Wood Truss Bearing Options

[Gopher13]

I am designing a truss composed of 4 x 8 wood members (or something like that). The attached sketch shows two different bearing options. The first one has the bottom chord bearing on the wall right where the top chord runs by leaving only a small sliver left. The second one has the top chord notched and bearing on the wall. I do not like the notch. I also do not like the small sliver left at bearing, but I don't really know why. There is not really any shear in the bottom chord so the reduced section height shouldn't matter. Which option do you like? Or maybe you have a better idea?

 

[BAretired]

The first option gets my vote. It's cleaner and easier to fabricate. Truss plates must be adequate to fully develop both top and bottom chord. I can't see a problem with the small sliver above the bearing.

BA

 

[jayrod12]

Most definitely option one. Do you not have minimum heel requirements in your area? Where I practice 14" heel is a minimum, in fact now 16" is much more common.

 

[dhengr]

Gopher13:
You could leave a couple inch high vert. end cut on the bot. chord in option #1. The bot. chord length matches the out to out of the bearing wall studs or sheathing. Then, the rafter chord sits on the sloped cut, as you show. You still need hardware of some sort to transfer the thrust at this joint. You could use a shear plate or split ring connector dapped into the sloped surfaces of the joint. In option #2 you are hanging the bot. chord and must transfer the trust too, a more complicated connection. The birds mouth bearing cut is most practical and effective in light framing where you side lap the rafter and the ceiling joist. It gives you horiz. bearing on the top pls. for both members. Sometimes you can change/improve this joint’s detailing by using a larger top pl. on the brg. wall. I’d make minor changes to option #1.

 

[Ron]

Option 1 is better. Option 2 puts more stress on the top chord to bottom chord connection, unless you "bucket" the bottom chord at the wall.


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[Gopher13]

Thanks for the replies!

Do you not have minimum heel requirements in your area?

I am not aware of such a requirement. You are referring to the horixontal distance from the outside face of the wall to end of the truss, no?

dhengr: I will play around with your ideas a little bit and see what happens. I do like your vertical cut on the bottom chord idea. Thank you!

 

[azcats]

Heel Height: [URL unfurl="true"]http://www.rooftrusses.ca/How-To-Measure-Heel-Height.html[/url]

 

[Gopher13]

Thanks azcats…...I was way off. There is no such requirement for what I am designing in the American state this is at.

 

[KootK]

What is your anticipated heel connection typology here? The answer to that question would affect my preference. Based on your 4x8-ish chord sizes, I'm guessing that this is not pressed, toothed plates but, rather, something more like steel gussets and bolts.

I see the pros and cons of the two alternatives a bit differently from others:

1) Option 2 is a problem if the connection will be pressed, toothed plates. Too much cross grain tugging on the top chord.

2) If your connections will be something like bolted gusset plates, rather than pressed, toothed plates, then I don't see that Option 2 is really any worse off than option 1 in terms of connection performance. In fact, option 2 may actually be better as I'll explore next.

3) As shown below, the truss joint has less eccentricity in it with option 2. The pre-eng truss guys use special, faux heel joint models to deal with this for option 1. EOR's seem to mostly ignore it in my experience.

4) As shown below Option 2 orients your truss bearing surface such that you're partially utilizing parallel to grain stresses which is always nice. Of course, if you're wall top plates aren't giving you any trouble, your bottom chord, perpendicular to grain bearing stresses likely aren't either.

5) I also recommend the vertical cut that dhengr mentioned. In the truss world, that's called a butt cut on the scarf cut. The pre-eng guys like this to be at least 1/4" tall. For heavier members like this, I think that I'd want a butt cut at least 1" tall. As the guy who used to make these scarf cuts, I can tell you that:

a) at lower pitches, it's almost impossible to make the cut and nail the member length at the same time without the butt cut. The butt allows you to accurately establish the length.

b) a low pitch, fade to nothing scarf cut usually ends up looking crappy where it goes to zero depth. The tip gets all hairy and often just breaks off or, generally, looks like butt. I suspect that similar concerns are what created your visceral dislike of the sliver and I feel the same way. Is this an exposed truss?

6) For option 2, I think that we all dislike the notch. It creates a stress concentration and a reduced section for overhang flexure. However:

a) The whole bird's mouth cut thing makes for a nice, old-timey look that some folks crave.

b) For most applications, overhang flexure is minimal enough that it still works at the reduced section.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[bhiggins]

The configuration of the truss bearing can have a significant effect of loading on the individual members. If the truss is bearing on the top chord it can cause higher loads perpendicular to the grain for the bottom chord:

 

[KootK]

@bhiggins: are you able to tell us what publication that comes from? I'm a sucker for hand drawn sketches.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[BAretired]

Why would a code specify a minimum heel height? I'm guessing it might be to maintain enough depth for loose fill or batt insulation.

BA

 

[KootK]

Why would a code specify a minimum heel height? I'm guessing it might be to maintain enough depth for loose fill or batt insulation.

Precisely that. Energy stuff.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[Gopher13]

I'm guessing that this is not pressed, toothed plates but, rather, something more like steel gussets and bolts.

Yes. Steel plates each side and bolts. The building is going to be a non-heated/cooled cabin on state land that people can rent out and stay in. The trusses will be exposed so they are not going for the pressed plate look.

the truss joint has less eccentricity in it with option 2. The pre-eng truss guys use special, faux heel joint models to deal with this for option 1. EOR's seem to mostly ignore it in my experience.

I was going to ignore it, but now that you have drawn it out, there is a decent amount of eccentricity.....Walls are going to be solid logs so I can't imagine the eccentricity will be an issue, but I do not have any experience with solid log walls so who knows. I was going to figure that out after I get a handle on the roof.

I also recommend the vertical cut that dhengr mentioned

Noted. Thanks!

a low pitch, fade to nothing scarf cut usually ends up looking crappy where it goes to zero depth. The tip gets all hairy and often just breaks off or, generally, looks like butt. I suspect that similar concerns are what created your visceral dislike of the sliver and I feel the same way. Is this an exposed truss?

Yes and yes.


bhiggins: Thanks for the diagrams.

Precisely that. Energy stuff.

With spray foam, I would think the required height wouldn't be very tall. jayrod12 mentioned 14 inches. Seems like a lot?

 

[jayrod12]

We require R-40 or R-50 depending on the region.