How to deal with these roof trusses

Relaxia · 4 Mar 2019 at 4:33 PM

We are planning to expand with a low-pitch roof. Span 7.2m, total length 8m. Shed truss.

The roof will also have two roof domes 1200x1200.

The designer for the extension proposed a glulam truss (56x495mm). That dimension is not standard. Should I glue together 56x225 and 56x270 beams?

I gave the measurements to a nearby truss manufacturer who designed a truss with their program. The problem with that truss was that they were to be set at c-c 600 instead of the intended c-c 1200, conflicting with the roof domes!

His suggestion was that the trusses could be doubled up, but the program he uses couldn't calculate the nailing.

What do I do now? Can I sort of screw the trusses together in pairs and go ahead?

mexitegel · 4 Mar 2019 at 4:52 PM

Can't you ask the designer to use standard dimensions?

sir_daniel · 4 Mar 2019 at 5:02 PM

Also calculate with a steel beam instead of glulam.

Autodidak1 · 4 Mar 2019 at 5:37 PM

R Relaxia said:
We are going to extend with a low-slope roof. Span 7.2m, total length 8m. Shed roof truss

The roof will also have two roof domes 1200x1200.

The designer for the extension came up with a truss in glulam (56x495mm). That dimension is not standard. Should I glue together 56x225 and 56x270 beams?

I gave the measurements to a nearby truss manufacturer who came up with a truss with their program. The problem with that truss was that they would be set c-c 600 instead of the intended c-c 1200, which conflicts with the roof domes!

His suggestion was that the trusses could be doubled, but the program he uses couldn't calculate the nailing.

What do I do now? Can I screw the trusses together in pairs and proceed?

I don't really understand the problem, let the designer come up with a standard beam. Works perfectly with both LVL (Kerto) or glulam. Redo and get it right...

Relaxia · 4 Mar 2019 at 6:09 PM

Ok, I understand you. Try again, do it right, etc.

Due to various reasons, I won't be contacting the designer who created the blueprints for the building permit again.
You would think the truss manufacturers should be able to produce sensible trusses themselves.

I guess I need to check with more manufacturers. I thought it would be good to contribute to the local industry.

Perhaps someone external could design a better truss that the local manufacturer can produce.
Do you have any recommendations for someone?

Autodidak1 · 4 Mar 2019 at 6:37 PM

R Relaxia said:
Ok, I understand you. Redo it right, etc.

The designer who created the drawings for the building permit I will, for various reasons, not contact again.
The truss manufacturers themselves should be able to produce decent trusses, I think.

I guess I need to check with more manufacturers. I thought it would be good to support the local industry.

Perhaps one can see if an external party can design a better truss that the local manufacturer can produce.
Do you have any tips for someone?

The simplest way is to go to your local dealer, show them the drawing, and say that you want a glulam/plywood beam at cc1200 and ask what you need. Their suppliers (Moelven/Martinsson etc.) calculate this for free so that they can make a sale. It takes no more than 5 minutes in a program. I would guess that a plywood beam 45x450 mm might be suitable for your needs.

PS. Then it's smart to have skylights that are 1100x1100 (or 1150x1150) so you can have the same cc on all trusses and don't need to reinforce.

justusandersson · 4 Mar 2019 at 9:05 PM

Converting a 56x495 mm glulam beam to another dimension is very simple and something that anyone knowledgeable in structural engineering can handle. You calculate the beam's moment of inertia using the formula bxh^3/12 (b=width and h=height) and look for a beam that has the same or higher moment of inertia. In your case, you can take 90x450, which is guaranteed to be a standard size.

Autodidak1 · 4 Mar 2019 at 9:19 PM

J justusandersson said:
Converting a 56x495 mm glulam beam to another size is super easy and something that everyone knowledgeable in structural engineering can handle. You calculate the beam's moment of inertia using the formula bxh^3/12 (b=width and h=height) and look for a beam that has the same or higher moment of inertia. In your case, you can go for 90x450, which is definitely a standard size.

I agree with you, but I wouldn't use the original figures at all...

marka · 4 Mar 2019 at 9:23 PM

Unfortunately, I have nothing to add to the thread itself, but I have seen Justus helping with calculations for various constructions in several different threads, and it just makes me so happy to see someone so generously sharing their knowledge. I sincerely want to thank you for helping to keep the forum vibrant! There are lots of other members who also share their knowledge, who of course deserve just as much praise, but today is your turn!

justusandersson · 4 Mar 2019 at 9:30 PM

It's always nice to receive praise even if I blush a little...

Relaxia · 5 Mar 2019 at 7:50 AM

I appreciate your suggestions, and if I were to calculate the beam for my own project, I would probably recalculate it myself as suggested.

But now it's an extension with building permits and a construction manager, and if I'm going to deviate from the original construction drawing, I want documented proof that it will work.

Right now I'm waiting for a quote from another roof truss manufacturer. Let's see if they also want to set c-c 600.

Autodidak1 · 5 Mar 2019 at 8:51 AM

R Relaxia said:
I appreciate your suggestions, and if I were calculating a beam for my own project, I would probably re-calculate it myself as suggested.

But now it's an extension with building permits and a construction manager, and if I'm going to deviate from the original construction drawings, I want documentation to ensure it will work.

Right now I'm waiting for a quote from another roof truss manufacturer. We'll see if they also want to set c-c 600.

If you let the building supplier calculate, you'll get documentation for it...

It's the supplier's engineers who calculate it for free...

Relaxia · 5 Mar 2019 at 9:42 AM

A Autodidak1 said:
If you let the builders' merchants calculate, you'll get it on paper...

It's the suppliers' engineers who do the calculations for free...

That could be a solution, but who converts the beam into a truss?

Ok, you could probably do it yourself, but it would mean I have to cut the beam as it needs to be 300mm at the front and just under 1m at the back (8m long, 7m span). Quite a low story.

I'd rather have someone else cut it and take responsibility for it.

Autodidak1 · 5 Mar 2019 at 10:05 AM

R Relaxia said:
That could be a solution, but who converts the beam into a truss?

Okay, I could do it myself, but that would likely mean I have to cut into the beam since it needs to be 300mm at the front and just under 1m at the back (8m long, 7m span). Quite a low profile.

Preferably I'd let someone else cut and take responsibility for it.

You are creating problems that don't exist. If you buy a beam that is, for example, about 450 mm high, the builder will cut the truss tails the way you want them. It is a common method when using beams, although you can do it differently.

Relaxia · 5 Mar 2019 at 10:36 AM

A Autodidak1 said:
You are creating problems that don't exist.

Story of my life

But why does it have to be so complicated then?

I want a bunch of trusses at c-c 1200.
There are truss manufacturers who both calculate and manufacture.
The manufacturer nearby has wood for trusses at c-c 600. He suggests putting them in pairs to achieve c-c 1200 where I want, but doesn't know at what distance they should be nailed together...

I say - does it matter?

You say - don't complicate it!