Steel Box Girder Bottom Flange Access Opening Fatigue Category

[bridgedesign222]

We're designing a bridge per the LRFD specifications. Our DOT requires access openings in the bottom flange spaced at not more than 300 feet. For our 3-span bridge this requires openings in the second span. LRFD says to place these in low stress regions. Clearly, anywhere in the second span is not going to be a low stress area. We've developed a ring to place around the opening to reinforce it. There is a grab bar welded to the ring and the bottom flange. The ring is bolted to the bottom flange. The bolts would be a category B detail. The weld to the ring and the bottom flange would be a category C detail. Looking at the fatigue detail sketches, there is an eyebar detail which shows the fatigue category on the net section is a category E detail. Is the bottom flange access opening the same as an opening in an eyebar? If so then it would be a category E detail. Fatigue is governing the design of the ring so determining the correct fatigue category is critical. I'd appreciate your thoughts on this. Thank you!

 

[graybeach]

If you are near a pier, wouldn't the bottom flange always be in compression and fatigue not be a concern?

 

[civilperson]

Assuming a box girder with interior diaphragms at the support, I agree with graybeach about lack of fatigue in compression region near the piers.

 

[bridgebuster]

Ditto on what graybeach wrote.

Also, below is a link to NSBA's guide to box girders. They provide various details including an access hatch.

http://www.aisc.org/Content/ContentGroups/Documents/NSBA5/TubGirderBookLink.pdf

 

[bridgedesign222]

Since the opening is located near the contraflexure point, the dead load stress is essentially zero. The fatigue stress at the opening location assuming there are no holes in the bottom flange is about 2.3 ksi. For a plate infinitely wide with a hole in it the stress riser is about 3. This would result in the stress at the hole location equal to 6.9 ksi. (Since our plate isn't infinitely wide we've done finite element analysis to determine the actual stress.) If it's a category E detail then we need to limit the fatigue stress to 2.25 ksi. If it's a category C then we need to limit the stress to 5.00 ksi. As you've indirectly indicated, the solution might be to move the opening closer to the pier such that the bottom flange is always in compression. It's likely doing this will change the governing condition from fatigue to compression. Doing this (going to a higher stress region) would go against what LRFD states. Thanks for your thoughts!

 

[VoyageofDiscovery]

The Canadian Code (Limit States Design) and similar to LRFD for bridges, states that fatigue shall be disregarded when the permanent compressive stress is at least twice the maximum tensile live load stress.

Perhaps you can place the opening in this region.

VOD