×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Bridge Truss Connection Design Loads

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!

RE: Bridge Truss Connection Design Loads

(OP)
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.

RE: Bridge Truss Connection Design Loads

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.

RE: Bridge Truss Connection Design Loads

(OP)
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.

RE: Bridge Truss Connection Design Loads

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.

RE: Bridge Truss Connection Design Loads

(OP)
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."

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login



News


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close