Staggered top and bottom joist chords?
Staggered top and bottom joist chords?
(OP)
Has anyone here ever seen a type of steel joist roof system in which the top and bottom chords of the joists are offset horizontally from one another, such that the plane of the webs is inclined at an angle rather than being vertical? I have been told it may be referred to as a delta joist system? The top chords are spaced at 4'-0" o.c. and the bottom chords are spaced at 4'-0" o.c., but the bottom chords are staggered 2'-0" horizontally from the top chords. It probably sounds crazy if you have never seen it before. The top and bottom chords are double angles with the two angles being screwed together. The webs are continuous round bars welded to the chords. When all of the chords are screwed together, you end up with a giant space frame as your roof structure. If anyone has any experience with a system like this, I would appreciate any info on who may have designed/manufactured it, and any recomendations as to how to analyze it for additional roof loads. Thanks.






RE: Staggered top and bottom joist chords?
RE: Staggered top and bottom joist chords?
The architect wanted something that looked like a space frame. I told him that there were problems with that type of construction and suggested that the trusses be angled. We used crimped members for the webs... welded into the fillets of the chord angles.
Dik
RE: Staggered top and bottom joist chords?
John Southard, M.S., P.E.
http://www.pdhlibrary.com
RE: Staggered top and bottom joist chords?
RE: Staggered top and bottom joist chords?
RE: Staggered top and bottom joist chords?
RE: Staggered top and bottom joist chords?
The drawings only show them schematically, but have a stated meet or exceed live and dead load for the joist supplier. Our current approach is more of a shell game, in that the snow load required under the current building code is less than that stated on the drawings so we have the difference between the two, and we can justify a slightly smaller dead load than the roof appears to have been designed for. It would be nice to show them capable of the bare minimum stated... a full blown analysis of every member, screw and weld is beyond our current scope-
My apologies for appearing to jump in late, I only just now found your thread-
MB
RE: Staggered top and bottom joist chords?
Dik
RE: Staggered top and bottom joist chords?
Thanks in advance,
MB
RE: Staggered top and bottom joist chords?
RE: Staggered top and bottom joist chords?
What you have is Butler's old SpaceGrid product. This was a 100% cold-formed product whereas the newer DeltaJoist in the 4' spacing was equivalent in concept to bar joists. If you can give me a little more information about exactly what you are needing I'll see what I can scrounge up from our archives.
Al..
RE: Staggered top and bottom joist chords?
Science, Engineering, and two Classroom Buildings. There may have been a fifth building (Business) under the same contract.
Federal Designation-Project No. 3-1-00214-2
(Mass?) Project EJ66-2
Drawing dates 10/14/69 and 02/04/70, no revisions noted
Architect: Crimp, Brown and Fisher in Boston, MA (long gone)
-Unfortunately I have no information on the G.C., or who purchased them...
The members are approximately 36" deep on 5 foot centers, and spans are usually either 20 or 25 feet. The supporting steel members are called out as "Beam Trusses", supplied by the same manufacturer, and most closely resemble typical joist girders spanning up to 60 feet.
The design drawings note a typical (meet or exceed) snow load of 40 psf, except in drift areas where it is noted as 50 psf. A superimposed dead load of 14 psf is also noted. Basically, since these elements appear to be a system, we were hoping there would be load tables akin to open web steel joists, but with only a performance spec and no size designations we had thought that they may lend themselves to being engineered in house as you describe above. Our client wants to add between 8 and 12 psf in ballasted rack photovoltaic panels, and it would be helpful to know if the roof is capable of more than the stated 40+14 (much like selecting an o.w.s.j. size that exceeds design requirements).
Again, any help would be appreciated. You've already given me more than I had, especially in that I had looked into Butler but had found no reference to "Spacetruss" as noted on the drawings, and had not pursued it further. If you'd rather I go through regular Butler channels I obviously have no problem with that-
Mike
RE: Staggered top and bottom joist chords?
What you are looking at is a proprietary spaceframe system, and these are usually intended to span as two way structures. I have no direct knowledge of this one, but ajh1's identification as a Butler system sounds right.
Analyzing the behaviour of cold-formed systems like this is very difficult, especially the connections.
RE: Staggered top and bottom joist chords?
al..
RE: Staggered top and bottom joist chords?
Hokie66- I'm not convinced it's a two way system, ajh1 could correct me on this, but they appear to be triangular joist elements spanning from beam line to beam line, set side by side. There are primary column lines, but no provision for intersecting column lines (or a quad grid, if you will) that I would expect in a space frame. And I agree, analyzing this system cold formed and connections etc. would be a chore at best, and is beyond our current scope (thank goodness
In the end, I'm just hoping that some additional information is out there somewhere. Taking the stated live and dead loads on the drawing as gospel, and then playing off what the code now requires vs. what the roof was designed to be capable of at a minimum is more of an investigation than an analysis, if you know what I mean. But by the same token, I wouldn't have an issue going to a joist chart and looking up a specific joist and taking that capacity and running with it, without having to measure chord sizes and prove the joist capable.
MB
RE: Staggered top and bottom joist chords?
RE: Staggered top and bottom joist chords?
A big public thank you to ajh1 for helping me out, for even taking the time. His insight into what I am dealing with is invaluable in this instance- Thanks, Al!
I don't post often, but read a lot (and learn a lot here)... quite a resource.
MB
RE: Staggered top and bottom joist chords?
dik, I'm wondering if you alternated the slopes of the joists, because the bridging and the floor would have to pick up the horizontal components; the joist can only handle in-plane loads
I just wondered because, if they were sloped at sixty degrees, that would pump half the vertical load into the bridging and need quite a big bottom chord truss to hold it.
Michael.
Timing has a lot to do with the outcome of a rain dance.