Er. Vivek Abhyankar describes the steps and aspects of load testing of various types of superstructures, along with some cautionary measures and points to ponder upon by the designers, site engineers, and testing agencies. The information provided will be a handy reference for civil, mechanical, electrical, and instrumentation engineers working in the infrastructure construction industry.

Load Testing of Various Types of Bridge Superstructures

Full scale load test is required to be performed on a selective specified number/s of actual structure/s - Bridges / Infrastructure works, as per the requirements of MORTH, IRC guidelines, NHAI, METRO, Railway tender specifications to ensure structural adequacy (mainly) of newly constructed bridge, as well as for rating (to ascertain the integrity / ability to function / adequacy / capability) of the old or newly repaired bridges subjected to modern highway loading.

In India, Load tests are performed on road bridges as per the guidelines of IRC:51 standard. Load tests are performed on railway and metro bridges as per the IRS / RDSO as well as IRC guidelines. The application of loads could be static or dynamic depending on the case to case or client requirements (generally static in road bridges and dynamic in railway bridges is followed). The critical / governing load case as per design calculations is simulated at site in various ways; like - using loaded vehicles and trucks; directly using heavy concrete blocks and sand / gunny bags on the deck; or using steel kentledge platform loaded with blocks or bags (to exactly simulate the footprints of wheels of design vehicle position); marching special vehicles (like military tanks or loaded wagons etc.); or use jumbo size water bags etc. to make the deck to deflect and compare the actual deflection with predicted calculations.

Load Testing of Various Types of Bridge SuperstructuresPhysical load testing on - (Left) Viaduct over river Tajo, Cáceres, Spain (2019); (Right) 160m long Steel Bow string Arch bridge at Kalikuto, Central Java.
Figure 1: Typical Bridge load test on a concrete and steel arch bridges shown above

The actual deformations are measured using survey instruments like theodolite, total station or strain / dial gauges etc. Then the bridge deck / superstructure is unloaded in a reverse sequence; the elastic recovery of deformation is monitored. Based on the percentage of recovery further decision is taken. Certain corrections need to be applied on the measured readings as explained below.

A typical road construction (highway or expressway) project comprises of following phases:
  • Project need identification & feasibility studies or recognizance
  • DPR (detail project report) phase
  • Bidding (& negotiations if applicable in some cases) followed by Award of work
  • Actual detail design & construction phase
  • Commissioning phase
  • Operations & maintenance phase
  • Repair, destruction, reconstruction phase.
These phases depend on the Time’ (age) and ‘Adequacy’ (ability) of various components in the road project; ‘Technology’ (developments at that time) and the ‘Budget’ with the owner. Broadly there are three main components in a road project:
  • Highway
  • Structures
  • Facilities
Structures include culverts, bridges, ROBs, flyovers, underpasses & overpasses etc. on which the vehicles can cross a gap (river, valley, road, rail, water, gas pipelines, separated grades etc) and are therefore significant from the safety point of view. From the history of infrastructure construction sector, it can be observed that majority of the construction accidents (i.e. collapses) have occurred on the ‘structures’ portion (bridges) than the ‘highway’ portion (except retaining walls, landslides) or ‘facilities’ (toll plaza, roadside malls etc). Note here that the accidents due to skidding of vehicles due to rash driving are not intended (because it is drivers’ mistake) but the head-on collisions at blind spots may be considered as flaws in highway design. Also, failure of structure takes a death toll on several people at one go, and takes a long time to reinstate the collapsed bridge or structure than the road portion to restart normal functioning of the road.

The recent bridge collapse at Nagpur Kalamna area (20th October 2021) teaches us that the superstructure can collapse even after several days / months after launching (if any minor error is left in design or construction) – this is like a ‘slow passion failure’. Hence it is extremely important to ascertain the capability of structures to withstand the structural actions (mainly highway or railway loadings) for a long term, by means of ‘Load testing’ on a few selective numbers of structures (remember all spans can never be tested!).

Two key factors that influence the capability of structures
The ‘capability of structure’ or the ‘ability to meet the intended structural performance’ (i.e. bending, shear, torsion, compressibility / tension in various components, elasticity & deformability) permanent and recoverable (both) when subjected to design loads / load combinations; extreme natural or artificial events (like scouring, differential settlements, strong base motions, earthquake, blasts, fire etc.) all depend on two main factors that affect the performance are:
  • Design & drawing accuracy
  • Workmanship (execution)
In the Infrastructure Industry, both these factors have their independence scrutiny process from start till end; however once constructed, both the factors (Design & Workmanship) get blended into each other in inseparable ways; it is extremely difficult (or practically impossible) to segregate them in the shortest time and verify conformity of individual factors and their subfactors (listed in table 1) and combined both factors ‘together’**; and commission the structure / release it for public use.

