MDX Bracing Connection Plates Fatigue Overstressing

[CalebA]

I'm designing a curved steel plate girder bridge that is continuous over the piers. I'm using MDX to model the bridge. I designed the flanges with varying widths and varying thicknesses between field splices. I did not specify any transverse stiffeners, but MDX defaults in the connection plates for the bracing. I am getting fatigue overstressing in these connection plates and am having issues resolving them. The thing that has the largest effect is changing the the bottom flange widths in the positive moment regions. Changing the web to connect distance, height of bracing, and size of bracings members themselves also have some impact. Is there another factor I could change here other than increasing the flange width? My spans are 100'-100'-100'-90'-90' with 9.25' girder spacing. I have bracing at about 20' spacing w/ 2 10' spaces on either side of the piers.

 

[SWComposites]

With the safety factors on ultimate loads, is it typical the bridge girders are critical in fatigue? Something doesn’t make sense.

 

[bridgebuster]

Full disclosure: I haven't used MDX in many years (based on some recent posts here, it reaffirms my belief that MDX is lacking in many things.). However, we ran into a somewhat similar problem a few months ago. It was a design build proposal for rehab of a curved girder viaduct (5 units with 3 span continuous girders, each unit was about 380'; curve radii were fairly large).The RFP indicated that some girders required strengthening.

In our case it was bottom flange compression in negative moment regions near the piers was controlling. The results were very sensitive to diaphragm spacing. There could have been lateral bending stresses being added. Since this was only a proposal, we decided to add diaphragms and if we won the job (we didn’t) we would do a refined analysis in CSiBridge.

One thought, can you widen the flanges and eliminate the lateral bracing? Some agencies we work with insist on lateral bracing and others prohibit it.