Bent truss chord connection

[andrewpaskman]

Hello all, my question is to do with an open web style truss as shown in the attached image

http://files.engineering.com/getfile.aspx?folder=10e0c85d-4b86-4216-b107-91d697f91741&file=truss.jpg

I would like to use a bent CHS as the internal truss chord for ease of manufacture but was wondering if there are any special considerations that should be made regarding the compressive load distribution into the chord? Can someone please provide some guidance or a reference for any axial load offset that may need to be considered when evaluating the buckling of the internal truss member.

Thanks Heaps.

 

[paddingtongreen]

I had to research to find what a CHS was, (it's the UK term for a circular HSS). I do wieh people would spell oyut what they mean in this multi-national forum. I would be dubious of this procedure, how do you intend bending the CHS?. They used to make some of the open web joists that way with square bars here in the States, but I think they tested them.

Clearly, there are eccentricities to take care of.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[asixth]

Will the fabrication be easier?

Conventional trusses fabricated from CHS (circular HSS/tubes) will require each chord & intermediate to be cut (circular cut) and welded. But the detail you have provided will require the CHS to be bent.

My concern would be ensuring that there is sufficient weld to transfer the forces from the diagonals to the chord members.

I would also ensure that the centrelines of all members align at the nodes.

 

[andrewpaskman]

Sorry guys, I should clarify. I will be bending up some circular hollow section to use as the internal truss chords and the main top and bottom) truss beams will be made from square hollow section. The circular hollow section will be pipe bent (no mandrel) and the connection detail will be a J butt weld between the circular and square sections. The bend in the chords is sized sufficient to transmit the required loads.

Fabrication by this means is definitely quicker and cheaper than conventional means. You can produce one single span of truss chords in one go and essentially make a truss from 3 members (bottom member, internal bents chords and top member). I have seen this form of truss manufacture used in high voltage tower constructions as well as other miscellaneous one offs.

 

[hokie66]

andrew,

The members you are thinking about bending are webs, not chords. The CHS members would be bent in a similar fashion to the way steel joists (bar joists) are fabricated in the US, but they use solid bars rather than hollow, and the webs are typically nested between double angle chords. In the case of steel joists, I think the designs have evolved based on a lot of testing to supplement analysis.

 

[dhengr]

Andrew:
You would do well to study what the manuf’rs. of open web steel joists are doing now to make their products. They are not in the business to lose money or take too long making their trusses, nor do they use materials for various truss components which are to expensive or difficult to fabricate with. Why do they do what they do the way they do it, with the mat’ls. they use, etc.? You may not be plowing new ground here. Get some of their older catalogs, which showed a lot of detail, get some of their specs., analyze a few of their trusses to see the member loads and stresses that conform with their load tables. Round and square tubing is not less expensive than angles and bar stock, nor are they easier to use in terms of fabrication and welding details, and they come in fewer stress grades . Learn the correct terminology for trusses, and the limitations involved in their design, analysis and fabrication. I suspect you will have some trouble bending continuous round tubing and maintaining truss depth and final truss length. The fairly sharp bends will not lead to a clean, consistent, weld joint, whatever a J butt is. Weld size and tubbing wall thickness and shape will likely cause you problems. It could be that the one-off’s you’ve seen didn’t do this type of study, or had some special reqr’mts. for their fab’ed. configuration, other than economy or efficiency.

 

[ToadJones]

terminology is throwing me through a loop!!!
I saw "bent truss" in the heading and starting thinking a "truss bent"....
Anyway, you're connection/weld details are seemingly going to be quite messy and might indeed be plowing new ground.

 

[csd72]

Andrewpaskman,

I would expect that you are introducing a whole new set of deformations and potential buckling modes by using a CHS. CHS's do not tend to react well to tight radius bends and tend to deform into an oval shape in the region of the bend.