                   
Table.1- Showing Two factors and their sub-factors
Factors Detail Design Construction Workmanship
Subfactors a.   Design Codes & version followed a.   Tender specifications & type of contract.
b.   Intended Design Life b.   Assumptions in Bidding / Risks / errors / omissions / strategies.
c.   Conceptualization & assumptions / parameters c.   Rates & material Specifications considered.
d.   Design Forces & combinations d.   Previous experience of agency in similar construction.
e.   Member sizes
f.   Analysis model & Software / IT loopholes, glitches
g.   Analysis model & Software / IT loopholes, glitches
e.   Degree of construction Supervision / PMC – mix design, compaction of concrete, curing, testing frequency, construction tolerances
h.   Permissible stresses & Strains / FOS f.   Resources – (4Ms) man, material, machinery, money (funds) & time.
g.   Foolproof / robustness of Method statements.
i.   Permissible over stresses
j.   Detailing / drawings – errors left in (if any)!
k.   Internal drawing checking and multi-layered scrutiny process.
l.   Third party Proof checking of design drawings.
m.   Authority Engineer’s / GC / third party (IIT etc.) review.
n.   GFC issued.
h.   Actual Material, Strength / test records (and if any alternative materials used in case of non-availability of required / specified materials).Material brands / suppliers – possible low grade / adulteration for spontaneous lucrative monitory temptations.
i.   Physical dimensions / details (rebars, bolts etc.) – any honey combing in concrete, any reduced size plates used, erroneous rebar placement, wrong clear cover, negative rolling margin.
j.   Misinterpretation of codes / clauses / drawings
k.   Technology and skills / use of new materials / bought out materials etc.
l.   Thus overall confidence level to turn the design into reality.
o.   Storage and retrieval mechanism for design / drawings, future use.
But unless both the factors are ascertained and certified by the EPC contractor / concessionaire / PMC / Client or owner / any third party appointed by client, it is difficult to put the structure for final use with full confidence or certify the structure. Thus, a need arises for one single safe technique which largely imbibes both these factors together and majority of their subfactors (at least if not all the sub factors) and boost up the confidence to start use, at a shortest possible time. This need is fulfilled by the technique called as a ‘Bridge Load Test’ (BLT), which does not go back to trace the origin of the design or workmanship (any pitfalls cropped up, if any) but simply targets on the end result / aim of both these i.e. ‘Safe Final Structural Performance’. The bridges are designed and constructed for an expected life of 100years; this depends on accuracy of design, specifications and good construction practices and regular maintenance (and retrofitting if needed in certain cases).

Load Testing of Various Types of Bridge Superstructures(left) Stone Arch Bridge; (right) PSC box structure with severe cracks.
Figure 2: Bridge Load Test on old Structures.

** In many of the old bridges (say 30yrs old) the design calculations or detail reinforcement drawings, as well as workmanship records (material brands used, their quality and cube strength results, as-built drawings, construction method statements MS etc.) are not available with the most of clients / owners / consultants or even contractors (or not shared). In such case regenerating them freshly from scratch is neither possible in short time nor affordable from budgetary point of view to client (unless that bridge has historic).

If any selected structure meets the end-conditions (performance specifications) laid by the ‘Load Test’ then it is said to be complaint / or broadly meeting the end requirement (safety and structural adequacy) and hence the two key factors (i.e. Design adequacy and Workmanship) too; and then they can be put to service. It is worth noting here that BLT is only for ensuring the safety and structural adequacy of such structures and not for other secondary requirements (like - efficiency of drainage / economy / ecology / aesthetics / adequacy to accommodate traffic / acoustics / lighting / vibrations etc. which has their own importance from serviceability point of view and to be assessed separately i.e. not with BLT!)

