Long Span Wood Truss Roof/ Overall Stability

[mijowe]

I have a structure where a few of the common discussion on here are converging on each other and I wanted to get opinions on my approach.

• I have a long span (70’) mono sloped trusses with the top and bottom chords at different slopes. The trusses are bottom chord bearing and spaced at 7'-0" on center. The truss supplier is modeling them as pinned roller, I will be making a positive attachment at each support.

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• I have T&G deck topped with plywood to create a diaphragm. The trusses are exposed from below, so there is no ceiling diaphragm.
• The trusses are supported at each end by a wood wall. Because the of the slope the wall on the high end is a level up from the low end.
• Wind on the walls, ends of trusses, and suction on the top chord are producing the lateral loads on the system.
• The room is 126’ wide and I have shear walls at each end. My lateral load path is into the trusses, then the diaphragm and then the shear walls.

The load path for the lateral load on the walls is dubious to me, it needs to travel through the bottom chord then the webs and into the diaphragm.
Just looking for thoughts. Am I missing anything? Are there ways to add redundancy without adding too much additional cost?

 

[manstrom]

Wood truss? 70' is a tough one to put on a truck.

How is the sheathing going to span 7'?

I'd worry about truss bracing too. What will be required for the uplift and probably diagonals.

Wind loads from the walls will go from the bottom chord up to the top chord, then diaphragm.

This is a big structure and it needs to be well detailed and well thought out. I do a lot of wood, but never a roof this wide.

I'm sure you are considering the sloped bearing conditions.

 

[DaveAtkins]

In my opinion each truss can transfer wind load from the bottom chord to the top chord and diaphragm through the truss webs. I don't think this will be a problem for each truss, but it is probably a good idea to let the truss supplier know about this additional load.

DaveAtkins

 

[mijowe]

70' on a truck is bad, so is the road to the site. The GC says they can do it.

For the roof deck we have 2" T&G plank that works for gravity loads, and 3/4" plywood on top for the diaphragm.

Truss bracing is another challenge, because it is exposed the architect has their own ideas on what they want to see.

The sloped bearing is a detail in progress, but defiantly being considered.


Thanks for the input!

 

[dik]

Can you use a tapered top plate? Are your diaphragm forces high enough that you need plywood or even 3/4" plywood? or even plywood? You could have 5 or 6 lines of bracing. The truss supplier should be able to tell you what you require (subject to artifact approval). The end verticals, being in compression with a 'zero member' attached could be cut square and bear on the top plate.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JStructsteel]

How are the loads being transferred from the diaphragm to the bearing walls? I assume your walls go up between the trusses?

I cannot see this being built w.o bracing, or without spending a ton of money on the trusses.

 

[RontheRedneck]

Several things come to mind for me.

One is that shipping 70' trusses is no big deal. We have stretch trailers that can haul 80'+ trusses.

Someone mentioned sloped bearing conditions. We would put a flat seat cut on the lower end, and run a vertical through at the high end. No need for a sloped bearing.

Wind loads - I don't see a problem with that as long as you communicate what you want to the truss supplier.

The truss designer should specify the BC bracing needed in wind uplift load cases. In the software we use, there's a "max" setting. It calcs the BC purlin spacing that's needed in wind uplift cases.


My biggest concern by far for this truss is erection bracing.

(Yes, I know the word "erection" has another meaning. I'm using it in the context of getting the trusses set in place. No double meaning intended)

I've been out on far too many jobsites where the trusses were lying on the ground due to inadequate erection bracing. That costs everyone money and time. People get hurt - Sometimes seriously.

But when I try to talk to contractors about it, their eyes quickly glaze over. "We now what we're doing". They don't want to hear it.

Back in 2019 I did trusses for a job that had long span parallel chord trusses roughly like the one in the OP. When I did sealed drawings I sent a letter to the GC along with them. I explained my concerns about bracing long span low sloped trusses. In return for my efforts I got the usual attitude thrown back at me.

After we delivered the job I saw this on the news:

https://newschannel20.com/news/loca...f-building-collapses-near-carle-at-the-fields

I hope you'll take my concerns into consideration, and push the contractor to use adequate bracing.

 

[mijowe]

The roof diaphragm does not work with only the T&G plank so I added the plywood on top. The thickness of the plywood is a result of getting the right nail length and not having them poke through the exposed t&g plank.

We are working on the bottom chord bracing; the Architect wants every other panel point.

I would like a square cut of the chords, as it stands now the truss supplier will not provide it that way.

Between the trusses I have infill walls to extend them to the roof sheathing.

 

[mijowe]

As far as truss bracing goes, the contractor has been alerted of the issues and is taking them seriously. The truss supplier has conveyed this concern as well. Ther is currently language in the IBC concerning special inspections for trusses over 60' and so the bracing is suppose to be inspected as well. At least for now, this is getting the attention it deserves.

 

[BAretired]

• I have a long span (70’) mono sloped trusses with the top and bottom chords at different slopes.

It doesn't appear that anyone would even notice the difference in slope. Varying the slope between chords has no structural advantage. Why not make it a parallel chord truss for simplicity in fabrication?

 

[jayrod12]

I would like a square cut of the chords, as it stands now the truss supplier will not provide it that way.

This seems a bit ridiculous. Give your contractor the option of the sloped bearing plates (keeping in mind the required fastening to prevent it from sliding down the slope), versus a flat wall plate with a modified bearing detail. He should realize the simplicity with the truss accommodating the slope with a modified bearing detail and be forcing the truss supplier to accommodate.