Staggered top and bottom joist chords? 1

[gte447f]

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.

 

[hokie66]

Three chord trusses are quite common, but usually as custom designed structures or proprietary space frame systems, not using bar joists as you describe. I would think the inherent eccentricity in the web to chord connections would be problematic in such a system.

 

[dik]

I've done that, and it works like a charm... check the roof trusses at Limeridge Mall in Hamilton, Ontario.

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

 

[southard2]

I would not do this. Never seen it before and I used to work at a joist manufactur. My guess is that at some point you would have to switch which way the webs were staggared and design the bridging as a permanent compression strut. But what if somewhat cut a strut......BOOM.....down she goes.



John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[ajh1]

Butler Manufacturing manufactured a product such as you describe under the "Delta Joist" tradename in the late 1990's / early 2000's. It started out as you describe with solid bars then migrated to a 5' spread system with hollow tube webs. It was discontinued in the last steel shortage due to unavailability of the component parts from the suppliers. While the system worked well in heavier load regions, it was uneconomical in lighter load markets because you are in effect putting in twice as many joists as would have been used for normal condition. It did provide a full lateral bracing configuration to allow a floating standing seam roof to be placed on top without the need for other bracing. Extensive testing was done on the product throughout the course of its lifetime. In addition to good performance it also had a nice constructibility feature in that you could preassemble large blocks of trusses on the ground and hoist as a unit minimizing in-air work by the erector and thus reducing construction hazards.

 

[gte447f]

ajh1, I believe the system I am dealing with is a Butler Delta Joist system as you suggest. I intend to speak to a gentleman at Butler today to gain more insight. Do you know if these systems were typically specified by the EOR as a SJI product would be, or was everything done by Butler. The reason I ask is I am curious about whether Butler made load tables available to specifiers. If not, it seems that any analysis for new/additioanal loads would have to be done by Butler.

 

[ajh1]

I didn't work directly with the product as I was in the Research group at the time. I would say in general everything was designed and analyzed by Butler. We don't generally produce load tables for direct specification by others, preferring that our own professional engineering staff be responsible for correct design and selection.

 

[mbullism]

gte- I'd be very interested in any info you may have gleaned from butler... We currently have four buildings we have to "analyze" for possible addition of ballasted solar panel arrays. We have original construction drawings from 1969 which call them "space trusses", but they are effectively lightgage triangular trusses set side by side and connected together similar to what you describe.

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

 

[dik]

southard2... it works well, and as analysed and as designed.

Dik

 

[mbullism]

I'd appreciate any input from someone familiar with these. They are decidedly utilitarian, and look to be more of a system than a "one of"... They are in several roofs in western MA-

Thanks in advance,

MB

 

[mbullism]

Close up of the web to top chord connection-

 

[ajh1]

mbullism:
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..

 

[mbullism]

Thank you, that would certainly be greatly appreciated. Basically, four buildings designed and built under the same contract for Massasoit Community College in Brockton, Ma., all have this type of roof system. Title block information as follows:

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

 

[hokie66]

mbullism,

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.

 

[ajh1]

Give Butler's engineering office in Annville, PA a call and mention SpaceGrid. Give them your contact information and tell them to forward it on to Al in Product Development in KC. They will know who are talking about. We'll take this off-line so we can have some direct communication without getting hung up with phone numbers and email addresses here in the forum.

al..

 

[mbullism]

Thanks, Al- will do. I appreciate it.

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

 

[hokie66]

Yeah, you are correct. I looked at your photos again, and the chords only go in one direction. Fooled me at first, because I once reviewed a similar system where the chords went both ways. I'll go into my corner now while you two guys sort it out.

 

[mbullism]

LOL,... No reason to head for any corner ;-) I've never come across a system quite like this, and from that standpoint it's definately interesting. Turns out, they set them side by side and bolted them together at intervals which gives the some loadsharing, diaphragm and general continuity before the deck goes on.

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

 

[paddingtongreen]

Now that gte447f and mbullism are sorted out, I want to go back to dik's design.
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.