"Remote" assessment of bridge footings 4

[GastonLagaffe]

Hello,

I am currently estimating on a design-build for a pedestrian footbridge that will be installed on the abutments of a (now removed) railroad bridge. The abutments go up to about 30 ft high of stone with vertical faces. I have no information as to when the abutments were built / when the railroad was removed. I would love to go out and take measurements but unfortunately my access is limited by the border closures... From photos, the walls look in good shape so I am not too concerned about degradation.

My gut feeling is that abutments that can support a railroad bridge will be plenty to support a light pedestrian bridge at full live load. However, I would feel a lot better if there were any resources that could support my instinct here. Would you consider acceptable to simply show that the total load on abutments from the pedestrian bridge is significantly lower than the design railroad bridge loads? Alternatively, are there any resources on quick conservative assessments of existing bridge abutments?

PS: My background is much more on the structural side - and typically our pedestrian bridges are far too light for geotech to be a major design consideration. I would however love any recommendations for courses / resources I could use to bring up my knowledge on the basics of bridge foundations.

Thanks!

 

[bridgebuster]

I don't see a problem with comparing loads.

In terms of analysis, it sounds like a stone gravity wall. All you can do is a stability analysis. One thing I learned over the years when it comes to abutments and piers, our ancestor were what I call "middle third guys", ie, the always put the resultant force within the middle third and that was it. You can search Google Books for some ancient texts on the subject. From those books you can get an idea on dimensions, proportions, design loads, etc. "Foundations of Bridges and Building" by Henry Jacob, 1914 is a good resource as is "A Treatise on Masonry Constrution" by Ira Baker, 1914. JAL Waddell's "Bridge Engineering, Vols. 1 & 2", 1916, another great resource. Waddell was one of the leading bridge engineers of his day.

 

[MIKE_311]

are these concrete foundations? can you take some cores and determine the concrete strength?

Do you have the original design plans? Can you back calculate the loading to find the reactions the foundation was expected to support?

 

[oldestguy]

I once had the job of evaluating the support capability of a corrugated galvanized culvert pipe of damaged oval shape used as a pedestrian tunnel within the backfill of a railroad bridge. There was measurable deflection of this culvert pipe as a train passed over. Amazing that the diesel locomotive weighed less than the rail cars with iron ore. I deduced this from the strain gauge measurements of pipe interior horizontal and vertical dimensions with stationary loads of a stopped train and also moving train.. After the train passed the culvert pipe returned to original shape, no permanent deflection. From this is deduced that the added loads on the cut stone units of the abutment did not permanently displace with these loads.

For the subject case I'd expect no problem with the lighter loadings.

 

[BridgeSmith]

The axial (bearing) load shouldn't be problem. There's no way a pedestrian load will come anywhere close to the 143 tons per railcar. The pedestrian load would almost certainly be less than for a light rail system, also.

The only loading that could possibly be more critical for a pedestrian bridge (and still very unlikely) is lateral, due to increased wind load on a larger projected area, once fences, etc. are added. The piers would have to be very tall and slender for that to present a critical loading case.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[GastonLagaffe]

Hi guys,

My apologies for the late reply. Your comments are all very well appreciated and I'm glad to hear I'm not the only one with the same line of thought. bridgebuster, these books look interesting and I will definitely try to hunt them down.

We have no original plans or really any information at all about these abutments. Going by historical records, looks like the original track was put down about 120 years ago. Even then, locomotives were undoubtedly much, much heavier than modern pedestrians The main concern then becomes about their condition - but the client has decided to hire a local engineer for an assessment. I feel a lot better about this now.

Thanks!

 

[3Fan]

We're working on a very similar scoped project right now. The client wanted us to verify that the abutments were sufficient to carry the pedestrian loads. No existing plans and no way were were going to dig down to see what the footings or piling was. We did not core the concrete or the stones. We compared what we guessed was the original design train load (based on a assumed year of construction) to the proposed pedestrian load. It was something like 18%. Everyone felt comfortable with that comparison. On the abutments that looked externally in bad shape, we are going to shotcrete all exposed surfaces with doweled in J bolts and some WWF.

 

[GastonLagaffe]

Thanks, 3Fan. I agree that this is the most reasonable course of action. Good luck on the project!