strength assessment of existing bridges for bridge

Advances in Bridge Engineering, March 24 - 25, 2006

STRENGTH ASSESSMENT OF EXISTING BRIDGES FOR BRIDGE MANAGEMENT SYSTEM R.K. Garg1 and Ram Kumar 2 1 Scientist, 2 Area Coordinator. Bridges and Structures Division, Central Road Research Institute, New Delhi (email: [email protected] ) ABSTRACT Strength assessment of bridges is an important activity in the Bridge Management process. Various aspects related to assessment of current strength as well as in future are discussed. Methodology suggested by IRC is also discussed with the help of a case study. The discussion would provide to adopt appropriate methodology for assessment of load carrying capacity of the bridge(s) for the purpose of Bridge Management System suitable to Indian conditions. INTRODUCTION Bridges are lifelines of a Nation’s infrastructure and massive investments are being made in the Highway Sector year after year. During the last 50 years, a number of Reinforced Cement Concrete (RCC) and Prestressed Cement Concrete (PCC) bridges were built across the country and it is estimated that about 25% of them are in some form of distress. Presently, maintenance of bridges is in a rudimentary state and only a few important bridges are closely examined on case-to-case basis. Considering that the number of bridge assets is increasing quite rapidly, there is a need to develop a scientifically designed Bridge Management System (BMS) in the country (Merani, 1990, OECD, 1992, Tamhankar, 1997). Maintaining bridges to a serviceable level is an important task of road transport network. Apart from assessment of load statistics, the condition assessment of the bridge is required for proper Bridge Management System (BMS). There may be complicated situations where the bridge is deteriorating. Sometimes the design data are not available or the material properties have large variations. The actual load carrying capacity of the bridge would require knowledge of diagnostics through field tests, future load predictions, material properties of components. Different methodologies have been adopted by various researchers (Akgul and Frangopol, 2005) and employed in Bridge Management System of respective countries such as USA, Norway, Germany, Japan, and UK. Deterioration model suitable to Indian conditions is being studied applicable for common types of bridges covering slab, Tbeam, PSC girder. Various aspects of strength assessment related methodologies are reviewed and a case study highlighting IRC codal recommendations are presented.

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R.K. Garg and Ram Kumar

BRIDGE PERFORMANCE PARAMETERS AS PART OF BMS Necessary information is to be assembled (OECD, 1979) based on reports of inspection, assessment, testing etc. starting from element or component level to the project level (i.e. for one bridge) and to the network level (i.e. for a group of bridges) and lastly to the route including pavement details and is described as follows and shown schematically in Fig.1. i.

Inspection and basic data,

ii.

Current condition state of each of the component called elements,

iii.

Current condition state of the bridge,

iv.

Assessment of the load carrying capacity of the bridge,

v.

The rate of deterioration of various components,

vi.

Future condition of state of components and of the bridge,

vii.

Assessment of the load carrying capacity of the bridge at a specific time,

viii.

Period when the load carrying capacity falls below a threshold value,

ix.

Maintenance requirements: assessment and repair,

x.

Modifications in cross-section or load carrying capacity due to repair,

xi.

Strategic decision leading to “optimized maintenance”.

BRIDGE

In similar manner information related to all the bridges falling within a road network can be assessed based on statistical approach to arrive at prioritized programme. Other components like pavements, earthworks etc. may also be added to this network to achieve a route management system. Condition without repair Repair

Acceptable Performance

A B

DURATION

Fig. 1: Condition Status of the Bridge with Time

302

C


Correlating Damage Data with Condition States The visual inspection data of damages to various elements is to be correlated with the condition states, which would form the basis for further assessment of strength of the bridge. The presence of cracks forms the vital visual signs of distress on bridges. The allowable crack width in concrete structures is 0.2 mm to 0.3 mm (IRC-21). A correlation of extent of cracking due to ASR has been reported by Godart et al (1989) and is given in Table 1. The size, extent and location of cracks would provide information on the possible causes. Once the cause is determined it would require detailed inspection data to ascertain the present condition and help assessing future status. The condition states for deck as suggested by Colorado Department of Transport are shown in Fig. 2. Table 1: Classification of Cracking (Godart et al, 1989) Cracking Indices: mean total opening (mm/m) 0-0.5 0.5-1 1-2 2-5 5-10 > 10

Extent of Damage Negligible Low Moderate High Very High Considerable

Table 2: Condition State Rating for Bare Concrete Deck (CDOT, 1995) Condition State 1 2 3 4 5

Description No repaired area, no spalling/ laminations exit Repaired area/ spalls/ delamination area is 2% or less of deck surface Repaired area/ spalls/ delamination area is 10% or less of deck surface Repaired area/ spalls/ delamination area is more than 10% but less than 25% of deck surface Repaired area/ spalls/ delamination area is more than 25% of deck surface

The US National Bridge Inventory describes various condition states on a scale from 0 to 9 as described in Table 3.

