Top Down Bridge Construction

[chrissyboy10]

Hi,
I'm about to construct a road bridge using a Top Down method of constuction, i.e. casting the deck on the ground, then excavating out underneath at a later date.
I was wondering if anyone had any experience of this method before and if so had any ideas on an alternative to a sacrifical plywood soffit shutter to cast the deck on.
The client is not too concerned it seems with quality of finish on the underside of the bridge deck when finished, though i am wary he will be when he sees the finished product!!

Cheers

 

[bltseattle]

I've never tried this, but if you have mostly incompressible bearing soil maybe you could use plastic sheeting, if thick enough it might stretch a little instead of tearing to take up areas of lesser bearing.

 

[jheidt2543]

I saw an article regarding this kind of construction within the last 4-5 years in one of the trade magazines like ENR, ACI International. You might try a Google search and see if you can find info. out on the Internet. (You might have to ignor all the hype on Google's IPO - just thought I'd mention it!

 

[daaro]

I have done a few of these. The jobs were grade seperations or underpasses. As for the soffit appearance, i used a thin PCC waste slab in each case. The owner was very concerned about appearance in my case. Another reason the waste slab was prudent, is grade control. With falsework, adjustments can be made by jacking falsework bents and with the at-grade method you cannot.(apply a bond breaker to the waste slab prior to stem & soffit pour. If bent footing(s) are spread footings propose a CIDH type of bent.

Benefits to this method are: no falsework, no shoring for bent construction (if CIDH can be used). easy access to superstructure construction(less crane time$$),no work over traffic.

Downside or risk: digging underneath bridge is not as productive as, owners are leary as this is a new method to most.

Good luck.

Daaro

 

[DRC1]

What abot casting the deck on galvized stay in place forms resting on the gound ?

 

[cvg]

many state highway departments do this all the time. contact Arizona Dept of Transportation bridge department for information on their methods