Approach Slab of a Concrete Bridge 1

[saadeh]

Hi all!

Last time I provided shop-drawings the Consultant of my project (which is a concrete pre-stressed girder bridge) he argued about the location of the approach slab with reference to the abutment mirror (the abutment vertical stem which has a lesser thickness than the abutment main load carrying stem) he insists that the common practice of bridge engineers is to locate the approach slab at a depth of 1.5m below the road way. is that really a good practice ? because I have never seen something like that in drawings, bridge construction practices, or textbooks. Plus, if the main purpose of the approach slab is to prevent severe differential settlement between the embankment and the super-structure and to provide a smoother transition across them, wouldn't lowering it below the road level create problems in transition smoothness of the approach slab and alter it real function ??

So is it correct to locate the approach slab well below the asphalt level (about 1.5m below road level) ??

Regards.

 

[bridgebuster]

In the US, many DOT's(if not most I would guess) place the top of the approach slab flush with the top of the abutment backwall. However, I recall that the MASS DPW used to bury the approach. The current MassDoT detail shows it as flush with the top of backwall. Unfortunately, I don't have a complete set of their old manual; only have the general view of the abutment.

To me, 1.5 m seems excessive.

 

[IDS]

I have worked on bridges for over 40 years, over which time use of approach slabs and their details have gone through just about every possible permutation, and as far as I know there is still no generally accepted consensus on their use. I just found the paper linked below, so I haven't read it in detail, but it looks like a good examination of the options, their usage (in the USA), and advantages and disadvantages.

http://www.virginiadot.org/vtrc/main/online_reports/pdf/00-r4.pdf

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[bridgebuster]

IDS: Thanks for the paper; they had the Mass DPW detail I was missing.

In addition to what you wrote, there's no consensus on length or thickness either. I've been around not quite as long and have seen lengths change. I remember 10' approach slabs. Some years ago, one DOT changed their length criteria to equal the height of the abutment. Of course if you have a wide road, a tall abutment, and a sharp skew, you end up with a very long slab.