Making a Steel Truss with Circles and Not Triangles 1

[Ron247]

I have an architect who wants a front drive thru that extends about 21' from the building. He has "envisioned" 3 roof trusses (2 ends and a center) to support the drive thru roof that he wants to be made out of circles and not triangular pieces. I have made several suggestions how to get a similar appearance but he wants to be able to see through the circles as part of the look. In the drawing below, the circles are hollow even though the shading indicates they are solid. There is no snow to design for. The roof DL+LL is about 21 psf, so there is no excessive loading.

Any thoughts on a method to achieve this appearance but make a structure that fits his "vision"? Also, is there a way to may a truss with circles? If there is, I missed that day in Truss 101. The circles are about 18" in diameter in the picture. There is only about 2" of solid metal between circles. From a steel beam depth perspective, the beam would be about 21" tall or more. As usual, the cost is no issue now, but I know it will become and issue when someone finds out how much this is going to cost. Rolling a beam will take 3 pieces rolled that then have to be welded together to form the roller coaster part of the roof. The cost for rolling only is $5k to make all 3 frames. Does not include the cost of the beam and the welding later.

 

[Celt83]

Castellated Beams I believe is the term: an older AISC article on the topic

If you have the Blodgett Book Section 4.7 goes into the analysis and design in detail.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[Celt83]

There is also AISC Design Guide 31

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[dhengr]

Ron247:
Probably the best way to fabricate something like that is to weld up a WF shaped, built-up member. The flanges are roll formed or brake formed to fit a wooden template and the web is cut to shape from stl. pl., with the holes cut into the solid web pl. The holes have to be several inched from the inside of the flgs. and the spacing of the holes will likely have to be 3-4” or so, and the Arch. may have to get used to that if he wants a spanning structural member. You can think of that as something of a Vierendeel truss or arch, certainly FEA software should be able to handle those members. Also, look up castellated beams. They get crazier by the day.

 

[Ron247]

While the final product will perform like a castellated beam, it cannot be fabbed like one. Has anyone ever rolled a castellated beam after the holes were made? I have never done that. I feel like the rolling process may deform the holes significantly and may not hold consistent radius.

I can roll a WF and then torch holes in the web or fab my own WF by burning a web plate and using rolled or broke flanges as dhengr noted. I have the feeling the architect thinks the cost of this will be similar to traditional bar joists that are sloped but not curved.

 

[rb1957]

this is "just" a web isn't it ? with holes in it ?

you could use flanged (or unflanged) lightening holes like we use (in aircraft). I'm used to punching these holes in Al, but I'm sure you do the same in Steel sheet.

another day in paradise, or is paradise one day closer ?

 

[Ron247]

Right now it is just a "vision" to the architect. He has no real idea how to get this look. As I stated in the post, he thought this would "just be a truss with circles instead of triangles". It is more than a web only. I am assuming it needs flanges to be stiff enough to span the distance.

 

[JAE]

Ron247,
I'd first call a few of your local fabricators to see what their thoughts are instead of trying to dream up a self-designed product by yourself. Tap the knowledge of others for this.

I would think that rolling a wide flange and then afterwards cutting the holes is the best way. Your fabricators may see the holes as no big deal to cut (plasma cutters).

 

[Ron247]

JAE, that may be the best option at this point. Now I am told they are using one particular fabricator. Whenever that happens, it is best to consult them.

 

[Ron]

This seems to be more of a variation of a Vierandeel truss. Shouldn't be an issue except a little difficult to fabricate.

 

[phamENG]

If bending and then cutting isn't an option, you could do a built up shape with steel plates. Fabricator could laser cut the curved web and holes on a CNC, bend the flanges (easier than bending a beam), and then weld it.

Either way, this isn't a new idea. Search for images of the castellated beams in O'Hare International airport in Chicago.

 

[JAE]

 

[phamENG]

Thanks, JAE. Exactly what I was referring to!

 

[Ron247]

Just got off the phone with the Fabricator and later the Arch. For this project, we are going to roll the beam sections and weld them together rather than burn the web option. Fabricator prefers rolling to built-up members. Architect is allowing me to make the circles smaller and space them apart more than 2". Thanks JAE and pham for the picture. Sent it to the Arch and he can see the relationship of the circle diameter to beam depth.

Thankfully, the "truss made out of circles" is no longer on the table.

 

[BridgeSmith]

Could it be built up using a top and bottom flange plate and heavy pipe sections? Might be tricky to analyze, though.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Ron247]

BridgeSmith: The pipe sections cut into 4" long pieces was one of my "possible ways" to achieve this but I told the Arch that analyzing it would be a nightmare for me. I do not have a higher end FEA program to use. I was thinking of rolling a T for the flange/chord and then cutting arcs in the web to nest the pipe sections into. The Fabricator was not in favor of built-up members and preferred rolling so I dismissed the thought. I do think it could work but as stated, tricky to analyze.

 

[dold]

I saw some funky trusses in structure mag a while back. Binghamton University Energy R&D building You'd have some interesting bending in the web members. Maybe you could bend tubes in a sinusoidal/half-circle-wave whatever you call it to form the web, and either direct weld web to chord or knife plate them, whatever the numbers tell you. Then add in filler pieces to make a full circle. I guess it wouldnt be too much different than any other truss aside from bending in the webs. As for the actual profile of the truss....it wouldnt be all that difficult to model it in RISA or whatever you have. But the circles being so close together will make that a challenge. I'd vote for castellated beams. Cut the circles after rolling.


O'hare is one of my favorite airports (because of the framing). Reagan int'l in DC also has really cool framing in the main concourse. Huge cruciform columns and moment frames galore.

 

[Agent666]

In the UK the castellated beams are called cellular beams, might help you find a whole raft of freely available information from the likes of SCI the steel construction institute. Link

The openings in the picture look way to close though, virtually no web post.

 

[MIStructE_IRE]

We call them cellular beams here. You’ll find free software from Westok Kloeckner Metals for the design.

They essentially act as beams with vierendeel action around the opes. The width of the remaining web posts is critical so the architects original sketch above would be difficult if not impossible to justify.