Deck overhang width

[gwynn]

A topic of much (well some anyway) debate in the office. Why does the width of deck overhangs for cast in place bridges with pre-cast girders continually increase?

With girder centres at 8-10 feet, going from a 2 or 3 foot overhang to a 5 or 6 foot overhang has saved at most 1 girder. At most, because in some cases larger girder spacing could be accomodated, meaning the same number of girders and smaller overhangs.

We've ticked off overall cost as an unlikey reason. The cost of one extra girder - including materials, forming, crane rental and transport (excluding girders which need top flange post-tensioning for transport) is pretty much equivalent to the extra fab time and labour for installing temporary bracing and blocking/shimming overhang brackets against the underside of the girder flange. In cases where a larger centre to centre girder spacing could be used the costs seem to make the larger overhangs more expensive.

So, am I missing something, or are these changes purely for aesthetic reasons?

 

[Qshake]

A good rule of thumb to use is that the overhang should be about 40% of the girder spacing. This helps to economize the concrete deck thickness and reinforcing. The overhang is the most likely controlling feature for bridge deck design.

Regards,
Qshake

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[shin25]

Gradually, most of the DOTs are increasingly using bulb-Ts, which have very wide top flange. This probably has made the wider deck over hang possible compared to what used to be in the past.