super long span 9steel trusses0

huangyhg

super long span (steel trusses)
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
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!
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.

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.
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.


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
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.
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.
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   
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:  
just a reminder that the roof