HSS Truss Joints: Gusset or Direct Weld?

[Rainbowtrout]

Hi All!

I am designing a roof truss with HSS members. The truss spans 150 ft. I am modeling a duo-pitched warren truss. My preliminary results use HSS 12X8 for the chords and HSS 5X5 for the web members.

My question is, what is the economical, efficient way of connecting the chords and the web members. AISC design guide 24 seems to point you at direct weld connections for HSS trusses, without any comments on gusset plates (slotted end of web members welded to gusset plate welded to the chords). Is it the industry preference based on experience? Why?

Thanks!

 

[BadgerPE]

I don't have any first-hand knowledge as to why directly welded is the predominate connection method but I have a couple theories:

1) Usually HSS trusses are exposed and a direct weld looks cleaner. I would think W-section trusses would more readily employ the gusset option.
2) If you are going to gusset/slot web members/weld all together, there are a lot of steps and labor involved too, so directly welding may be the best option.

Your best bet may be to talk to a local fabricator and discuss both methods with them. You will get a better feel for what they recommend and what is done in your region.

 

[bigmig]

I designed one a lot smaller last year. I close inspection of the joint forces revealed that I was having trouble getting the weld lengths to work for the required loads when I was trying to use the tube to tube welded connection. I had to go to the gussets to get more purchase. In addition, the gussets help avoid a high concentration of stress that can have the potential to buckle the wall of the supporting chord.

 

[KootK]

I think that Badger's first argument is the crux of it:

1) If the truss is an exposed feature, gussets aren't sexy enough.
2) If the truss is just an elegant work horse, HSS isn't efficient enough.

Technically, one difficulty with gussets on HSS chords that you're always delivering your axial load eccentric to the chord centroid unless you run the gusset right through the HSS chord which is expensive and not particularly attractive.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SAIL3]

As pointed out direct welding is probably more cost effective if there is no field fit-up problems.
Direct welding presents it's own problems with welds sometimes being done on the radius of the members.
From my own experience, it pays off to choose the size of the members with the ensueing connections in mind especially the width of the members. My first preference, if practical, is to have all the members the same width so I can lap the joints on the outside with gusset pls. If this is not viable, then I would try for a difference in widths that would result in welds that are as simple as possible. Otherwise one is dealing with partial penetration goove welds that can be difficult to effectively achieve.

 

[BAretired]

The book entitled "Hollow Structural Section Connections and Trusses" by Packer and Henderson is an excellent reference on this topic.

The most economical joints are gap joints with fillet welds all around the web member at the web/chord junction. If web members and chord members are the same width, uneconomical flare bevel welds are required, so it is better if the web members are slightly smaller than chord members permitting fillet welds all around the web members.

BA

 

[Ron]

Agree with BAretired....also, if your stress levels indicate gussets, use them. If not, don't. I've been involved in the design and inspection of numerous T-K-Y joints and they are not that difficult to construct.

 

[jike]

As others have said:

1) Web members with should be slightly smaller than chord width
2) Diagonal web member WP's should be offset so that diagonal members can be cut parallel with the chord and have sufficient gap between diagonal and vertical to facilitate straight fillet welding.
3)HSS trusses are generally exposed and therefore do not have gusset plates for a clean look. If truss is not exposed, why not use cheaper members? (wide flanges)

 

[Rainbowtrout]

Thanks all. We decided to go with direct welds because the geometry was ideal and the loads are OK. I did think of the aesthetics but didn't think too much about fabrication costs until many of you pointed out. Packer is definitely the expert on this - I used his charts to rough check the configuration before using DG 24.