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Table 3: Condition State Rating (NBI/FHWA, 1995) Condition State 9 8 7 6 5

Condition

Physical Description

Excellent Very good Good Satisfactory Fair

4 3

Poor Serious

2

Critical

1

Imminent failure

0

Failed

A new bridge. No problem noted. Some minor problem. Structural elements show some minor deterioration. All primary structural elements are sound but may have minor section loss, deterioration, spalling or scour. Advanced section loss, deterioration, spalling, scour. Loss of section, etc. has affected primary structural components. Local failures are possible. Fatigue cracks in steel or shear cracks in concrete may be present. Advanced deterioration of primary structural elements. Fatigue cracks in steel or shear cracks in concrete may be present or scour may have removed structural support. Unless closely monitored it may be necessary to close the bridge until corrective action is taken. Major deterioration or loss of section in critical structural component or obvious vertical or horizontal movement affecting structural stability. Bridge is closed to traffic but corrective action may put back in light service. Out of service. Beyond corrective action.

Health Index Apart from condition state of an element like protected, exposed, attacked, damaged, failed, an indicator of single integral of the structure health can be expressed called Health Index (Bulusu & Sinha, 2005). Health Index (HI) varies from 0% (the worst condition) to 100% (the best condition). As a new bridge an element will have 100 points and may show decreasing value corresponding to the possible degradation during its life





time. HI is expressed for an individual element (e) as, He (%)= ∑ks qs ∑qs100-----(1).

s

s



Here, ‘s’ denotes the index of the condition state, ‘q’: the element quantity in the s-th condition state, ‘k’: for health index coefficient corresponding to s-th condition state and is a fractional value as defined in Eqn. 2.

k s = (n − s ) (n − 1)

--- (2).

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The value of ks is computed for s = 1,2,----n, and represents the number of condition state depending upon the type of CORE element. Further, HI of the entire bridge can also be computed by

H = ∑ H e Qe . W e ∑ Qe W e e

--- (3).

e

Here, ‘e’ is index of an element, We is a weighing factor of element-‘e’, Qe is the total quantity of elements in the bridge. We are computed as failure cost or an empirical value. Condition/ Structural Behaviour of Elements Concrete structures possess considerable reserve strength; therefore, the load capacity evaluation should be different from the design method. It may be noted that for an existing bridge, several design parameters such as dimensions, dead load, distribution and frequency of traffic etc. are better defined. Therefore, some of variability as considered during the design process has been ascertained. Based on several studies on deteriorated concrete bridges, Beal (1989) has suggested that many a times cracks are cosmetic in nature and do not reduce strength of the member significantly. However, in case of extensive loss of cover on tension steel, of the order of 75% loss in area as caused by corrosion, the member suffers considerable strength loss. ASSESSMENT OF PRESENT LOAD CARRYING CAPACITY Strength parameters like flexure, shear forces, torsion, buckling and serviceability aspects are required to be assessed. Assessment for shear is critical because of its brittle characteristics of failure. Various factors related to shear capacity are described as follows. Assessment of Shear Capacity The shear capacity may be expressed as combination of shear taken by steel and the concrete as per relevant codal provisions. Appropriate capacity reduction may be considered for taking into account any degradation of the structural members. It would be appropriate to ignore shear capacity of the concrete if severe diagonal cracking is observed (Klein and Popovic, 1985). To help proper assessment of strength, precise information of crack mapping, measurement of stirrup spacing and the location relative to cracks, spalling, corrosion, delamination etc are required. Inclined shear cracks may be of two types namely web-shear cracks or flexure-shear cracks. Web-shear cracks are formed due to principal tensile stress in an uncracked beam web. Flexure-shear cracks are formed in a beam having already flexure cracks. Further, secondary cracks may also be present which are formed due to splitting forces induced by slip between the concrete and

