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Dead load deflection calculation for bridge design

Dead load deflection calculation for bridge design

Dead load deflection calculation for bridge design

(OP)
I had a question regarding dead load calculations for bridge design. The specific bridge type is concrete deck on steel beams with concrete parapets.

Per IDOT, we have to show top of slab elevations on the plan sheets, after adjusting for dead load deflections. While calculating these dead load deflections, should the weight of parapet be included or do I only have to include the weight of the concrete deck? The parapet weight is considered a non-composite load (DC2), so I wanted to check if this needs to be included for the dead load deflections.

The concrete deck weight will be resisted by the non-composite section while the parapet weight will be resisted by the composite section, so this will require 2 separate calculations, is that correct?

Thanks.

RE: Dead load deflection calculation for bridge design

t230917 - you should have posted this in the bridge engineering forum. However, not a big deal.

I'm puzzled on how the concrete barrier is non-composite but yet is a DC2 load? You may want to double check your statement. Typically, the deck is poured, has some cure time, then the concrete parapets are poured next.

If this is the case, you need to determine the dead load deflections of the non composite beam due to any dead loads applied before the deck is hardened- DC1. Assuming the parapets are cast after the deck is poured, then you are dealing with what is commonly referred to as the superimposed dead load or, in AASHTO LRFD - DC2.

The DC1 deflections are based on the noncomposite section section properties for the steel beam.

The DC2 deflections are based on the long-term composite section properties. This is typically found by transforming the slab and flange width into steel by reducing the widths by the ratio of 3n where "n" is the modular ratio of the concrete and steel.

Some folks ignore the flange width which is fine since it doesn't really impact the deflection calculation significantly.

RE: Dead load deflection calculation for bridge design

(OP)
Thank you for your detailed reply. This is what I was thinking in my head but you explained it perfectly.

Sorry, I meant that the parapet is a DC2 load, acting on a composite section and the concrete deck is a DC1 load, acting on the non-composite section.

RE: Dead load deflection calculation for bridge design

The girders are cambered for the own weight + diagrams.
They have additional camber for DC1 loads.
The deck hardens and then you place DC2 loads. In this stage you have a different I, since your deck is now composite. The other part here is how do you distribute the load of the barrier to the girders? Some agencies say 75/25 to the exterior/first interior, others says evenly distribute to 2 or 3 girders. What ever you determine, that's the camber that gets applied for DC2 loads.

then you have vertical curve camber...

RE: Dead load deflection calculation for bridge design

As Mike pointed out, don't forget the vertical curve camber otherwise known as geometric camber. If your interior piers are higher than abutments you will have this. It may have little effect if the elevation difference is small but can contribute if there is a large vertical curve.

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