Bridge Truss Connection Design Loads

[emwell]

Hi All!

I drew the lucky straw to design the connections for a steel truss pedestrian bridge composed of W sections. Where I'm running into problems is how to appropriately apply the requirements of AASHTO 6.13.1 to figure out the design loading for each connection. We have the truss modeled in RISA with all of the applicable load combinations, so I have the member reactions at each connection to work with. I understand that I need to design for the the maximum of the either 1) the average of the reactions due to the applied loads and the member resistance or 2) 75% of the member resistance. My question is this - since many of these connections see combined axial and shear reactions OR axial, shear and moment reactions, how do I account for these combined effects using the AASHTO load requirements?

For example, at a fixed connection where the connection sees simultaneous shear, axial, and bending moment reactions the 75% of member resistance condition controls. Do I design for each of the 75% of member capacities (shear, axial, moment) separately? Do I apply all three at the same time?

Help!

 

[bridgebuster]

Typically, when we design a gusset plate the connection of each member on the plate is a discrete element. See attached FHWA publication for load rating gusset plates, it is applicable for design.

 

[emwell]

Thanks for the reference bridgebuster, it will definitely be helpful when I get into designing the gussets! Unfortunately it doesn't explain how the loads from each member on the plate are determined. THAT is what I'm having issues with at the get-go. I can get the reactions from each member from my RISA model, but I'm trying to figure out how to meet the requirements of AASHTO 6.13.1.

 

[bridgebuster]

emwell - the member forces are axial. They're obtained by truss analysis. Once you determine your member forces you design for the greater of the average of (member force + member capacity)/2 or 0.75 x member capacity. The FHWA example omits the truss analysis because their focus was on gusset plate design. Each member is a discrete connection.

 

[emwell]

I'll have to look at my model/truss analysis again. I know I have some locations (e.g. at floor and roof beams, some verticals) that have both shear and axial reaction components or even shear, axial and moment reactions.

 

[KootK]

Is this a vierendeel truss or some other truss form not dominated by axial load response? I suspect that the code provisions being discussed were geared towards joints with incoming members predominantly loaded axially. Dealing with 75% of a member's shear capacity, for example, would place an enormous and unreasonable demand on your connections in many instances.


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.

 

[emwell]

KootK - Nope, nothing so fancy! The truss itself is dominated by axial loading, however, at some members we do have shear and axial components to our reactions. For example, the floor beams tee into the bottom chord of the truss. While they are predominately in axial compression, they are also transferring the dead and live load from the deck in shear.

I think (from re-reading the second paragraph of Section 6.13.1), that I can design the floor beams for the calculated member force effects rather than the 75% rule since they were included with the structural model. Hooray!

But now I have a new problem - my vertical and diagonal members are W sections and need to be connected to the gusset plates through their webs (they are rotated 90 deg from a typical truss bridge orientation ... architect preference ....) The problem is - the web alone can't carry 75% of the axial capacity of the member in shear! Anyone know if the following clause would apply in this situation?

"In the case of connections that transfer total member end shear, the gross section shall be taken as the gross section of the connected elements."

 

[EQguys]

emwell:

Take a look at the following publication by FHWA. It might give you an idea for how to come up with the connection.

http://bridges.transportation.org/D...anceandExamplesforGussetsFebruary2009rev3.pdf