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reinforcement or due to dowel action of the bar transferring shear across cracks. Normally, flexure-shear cracks precede web-shear cracks. It is important to ascertain the cause(s) of cracking to appropriately take corrective measures. For beams with web reinforcement, the failure load may be greater than the inclined cracking load; however, beam without web reinforcement may fail at inclined cracking load (Bresler and MacGragor, 1967). Presence of secondary cracks in either case may indicate loss of effectiveness of stirrups showing possibility of near failure state. There are several parameters other than strength reduction factor such as low amount or cutting-off longitudinal reinforcement, degree of shear reinforcement, presence of axial tension, size effect (aspect ratio effect), T-beam effect and role of repeated loading influencing the shear strength of the bridge and are not considered in IRC:SP37. Beams having low longitudinal reinforcement (< 1%) show reduced shear strength compared to codal values. The cut-off region of longitudinal bars is to be checked for the possible low shear capacity if enough stirrups are not present. In this region, high local shear and bond stresses are developed leading to flexure-shear cracks. Axial tension might be caused by restraint shrinkage or thermal contraction and causes early development of cracks, which may influence more verticality of shear cracks. As the depth of the beam increases the shear stress at failure reduces; aggregate interlock decreases with greater depth leading to reduction in the shear capacity. Beams with increased flange width show greater capacity. Experiments have suggested that fatigue strength of a beam without web reinforcement is less than under the static case. IRC Method The guidelines are provided by IRC: SP37 to assess the present load carrying capacity of some of the common types of bridges. Need of proper inspection and condition data is stressed which may be required during the process. Use of Load and Resistance Factor (LRF) method is allowed apart from working stress method. However, LRF method is to be used with caution by way of considering complete calculations before upgrading the bridge. . The capacity reduction factor is to be taken appropriately and may vary from 0.7 (damaged state) to 1.0 (new bridge). A procedure is outlined and is reviewed as follows. Consideration of Reserve Strength/Load Factors Increase in strength in form of increased permissible stress for working stress method is allowed by IRC. For steel, 45% additional to the relevant stresses (with a limitation for older steel) and for concrete and masonry a value of 33.3% is allowed. The load factors may be obtained from Table 4.

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Table 4: Load Factor for Assessment of Strength Load Action For Strength Evaluation Dead Load (Action additive to Live load) Dead Load (Action opposing Live load) Live Load (including Impact) Serviceability Evaluation Dead Load or Live Load

Factor 1.1 1.0 1.8 1.0

Based on visual inspection, rating of status of each major component is given and a cumulative weightage factor is computed. The weightage factors for different components for a T-beam bridge are deduced as 0.40 for deck slab (including footpath slab with 0.07), 0.55 for girders (including transverse girder with 0.15) and 0.05 for expansion joints and bearings. Case Study: As an example computations are made for a T-beam bridge based on the IRC (SP37) method and are summarized in Table 5. Table 5: Condition Status and Rating of Bridge Components (IRC Method)

Main slab

Full Points (out of 100) 33

Assigned Points based on condition 25

Footpath slab Longitudinal Transverse

07 40 15

07 35 05

Expansion joint/ bearings

05

03

Total

100

75

Component

Deck Slab Girders

Physical Description Minor damage with minor cracks No damage Minor cracks Badly damage with large cracks Bearings tilted & need cleaning. Expansion joints with large gaps or collided as visible at superstructure.

Weighted Score

76 100 88 33 60

The condition state of various components of the bridge under study is described based on the Inspector’s reporting as shown in Table 5. The weighted score for various components varies from 33 to 100, reflecting varying degree of damage to components (healthy being awarded 100). The awarded score of the bridge is 75 points; however, a new bridge would have been assigned 100 points. The score is also not below some 307


threshold value say 50 or 60. Therefore, the life of the bridge can (economically) be extended. The same global score of 75 may also be considered as the ‘Capacity Reduction Factor’ (of 0.75) for computation of load carrying capacity of the bridge. ASSESSMENT OF FUTURE CAPACITY Degradation Process and Modeling Time-independent statistical properties of materials e.g. concrete, steel, masonry etc are to be considered (IRC: SP60) to simulate the realistic degradation behaviour. The location, extent and severity of defects will determine the strength of components. It is also of interest to ascertain the service life of the bridge for certain predetermined period of time. Over the period gradual reduction in strength of the bridge take place even if it is not subjected to external damaging agents. The degradation of material may be caused by several factors (Mallet, 1994) such as loss of cross-sectional area of concrete or steel, carbonation, chloride ingress leading to corrosion of reinforcement, sulphate attack causing expansive reaction with cement, attack from alkalis & acidic substances, ASR, freeze-thaw cycle, creep-shrinkage-relaxation of material, thermal cycles, fatigue and fracture cycles etc. Regression analysis (Morcous et al, 2002) may be adopted to model the degradation process due to the relevant degradation agent(s). There is paucity of field data for Indian bridges performance for the purpose of studying the degradation process. Although degradation process is non-linear in nature but the behaviour can be simulated to an acceptable degree using stationary stochastic process. The Markovian Chain approach yields close simulation between the simulated and the observed condition states of elements. Markov Chain Approach At element level a Markovian Deterioration model is used by BRIDGIT & PONTIS (USA; Hawk, 1995), OBMS (Canada), SIHA (Finnish), KUBA-MS (Swiss) and many others. The element condition data is recorded on each of the inspection which forms input to the Markovian Model. They represent an average bahaviour of the inventory. The discrete form of condition state as modeled by Markovian Model is similar to the inspection program recorded at discrete time frame. It needs only two successive condition states. The present condition state of an element is considered without its past history is one of its shortcomings. Nevertheless, the condition of an element is in relation to the whole bridge and provides acceptable simulation for the network level study. It is also possible to apply a factor at the project level to obtain deterioration probabilities derived from knowledge-based models reflecting past records. In fact, two types of information is required to be deduced for a BMS and are the distribution of bridges for various categories of condition rating and varying condition of the bridge at various time intervals. Jiang et al (1989) and others have used Markov

