Structural Audits of Bridges and Infrastructure Works

PART – II

Structural Audits of Bridges and Infrastructure Works

General:- Part-I of this paper was published on 5th January’2021 (although I had started composing it in the month of September’20202 it took five months to give it proper shape). After getting many comments, queries and suggestions from you all, I started composing part-II after 15th January’2021 it got almost ready by end of January’2021. But I fell ill and it took some time for this article to take a shape. Hope you will find it useful.

In first part of this paper, general methodology and fundamentals of performing structural audits, fees for each assignment, do’s and don’ts about NDT, interpretation of data, report preparation, was discussed keeping main focus on the residential and commercial buildings. A few case-studies and cautionary advises were also given. Now in this part-II of this series, thoughts on ‘Structural Audits of Bridges and Infrastructure Works’ are presented. This is mostly useful for the government transportation departments like – NHAI, MSRDC, PWD, DMRC, CIDCO, local municipal corporations, ZP and infrastructure consultants, rehabilitation contractors etc. during upgradation / repairs of the old roads or while preparing DPR of new roads / railways / metros or commissioning.

Keywords:- IRC Codes, Audits, Exposure condition, damage mechanism, retrofitting, carbonation, corrosion, protective coatings, AASHTO, FHWA, MORTH, overloading, DPR, SHM, CD (cross drainage) structures.

Introduction:- Bridges (MNB, MJB, ROB, FOB), culverts, elevated viaducts, grade separators (flyovers, underpass, overpass, RUB) etc. form an inseparable part of any transportation project due to ground characteristics and crossing utilities etc. A complete cycle of feasibility studies, planning, bidding, detail design & approvals, construction, maintenance, widening, repairs, retrofitting, dismantling and new construction of bridges and infrastructure works is done according to MORTH guidelines and IRC / IRS codes and a few IS standards and some specialists literature (RDSO. AASHTO, FHWA etc. as per case). A few authorities like PWD Maharashtra, DMRC, MSRDC have their own internal procedures for audits. Following are the important codes and standards to know before entering into the field of infrastructure audit :-

·      IRC-5 – standard gives definition of various types of bridges / culverts and their components. 

·      IRC-6 – standard defines the highway loads on the bridges and culverts.

·      IRC-78 – standard describes the guidelines for foundations and substructure.

·      IRC-SP:13 – code defines guidelines for culverts and CD structures.

·      IRC-83 (part I to IV) – give the guidelines for bearings and IRC-SP-69 elaborates the bridge expansion joints.

·      IRC-112 – Code of Practice for Concrete Road Bridges.

·      IRC-SP-114 – Guidelines for Seismic Design of Road Bridges.

·      IRC-SP-115 – Guidelines for Design of Integral Bridges.

·      IRC-SP-51 – defines the bridge load tests.

These are the basic codes one must know before entering the domain of bridge audits. There are several other codes under IRC / MORTH / IRS / RDSO and guidelines which one can master gradually. But a few important codes are as below :–

·      IRC-SP-18 – Manual for Highway Bridge Maintenance Inspection.

·      IRC-SP-35 – Guidelines for Inspection and Maintenance of Bridges.

·      IRC-SP-52 – Bridge Inspector Reference Manual.

·      IRC-SP-74 – Guidelines for Repair and Rehabilitation of Steel Bridges.

Provisions in Manuals for – two / four / six lanes and expressways (IRC-SP-99) should also be known.

Extent of damage in Buildings vs. Infrastructure:- In general, the process of ‘aging’ or deterioration of engineering materials happens faster in infrastructure works as compared to building structures due to permanent exposure to vagaries of the weather, dynamic nature of loading, and often the poor quality of construction by lowest bidder (L1) and off course inadequate maintenance! Considering these aspects; the auditor should know about the ‘exposure conditions’ defined by IRC codes and the qualifying criteria is based on the probabilistic damage (relative humidity, distance from seashore, contents of pollutants in the air, ingredients of the masonry (grade of stone / aggregates / cement), clear cover to reinforcements bars, any protective coating etc.); hence exposure condition is elaborated in detail, ahead in this paper. The structures are designed for hundred years life, the degree of maintenance also matters a lot for extent of dilapidation during this period.

-         Imagine, two healthy humans with same level of immunity; one fall sick but does not take medicine and another takes proper medicine when sick, so who has a change to live longer? – naturally the second one, isn’t it! same is the case with maintenance of structures.

