Prestress I-Beam Girder w/raised concrete median load 3

[KMA]

I am looking at the AASHTO Standard Specs Section 3.23.2.3.1.3 for design of a Prestress I beam girder type IV. I am looking at an interior beam carrying some of the load of a raised concrete median. I have always thought of a raised median and a sidewalk as being similar (concrete load on the composite section), except for the sidewalk LL provisions. My question is why is an exterior beam carrying a sidewalk LL designed as per AASHTO standard Specs 3.23.2.3.1.3,overstress allowed at 125%. But no similar criteria for interior beams? Any guidance would be appreciated.

 

[broekie]

My interpretation of that part of AASHTO is as follows:

1. All composite dead loads (railing, sidewalks, wearing surfaces, raised medians, etc.) get evenly distributed among al of the beams per AASHTO 3.23.2.3.1.1.

2. Sidewalk live load assumed to be carried by the exterior girders. This is the part I'm not exactly sure of. I have sometimes also distributed sidewalk live load among all of the beams and ignored the 25% allowable stress increase.

3. If you assume that the sidewalk live load is carried by the exterior girders only, you can increase the allowable stress by 25% for the condition of sidewalk live load and traffic live load acting on this beam.

4. If for some reason the sidewalk was down the center of the bridge, I think this provision could be applied to an interior girder.

Hope this helps.

 

[wiktor]

Typically exterior beam would carry both highway (truck) loads, and pedestrian loads. Probability, that both (or even the full pedestrian load) would occur simultaneously is very low, so designing the girder for these loads is very conservative. In some cases (mountable curb, or lack of barrier separating vehicular load from sidewalk) the girder should be checked for full truck loading, with 125% overstress provision.

 

[Qshake]

I agree with both posts as noted above. My design experience follows exactly with what broekie has noted.

Regards,
Qshake

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