If any structure (old or new) fails to demonstrate / fulfill the prescribed conditions (performance specifications / criteria) of the load test, then it is said to be ‘deficient’; in such case the decision has to be taken for – (i) whether to perform test again or (ii) retrofit the structure to upgrade its performance or (iii) to demolish the structure and construct a new at its place. This decision depends on the degree of deficiency observed during bridge load test (termed as BLT or LT here onward) and underperforming structures in LT / BLT may be pursued with rigorous SHM (structural health monitoring) and repairs / rehabilitation / retrofitting cycles based on following factors:
  1. degree of damage / inability / under performance / cracks / vibrations / corrosion etc. (as applicable)
  2. public-emotional, cultural or historic value of that structure
  3. off course budget. It is interesting to note that in UAE irrespective of structural conditions they demolish and reconstruct the infrastructure (bridges) after 25years period. (but India is a developing country; also such early age demolition is like under estimating the legacy of concrete to stand for hundreds of years least (if not thousand like roman constructions) – and one more thing to remember towards national economy of any developing country is ‘money which goes wasted in bureaucracy / late or untimely decisions / corruption’ – better to keep silence and concentrate of technical aspects of the subject!
Fig.1 to 4 show BLT on various types of old & new bridges (Fig.1 shows a typical load test on a concrete & steel Arch bridge. Fig. 2 shows BLT on old bridges. Fig.3 shows Load testing at Erzsebet Bridge 1964 (Hungary) & Load Test on a Curved Balance cantilever span. Fig.4 shows BLT on Durgam Chevuru Cable stayed bridge, Hyderabad).

Load Testing of Various Types of Bridge SuperstructuresFigure 3: (Left) BLT at Erzsebet Bridge 1964 (Hungary) (Right) BLT on a Curved Balance cantilever span

Load Testing of Various Types of Bridge Superstructures(Left) Load testing of deck using loaded trucks; (right) Bridge after inauguration
Figure 4: Durgam Chevuru Cable stayed bridge, Hyderabad

Identification of Structures & Spans to be tested
Usually, the structures (chainage) and particular span to be tested are decided by the client / owner / PMC based on previous incidences / records (if any – like lesser concrete cube strength or pile failure or PT strand slip while post-tensioning or hammering effect caused by any equipment falling / hitting the concrete superstructure leading to micro hairline cracks etc. etc.); contractor has to perform BLT as per contract conditions on those spans. A good contract document provides clear guidelines to identify number of spans to be tested at beginning of the project itself (so that proper estimation / costing can be done in the Bid stage itself). Some of the clients insist for independent testing and certification by any NABL accredited testing agency / labs & external institutes (like IIT / NIT). While selecting the span/s, the uniqueness of structural geometry (large span / skew / curvature etc.) also places a vital role in selection of structure as explained ahead in this paper. As per guidelines of IRC-SP-51:
  • For new multi-span bridges ‘one’ out of 15 number of spans could be chosen for load testing.
  • If number of spans exceed 15 then max ‘Two’ spans to be chosen.
MORTH circular (NH-34066/8/2016-S&R(B) dated 6th May’2016 on Safety audits of bridges during design and construction phase) recommends a third-party proof checking & test (in addition to regular scrutiny matrix i.e. PC and AE) from IIT/NIT, for bridges having span larger than 15m. Usually smaller individual spans (i.e. less than 15m as per MORTH Circular) or box type culverts, series of boxes, slabs etc. are not considered for LT or third party proof check. LT/ BLT on any additional spans other that contract provisions may be charged separately by the contractor to client – here sometimes a dispute arise between contractor and client (as client representatives want the contractor to do anything and everything for them during contract period without bothering / spelling out anything about the reimbursement). The sorry state is to avoid fights or arguments many local illiterate contractors do as what ‘Sarkari Babu’ (client officer) says!

Types of BLT
Going into dept, the tests can be classified further based on the intended purpose. Following are the key types of tests performed professionally:
  • Behaviour Tests - Carried out to verify the results of any method of analysis or design with equal or lower design load.
  • Proof load tests – These tests are done on new structures which have design or construction problem or to rate an existing structure/s.
  • Stress history test - Carried out to find distribution of stresses in fatigue prone area of bridge. Data of regular traffic is used.
  • Ultimate load test - Performed to predict structural performance. Used when theoretical knowledge is not available. Provides information about sequence and mode of failure i.e. structure / part of it / material is tested till collapse / complete failure (usually performed in laboratories & or for academic researches).
  • Diagnostic test - To monitor behavior of each component of bridge.
(NOTE: The type, magnitude application and duration of different tests is decided based on objective and valuation procedure.)

