone to toppling they are during erection.

 

[jayrod12]

The shallower they are, the more prone to toppling they are during erection.

This seems very counterintuitive.

And that's only because taller thins are generally more unstable. But I get how compressive stress would be far higher under only self weight case and therefore more unstable. Just still feels counterintuitive.

 

[RontheRedneck]

"I get how compressive stress would be far higher under only self weight case and therefore more unstable."

You stated the problem perfectly. What's counterintuitive about it?

 

[jayrod12]

My tall thin unstable things comment is why it seems counterintuitive at first.