Steel Box Girder Access Opening

[TampaBridgeDesign]

The DOT we're designing a steel box girder bridge for states that access opening in the bottom flange shall not be spaced more than 300' apart. We have a 3-span bridge with a total length of 600'. The DOT won't let us put the access opening over traffic in a positive moment region so it's going to end up being in the compression zone near the pier. Has anyone designed the area around this opening in the compression zone? I assume that some kind of additional ring plate or longitudinal stiffeners will be required. Is there any literature for this situation? Thanks!

 

[cooperDBM]

In Ontario we put the access openings in the web near the abutments where traffic usually doesn't have to be interrupted for inspections. The web plates are generally much thinner than the bottom flange and can be stiffened vertically and horizontally as required. Intermediate accesses are only used for very long bridges. Perhaps Florida(?) requires more access openings for ventilation during an inspection in due to it's hotter climate.

I can't imagine trying to put an access in the bottom flange except at the abutments. Near the piers you would have very thick bottom plates and probably long. stiffeners.

Is the web an option for you?

 

[TampaBridgeDesign]

Unfortunately the web is not an option. The bottom flange access openings near the abutments are 3' in diameter and the opening we'll have near the pier only has to be 2' in diameter. Our b/t ratio at the pier section is such that no longitudinal stiffeners are required.

 

[wiktor]

For a three span bridge, 180' each, I've used reinforcing collars around the openings in the bottom flange, welded.
The collar was dimensioned to partially substitute for the material lost due to the opening, as detailed calculations for the section were done. As the opening was offset from the pier by 8', the negative moment could be carried by weakened section without reinforcing ring, but one was provided to minimize change in the stress level.

 

[TampaBridgeDesign]

wiktor-

Did you perform a FEM analysis to check for buckling? I guess that's our biggest concern.

 

[EngTampa]

TBD,

Did you find any literature regarding this topic? I'm running into the same situation with FDOT. Similar bridge length as yours 185-245-185.

 

[TampaBridgeDesign]

Unfortunately, we haven't been able to find any literature related to this topic. More than likely a finite element model will be set up to determine exactly what is going on at that particular location. Have you guys had any luck finding anything that would help with this type of design?

 

[wiktor]

No, we did not perform FEM analyses, but the stresses were not so high in the specific location.
I will strongly encourage performing FEM analyses of the area, as simplified model is easy to construct and assess.