Different methods for Application of Load
The highway loading configuration (IRC Class- A, B, 70R, foot path live load) specified in IRC:6 are only the ‘representative’ one to simulate the worse effect caused by the conventional commercial vehicles / loads, observed (& available) in real life conditions (and are often more). These loadings were specified first in 1966 and kept ever updating since then (still the difference is observed between the actual commercial vehicles and the IRC (as sometimes IRC vehicles are over conservative and, in some cases, it is not!). But it is practically impossible to simulate the effect of thousands of types of commercial vehicles (two wheelers, three wheelers, four-wheeler cars, multi-wheel trucks, dumpers etc.) and their permutations.

Anyway, during load test an attempt is made to simulate the loading to generate effect (design BM / SF / TM) closest to the critical design load combination (inclusive of impact factor of load). As stated previously, the loading is applied using – (i) Trucks (ii) Concrete block or Sand Bags (iii) Steel Kentledge (iv) Loaded wagons, or (v) loaded special vehicles (like military tanks) (vi) inflated jumbo rubber bags filled with water etc.

Load Testing of Various Types of Bridge Superstructures(up) Application of Test load using (Left) Concrete Blocks (right) Loaded steel Kentledge

Load Testing of Various Types of Bridge Superstructures(up) Static Load test on High-Speed rail bridge using loaded Trucks
(Ref. ASCE Journal Journal of Bridge Engineering - Volume 20 Issue 2 - February 2015)

Load Testing of Various Types of Bridge Superstructures(left) Conventional Static weigh bridge (right) Modern Portable digital load cells
Figure 5: Application of Test load using (Left) Concrete Blocks (right) Loaded steel Kentledge

Load Testing of Various Types of Bridge SuperstructuresFigure 6 (b): Bridge deck – load test using gunny bags
(Figure 6 Ref:- Geocon India & Associates) Deflection Test at Kharagpure (South Eastern Railway), Carried out Deflection Test according to Clause 10.5 of IRS Dridge Code for PSC Slab of Spain 32.75 Mts and 13.4Mts of Test Load of 644 MT and 285 MT respectively for Proposed 3Rd Railway Line between Panskura and Kharagpur in South Eastern Railway for Rail Vikas Nigam Ltd.

Load Testing of Various Types of Bridge SuperstructuresFigure 7: (Left) Load test using gunny bags on PSC U-Trough
(Right) estimated deflected shape using manual calculation & 3D-FEA software program **
(Ref. – paper by V Raju1 , Devdas Menon)

Load Testing of Various Types of Bridge SuperstructuresFigure 8: (Left) Bridge load test with special military vehicles (Right) Danube Multi modal transport Bridge, Load testing the bridge using several types of vehicles - February 15, 2015 (Photo by Tamás Simonyi)

Load Testing of Various Types of Bridge Superstructures

Load Testing of Various Types of Bridge SuperstructuresFigure 10: February 1932 - Rail Load Testing Sydney Harbour Bridge (Photograph by Mary Evans Picture Library) (right) Sydney Harbour Bridge after commissioning.

When the vehicles to be used for the test are finalized, then reaction on each wheel / tyre shall be recorded in advance with full pay load. Commercial vehicles are recommended by IRC-SP-51 and IRC-37. The reaction / tyre loads are measured using load cells or weigh bridges. Nowadays due to remarkable growth in IT/Digital & electronics technology, tiny portable load cells exclusively for monitoring of tyre loads are available in market.

Application of Load Increments in BLT
The load is applied in small increments of 10% or 25% reaching till 100% of the intended design load (causing critical forces / stresses / deformations); each increment is sustained for about 4 to 5 hours or till the dial-gauge reading gets stabilized; when final load i.e. 100% design load is reached it is maintained for 24hrs. Deformations are measured at key points (explained ahead in this paper) at every 2 to 3 hr intervals (thus 8 to 12 readings are obtained in 24 hours duration with final loading). Now the unloading is started, in reverse sequence; each unloading sequence is maintained for 2 to 3 hrs duration and at least two readings are taken during each unloading step also. Table 2 shows this this in summarized format.