-         Now imagine a third person who has severed ‘inborne’ nutrition deficiencies. Such person has more likelihood of falling sick when subjected to vagaries of weather. Medicine can merely help him to just sustain a life of couple of years – same is the case with improperly constructed structures !  

It may be noted that the maintenance of buildings is much feasible than the culverts and bridges which situated located often either in lonely jungle areas or in too densely crowed busy roads where in both opposite cases the maintenance is either not possible or neglected! (no point in saying that even the building owners also neglect the maintenance, but the internal shadows, external claddings / facades / plastering protects the building components to a great extent as compared to infra works. But now a days there is a myth within the clients / DPR consultants / design consultants / proof checkers regarding the protective acrylic based elastomeric paints / coatings that they protect bridges !!

Another important point to be noted is ‘type of construction materials’ – stone, bricks, timber, PCC, RCC, PSC, HPC, UHPC, steel, aluminium, FRPs, composites, alloys etc.

When we say bridge and infrastructure works, then the name has a wider coverage – apart from various types of bridges and culverts it also envelopes jetties, trestles, dolphin structures, dams, tunnels, metros (underground and elevated), runways, water treatment and retaining structures. Hence for the convenience these are classified as :-

1.      Transportations Infrastructures – Roads / Railways / Metros – which includes bridges, tunnels and culverts and other related.

2.      Water and waste-water conveyance infrastructure – this mainly include treatment plants and pipelines, channels / canals, storage tanks (OH, Gr and UG tanks).

3.      Hydraulic / irrigation structures – dams, barrages, pumping stations, turbines, hydro-power stations etc.

4.      Marine and off-shore – jetties, berthing and mooring dolphins, shore protection works, ports and harbors, etc.

5.      Power Infrastructure – Thermal / nuclear plants, cooling towers & chimneys, transmission towers, electric lines, grids, power stations, distribution centers etc.

Power. Off-shore, deep off-shore, oil-&-gas fall separately under industrial infrastructures. In present paper, focus is kept on the transportation infra with brief coverage on water, hydraulic and marine structures.

Components of Bridges and Infrastructure:- before entering into auditing procedure for transportation infrastructure it is important to know the important components of bridges. Infrastructure & Bridges are broadly classified into two main classes –

·      Box type structures / Culverts – span < 6m (often box barrels or slab type structures).

·      Minor bridges – span > 6m & < 60m (box / slab or girder type structures).

·      Major bridges – span > 60m (predominantly girder / truss type).

·      Pipe structures – diameter tanding from 600mm to 1800mm.

The main structural components of a bridge are - foundations (piles, pile-caps, isolated footing, wells, caisson etc.) substructure (piers, abutments, caps & pedestals, arrestors, bearings) and superstructure (girders, deck slab, diaphragms, blister blocks etc.). Apart from these three, there are approach slabs (to connect to road), crash barriers (to avoid falling of vehicle outside carriageway), wearing coat (to render comfortable ridding surface and protect the bridge slab), drainage arrangements, inspection chambers / platforms, lighting arrangements etc. The bridges spanning across a water body – stream, river, nala, canals are called ‘CD’ (cross drainage) structures.

Structural Audit procedure:- Unlike residential or commercial buildings (covered in part-I) where the audit was recommended from top to down, here in case of bridges (part-II of paper) the audit is to be performed from bottom up i.e. from foundation to piers from starting first abutment till end abutment, inspecting each pier and foundations one by one and then move on to the superstructure, bearings, approach slabs, barriers etc. it is worth noting that in case of CD structures major damage could be found in the foundation and substructure due to – scouring of soil, wash-out of the protection aprons, damages to stone quarter-cones & pitching, accumulation of debris between piers / walls, ponding of small vessels (in case of navigational channels), liquefaction (in soft soils in higher seismic zones), illegal dredging in the river closer to bridge foundations, constant exposure to flowing waters, construction of new foundation adjoining the existing foundations (leading to changes / obstructions to existing hydraulic flow pattern). Following are a few images which show above phenomenon.

  (left) pier hit by floating debris           (right) uneven settlement in transverse direction.

(left) scouring of soil                (right) damaged bridge pier due to vehicle hit.

   (left) vegetation growth inside pipe culvert due to poor maintenance.

(right) organic debris accumulated inside a pipe culvert.

