Live Load Distribution for Curved Bridges

[gnault]

I am trying to determine a live load distribution factor for an exterior girder of a curved bridge composed of straight precast girders with a composite CIP roadway slab. The roadway slab overhangs the exterior girder creating a slab cantilever. The length of this overhang varies along the span due to the curvature of the bridge (3.7' MIN @ the end of span to 5.3' MAX @ midspan).

My question is what should I be using as my slab overhang length since the length varies along the span? Is it common practice to just use the maximum overhang length? It seems to me like this would yield an overly conservative value for the entire span. Is it justifiable to take an average of the MAX and MIN overhangs?

I am trying to determine how much of the overhang to consider in order to use the equations in Provision 4.6.2.2 in the AASHTO LRFD, which allows the use of the approximate methods of analysis providing that the curvature is below a certain amount (which this bridge is). I understand that there are more advanced analyses that can be used to model the geometry, etc. however I am trying to stick with the simplified approach.

If anyone has had any experience with this or knows of a reference that addresses this type of geometry, please let me know.

Thanks for the help.

 

[bridgebuster]

Average it.

 

[Qshake]

Just a note. Check your agency requirements for slab overhangs with chorded girders on curved alignment. There are several agencies, DOT's that have limits less than the 5'3" max you noted.

One problem with this is not necessarily related to final design of the slab per se is the construction loading. Depending on a contractor's method of forming and placing, such a cantilever places a large torsion on the exterior girder and prestressed I-girders are not well braced (as say steel is) so despite having robust section properties, the exterior girders have a tendency to rotate out of plumb and since their underall the form work it isn't noticed until it's too late.

So you may want to consider temporary bracing requirements or different cantilever lengths (pending agency requirements).

Regards,
Qshake

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[rscassar]

I would be using the maximum slab overhang when calculating distribution to the exterior girders.