150 feet span roof 13

[dgkhan]

I have to design a circular roof 150 feet diameter. Shall I use two way steel truss. by the way what is space frame, is it two way truss? (the roof is flat so nothing parabolic)

 

[dik]

Do you need a space frame? Can you use a clear span truss at the diametre and then frame into it with smaller trusses (7 or 8 different truss lengths) at say 10' centres (using 3" roof deck). The main truss panels can be spaced to match the smaller trusses.

Dik

 

[msquared48]

I would consider radial trusses, all moment connected to a central steel cylinder of the same depth as the trusses.

Mike McCann
MMC Engineering

 

[Ron]

Agree with Mike. Also...don't build a flat roof! You can slope the trusses or specify tapered insulation on a flat deck, but DO NOT let them build a flat roof. The ponding probability, deflection and progressive ponding collapse are all high...and a round roof section doesn't help things!

 

[dik]

That's likely the pricey way to do it, and the way to do it if it's exposed and he needs a space frame.

Dik

 

[dik]

...and, don't build it flat as Ron noted.

Dik

 

[dgkhan]

what is a space frame

 

[msquared48]

It's a frame in space - three dimensional - that would cover the whole area of the top of the dimension. Spans all directions. If you are looking for a light roof, this would be the way to go.

Mike McCann
MMC Engineering

 

[hokie66]

To elaborate a bit, space frames are usually two way trusses as you suggested in your first post. The top and bottom chords are offset, so the whole roof is formed by a maze of triangles. There are a number of proprietary space frame manufacturers. Most use circular hollow sections for all members. The joint design is the tricky part, and again, there are quite a few proprietary solutions, the most elegant of which are also the most costly.

What do you or your architect intend as the roofing material? The selection of a structural system would depend to a degree on how the roof slopes, the amount of secondary members required, etc.

 

[msquared48]

Unistrut makes classical space frames.

Mike McCann
MMC Engineering

 

[a2mfk]

Do we want someone designing space frames spanning a 150ft circle if they don't know what one is?

I followed this whole post and don't really want a piece of that design. Maybe if you guys were all in my office.

 

[JAE]

a2mfk, you said what I was thinking, too.

 

[hokie66]

No, we don't want someone designing a spaceframe if he doesn't know what it is. But he wouldn't be actually designing a proprietary spaceframe, so hopefully the vendors would know what they are doing. Hopefully again, the OP would be able to incorporate a proprietary spaceframe into a building...if not, he probably can't design the roof at all.

 

[MiketheEngineer]

Get some help - I agree with the above posts.

Have done things like this using three or four "main" trusses running parallel to a diameter. Cuts down on the total "central" truss load. Obviously they are different lengths.

 

[ztengguy]

Everyone thats an expert at designing space frames started with their first one. I think he is reaching out for help, but I agree OP should do more research before coming here. Pretty aggressive span too, would like to see final design

 

[SAIL3]

As pointed out, definitely not a flat roof.
Radial trusses from a central ring is generally an elegant solution. The only drawback with this concept is that it depends solely on the integrity of the central ring and it's conn to the radial trusses. If you are confident of quality fabrication/erection and effective maintenance, then it would be a viable option. Unfortunately, in these low-tech projects, one or all of these requiements may be lacking.
With that in mind, I believe I would tend to go with the parallel trusses as it results in a more redundant and robust structure and less sensitive to the potential shortcommings mentioned above.

 

[dik]

I was thinking of something like the attached... 1 long span truss, and 7 x 4 smaller trusses. Depths vary depending on loading, and the use of 3" deck to maximise the spacing... might be able to go to 12' or more.

Dik

 

[a2mfk]

dik- seems like that would work just fine. KISS. And outside the main girder the other joists could be pretty typical long-span joists.

 

[dik]

It's probably the least costly way to do it... not very elegant.

Dik

 

[hokie66]

When we talk about a "flat" roof, nobody really means flat...I hope. Selection of the system should depend on the roofing system to be used. Where I am, steel roofing would be used on about a 2 to 5 degree slope, supported on steel purlins. Dik's structural system would work, but I would prefer more of a two-way load distribution.