steel canopy possible issues 2

[mats12]

Im dealing with a new steel canopy (above entrance of an existing building.) I want to talk about possible issues regarding this design.

Primary steel beam is H section (8 m in span).
Secundary beams are hollow/box profiles that are connected in the middle of primary beam (it has to be in the middle).

Since cantilever is 1,40 m long I suspect that there will be quite a torsion acting on a primary H beam, but this is solved by anchoring in existing structure on the other side of secondary beams (support reaction at the end shows upward). Is that correct?

Since its a cantilever, a connection of box profile to H profile should be fixed (moment) connection. Bolts will be in tension. Im wondering what road should I choose/you suggest:
Connection 1 or connection 2.

Im worried about 1 since cantilever is connected directly to the web of H section (but there is no need for additional plates that way).

tnx for help

 

[mats12]

This is a load path I came up with...

cantilever causes rotation (torsion) of H beam. So the moment M is transfered through cantilever beam to bolts and from there to flanges of H beam. I have to prevent rotation of H beam so that means that top flange needs to be supported in my opinion. So the beam on the right is in tension... Is that right?

But what happened if we place this beam in the middle of H section (like cantilver beam in the right)? how do forces gets transfered?

I really need to prevent any rotation of H beam since any small rotation causes in large deflection at the end of catilever beam.
I also want to prevent any rotation because of the connection at the end of H beam (connection with column - weak axis).

 

[Ron]

First....a bit of terminology clarification. You mention an "H" beam. Is it truly an "H" section, which is normally used for columns or piling, or is it a "W" section (wide flange).

What is your loading? Is it wind only? Is there an additional live load on the canopy? Will it support people? What is the spacing of the HSS sections?

As for your connections, No. 2 will yield less deflection from beam rotation assuming the web of your wide flange beam is thinner than your flanges; however, it will require a different welding technique (overhead position) which is more difficult in the field. You could also consider a different type of connection that would include a gusset for the beam to help with resisting torsion.

Your framing system will also be more difficult to add roofing since your HSS outriggers are not at the same level as the top of the beam. Why is that?

 

[KootK]

I feel that your original detail #1 is the way to go. It should be cheaper to fabricate and will eliminate any concern for torsion in the supporting beam. That said, I'd give at least some passing attention to the fact that your steel frame will be exposed to the elements and, thus, growing and shrinking a bit relative to the adjacent wall.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mats12]

tnx for replies

@Ron:

What is your loading?
self weight, wind and snow

Will it support people?
NO.

What is the spacing of the HSS sections?
0,90 m

 

[jayrod12]

Can you not just have your Hollow sections run over your beam? that would simplify everything.

 

[hokie66]

I see no reason for concern about torsion, as the outriggers have a backspan, so they are propped cantilevers.

If you can't weld the whole thing as an assembly in the shop and transport it to site, the connection 1 is best, provided the beam is deep enough for access for bolting.

 

[WARose]

#1 seems like it would be easier to build. Which ever one you select, I don't know that you've necessarily [100%] "solved" the torsion issue by such a frame. It certainly will restrain/reduce it......but from the bending of the plates and/or the anchors giving a bit, you will induce a bit of twist in the beam. It will likely be minimal, but it doesn't take much twist/displacement to induce significant stresses in a H/W shape.

 

[KootK]

Can you not just have your Hollow sections run over your beam? that would simplify everything.

Tough to argue with that logic. Another attractive way to do a canopy like this is to attached the hollow sections to the underside of the beam. Is this a glass canopy?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

The OP did say that the outriggers had to be "in the middle", presumably for some architectural reason.

WARose,I fail to see a torsional concern. Significant twisting displacement in a closed section, a lot of stress. But in an open section, the same twisting displacement results in much less stress. Stress follows stiffness.

 

[WARose]

WARose,I fail to see a torsional concern. Significant twisting displacement in a closed section, a lot of stress. But in an open section, the same twisting displacement results in much less stress. Stress follows stiffness.

Depends on the circumstances. But I've handled enough torsion problems to know that it doesn't take too much displacement to get some decent stresses in wide flanges. If someone is designing to the edge....it could put them over.

Granted, it's not the first thing that comes to mind that needs to be checked in this. (What Connection #1 will do to that web is the first thing that jumped out at me.) But the OP sounded like he thought this detail eliminated it.....so I just wanted to make that point.

 

[BAretired]

You could cut square holes in the web and run the HSS members through, fillet welding them to the web on both sides. Ideally, the beam and outriggers would be fabricated as a unit in the shop to avoid field welding.

BA

 

[hokie66]

I agree that shop fabrication of the whole assembly would be better, provided it can be transported and erected.

But I can't for the life of me see the difference between the end plate connection both sides and the pass through connection, except that the end plates would stiffen the web more.

 

[WARose]

@the OP: Why not just make the whole thing out of HSS members? Potentially that would be cheaper than all those [plate] connections.......and you could shop weld all the outriggers, and only have to field weld to the columns (and possibly omit the connection to the existing structure).

I don't know if the numbers would work out or not (you would probably have to consider a unbalanced snow load). But it's something to consider.

 

[jayrod12]

Regardless of what the OP said about it having to be in the middle, we must do our best to convince them to see the light. Performance of the structures we design are a direct reflection on our abilities. Think of how bad that thing is going to leak, it's going to be a battle to waterproof it.

 

[dik]

Can you run your little cantilevers over the top of the W sections? In Canada, generally W sections are used for beams and columns and H sections for piles.

Dik

 

[BAretired]

I agree that shop fabrication of the whole assembly would be better, provided it can be transported and erected.

But I can't for the life of me see the difference between the end plate connection both sides and the pass through connection, except that the end plates would stiffen the web more.

The pass through connection has a cleaner appearance and eliminates localized bending of the end plates but otherwise has no particular advantage.

BA

 

[mats12]

Thank you for replies!

Well this is the only other option that investor agreed upon:

I think thats much better.

 

[jayrod12]

Definitely better,

 

[KootK]

Maybe go with two bolts so that the HSS can LTB brace the wide flange.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.