Steel truss gusset plate connection
Steel truss gusset plate connection
(OP)
I am designing the connections for a simply-supported steel roof truss. The EOR specified the geometry, member sizes, and member axial forces. It is a Pratt truss with a configuration similar to this:
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The chords are WT sections and the webs are double angles that are lapped and welded to stems of the WT's. My question is at the bearing location at the ends of the truss (it bears on WF steel beams). The architect and EOR require a specific heel depth over the centerline of the beam, so the work point where the neutral axes of the WT chords intersect is ~16" outside of the centerline of the beam support.
My plan is to put a trapezoidal-shaped gusset plate welded to each side of the WT stems at the supports, and to add stiffeners directly over the support as needed. My questions is how to evaluate the gusset plates with the internal moment that results from this eccentric work point configuration. I have checked multiple resources, including all the AISC design guides, text books, and even Blodgett, and I can't seem to find a good discussion or example of how to look at this situation, but I know I am not the first person to run into this since I see it all the time in old buildings. Any guidance would be greatly appreciated.
Thanks!
._______
/|\|\|/|/|\
The chords are WT sections and the webs are double angles that are lapped and welded to stems of the WT's. My question is at the bearing location at the ends of the truss (it bears on WF steel beams). The architect and EOR require a specific heel depth over the centerline of the beam, so the work point where the neutral axes of the WT chords intersect is ~16" outside of the centerline of the beam support.
My plan is to put a trapezoidal-shaped gusset plate welded to each side of the WT stems at the supports, and to add stiffeners directly over the support as needed. My questions is how to evaluate the gusset plates with the internal moment that results from this eccentric work point configuration. I have checked multiple resources, including all the AISC design guides, text books, and even Blodgett, and I can't seem to find a good discussion or example of how to look at this situation, but I know I am not the first person to run into this since I see it all the time in old buildings. Any guidance would be greatly appreciated.
Thanks!






RE: Steel truss gusset plate connection
If you must have a 16" eccentricity for architectural reasons, the bottom chord must be capable of resisting a moment of 16R for the end panels of the truss. If the EOR has specified member sizes, he should have already taken the eccentricity into account.
BA
RE: Steel truss gusset plate connection
1. Use better details (than trapezoidal-shaped plates mentioned above) to avoid the problem.
2. In general, design gussets thicker so that they can be smaller... this makes gusset plates a better approximation of a pinned joint. Therefore, secondary stresses are lower.
Within ten years, or so, of the this paper's publication, the design of steel trusses was revolutionized. Improved gusset plate design largely displaced true pinned joints in steel trusses.
Figure 7, shown below, is probably a good detail for your use. The 1909 paper (PDF) is attached.
Note: Don't discount this paper because the joints discussed were riveted. The geometry is important, not the type fasteners (e.g. modern bolts or welds).
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Steel truss gusset plate connection
Here are my thoughts regarding your particular situation:
1) As BA mentioned, you'll have chord moments. In my opinion, they'll manifest in both the top and bottom chord in proportion to stiffness. I've shown a simplified method for estimating the moments below. It draws from the fictitious heel joint method that I mentioned above.
2) Before sinking to much time into your design, I'd query the EOR on the joint. Two things make me suspect that the EOR has failed to consider the joint eccentricity. Firstly, they didn't provide you with the moments that you need to design the connection. Secondly, a WT is a piss poor shape for the top chord in this application. The web of the top chord WT will be in flexural compression and will be unbraced between panel points. Not necessarily a deal breaker but, in my estimation, deserving of some attention.
3) I've proposed a design method for the gusset plate connection in the sketch below. This is just me making stuff up first principles wise. I know of no accepted reference that would explicitly support my proposal.
4) I love the stiffeners that you've proposed for the gusset plates. When you review my proposed method, you'll see why. It may not be the lowest cost approach from a fabrication standpoint but, man, does it feel good mechanically. I'd weigh the pros and cons of the particular situation. If there are 100 of these things, spend the time to design the stiffeners away. If there are two, keep 'em and move on.
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.
RE: Steel truss gusset plate connection