Unreinforced concrete arching action

[mznstr]

Hi all,

I am currently looking at an unreinforced concrete arch bridge, which I believe developed some transverse cracking in the past and therefore has been strengthened with steel beams and brick diaphragm walls @ 1m c/c as shown on the photograph below. The transverse section of the structure effectively looks similar to a steel beam and infill slab bridge, however, the unreinforced arch, which acts as the infill slab must transfer the loads onto the steel beams somehow.

This article gives some ideas and refers to UK departmental standards (CD 360 is the current version of the standard), however, the standard only deals with reinforced concrete.

My idea is that the arching action in this concrete infill slab can be checked by imagining a mass concrete slab within this 'infill' slab when looking at it in cross-section with the rest of the concrete on the intrados being non-structural.

Alternatively, I can imagine two compression struts inside the arch pushing the beams apart.

Could you please advise on how'd you approach this?

 

[le99]

It looks like a stone arch to me. Are you sure it is concrete? The two behave quite differently.

 

[mznstr]

It looks like a stone arch to me. Are you sure it is concrete? The two behave quite differently.

Yes, the arch barrels are mass concrete. There is no information on the structure whatsoever, but we've done the investigation works to confirm this - the original stone abutments were retained, so as the spandrel walls, but the barrels were reconstructed in concrete.

 

[le99]

Sorry, I don't understand what is the problem and your objective, can you try make it clearer?

 

[KootK]

1) To my eye, the proportions of the steel beam don't seem appropriate relative to the scale of the arch for that beam to be working primarily as flexural reinforcement.

2) I wonder of the real purpose of the steel beams might have been to act as a tension tie in making the bridge function as a tied arch. This would raise two questions in my mind"

a) Do the steel beam connection posses the ability to transmit significant axial loads and;

b) Why build that bit of masonry wall on top of the beams? Aesthetics? Bird deterrence?

3) With regard to your proposed approach, I would be leery of any strategy that denies the structure the behavior associated with its true form: an arch of some kind. In generally, doing so tends to lead to serviceability problems as the structure has to pass through its "preferred" mechanism of resisting load on its way to your "designated" system of resisting load.

 

[HTURKAK]

Better to consider two compression struts inside the arch pushing the side walls apart..

My approach would be,

i= calculate the horizontal thrust which can develop at side walls ( OT resistance + soil thrust at rest case )..

ii= calculate the resistance of arch to uniform load with this available horizontal thrust ..( you will see that, the design vertical load is more than the resistance of arch with horizontal thrust ( OT resistance + thrust at rest case )..then transverse cracks developed , the retaining walls probably developed passive thrust ( or inbetween case )

iii= When transverse cracks developed ,the arch has been strengthened with steel beams .. so the total resistance could be approximated with adding the sistering effect of steel beams with bending..

iv= Calculate the max. horizontal thrust which can develop with passive thrust and calculate the max. resistance of arch using this horizontal thrust and add the bending resistance of the steel beam..

v= apply a reasonable F.S. to the total resistance to get the safe service loads.

P.S. You can get the formulas for arches from a structural handbook.