Table 2
Load Test Duration Number of readings Loading phase (32hrs to 36hrs)
No load 2 to 3hrs 2 readings
25% 2 to 3hrs 2 readings
50% 2 to 3hrs 2 readings
75% 2 to 3hrs 2 readings
100% 24 hrs 8 to 12 readings
75% 2 to 3hrs 2 readings Unloading phase (8 to 12 hrs)
50% 2 to 3hrs 2 readings
25% 2 to 3hrs 2 readings
Unloaded 2 to 3hrs 2 readings
Key points & Measurement of Deformation
Load Testing of Various Types of Bridge Superstructures
The Positions or the key-points (in the deck plan), at which the deformations are measured, shall be decided in advance in consultation with the structural engineer (bridge designer). These points depend on following factors:
  1. Type of structural configuration in longitudinal direction (viz. Portal or box type structure / Arch / Truss / Beam, Slab, Box / Cable stayed / Simply supported or Integral)
  2. Type of deck cross section (slab / girders / box / multiple box / cross-girders / trusses on both road edges).
  3. Type of Plan - Skew or straight.
  4. Type of central axis - Curvature in plan or straight.
  5. Different permutation / combinations of above factors.
  6. Continuity or simply supported / integral etc.
Diagram on right hand side illustrates straight / skew / curved bridge deck configuration (in plan). As sated earlier, a structure to be load tested need to be first identified based on design & construction complexities, construction records (pile load test, cube tests etc.), any visible cracks or vibrations (especially in case old structures) and the statutory requirements / contractual specifications (bridge crossing over other services / utilities need to be tested viz. ROB, gas pipe-line etc).

After finalization of said structure, specific span needs to be identified on which load tests is planned to be performed (note:- all spans cannot be load tested as it is practically impossible to do so; hence a logical and statistical approach need to be followed). Usually long spans, special spans with above stated 6 complexities are selected. IRC-SP-51 states certain guidelines for selection as stated previously in this paper; but note that they are the minimum numbers! In case the spans are standardized all through the project they testing one or two out of several can give confidence to client (and test of remaining spans / numbers, is just a procedure to fulfill contractual requirements) A judicial decision has to be taken on case to case / project basis, as explained below with an arbitrary example:-

Let’s imagine, there is a long viaduct connecting two far points in a city having 6km length with total 157 spans on varying length and type of superstructure. Suggested number of tests (one per variable) are as shown in table 3.

Table 3

No. of Load Tests

Nos

Typical Span Length (m)

Total Length (m)

Configuration of Super Structure

straight

skew

Curved

Total

104

40

4160

PSC Box Girder

1

1

1

3

50

25

1250

Pre-tensioned I Girder

2 **

 -

2

2

250

500

Concrete Extradosed

1

 -

1

1

90

90

Steel Truss

1

 -

1

157

Nos

6000

m

7

Nos.

In BLT what we measure are basically the ‘Flexural Deformations’ at key points. The torsional deformations (rotations) are interpreted indirectly from the differential flexural deformations on the two opposite edges of the deck (the bridge experiences torsion due to plan curvature or eccentric loads in C/S or large skew. In BLT shear deformations / axial effects (shortening of elongations) / higher order deformations etc. can’t be measured and neither it is expected. Fig. 11 below shows the common reasons of torsion in the deck / superstructure proper.

Load Testing of Various Types of Bridge Superstructures(left) Bridge with large curvature in plan (right) bridge deck with large skew

Load Testing of Various Types of Bridge SuperstructuresBridge deck with eccentric loading (left) Box girder (right) beam Grillage

Load Testing of Various Types of Bridge SuperstructuresExaggerated view of twisting / torsional rotation in a wide deck steel box girder (shown in red dotted line)
Figure 11: Different cases those generate torsion in the bridge cross-section.

Usually, the measurement points are the points where governing / critical bending moments (and / or deformations) occur in the superstructure due to governing load cases (say LL + Class-70R etc). Generally, the deformations are measured at L/3, L/2 (i.e. mid span) and 2L/3 along the length and ‘extreme edge + central girder’ in cross section direction; for skew deck on the extreme corners (to monitor uplift) and for curved decks on the inner and outer curved edge, additionally. Measurements are taken using Dial gauges / strain gauges or sensitive surveying instruments like ‘Total station’ or ‘theodolite’ etc. or even in combination to confirm independently. A framework of steel staging cribs / towers is erected below the deck for supervision team to access the dial gauges frequently and easily. Safe access ladder (covered with green safety-net / cloth) with handrails from ground till top to access the staging, with safety belts, PPE kit, halogen lights for night vision etc. shall be arranged at site for hassle free operations. Figure ahead shows such staging and dial gauges by various agencies. Nowadays many good PMCs deploy CC tv cameras at test location / site so that no one can tamper the readings. Even the dial gauge assembly is sealed at end of the day work every day. Following images show the methods of measurement of deformation by various testing agencies locations prescribed in IRC code.

