Super Long Span (Steel Trusses)

[ash060]

I am currently involved in a job that requires a 250' clear span for the roof. I have looked into the possiblity of using steel joists. I am waiting to hear back from a couple joist engineers.

The roof is about 32' to the eave and has a 6 on 12 slope. It is also a open structure. It is a big horse show arena. The location is south florida so earth quake is not an issue.

The architech wants a sloped soffit as well so the joist or trusses will have to be scissor type. i can go about 20' deep at the center.

my question is would steel trusses possibly be more econmical than joists?
what would be a good spacing for the trusses, i was assuming about 30' c/c?
What would be the best type of section for the truss parts (Wide flange, Tees, HSS)?
How much will thermal be an issue?
Any guidance would be a great help

 

[jike]

I have done a couple of 180 to 185 feet spans with joists in Wisconsin. If I recall correctly, manufactured joists will be limited in depths to about 120" (largest they can get out of their door).

Sounds to me like you will be using fabricated main trusses with joists spanning between them.

Good Luck!

 

[SteelPE]

I believe steel joists stop at 144’. Then you end up in engineered steel trusses (done by a fabricator not a joist manufacturer). Shipping is going to be an issue not matter what you pick.

What about a metal building? With this type of system you would have massive foundations.

 

[jike]

You most likely will have trouble shipping 20 foot deep trusses without having them broken down into pieces unless you can find a nearby fabricator with no shipping restrictions.

 

[jike]

I would also suggest checking into a metal building. This might be the cheapest option.

I did not see your questions at the bottom of your email:

Steel joists are always cheaper than fabricated steel trusses

25 foot to 30 foot spacing

Keep truss connections as simple as possible, therefore, use angles first, tees second, WF next and tubes last unless there is some architectural consideration

Thermal should be investigated especially if the building has minimal insulation, ventilation or A/C.

 

[ash060]

Metal Building contractors said the max they could go was 180'. I just spoke with the joist engineer and he said that joist could go 250', so I will probably use joists.

Thanks for the help

 

[ronster]

If you use a scissor truss on a 250 ft span you are going to have a large horizontal deflection or lateral outward force to deal with and only a cantilevered wall or column to hold it back. Good luck with that. Also, while South Florida does not have earthquake loads, you may find yourself with some massive uplift forces from wind to deal with.

 

[hokie66]

250' joists may carry the vertical component of load, but what carries the horizontal? I would be looking at portal frames with the columns and rafters both truss sections. I think the chords will be wide flanges, column type, with web horizontal.

 

[Lion06]

One thing I would like to note regarding trusses is that you should make sure the bottom chord is braced appropriately. This is not just for any bending that might take place in the bottom chord from making joints rigid, but more importantly the bottom chord is the bracing mechanism for the compression diagonals, so it must have adequate strength and stiffness to brace those compression members.

 

[WillisV]

StructuralEIT - good point, but note that there are many dissenting opinions on whether that is really necessary or not - see this thread where we hashed it out at length a few years back: thread507-124363 (as well as all of the dissenting opinions to the original Fisher paper)

 

[jike]

Just a reminder that the roof members should be designed and braced for wind uplift.

 

[JedClampett]

Let me dissent from some of the other posts. My experience with Metal Building and Joist manufacturer's has been less than stellar. They say they can do great things, but trying to get their attention is difficult. It's pretty painful and then they put your project in their brochure.
Why not dust off your old design book and design them yourselves? It's really not that hard. With trusses this deep, the members are not going to be very heavy. You can space them at twenty feet or so, put in light purlins and do a pretty economical design. Note that you can vary the chord and diagonal sizes to optimize the design.
This sounds like a once in a lifetime chance. Why not take the design and do it yourself? It's much more satisfying than writing a specification. And it doesn't cost that much more.

 

[ash060]

Well I would not mind doing the design myself, but I do not think that steel trusses are the most economical way to do this, being it is a hip roof and would require at least 3 truss types by the look of the plan.

But does anyone have an idea as to how I can detail the end truss connection to allow about 4" of movement in the horizontal.

I tried to restrain the truss at both ends but the force gets to be about 350 kips, and that is crazy.

If I go with pin and roller I get 4" of horizontal deflection but only a 30 kip reaction (which is doable). If anyone has any ideas it would be a great help

 

[strguy11]

You are getting 4" of defl over 250'? Isnt this like L/750?

 

[SteelPE]

When you modeled your truss with a fixed end did you account for any flexibility due to the fact that you truss will be supported by columns?

 

[ronster]

Unless you have rigid stucture on one side (such as a reinforced concrete wall) then I would not count on coming up with a connection that will allow the end of the truss to move 4 inches without the top of column or spandrel beam moving and taking a large lateral force. That is why i said good luck in my earlier post. I would use a rigid frame similar to a prefab metal building with purlins or convince the client to use a flat bottom chord on the truss.

 

[JedClampett]

ash060, I wouldn't make to much of the price advantage. For trusses this large, there's no mass production advantage for the design or the assembly. The joist manufacturer will have to do the same calculations you would. And steel costs them the same as it costs any other fabricator. It's pretty much a commodity price. If you can match weights per square foot you'll price out the same.
A disadvantage of having these done by a joist supplier is that you don't really know where the conservatism is in their design. When Joe (or Jane) mechanic wants to hang a 500 lb widget from the joist, you're going to have a tough time saying yes or no. The joist supplier calculations are very tough to fight through, if you can at all.