Converting Double Channel Plate to Rectangular Hollow Section

[pratamahrs]

Hi all,

I have a problem. I have proposed the Steel RHS for the mullion but the client said it is too expensive. So, they want to change the section from combining the channel section. They said it's cheaper.

Do you guys have any thoughts of this? Does this section is possible to build?

Hereby I attached the drawing of the section.

Thanks in advance

Pratama H.R.S., B.Sc, M.Sc
Facade Engineer

 

[tempeng]

Is this something they have done before? I only had a quick look at the image and whilst it's probably OK, I can't decide if I like it or not.

 

[pratamahrs]

No, they haven't. It's just an idea without any test or academic reference from client.

Pratama H.R.S., B.Sc, M.Sc
Facade Engineer

 

[Once20036]

I assume that you're talking about a pair of bent plates, in lieu of a channel section?
Channel sections (C & MC, in the states) have some taper to the flanges, so the tip of one flange will not line up nicely with the heel of the other flange, as you're showing.

Overall, I don't really like it, but can't quite quantify as to why. I suspect you feel the same or you wouldn't have posted here...

 

[WinelandV]

For something lightly loaded, it's probably ok. If you're going to load it significantly, then I would analyze it as only 2 channels (with flanges possibly braced against LTB), instead of a box section.

 

[KootK]

As long as the detailing reflects the physical reality of bent plate, I think it's a great solution. You absolutely have a structurally effective tube of 1 X thickness that you can rely on for flexure and LTB. And, depending on th b/t ratio of the outstanding leg, you've probably got the secondary, interior flange available for strong axis flexure as well.

In terms of pure structural performance, this solution is better than the RHS. If the client prefers the double channel, I would certainly allow it. KootK approved.

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.

 

[CELinOttawa]

I take issue with this detail if you try to avail of plastic strength or if there is a torsional case (which, at least for the torsion, is not typical for a mullion). As long as this detail receives effective bracing against LTB, and you design elastically, I think you're fine. Otherwise I think we're taking a little too much for granted saying that this will perform equal to a RHS under cross section shear flow.

Reminds me of the eccentric brace knee connection I worked on as a forensic engineer while in NZ; It was designed to be a pin connection, looked entirely reasonable, and caused structures to fail at a fraction of the required design load.

https://www.building.govt.nz/buildi...d-eccentric-cleat-connections-in-compression/

No one expected that, but it still happened.

Sometimes good engineering practice is doing our design with as little innovation as possible. But, somewhat ironically, I also like the alternative detail of the two PFCs nested into one another... I would just be likely to design this elastically, rather than allow plastic flow to initiate in an untested section.

 

[BUGGAR]

I suggest designing your internal flanges for their individual b/t ratios and with unsupported edges. Will this put you into light gage design (AISI) instead of AISC?

 

[XR250]

I don't see how this can be a cheaper than a piece of tube steel

 

[JAE]

Tolerance fit-up might be a problem too. Those plates would have to be on the money to fit like that and I'm not sure they always would...might require a bit of prying to get to fit.

I agree with XR250 - The fabrication labor here will really add to the unit price of these things.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[pratamahrs]

Thank you for your suggestion. I design it elastically.

The wind load is 1 kPa. The width of the panel is 1350mm. I think it's a lighlt loaded since the uniform load is about 135 kg/m2.

The thing is fabricators in Indonesia don't produce much hollow section so hollow section become the unusual section to fabricate. Maybe they don't even have enough technology to produce it.

By the way, should I check the profile for 2 C's or just RHS is enough?

Thanks

Pratama H.R.S., B.Sc, M.Sc
Facade Engineer

 

[dhengr]

Pratamahrs:
Given the way that structural mullion will fabricate and assemble, you might be better off if you made it up from one channel shape, on one side, and the other web (side) being a flat strip/sheet a bit wider than the channel is high. This will allow you to make both connecting welds down hand fillets from the same clamped assembly fixture position. Then rotate the fixture 90̊ and weld on one of the small trim attachment channel shapes to the main structural mullion flange, and rotate it again, 180̊, to weld the other small trim attachment channel shape to the main structural mullion. The above approach should improve fab. and assembly and present fewer fit-up problems. This approach might require that you use slightly thicker sheet material for the structural mullion portion, for both the formed channel and the flat strip/sheet web piece, to get the same section properties.

 

[CELinOttawa]

Not to mention durability issues. I like dhengr's suggestion most of all, and would say that if you don't use full length welding (which you shouldn't, given the likelihood of heat warping longer runs), require a post-main weld seal weld. IE: Make your weld what you need for strength, say 6mm 75-200, and then come back with an interweld pass of 1.5mm 125-200 between.

 

[KootK]

By the way, should I check the profile for 2 C's or just RHS is enough?

I would check it as an RHS with a constant wall thickness. If that didn't work, I would check it as an RHS with the additional flange material included and checked appropriately for local buckling. I wouldn't check it as two independent channels as lateral torsional bucking would likely not work for that condition unless one were to make the somewhat dubious assumption of continuous rotational restraint being provided by the glass, gaskets, and pressure cap assembly.

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.

 

[thaidavid40]

If it must fit closely and true with the window glazing, then I recommend that you provide some means of adjusting the finished section for twist. All of that welding will cause the section to build up a LOT of residual stress, and will likely cause it to twist more than your window designer would be willing to tolerate.
Dave

Thaidavid

 

[pratamahrs]

I checked the profile by the RHS.

I don't really understand about the residual stress caused by continuous weld.

If it must fit closely and true with the window glazing, then I recommend that you provide some means of adjusting the finished section for twist

Can you explain the twist? Why will it happen if I do the continuous weld?

Given the way that structural mullion will fabricate and assemble, you might be better off if you made it up from one channel shape, on one side, and the other web (side) being a flat strip/sheet a bit wider than the channel is high. This will allow you to make both connecting welds down hand fillets from the same clamped assembly fixture position. Then rotate the fixture 90̊ and weld on one of the small trim attachment channel shapes to the main structural mullion flange, and rotate it again, 180̊, to weld the other small trim attachment channel shape to the main structural mullion. The above approach should improve fab. and assembly and present fewer fit-up problems. This approach might require that you use slightly thicker sheet material for the structural mullion portion, for both the formed channel and the flat strip/sheet web piece, to get the same section properties.

Can you please show this mechanism in the drawing or sketch? I'm a little bit confused. Sorry

Thanks

Pratama H.R.S., B.Sc, M.Sc
Facade Engineer

 

[CELinOttawa]

Lookup a text on flame cambering... The problem being discussed is effectively a minor version of purposefully deforming the sections....

 

[Ingenuity]

I don't really understand about the residual stress caused by continuous weld.

Link