Load Testing of Various Types of Bridge Superstructures(1) Radhey Testing & Consultant Pvt. Ltd. (2) Geoties Infratech Pvt Ltd, & (3, 4) Span load test by M/s Geodynamics

Load Testing of Various Types of Bridge SuperstructuresDial gauge used for measurement of deflection during load testing.

Load Testing of Various Types of Bridge SuperstructuresMeasurement of deformation using Survey instruments

Load Testing of Various Types of Bridge SuperstructuresFigure 12: Methods of measurement of deformation.

Temperature and other corrections
As the load test on one structure, consumes almost 36 to 48hrs duration i.e. almost 2days; the temperature variations (over day and night) affect the dial gauge readings due to thermal expansion and contraction of concrete and supporting components / materials (& even dial gauge itself). It is difficult to apply the temperature correction on the intermediate incremental loading / decremental unloading stages; the correction is usually applied only on fully loaded stage which is retained for 24hrs based on the maximum and minimum reading obtained (at night when temperature falls, concrete and other materials contract and in maximum day temperature they expand). The figure above is from old version of IRC-SP-51 which briefly explains how to apply the temperature correction (for details refer code). Applying this correction is very important.

Load Testing of Various Types of Bridge Superstructures

The changes in humidity, surrounding sounds, lights usually do not affect the readings and hence are ignored. It is obvious that during BLT the particular structure has to be made inoperative (i.e. free from regular traffic). But the minutest vibrations due to traffic on adjoining carriageway may influence the readings of sensitive instruments in certain cases (viz. small median / touching bridges / pile driving on adjacent road etc.); in such case the traffic must be suitably diverted to other road for two days (or at least 24hrs – during fully loaded stage).

Another correction required is in case of elastomeric / neoprene / rubber bearings for minute axial compression of bearings. In case of POT-PTFE or spherical bearings the axial compression is negligible. Often this correction is forgotten by professionals in practice. Fig.13 shows these various aspects / causes those lead to some variation in results as compared to theoretical estimates.

At design stages, the designer assumes certain material properties and member sizes (stiffnesses) like – Concrete and reinforcement strengths (Fck, Fy), modulus of elasticity (Ec, Es), gross concrete area / reinforcement steel areas (rolling margins) / HTS strand areas, friction coefficient between strands & post-tensioning ducts etc. but during construction stage as per permissible tolerances specified in design codes and actual material properties and dimensions may slightly vary (on higher or lower side) also in case of old structures due to dilapidation (carbonation / corrosion / honey-combing / cracks / alterations etc.) over decades of exposure, the effective material areas to withstand stresses may have got reduced. NTD must be performed (rebound hammer / core test / rebar locator to trace rusting) to estimate the effective material strengths and stiffnesses in advance! Otherwise, the test results are found to be grossly different that the theoretical estimates.

Load Testing of Various Types of Bridge Superstructures(Left) Cracked concrete of Pfeiffer Canyon Bridge beyond repair, (right) crack in articulation of Chicago steel bridge

Load Testing of Various Types of Bridge Superstructures(left) Rusted reinforcement in solid Slab type bridge deck (right) Corrosion of railway steel arch members.

Load Testing of Various Types of Bridge Superstructures(left) honey combed concrete (right) largely corroded reinforcement in cantilever pier.

Load Testing of Various Types of Bridge SuperstructuresHighly corroded grouted external Prestress Cable / strands.
Figure 13: Reasons of variation in readings than theoretical estimates.

Types of Bridge Super Structures / Cross Sections
The governing load case/s and their positions (key points to be tested) depends on the deck configurations (arch bridge / cable supported / beam type or truss type) following figure 14 shows important types of structural configurations commonly used by designers.

Load Testing of Various Types of Bridge SuperstructuresFigure 14: Different type of bridge superstructures

Apart from the type of structural configuration along length, the type of cross-sectional configuration is also important (solid slab, voided slab, I-girders / beams, box, multiple boxes, strutted boxes etc. are popularly used types as shown below). Usually in case of slab type bridges the long edges are considered for measurement of deflection; for girder type bridges naturally the middle and extreme girders are suitable; for truss bridges the truss joints (for trusses on two extreme edges) are selected; for box girder cross sections the readings are taken usually below the webs of bix. In Fig.12 the points of measurement of rotation in a balance cantilever type bridge are indicated (ref. IRC:51)

Load Testing of Various Types of Bridge SuperstructuresFigure 14: Different type of bridge Cross Sections

