Estimating Bridge Remaining Service Life - Available Guidance? 2

[EngDYIGuy]

Any guidance available for estimating the remaining service life of existing bridges? Customer wants this information to program future repair/replacement funds.

I have a 75 year old railroad bridge with steel stingers controlling the load rating in flexure, with no section loss or corrosion, and concrete bents with spalling/delaminated concrete everywhere. All the spalling is in the concrete cover only. Marine environment. Spalled areas show reinforcement has surface corrosion. Substructure rating is "Fair", Superstructure rating is "Good" by inspection report. Concrete features have had capacity's reduced for load rating do to condition factor, but are far from controlling the load rating, compared to the stringers in flexure.

 

[TehMightyEngineer]

AASHTO Manual for Bridge Evaluation and your states Bridge Design Manual. Since it's a railroad bridge there might be an equivalent AREMA bridge evaluation manual but I've never had to deal with it.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[Lomarandil]

I don't know if there are resources to quantify a remaining service life. (I've read a lot of the MBE and haven't seen it, but certainly not cover to cover).

Just my opinion, it sounds like the service life is indefinite at this point. Based on your description, I'd expect the bridge to be adequate until the spalling accelerates to begin controlling the rating (which sounds like it may be a while), some other decay begins to show, or systematic changes (heavier traffic) require a retrofit or total replacement of the bridge.

 

[EngDYIGuy]

Another monkey wrench to throw into the discussion. The bridge is in a high seismic area, and was designed prior to the implementation of seismic design criteria and grandfathered in. Load rating is for gravity loads only per AREMA. Risks for the potential of seismic damages is currently unknown, and will be captured in a future seismic assessment and liquefaction study soon.

 

[SlideRuleEra]

I really like Lomarandil's approach. Another concern is the connectors, e.g. anchor bolts. Have steps been taken to evaluate them? In a marine environment steel connectors can fail long before steel members.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[graybeach]

Another way to look would be to prepare a business case for whether or not to do a full replacement. You can include alternatives to do nothing, repair rehabilitation, and even decommissioning. Consider the broad range of construction, maintenance, risks, etc.

 

[Lomarandil]

Seismic is a good point to consider.

You could get into the betting game of finding the expected return period of an earthquake expected to damage the structure (beyond some minimum acceptable threshold). This would be similar to the logic behind bringing down the 6th St Viaduct in LA -- currently adequate, but likely to collapse within X years under a Y size earthquake. There are probably some public documents available from that project that would give you some framework for how to discuss the risk (after your technical evaluation is done).

Of course, then you run the risk of a non-technical manager getting upset if that earthquake happens to hit in 5 years instead of 10. Hopefully they all live in the same high-seismic region as your bridge and understand the basics of earthquake magnitude and frequency.