Is this truss profile manageable?

[E M]

Is this truss profile manageable?

65'-0" span with bearing points far left and right of the profile.

Need the end extensions so can work with some slope concerns with getting the truss elevated at the center. The thinking goes that basically just build in a parallel chord truss and piggy back another truss on top. But in reality just build the truss this way.

It's kind of bugging me in one way but also seems ok to me in another.

If can design this way then will eliminate the need for 2 additional beams and all the additional support structure if they were needed to be placed at the normal heel location.

Figured this was a good place to start with the question.

Thanks.

 

[RontheRedneck]

I want to address some comments that Lexpatrie made:

"the 65' span and the height don't sound particularly practical from a shipping context"

A 65' truss is no big deal. We have stretch trailers that go up to 70' or so. We build and ship trusses up to 80' on a regular basis.


"...the height, if it's shipped flat is in the 8' zone."

The height we can build is about 13' 2". So this truss may or may not need piggybacked.



"The end bearing can be problematic (bearing stress) and the wall top plate can be easily ignored in design and wildly overstressed."

The bearing area is checked on every truss design, every time.


"usually on these long trusses there's the temptation to use the lowest possible loads"

No idea why you would say that. Offhand I'd say the opposite is true. We're concerned about handling, so we're more conservative in design.


From the OP:

"How deep would you suspect bottom line min would be for the "tails". "

If they were 2' deep I think they'd work. Shorter than that and I'd have to run it.

 

[Brad805]

My crews would not like this. They have found that unless the truss manufacturer can hold a nearly perfect line their wall lines are not as straight as they like. Obviously this will come down to the local supplier.

 

[RontheRedneck]

Thought of a couple of things to add -

The flat areas at each end will most likely not have plywood on them. They'll need purlins to brace the TC.


Check totaldeflection on a span this long - Not just a deflection ratio.

Around here it's generally O.K. to have a total load deflection ratio of L/180. That's over 4".

 

[lexpatrie]

Ron, I don't mean your bearing check, I mean the load hitting the top plate when it doesn't match up with a stud. That puts the top plate into bending that is difficult to justify in an analytical sense (one could handwave that the sheathing will support it, but I'm not aware of any testing that demonstrate that actually works, and there is the occasional project with "no" sheathing (metal panels, for example).

My comments about loading don't refer to the truss designer, somebody shaves 5 psf off the truss and hands you the loads, you're designing for the loads specified. That's delegated design. I mention that for a reason.

To Align or Not to Align, Steimle, Structure Magazine, Feb. 2024.

65' x 2' spacing x say, 16 psf live + 15 psf dead? (31 psf TL, 62 plf TL, = 2,015 lbs. That's alright for the average 2x6 wall, but the point load spanning 16" between studs on a 2x6 top plate oriented flat (which isn't the truss suppliers job) feels pretty high. The way I was looking at that slope I figured it was something 3"/12" for shingles, 32.5' to center, 3 " per foot = 8' already without considering the flat ends, and the minimum depth at bearing. I'll just grab a 1' minimum depth for the flat end as a DD value and then we have 9' shipping height. I guess there's room given you folks are able to do 11'+.

 

[E M]

Thanks everyone for the education and interest. As an aside, I've come up with a way to elimnate the funky jog at the right end so simplifies this somewhat. Will be addressing the design with a fabricator soon.

Ron- I currently have drawn at 2/6 deep for the 'parallel chord" part.... for sketch purposes. Thanks for the insight and your take on 2/0 at least being something that the truss engineer could run numbers on. Also thanks for the real world build limits per your experience.

 

[phamENG]

phamENG, not sure why you said it would be a 2 or 3 part truss. It might need to be piggybacked, depending on the height. But it doesn't look like a big deal to me.

Ron - I could see a stretch trailer being fine in a rural area, but where I am most of the buildings going up can barely be accessed by a standard semi. So, around here, a truss of this size would need a splice and a piggyback to ship it. Just something for the OP to consider. Don't see too many trusses of this size, though. Except one church where they failed to install in the bracing correctly and, when I looked at 10 years later about 60% of the trusses had buckled and were a good 2 to 4feet out of plane.

 

[RontheRedneck]

Lexpatrie said: "I don't mean your bearing check, I mean the load hitting the top plate when it doesn't match up with a stud."

Gotcha. Although I haven't seen where it's a problem in the fields.



Lexpatrie also said: "My comments about loading don't refer to the truss designer, somebody shaves 5 psf off the truss and hands you the loads, you're designing for the loads specified."

We have minimum loads we design for regardless of what's called for on the plans. Although not everyone does that.



phamENG - I get what you're saying. It's definitely not an issue I have to deal with most of the time.

 

[KootK]

Although I haven't seen where it's a problem in the fields.

In the markets where I've worked, trusses are only aligned with studs where:

1) It's an accident.

2) It's a girder.

3) It's a special "advanced framing" project.

The norm for me is having to assume that trusses load top plates in bending as lexpatrie described.