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Performing Remaining-Life Analysis for Bridges & Infra

Fear of unknown prevails for bridges and infra. So it makes sense to Perform Remaining- Life Analysis of them not just Structural Audit .

I am going to assume a few things here. You are an infrastructure asset owner or a chief/executive engineer or a minister. You were thinking of the latest bridge collapse and the number of lives lost in it, and the future action plan to avoid such a tragedy. You are wondering why the bridge collapsed even after all the Structural Audit formalities were in place. Note, I am using bridges here, but the argument is valid for all critical infrastructures; Power Plants, Ports & Harbours, Dams, etc.

You know how bridges can suddenly collapse prematurely due to deterioration. In fact, after the Gokhale bridge incident at Andheri railway station in 2018, I was involved in a project with Railways for the rapid vulnerability assessment of rail bridges. I saw the pain of the officers and engineers very closely in this project; travelling to far off places from home, returning home very late, no time to spend with family, pressure from superiors and ministers, etc. I know how painful it feels being an officer/engineer caught in such a situation.

Imagine the shock and pain you have after your bridge has collapsed and someone or our own loved one is suffering grievously in such an incident. The quick and sudden requirement of funds to treat your family or for rehabilitation of the bridge is unaccounted for and that’s why you do not have sufficient money now. Since the Savitri river bridge collapse in 2016, there has been at least one bridge collapse every year in our country. Based on the statistics of national highways (NH) reported in 2017, it is found that, 6551 bridges are structurally deficient and out of these bridges, 86% are below the age of 49 years [3]. This statistic comprises of 5591 minor bridges, 664 major bridges and 296 extra-long bridges [4]. To give you a point of view, the NH bridges are merely 3% of all the bridges in the country; just imagine the vastness of the issue. You don’t know beforehand which one of these will collapse first or someone or our own loved-ones become its victim. Is there any salvation? Read on…

Deterioration

Cause of Failure

Leaving aside the deficiency in design and details, “deterioration” is the primary cause of such failures. When the structural performance falls below a specific threshold due to deterioration, is an end of life. Major deteriorations of reinforced concrete (RC) include rebar corrosion due to chloride ingress/carbonation of concrete, loss of concrete strength, loss of prestress and fatigue, etc. You would ask me, “If these are the factors, why do bridges still continue to collapse? Can’t we control them”, Off-course yes! But there is a fundamental block among the engineering fraternity in general…

The Real Cause of Failure

One of the most profound statements I have come across is this, “… RC structures are undergoing deterioration faster, because the mechanism of degradation was not understood well at the time of construction” [5] and was not taken into account in their planning.

In conventional engineering we were not taught to consider deterioration of infrastructures. Remember your 4th year of engineering, we studied ‘Design of concrete structures’. We were taught to design for a given distributed load, or a bending moment, but not taught to design a structure considering, it will corrode in 10 years or how to consider the effect of corrosion in design. Thus in essence, the conventional engineers consider the performance of structures as non-degrading over time, see Fig. 1. As such they are unable to handle deterioration at a detailed level. One has to do a long and detailed study or go for higher education, such as a PhD in the subject to understand it.

Figure 1: Structural behaviour taught in our engineering

To give you an analogy, suppose you did not account a particular load in design. What will happen when that load comes on the bridge — it fails. Since the engineers did not account for deterioration in design and they are not taught how to handle it — this is the real reason for infrastructure failures.

The Remaining-Life Analysis

In a common Structural Audit (SA), engineers perform tests on the bridge and evaluate only its instantaneous performance at time t1 (see Fig. 2). They consider the same structural capacity of the previous audit until the consecutive audit is done, at t2. The reason for this is the same explanation behind Fig. 1; which is, the inability to realistically analyse deterioration.

Figure 2: You gain no knowledge about the future from a Structural Audit

It is possible that bridge failure can occur before your next audit (t2) (Fig. 2), because you gain no knowledge about its future behaviour from a conventional structural audit. Mostly the decision to do an audit is initiated based on visual inspection. The New Zealand Transport Agency confirmed that ‘visuals can be misleading’, because the Sorell Causeway Bridge suddenly collapsed with no obvious visual distress [6]. This indicates that many bridges may already be in a severely weakened state while still passing a visual inspection test without detection of damage.

But now, it is possible to tell you just exactly when your bridge is about to fail or when it is due for a major rehabilitation, see Ref. [1, 2]. In a Remaining-Life Analysis (RLA) you are empowered through a deterioration analysis to “see” the behaviour of your bridge in the years to come and inform you proactively when you have a dangerous situation in future, see Fig. 3.

Figure 3: Remaining-Life Analysis proactively prepares you to avoid future calamities

RLA proactively tells you the future behaviour of your structure through strategic data fusion of SHM/NDT and performance model of the structure through Bayesian updating [1]. Both, the SHM/NDT and the performance model cannot stand on their own to analyse the future behaviour of your infra, they must be stochastically combined. Bayesian updating empowers you to realistically analyse the behaviour and lets you account for uncertainties such as design/execution deficiencies, deteriorations, extreme events and determine the balance life of your project with confidence.

The most important benefits you get from RLA is seen in Fig. 4.

Figure 4: Life-cycle gain from Remaining-Life Analysis, impossible from a Structural Audit

As you already know the behaviour of the bridge in the coming years you are forewarned of its end of life. Usually when there is a sudden collapse you are into panic and might make a hasty decision, unwise for the long run. But because you “see” through RLA you are prepared to combat future rehabilitation by accumulating funds, rather than digging a well when there is fire. Plus you can do proactive life cycle planning for your bridge (impossible in a structural audit), that can minimise your operation cost, and can achieve a target life extension with confidence. I always say, “Bridges don’t demand RLA, they deserve it”.

Conclusion

Structural audit is just ok for instantaneous testing, but it cannot provide us confidence about the future performance of the bridge. Remaining-Life Analysis (RLA) is about care and honesty, which can provide confidence and peace of mind to you. It benefits those who really want to see their Nation as a super power by avoiding its enormous losses from infrastructure failures. There are three qualifiers, to check if RLA is for you: 1) You are wise and knowledgeable, 2) You are proactive, and 3) You don’t compromise for the Nation. If you fit in all three criteria, the world is open for you to succeed.

References

[1] S. A. Faroz. Assessment and Prognosis of Corroding Reinforced Concrete Structures through Bayesian Inference. PhD thesis, Indian Institute of Technology Bombay, Mumbai, India, 2017.

[2] S. A. Faroz and S. Ghosh. Bayesian integration of NDT with corrosion model for service-life predictions. In Proceedings of IABMAS 2018, Melbourne, Australia, 2018.

[3] S. Joshi, N. Naga, and U. Rajesh. Experience of Management of Bridges Prior to and Post Evaluation of BMS on NH network of India. In Proceedings of IABMAS 2018,      Melbourne, Australia, 2018.

[4] S. Joshi and S. S. Raju. Indian Bridge Management System – Overview and Way forward. In Proceedings of IABMAS 2018, Melbourne, Australia, 2018.

[5] P. S. Marsh and D. M. Frangopol. Reinforced concrete bridge deck reliability model incorporating temporal and spatial variations of probabilistic corrosion rate sensor data. Reliability Engg. and System Safety, 93(3), 2008.

[6] R. A. Rogers, M. Al-Ani, and J. M. Ingham. Assessing pre-tensioned reinforcement corrosion within the New Zealand concrete bridge stock. Technical report, NZ Transport Agency research report 502, Wellington, New Zealand, 2013.

Authored by;

Dr. Sharvil Alex Faroz

Dr. Sharvil Alex Faroz, PhD IITB, CEO, Infrastructure Risk Management (IRM), Mumbai. Ph: +91 9970564547, Email: IRMS365@gmail.com, Website: www.IRM365.in

About the Author;

Dr. Sharvil Alex Faroz holds a PhD in structural engineering from IIT Bombay. He is the CEO of, Infrastructure Risk Management (www.IRM365.in), a company to help infra owners avoid unexpected failures by analysing their Remaining-Life and proactively implementing Service-Life Extension strategies. Dr. Sharvil is evolving with infrastructure projects of Remaining Life Analysis, Target Life Rehabilitation and providing Major Maintenance-Free Life for new infrastructure.

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