Bolted Crossframe Connection

[3Fan]

A few months back, we were investigating what analysis method to use on a new bridge project. For reference, the old thread is this one:

thread607-412835

To sum up this bridge, it is a single span plate girder (128.5'), resting on MSE walls. There are 8 beam lines. The beams are straight and parallel, but the horizontal alignment is curved. The overhangs vary across the bridge. The substructure is skewed at about 45 degrees. We have used MDX to model the system. We feel pretty comfortable with our model. We are at the point where we are detailing the crossframe connections to the intermediate connection plates. We understand with MDX output forces are high (in the magnitude of 2-3 times). Our structure is galvanized so all connections need to be bolted. We will either be using K style or X style bracing with a top strut. MDX is outputting forces in the 160 Kip range. When we check our bolts in shear, we are getting a lot of bolts. How have you guys/gals handled the bolted connections of cross frames to the girder with these type loads? The connection get rather large with all the bolts that are needed.

Thanks for the help and opinions once again.

 

[EQguys]

3Fan: Welcome to the world of MDX. I have two suggestions to reduce your crossframe sizes and forces.
a. Use a Plate and Eccentric Beam (PEB) type model and not a Grid model.
b. Manually add the Warping stiffness of the girder to the overall torsional stiffness. Contact MDX and find out how this can be done and let us know too

 

[OSUCivlEng]

Are your crossframes staggered or in a continuous line? Staggering the crossframes can reduce the live load induced forces. However, there are some fit up issues that come along with staggered cross frames, especially on a 45 degree skew. The steel detailer and/or fabricator just has to be aware of it. I have attached a pretty good NSBA document about fit that came out last summer.

Staggering the crossframes will also have some effect on the flange lateral bending stresses. Ask someone at MDX how they calculate that too. Im curious if anyone can find a decent explanation of how to quantify lateral flange bending stresses besides building a 3D Finite Element Model.

 

[3Fan]

Thanks for the replies. Our cross frames are continuous and perpendicular to the girders. Our DOT has a nice flow chart to guide us on how in-depth we are to get on girder designs like we have. If our girder twist is > 1/8"/ft of the girder height, we are directed to increase the girder weight by up to 25% in hopes of alleviating the twist. If after increasing the weight by 25%, we are then directed to implement an internal lean-on bracing system and a refined analysis (full 3D). For us, increasing the weight reduced the girder twist enough that we did not have to do the lean-on bracing. Our DOT states the contractor is to erect the girder plumb with all cross frames fully bolted. Which would be the SDLF (steel dead load fit).

We have used the Plate and Eccentric Beam model and it did reduce the cross frame forces some. Which helps a little. Still having a hard time fitting all the bolts in that are required.

I'm sensing some sarcasm with the MDX comments? I have not spoken with them about torsional stiffness directly, but he did mention MDX is working on a full 3D update to their program. It was in the testing phases. If he had time, he was going to use our bridge as a test rat. That was a month ago and I haven't heard back from them.

Thanks!