  (left) scoring of soil behind abutment            (left) Soil Slope failure at abutment.

   (left) corrosion and spalling of cover concrete           (right) vegetation growth in stone masonry.

 While inspecting the foundations of the damaged structures in case of river bridges people often use barges or small pontoons. Care shall be taken that the vessel (boat / ship / barge / pontoon) used is adequately safe and stable and equipped with lifeguards. Usually, the inspection to be planned in summer season when the river will have minimum water and no turbulence so that vessel can stand calmly near individual piers. The pontoons may be tied with a rope and held by one person firmly from top of the bridge. The cherry picker type inspection boat can be seen in adjoining images.

(left & right) bridge deck inspection vehicle from top.

A few photographs of bridge piers damaged due to earthquakes (partially or fully) can be seen ahead – damage due to earthquake is another area which requires immediate attention / audits (and if needed immediate stoppage of use). The damage could be of catastrophic nature as shown in the photographs below where there could be partial or complete collapse (due to hinge formation in pier base). In some of the partial damage cases, concrete could be rectified by fiber wrapping or jacketing. 

(left) concrete compression failure           (right) concrete shear failure.

(left) concrete core n shell separation - compression failure   (right) join failure.

(Left) Typical lateral flexural failure mechanism shown        (right) image from Kobe earthquake.

Above image on the right side shows complete lateral collapse of an elevated viaduct during Kobe earthquake episode. The structural audits of such collapsed structure is to assess the reason of damage and take a call on demolition etc. under a panels of experts only (** readers may refer a post by author of this article on ‘RVS technique’ published in www.sefinida.org).

Damage mechanisms in Steel bridges due to corrosion and fatigue:- In still bridges, lack of protective coat could be serious issue and may lead to severe corrosion. Another issue is damaged due to Fatigue and Temperature. Slenderness is yet another phenomenon for steel bridges (unlike concrete structures which are sturdy and voluminous). Images below show some of the damaged cases of steel bridges. Expert auditor can figure out the exact issue during audit by visual observation and a few calculations / tests if needed.

(Left) Typical damage at joint        (right) local damage to steel beam.

(Left) Typical lateral buckling of slender steel truss.

Exposure Condition to Concrete and design Life :- The environment to which concrete is exposed to, governs the magnitude of carbonation and corrosion. The dissolved gases, humidity demands certain minimum cement contents in the concrete, minimum grade of concrete and clear cover to rebars. The exposure conditions are classified as moderate, severe, very severe and extreme. For residential structures additional exposure condition is specified in IS-456 as ‘mild’ for the covered / protected / plaster surfaces. Typical exposure classes / damages in mild – moderate – severe and extreme conditions can be as seen below. Usually, an infrastructure located more than 30km away from seashore is classified as ‘moderate’, now a days the ‘relative humidity’ is followed by many consultants, but it is deceptive in authors opinion.

 (Left) Mild exposure        (right) Moderate exposure.

(Left) damage observed in severe class         (right) Typical damage observed in extreme exposure.

As per current situation towards meeting the exposure condition the emphasis is given towards increasing the cement content and grade of concrete in designs. The design life is specified as 100yrs in the code. But a few agencies land up neglecting actual healthy construction at site (w/c ratio, clear cover to rebars, curing, lack of compaction leading to honey-combing, rust free bars at the time of construction !), dense – defect free concrete (no matter even may be M20 grade) was found to be adequately performing in past cases. The bad quality of construction should not dictate / mislead to use of higher and higher material usage (and cause national wastage of resources).

Another misleading aspect toward concrete ‘design life’ is ‘protective coating’, it can not change the exposure condition nor protect the concrete for infinite time. The coatings itself has maximum life of five years or so; even the defect free coating (bi-directionally applied non-pervious coat) is important. Hence the original defect-free concrete is most necessary. (but now a days in many cases applying coating is just a cosmetic treatment based on minimum rate per sqft. area).

(Left) damaged steel truss ridge due to overloaded vehicle  (right) cracked PSC girder due to overload.

