Existing Wood Bowstring Truss Evaluation 1

[Jason T]

Hello,
I am currently working on a project with some wood bowstring trusses that span 120 ft and are 16 ft deep at the center. I'm assuming based on conversations with contractors in the area that these were constructed in the late 1950's or 1960's. I do know they were built on site and the top chord consists of a glulam SP member that is (6) 1 5/8" x 9 1/4" with an additional (3) 1 5/8" x 3 1/2" glulam section attached to the bottom of the larger section. The bottom chord is (2) 4 1/2 x 11 members. The panel points are 12 ft o.c. with a vertical 3 1/2" x 3 1/2" member at each location. Evert vertical has a steel plate at the bottom chord and top chord with a steel rod on each side of the chord. The diagonal members are strange, they consist of (2) 2x6's in an "L" shape nailed together.

The building has been vacant for roughly 15 years and a developer is bringing it back to life.

I'm using Risa 3D to model this and the web's are failing with just the standard roof loading. Adding the unbalanced loads makes it look as if this thing should never had been standing in the first place. The interior changes are adding additional supports under some of the trusses and others are getting full height walls built under them for support. This is causing a lot of web members to go into compression and reinforcement needing to be installed.

My question is, would this type of truss be designed as an arch with a tie? I'm treating it like a typical truss analysis. I can't find any sources of information regarding a truss design like this.

Any input, assistance or advice is greatly appreciated.

 

[JStructsteel]

any way you can run, run fast from this?

I did some bow string truss repairs a while ago. The contractor used tilt up braces to stabilize the truss and take the load why things were fixed.

It was a mess, the truss was so delicate they would fix one area, and that would cause some damage at other areas. Alot of the wood was dry rotted.

 

[Jason T]

Unfortunately I’m not able to run from this one.

 

[dik]

With that type of delamination, I'd seriously be looking at epoxy injection... and I think the gaps are too large for penetrating epoxy. I don't see another manner of reaffirming the horizontal shear capacity of the members (if needed). You will have a hard time doing that with mechanical fasteners... is there any heritage issue? It may be less costly to replace the roof. Is there a chance you can post a series of photos that show the elevation of the worst truss?

For epoxy injection, I don't think you have to get the 'wrinkles' out... it might take a little research, and the epoxy is a lot stronger than the original adhesive (if there was any) and a lot stronger the the horizontal wood shear stress. It's not the thing you want to 'run from'... it's the type of project that makes engineers...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Jason T]

Dik, could you explain what you mean by heritage issues? I’ll be discussing the epoxy option with the contractor. Trick will be finding someone who knows how to do it properly.

 

[dik]

Is there any historical merit to the construction... about 30 years back I did some repairs to some heavy timber trusses in a Town Hall for a city in Ontario... some were badly burned and I repaired the damage with plated steel... as was intended by the politicians... in hind sight, I could have provided a similar, less obtrusive repair that 'honoured' the vintage of the building, and I should have. As far as epoxy injection, you may want to talk to some contractors with concrete experience... I've used epoxy injection for timber (in particular, delaminated glulam), but it is more common with concrete... They also use polyurethane adhesives for injection, but my experience is that epoxy is more rigid. They also use epoxy injection and penetrating epoxies for boat repair caused by brown rot (aka dry-rot).

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

That's one of the first things you look for with restoration/repair work with wood.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik