Structure behaves as an arch or flexural member? 1

[AK4S]

I am working on load rating of an existing bridge structure; a reinforced concrete arch built in 1902.

The structure has a short 7ft clear span with concrete profile, with 16” thick arch ring which encases 10” deep rolled steel beams spaced at 2ft on center spacing.

See attached sections of the bridge and representative photos.

(A). Based on the arch profile (with nominal surface reinforcement) it appears that the structure should behave like an arch and should be analyzed as an arch.

(B) However, the presence of the embedded steel beam makes me believe the original intent might be to treat it like a flexural member, so consider the steel beam as the main load carrying element and analyze as beam with fixed ends. Evaluate for the positive moment at mid-span which is WL2/24 for uniform load. The negative fixed end moment, WL2/12 will not govern since the depth of the structure at that location is significantly large (abutment walls).

Any thoughts on how the structure should be best analyzed, A or B? Would appreciate, if you could direct me to any references which would provide guidance on why a particular behavior is expected.

There were no significant cracks (only few minor hairline cracks) observed at the underside of the structure in the crown area.

 

[AK4S]

I did a very simple sketch to show you a possible approach. Only you can say if it is appropriate for you project, or not. But it doesn't have to be very complex.

A first step can be to model the concrete arc (green) and the steel beam (blue), see figure. Use an appropriate thickness to get a reasonable stiffness and load distribution. The red lines around the beam is some type of interface, rigid/friction/free. Based on this simple start you can push the modelling further with the correct beam section, solid elements for the concrete and so on.

But the main question is, is it worth it in this particular project?

Thank you for sharing potential approach for a FE model. Unfortunately there is no budget for trial and error on a detailed FE model and analysis. We'll have to justify that a simpler approach may not yield correct results and that that the FE model developed works as indented.

 

[AK4S]

If you are looking how the structure was originally analysed , i would say other option (C). I guess the designer analized assuming rigid frame structure maybe used rigid frame formulas or energy methods. ( Moment distribution was not known at that time) .
The top slab or say top arch at midspan was designed as composite beam using the rules of WSD.
I had an old RC book there are worked examples but i could not find. I have seen some very old frame calculations and almost all were simple hand calculations using formulas .

View attachment 15966


Regarding your question; load rating of the structure, you need a full survey for concrete quality, reinforcement scheme , corrosion.. then model the frame and apply the code required railway loading together with lateral soil loading and see .

There is a past NDT report which provides concrete strength (estimated b/w 3300psi to 5000psi) and past field core info which provides the steel beam information, partial info on the wall rebar as well.

Thank you for your suggestion regarding a Rigid Frame Approach. I'll have to look around to see if I can find any info on the analysis from that era to see how the OG designer might have analyzed a rigid frame. Please do share in case you can find any info from your old reference book or elsewhere.

 

[ThomasH]

Thank you for sharing potential approach for a FE model. Unfortunately there is no budget for trial and error on a detailed FE model and analysis. We'll have to justify that a simpler approach may not yield correct results and that that the FE model developed works as indented.

FEM does not have to mean trial and error, but it can be. Still, with a limited budget you should work with methods that you are comfortable with.

 

[Tomfh]

Without confirmation that the backfill can resist the thrust without budging, I’d assume flexure.