Archie_M_notes

A seriously skew bridge

I have seen some skew bridges in my time but this one almost defies photography. The two views below might show what I mean.

 

This is looking more or less axially through the bridge and shows the semi-circular arch with a 20ft span (this WAS the canal era).

Looking square to the spandrel shows the enormous elliptical stretch.

The end view of the acute corner of the abutment below gives another emphatic illustration of the skew. Ballard had considerable trouble geting the centring out and transferring stress. There are bulges evident that probably date from that time, though the cracks he describes seem to have been repaired effectively.

Reinforced concrete arches

A common question in a slightly different form:

Hi Bill,

I am currently undertaking some arch bridges assessments for which I am using Archie-M (version 2.4.1). One of these bridges has a multi-span masonry arches widened with reinforced concrete arches. My query was – can we use Archie to assess a multi-span RC arches as well using appropriate RC material properties? If not suitable for reinforced concrete arch, can we use it assess plain concrete arches?

 

I will be grateful for your response.

 

Best Regards,

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Many bridges were extended in this way. They were designed assuming arching behaviour and assuming very low bending effects but modern requirements suggest elastic analysis. If you do that, the result is almost certain to be an indication of substantial overstress.

I have, in the past, somewhat facetiously, suggested cutting the reinforcement to turn it into an "arch". That is clearly unnecessary but I am sure the case can be made that if it would work as an un-reinforced arch, the reinforcement will not reduce the capacity.

When does an arch need a detailed survey?

Here is a lovely example of an arch that has distorted enough to need a detailed survey. It was almost certainly originally three centred. An ellipse certainly doesn't come near to fitting. I suspect that the pier to the left has settled and rotated a little to the right. The arch has then fleattened to the left, but there is some evidence in the bed joints in the spandrel at the right of the arch having been compressed and so forced up at the right hand haunch.

However, it is certain that nothing short of a proper survey, with perhaps 20 or 30 points in the span, will give any hope of modelling the arch effectively. Even then one should note that the joints in the left half of the span are nearly all open at the bottom giving a very much reduced ability to carry live load.

So here we have a bridge that should not be assessed except as a guide to how it might perform once properly repaired.