Curved Bridge - Bearing Plate

[Nulukkizdin]

All,

Looking into the design of a curved steel girder bridge. I'm trying to figure out the best way to deal with the fixity at either end of the span. For gravity loads, it would make sense to provide rotational fixity such that the midspan of the bridge does not "fall off" and rotate down toward equilibrium. When looking at thermal loads, however, common bridge practice would be to allow translational movement at one end of the span.

How do folks typically deal with this scenario? I know we (not I specifically) see curved highway bridges all the time, but I'm not seeing any examples that address this issue at the bearing ends. Is it safe to assume that any thermal translation will be along the tangent of the curve at the bearing pad? If so, can we achieve the required fixity (for vertical loads) while achieving the translational freedom by simply slotting bolts holes along the tangent of the curve?

Thanks in advance, any help is appreciated!

 

[bridgebuster]

FHWA and The National Steel Bridge Alliance have on line publications for curved girder design. The bearings are set on the chord of the curve to allow for movement.

 

[BridgeSmith]

The girders (straight or curved) are typically connected with cross frames or diaphragms to maintain them in a vertical position.

That said, I would not recommend taking on a bridge design, especially a curved-girder bridge design, without the guidance of an experienced bridge designer. There are warping stresses, centrifugal live loads, torsional stresses, etc. in addition to the typical moving live loads, braking forces, etc. of the various notional truck configurations, and combinations of loading. Fatigue and fracture considerations are also very complex for a straight bridge, and even more so for a curved bridge with aforementioned torsional stresses, etc. being added into the mix.

 

[Lomarandil]

You don't typically see slotted holes used for thermal movement in elements as large as bridge girders. I've never heard it said out loud, but I imagine there are concerns of reliability.

HotRod is right, jumping into curved bridge design is not something to be taken lightly. If you really need to get into it, read the NCHRP report (I think #563) first to get a handle on the issues unique to curved and skewed structures.

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The name is a long story -- just call me Lo.

 

[BridgeSmith]

"You don't typically see slotted holes used for thermal movement in elements as large as bridge girders."

We do actually have slots on the sole plates for anchor bolts, typically in combination with with a steel reinforced elastomeric bearing pads. The nuts are locked in 1/2" above the sole plate to allow movement. The system also includes keeper bars to ensure the pads don't 'walk' out. We do that mostly at the piers; most of our abutments are integral, where thermal movement is accommodated by bending or plastic hinging of the single row of steel H piles that support them.