Bridge Rating
As defined by IRC-SP-37, “Rating of a bridge is the safe permissible load carrying capacity of the bridges in terms of standard IRC loadings”. Rating and posting of bridges are desirable for all old and new ones. In the following situations it becomes necessary that:
  1. The design live load is less than that of the heaviest statutory commercial vehicle plying or likely to ply on the bridge.
  2. The design live load is not known.
  3. Where records and drawings are not available.
  4. The bridge, during inspections (routine or special), is found to indicate distress of serious nature leading to doubts about its structural and/or functioning adequacy.
Key phases in the rating:
  1. Inspection and maintenance
  2. Assessment of condition
    1. Preliminary Assessment
    2. Detailed Assessment
For further details refer the IRC-SP-37 code.

BLT performed at Nagpur Mihan Interchange
A BLT was performed on a twin cell – segmental box girder type superstructure at Mihan, Nagpur interchange span as shown below. Loaded trucks were used to obtain the desired forces (BM, SF, TM). Various parameters / options / experiences which were gained are shared in this paper. There were several other BLTs performed but not shared here considering the limitation of pages.

Load Testing of Various Types of Bridge Superstructures(left) key plan (right) plan of selected span

Load Testing of Various Types of Bridge SuperstructuresCross section of bridge (MCW & Ramps)

Load Testing of Various Types of Bridge Superstructures(left) Staff recording readings (right) loaded trucks.

Concluding remarks
Bridge load tests is an important and essential tool / technique for new as wells as old structures to gain confidence in shortest time and fair accuracy before putting them in service. BLT has been performed in industry since past several years to ascertain the structural adequacy and to rate them. There are several minute & important aspects to be investigated; the end results as covered in the current paper. If the structure is found to be failed in the BLT then before rejecting it from service / declaring for demolition route cause analysis is recommended; but if the structure is found to be satisfactory in BLT then it can be accepted provided the instrumentation and test procedure is rigorous and accurate!

Practice makes man perfect; budding professionals must perform at least ten to fifteen BLTs on variety of type of structures to gain exposure to various aspects and ultimately confidence in this technique. Present paper has only an attempt to give glimpses of various key aspects; still, it will inspire and help budding engineers to venture / explore further in this area. The literature listed ahead will be extremely helpful in this mission!

Reference
  1. IRC:51 – 2015 - Guidelines for Load testing of Bridges, by Indian Roads Congress (IRC).
  2. IRC.SP:37 – 2010 – Guidelines for evaluation of Load Carrying Capacity of Bridges (1st Rev)
  3. Ministry’s TECHNICAL CIRCULARS AND DIRECTIVES on National Highways and centrally sponsored ROAD and BRIDGE PROJECTS – Volume II (Published by IRC on behalf of Govt. of India – Section 3200/5 ‘E’ load testing.
  4. Public works department (PWD) announcements.
  5. ‘Load Testing of Bridges – Vol.1 & 2’ – book Edited By Eva O.L. Lantsoght (CRC Press).
  6. Analysis of Behaviour of U-Girder Bridge Decks – paper by V Raju1 , Devdas Menon, Int. Conf. on Advances in Civil Engineering 2010.
  7. Analysis of load test on composite I-girder bridge - paper by F. Huseynov, J. M. W. Brownjohn, E. J. O’Brien, D. Hester - J Civil Struct Health Monit (2017) 7:163–173.
  8. Load testing of Highway Bridge – paper by Petra Bujňáková, Jozef Jošt, and Matúš Farbák - University of Žilina (Research gate).
  9. Strength evaluation of prestressed concrete bridges by load testing – paper by Eli S. Hernandez & John Joseph Myers.
  10. Evaluation and Load Testing of 100yrs old Steel Transit Structure – paper by S G Pinjarkar – Transportation Research Records 1290.
  11. Analysis of Static Load Test of a Masonry Arch Bridge – paper by Jing-xian SHI Tian-tian FANG Sheng LUO (Oxbridge College).
  12. Bridge Load Testing - LL distribution, Rating, Service & Dynamic Response – paper by Dong, Bas – (published in Frontiers).
  13. ENCE717 – Bridge Engineering Long-Span Bridges – PPT Chung C. Fu, (http: www.best.umd.edu), University of Maryland.
About the Author:
Er. Vivek G. Abhyankar is Founder of SGAWings Civil Engg Consultants and Advisor (OPC)
Er. Vivek G. Abhyankar is Founder of SGAWings Civil Engg Consultants and Advisor (OPC); Fellow of Institute of Engg (India), Fellow of IaSTRUCE, Licensed Structural Engineer (MCGM) and Life member of various professional Institutes (IRC, ISSE, IIBE, ACI, ICI, ACCE, ISRMTT, INSDAG, ASCE, NICEE, SEFI). A Gold medalist from University of Mumbai in PG-Structures degree. Has over 22 years of experience in planning and design of various civil engineering structures. Was a visiting faculty for Structural Engineering at VJTI, SPCE. Has written more than 30 technical papers on engineering and contributed to chapters in top rated books, E-Learning, Knowledge Management, Engineers’ Day, standardization of construction inventory etc. (This email address is being protected from spambots. You need JavaScript enabled to view it.).
Girders with corrugated steel webs represent a new innovative system which has emerged in the past decade. These girders have thinner webs, higher fatigue resistance, and are more economical, easier to fabricate, more aesthetic, and provide faster construction

