Structural engineer type question.

[Bill Douglas]

Hi Guys.

I'm planning on building a per pergola type structure with a glass roof. I'll have six rolled Steel Joists 100mm x 45mm each doing a 5m span. They will be supporting six sheets of glass weighing about 50kg each. So each RSJ (I-Beam) will be supporting about 125kg.

The question... Is a five meter span OK for this amount of weight?

It will look something like the below pic' but bigger and different. I just googled for the pic to help explain the structure.

LOL sorry, here is the pic'.

[stevej37]

must be all glass...can't see it.

That works! Looks nice. What about cleaning the glass?

[Bill Douglas]

I won't get too fussy about the glass. It's a bit stained anyway. They were windows in an office building in town that had a failure of the e-coating or something. They are two square meter sheets 10mm thick and safety glass. I made a smaller six square meter patio roof for over the front door a a few years ago and love it. Light pours into the house while keeping rain and wind away.

[unclebilly]

I’d need to see the cross section of the rafters to make a determination of whether you are in the ball park. I’m guessing no snow load to consider...

[Bill Douglas]

Originally Posted by unclebilly

I’d need to see the cross section of the rafters

The rafters are an I-Beam shape. 45mm across the top and bottom. 100mm from top to bottom with a thickness of 4mm.

And no snow. A run off for rain so angled about 15 degrees.

Thanks for taking an interest you guys!

[unclebilly]

Steel or aluminum?

[Bill Douglas]

Steel.

[john70t]

Originally Posted by Bill Douglas

And no snow.

That changes a lot. You got it easy.

So only thing to worry about is wind shear.
What is the kit rated for?

You want the attachment points to be tight. Grade 5 or even Grade 8 bolts through both runs.
Some angle braces to keep it square.
Washers to spread the load.
The base bolts into concrete and/or backed up with deep ground implants using cables.

[john70t]

Steel will rust eventually.
Give all the pieces a few extra coats of enamel in your preferred color before installing it.

[Bill Douglas]

Originally Posted by john70t

That changes a lot. You got it easy.

So only thing to worry about is wind shear.
What is the kit rated for?

You want the attachment points to be tight. Grade 5 or even Grade 8 bolts through both runs.
Some angle braces to keep it square.
Washers to spread the load.
The base bolts into concrete and/or backed up with deep ground implants using cables.

It's sounding quite positive

It's windy here, but one side will be bolted to the house and another side to a concrete wall. The other side will have a large (high density PSI concrete) foundation sitting on the hard rock pan a couple of meters underground. I'll use 304 stainless steel 12mm bolts to hold it together and get it all hot dipped galvanized.

Things really rust here John. I get extra thick galvanizing done and use stainless fasteners wherever I can.

[john70t]

Around here, S.E. Michigan is all heavy clay soil which buckles and heaves with the snow and cold.
So nothing external attached to any solid foundation would be considered "permanent".

We also get the occasional 100degF summer and the -20degF winter.
(Probably less than Chicago and NYC or the upper East Coast though).

Sitting in sand, you probably want the frame pieces to be attached solid into the ground.
Make the flat sections disposable.
Any new drilling needs a shot of sealant spray.

[sc_rufctr]

Will the sides be open or closed up?

If they're open...
The Steel should be strong enough to hold the weight but I'd be concerned about the wind load.

[Bill Douglas]

Originally Posted by john70t

Around here, S.E. Michigan is all heavy clay soil which buckles and heaves with the snow and cold.
So nothing external attached to any solid foundation would be considered "permanent".

We also get the occasional 100degF summer and the -20degF winter.
(Probably less than Chicago and NYC or the upper East Coast though).

Sitting in sand, you probably want the frame pieces to be attached solid into the ground.
Make the flat sections disposable.
Any new drilling needs a shot of sealant spray.

We're pretty lucky here. In winter it doesn't get below 46degF and summer doesn't get much above 78degF.

There is a very weird hard rock pan a couple of meters down. It's like solidified iron sand that I can barely scuff the surface of with a crow bar. GF has engineers specs for a deck she is getting made for her house, so I thought I'd just copy then double the spec's for the footprint of the foundation. And have it on the rock pan.

[Porsche-O-Phile]

Wind load and possibly seismic are concerns. How are you addressing shear / lateral forces?

[Bill Douglas]

Originally Posted by Porsche-O-Phile

Wind load and possibly seismic are concerns. How are you addressing shear / lateral forces?

Nope. I wouldn't have a clue.

[john70t]

In extreme cases of wind:

If the vertical posts are very hollow and weak, add curved braces/brackets which extend below center-line of the post. Or at least a 1/3 down the post.
These can be made of 1/2" steel for example. Or 2x16 wood cut down. Or LVL. Whatever works. Anything is better than no triangulation.
The world is your oyster as to design.
Very strong but also decorative.

That will help keep things square to lateral/side forces.

An open top such as lattice will let air flow through but provide some shade.
A solid top will provide rain cover but might act as a sail and try to pull on the entire structure.

[Bill Douglas]

I bought some used steel that had come from powerline pylons. it's already galvanized too. It's U-Channel steel 150mm x 75mm and 7mm thick in the web. i'll be using this for the posts.

And with one side of the pergola attached to the house, the other to a concrete wall, i'm guessing it's good for lateral/side forces.

[john70t]

Just remember that new posts will probably want to sink a bit at first, and pull attached structure such as the house with it.

Boxed steel is much stronger.
U-Channel is open and tends to dig itself into the ground. Be warned.
You could weld brace tabs on later I suppose. Cut them fashionable.

Any vertical posts should sit on top of concrete-filled sono-tubes 4' below ground at the very least in frost/cold zones.
Something solid. Foundation is everything.
There will still be some minor shifting though.

[Bill Douglas]

I've got some 100mm flat steel that I was going to weld on the ends as "feet" then use 12mm SS bolts to gal' steel set in the concrete foundation.

I was planning on making the thing, fine tuning it, then pulling it to bits and getting the RSJ joists/rafters galvanized. So hopefully the bedding down would happen before everything was finished.

Thanks for being so helpful and sharing your knowledge John. Are you a structural engineer to know this stuff?

[john70t]

No not an engineer. Sorry to pretend to be knowledgeable on the internet.
(Please disregard this fool.)
I've learned from my many house mistakes twice-over, having been through it, and just trying to help out another.