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under Rating Category

Percentage of Bridge

Chain modeling to obtain these predictions. Based on Indiana Bridge stock, the population of bridges under a rating for deck was simulated and is shown in Fig. 2, which was close to the observed data. From this calibrated model, the condition of the deck is predicted for a period of time as shown in Fig.3.

40 30 20 10 0 1

2

3

4

5

6

7

8

9

Rating of Deck (0-9)

Fig.2: Percentage of Concrete Bridges by Deck Condition Rating (Jiang et al)

Fig.3: Performance Curve of Bridge Deck (Jiang et al)

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There are several other methods for modeling the future performance of elements and the bridge such as using reliability approach (Akgul and Frangopol, 2005). It is important to note that the rigorous methods to model the degradation process seem suitable at the project level i.e. for studying a few bridges. However, simplified approaches are preferred at a network level i.e. for studying a group of bridges on certain transportation route as part of BMS. CONCLUDING REMARKS Based on the review carried out herein, the following points have emerged for further discussion and implementation of preparing a BMS at CRRI. There are several factors affecting the strength in flexure or shear which are need to be considered for assessing the load carrying capacity of bridge components, thus requiring review of IRC:SP37. There is need to generate field data for Indian conditions for modeling the degradation behaviour particularly in different environmental conditions such as in marine environment. Rigorous methods like Markov Chain are suitable for modeling deterioration process of the particular bridge, however, the simplified approaches are adopted at the network level in a BMS. ACKNOWLEDGEMENT The authors gratefully acknowledge the kind permission granted by the Director, CRRI to publish the paper and support of DST for partial funding of the R&D collaborative project at CRRI. REFERENCES 1. Akgul, F. and Frangopol, D.M, 2005. Lifetime Performance Analysis of Existing Reinforced Concrete Bridges. 1 Theory, J. Infrastructure Systems, ASCE, Vol. 11(2), pp.112-127. 2. Beal, D.B., 1989. “Strength of T-Beam Bridges”, ACI-SP88, pp. 143-165. 3. Bresler, B. and MacGregor, J.G., 1967. Review of Concrete Beams Failing in Shear, J. Structural Engg., ASCE, Vol. 93, pp. 343-372.

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4. Bulusu and Sinha, 2005. Comp. Methods Predicting Bridge Deterioration”, Transportation Research Record-1597, pp. 25-32, TRB, NRC, USA. 5. Godart,B., Fasseau P. and Michel M., 1989. Diagnosis and Monitoring of Concrete Bridges Damaged by AAR in Northern France, Proc. 9th Int. Conf. on AAR in Concrete, London. 6. Hawk, H., 1995. BRIDGIT Deterioration Models, Record-1490, pp. 19-22, TRB, NRC, USA.

Transportation Research

7. IRC: SP: 37-1991, Guidelines for Evaluation of Load Carrying Capacity of Bridges. The Indian Roads Congress, New Delhi. 8. IRC: SP: 60-2002, An Approach Document for Assessment of Life of Concrete Bridges. The Indian Roads Congress, New Delhi. 9. Jiang, Y., Saito, M. and Sinha, K.C., 1988. Bridge Performance Prediction Model using the Markov Chain, Transportation Research Record-1180, pp. 25-32, TRB, NRC, USA. 10. Klein, G.J. and Popovic, P.L., 1985. Shear Strength Evaluation of Existing Bridges, ACI-SP88, pp. 199-214. 11. Mallet, G. P., 1994. Repair of Concrete Bridges: State of the art review, TRL, ThomTelford. 12. Merani, N.V., 1990. Bridge Management System: Panel Disc., Journal IRC, pp. 387-402. 13. Morcous,G., Rivard, H. and Hanna, A.M., 2002. Modeling Bridge Deterioration using Case Based Reasoning, J. Infrastructure Systems, ASCE, Vol. 8(3), pp.8695. 14. OECD, 1979. Evaluation of Load Carrying Capacity of Bridges, Road Research Group Report, Organisation of Economic Cooperation and Development, Paris. 15. OECD, 1992. Bridge Management, Scientific Expert Group Report, OECD, Paris.

16. Tamhankar, M.G., 1997. Assessment of Remaining Life of Bridge- An approach Document, Journal IRC, Vol. 58, pp. 387-402.

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