HSS to WF Truss Connection 1

[klipper]

I have been given a flat pratt type truss max tension web member force 350k and max compression web member force 250k. Bottom chord is a W12x65, top chord is a W18x60, web members are all HSS7x5x0.5. I need to come up with the connections. I started by using a 5/8" gusset plate with the tubes framing into the plate (knife plate type situation) and then welded with 5/16" weld along the 4 interfaces. I have checked buckling, yielding, shear, have added doubler plates to the chords. My web members come within 1" from the flange of the chord members. The working point is at the center of the chord members. I guess my questions is - are many of you utilizing this gusset type connection when connecting tubes to the flanges of a WF chord section or are you mostly welding the ends of the tubes directly to the flange? I did not take this approach because I thought it would be difficult to weld under the 45 deg. braces in a pratt truss.

Thanks for your input/info/help

 

[structuresguy]

Well, either method could be made to work structurally. I suggest you talk with qualified steel fabricators in the area where the project is located. They will be much better equiped to answer your questions on weldability than most engineers. After all, they do that sort of thing every day. Also, it has been my experience that fabricators are usually pretty happy to help you out, in hopes they will land the job. Or at least, if you work with the same fabricators a lot, they might hope that "one hand washes the other" so to speak. Help you out here in hopes that you will help them out later.

Personally I have asked basically the same question to several fabricators. The answers are generally split down the middle. Some say knife plate is easier, others prefer to weld tube direct to flat surface. It really comes down to what their shop is used to doing I think. Knife plate requires them to burn slots in the tubes as well as cut them to length. Direct weld is only one miter cut, unless the work point is such that the tubes would overlap, in which case a double miter is required, along with more careful fitup. You may want to shift the work point to avoid this, but you would need to account for the eccentricity. Also, knife plate can be difficult to put in place if field erecting. however, if your tubes are sized closely for the loads, penetration welding may be required, so that would increase the prep work considerably.

Aesthetically, the direct weld method results in much nicer appearance, so your architect may have an opinion on the matter as well.

 

[dik]

You may want to size the tension member so that it is stressed to approx 75%-80% of the capacity... makes for a much easier weld connection