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The iconic National Namaste Signature Bridge in Delhi over the river Yamuna appears languid in perpetual motion in the tradition of the Calatrava’s Bridges in motion, symbolizing nationalism in its visual form. V N Heggade, FNAE, Structural Consultant, Mumbai

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The former Prime Minister of India Shri H D Devagowda laid the foundation stone of the Bogibeel Bridge on January 22, 1997 at Kulajan (Assam). After 5 years of laying of the foundation, on April 22, 2002 the Prime Minister of India Shri Atal Behari Vajpayee

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As part of Northeast Policy, the Ministry of Railways took up the project of providing rail connectivity to Imphal in Manipur state. As per the detailed survey, the rail alignment takes off from existing Jiribam

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Brahmaputra is one of the longest river in Asia with a total length of 2880 km & out of which 920km (approx.) lie in India. The New Brahmaputra Bridge @ Tezpur is one of the most challenging one, in the attempt to bridge

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The road and railway crossing across the Brahmaputra on National Highway-52B is located at Bogibeel, about 17km downstream of Dibrugarh in Assam. The location is an area of high intensity rainfall (the annual rainfall ranges

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The world has progressed beyond imaginations. Similar to the field of IT, Civil Engineering has also taken several steps forward to bring out innovative technologies to meet challenges of this world’s aspirations. Going forward from

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The bridge lies in the state of Assam, but this location is usually referred to as upper Assam as it is far away from Guwahati and bordering China. The nearest airport is Dibrugarh. Then there is a drive of 65km into the

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The Infrastructure Industry is continuously evolving; new roads, elevated corridors, expressways, railways, dedicated corridors, metros, mono-rails are being constructed all the time. They also require constant

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The 6.5 feet wide walkway in the Hongyagu Scenic Area in Hebei province hangs 755 feet above the ground, and is the world's longest bridge, which opened in 2017-end. Designed by Tel Aviv architect Haim Dotan

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Prestressed Concrete (PSC) structures especially bridges are being largely used all over the world due to their multifarious advantages in terms of structural behaviour, economy as well as aesthetical

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Four Span Pipe Bridge
Distressed structures require necessary remedial measures in order to restore their original structural properties like strength and stiffness. Validating the effectiveness of the proposed qualitative remedial

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Foot Bridges
Footbridges have a longer history than road or railway bridges. Ancient footbridges comprised of natural materials such as roots of trees, ropes, stones, masonry, and wood. Modern footbridges are built

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Thane Creek Bridge
Load testing is done to determine the safe load carrying capacity of structures, determining if specific legal or overweight vehicles can safely cross the structure or needs to be restricted and the level of post

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Bridge in Central Kerala
Cement concrete is a versatile building material and is the second most consumed material in the world, after water. It is made using porland cement, coarse, and fine aggregates, water and one or more

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Concrete Road Bridges
Planning, design and construction of bridges and flyovers is very challenging at places with space constraints. There has been rapid development with the availability of high strength steel and concrete

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Integral Abutment Bridges
The conventional bridges use expansion joints and bearings to accommodate the thermal movements of the bridge. However, they don't completely eliminate the distress caused because

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Bridge Restoration
The use and advantages of Post Tensioning are very well known in the Civil Engg Industry as it has been in use for the last 40 to 50 years. Post tensioning is one of the best methods to induce stresses in the concrete

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Signature Bridge
Wazirabad Bridge or Signature Bridge Project is an initiative of Delhi Government, planned during the Commonwealth Games. It was conceptualized to be a landmark tourist icon that would also speed

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