Assessment of Damage due to Overloading / Changing axel loads :- In some of old bridges situated near the new ports / cargoes, it was found that the deck slabs experience micro cracking and even the girders were found to be overstressed due to increasing wheel load patterns over a decades / changing axel loads in modern vehicles. Considering this, IRC and MORTH standards have improved their guidelines considering the futuristic growth approach. However, some of the logistics / transport companies do not adhere to the loading standards and over-load the vehicles. PWD / MSRDC / NHAI has strict rules to monitor such activities at toll plaza areas by electronic weighing mechanisms and penalties! But once the damage is caused to a bridge it will not revert by penalties! (images on right side show a few overloaded vehicles and damage caused due to them). In olden bridges (mainly) or even in badly constructed new bridges the ‘tipping point’ or the ‘flash point’ occurs at a section with low strength (due to strength degradation over time or poor quality of construction) and exceeded force (BM / Shear / Torsion) meeting together!

From above photos and similar cases, it is clear that a few of the cases can be repaired if attended in time, however a few undergo instantaneous damage (mostly seismic cases) and may be difficult or even impossible to reinstate with repair / retrofitting or even if under regular maintenance (but here it does not mean that the owners / concessioners / maintenance agencies should escape from their responsibility of maintaining the structures – because well-maintained structures are likely to behave better than others!).

·      In case of stone arch bridges slipping of a key stone from arch rib is a common cause of failure. Growth of trees / weeds / shrubs is also common.

·      In case of reinforced concrete bridges, the corrosion, creep, overloading are the predominant causes.

·      In case of Prestressed concrete bridges, corrosion of strands / relaxation of anchorages / corrosion of anchorages are the commonly observed cases.

·      Apart from these the sinking of approach slabs due to washout of sub soils, damages to crash barriers or gaps in precast crash barriers etc. are the other observations.

During audits, the auditor must capture all these observations with photographs, measurements, GPS coordinates etc. and tally with the previous audit records while preparing a final report.

In CD structures / river bridges IRC-SP-18 has recommended audits twice a year, one before monsoon and another after monsoon; but during each monsoon the HFL, discharge etc. shall be recorded by the department. The river flow on upstream U/S as well as downstream D/S shall be carefully noted with pictures and sketches and survey data, especially at meandering locations to keep assurance about proper functioning of the bridge as wells as water flow below without any erosion. It is worth nothing here that the design life of infrastructure works specified by IRC codes is 100yrs. But often in practice most of the bridges / culverts start showing-up signs of dilapidation after 30 to 40years period and most of them give-up at early age of fifty to sixty. Proper design and construction followed by yearly cycles of audits and maintenance cycles can prolong the life to 100yrs. It is a big myth within many that, only by virtue of just proper ‘design’ (and even not construction or maintenance) the infra-works can see their century! Good design is just a good paper-work; defect free construction (with corrosion free rebars, good cement and aggregates, dense honey-comb free concrete, watertight formwork, 21days curing to concrete etc.) followed by regular maintenance cycles (removing the debris accumulated in culverts or piers; controlling the axel-loads on bridges; giving proper surface coating after thirty years once in a five year etc.), replacing damaged bearings and expansion joints - all three together, only, can give a long lasting infrastructure.

Fig. above shows wide range of Meandering in the river Allier, France over period of 62 yrs (1946 to 2008)

Damages to None structural components :- Apart from damages to main structural components. The secondary structural components may also get damaged. The damaged expansion joints and bearings shall be properly captured in the audit report along with the data of installation / repairs / replacement etc. and future recommendation. In infrastructure designers and maintenance workers, it is well accepted fact that the ‘integral structures’ (jointless / bearing less i.e. fully monolithic structures) are not only cheaper than conventional bridges but also perform well in the long run (provide creep, shrinkage and thermal forces are properly estimated and rebars are detailed accordingly); however Indian contractors / construction teams offer resistance to construct integral bridges due to complexity in reinforcement (and lack of trained man power at site).

Above pictures show the damaged bearings.

   Above pictures show the damaged expansion joints, same shall be captured during audits.

Above pictures show damaged metallic cash barriers due to vehicle collision.

Damaged stone arch bridges :- Arch is a structural form which sustains external loads by remining in ‘compression’ for lifetime. In olden days, arches were often formed from stone or brick masonry. But if any piece / part / segment from stone or a brick is displaced from the arch then it is extremely difficult to repair such arch (and mostly such arch collapses instantaneously). Cracks or growth of trees / shrubs loosen the joints in arch and endanger the stability. Images below show some of the cracked arch ribs of bridge under repair. 

