## Bridge Truss Connection Design Loads

## Bridge Truss Connection Design Loads

(OP)

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!

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!

## RE: Bridge Truss Connection Design Loads

## RE: Bridge Truss Connection Design Loads

## RE: Bridge Truss Connection Design Loads

## RE: Bridge Truss Connection Design Loads

## RE: Bridge Truss Connection Design Loads

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: Bridge Truss Connection Design Loads

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."

## RE: Bridge Truss Connection Design Loads

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/Documents/FHWA-I...