Vibrations in the Bridge Decks :- Some times (often in steel foot over bridges and other light weight bridges), the main structure of the bridge is safe and stable but only problem is vibration or oscillations due to people walking on the deck. Such vibration problem could be due to dynamic frequency matching closer to the frequency of live loads moving on deck top. This problem can be easily resolved by altering the frequency of bridge by simple technique as – (1) introduce cushion / damping (add rubber pads or bearings or sand cushion below tiles, or (2) change mass (add or remove some weight if permissible) or (3) changing stiffness by adding or strengthening a few bracings etc. The exact solution varies from case to case and must be analyzed in detail by a competent structural engineer.

Tools and tackles:- In part-I of this paper all the tools and tackles required for structural audits were discussed in detail. But unlike residential or commercial buildings (where the structural audit is mainly a visual inspection / condition survey), in case of bridges many additional tools and tackles are required apart from the PPE kit (safety shoes, helmet, safety belt, glowing jacket etc.). NDT tools, access ladders, strong ropes, torque wrenches (for steel bolded bridges) etc. are necessary to be carried, in addition survey instruments – total station / GPS / theodolite, ranging rods / staff / measuring tape etc. are needed to capture the geometry correctly. In some cases (like heritage bridges) the audit process become in long chain of monitoring and repairs, strain gauges become essential in such SHM projects. 

A FEW CASE STUDIES :- on part-I pertaining audits it was mentioned that apart from actual findings sometimes the political / social pressures influence the findings of the audit. It is harsh fact but a truth. A few of the recent bridge failures are worth studying !

Type of contract :- The type of contracts (EPC / Item rate / BOT / HAM / lump-sum / labor / petty) and the type of agency appointed by the owner for design and construction also influences the ‘Quality’. Quality is never Free. One can not expect the High grade of quality always by ‘L1’ bidder. Sometimes, the bidders do cost cutting / compromises in construction to meet the time and cost schedule and exploring the loop holes in DPR document. Then how can they guarantee 100yrs design life !! some intelligent contractors pass on the blame to clients saying that ‘they did not do proper maintenance’ (of course is many of the cases lack of maintenance is a fact) or to DPR consultant or to designer itself as the design drawings were not given in time or they were not error free.

SHM / Instrumentation program :- in modern contract documents provision is kept for detail instrumentation / SHM program. What matters is the location where to install the strain gauges, frequency to collect the data and analysis of the huge quantity of data generated in the computer. There are very less technically equipped agencies who understand and meet the varying project requirements.

Audit Report :- Unlike residential buildings the audit report for infrastructure projects could be very exhaustive covering the culverts / MNBs / MJBs and ROBs, Flyovers etc. and their components. Another main difference is the employer for residential buildings could be small cooperative housing society whereas for infra projects it could be government departments. In part-I of the paper various cautions, contents of report, back up data (photos / data forms / analysis models / NDT results etc.) etc. was discussed; these details need to be properly presented and preserved. A typical format of ‘Bridge Inspection’ prescribed in Appendix-3 of IRC-SP- 052 (Bridge inspectors’ manual) is as shown here.

Safety Consultant’s scope :- In highway projects a Safety Consultant is appointed at design stage for auditing the safety of overall project (mainly road, gradients, curves, junctions, black-spots, side slopes, barriers, median slopes etc.). one should not get confused between ‘road safety audit’ and ‘structure-stability’!

Concluding Remarks :- There is so much to write about the bridge audits but the space and attention span of the readers has a limit ?? Practice makes man perfect ! Hope you will find this paper covering all key aspects. Your suggestions / queries are most welcome. The main intension of this paper is knowledge sharing and not to hurt any individuals or organization/s.

References:-

1)     Various IRC Codes stated above.

2)     Various websites working in the field of audits.

3)     Part-I of this paper.

4)     Various papers published by author on the bridge engineering – design / construction (can be found in www.reserachgate.net)

5)     www.sefindia.org

About the Author

Er. Vivek G. Abhyankar – Fellow (IEI), BMC licensed SE, has more than 21 years of professional experience in the field of structural planning and design, detailing of various enabling and permanent works in Concrete and Steel (bridges, metros, buildings). He was also a visiting faculty for Graduate and Post-Graduate students in Structural engineering at VJTI, SPCE. Guide for PG and AMIE projects. Certified structural engineer of MCGM. Life member of various national and international professional bodies and contributed to more than 40 technical papers and 3 chapters in books and guided more than 10 MTech, AMIE projects. (email – [email protected])

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