CONDITION ASSESSMENT OF STEEL BRIDGES IN
SRI LANKA
K. A. D. K. Chathuranga,
Department of Civil Engineering, University of Morat 
(email: kamanthach@gmail.com)
C. S. Lewanagamage,
Department of Civil Engineering, University of Morat 
(email: sujeewal@uom.lk)
The majority of steel bridges in Sri Lanka have been built more than 100 years ago. 
Most bridges have been subjected to changes such as deterioration due to corrosion, mechanical 
damage due to fatigue and repair due to maintenance. Therefore condition evaluation of such bridges 
is vital to ensure public safety. This paper presents condition assessment of selected steel bridges in 
Kaluthara district. The task is pursued through visual observations. Two mathematical models have 
been developed in this research to evaluate the condition of bridges. The Analytical Hierarchy Process 
was applied to obtain the priority vector of bridge elements and important factor of bridge 
elementswere used to obtain the overall bridge condition index. The modified overall bridge condition 
index is employed to assess the bridge condition. The results obtained from four selected steel bridges 
are presented in this paper.
Keywords: Steel Bridge, condition assessment, visual observations, Analytical Hierarchy Process
(AHP), Modified Overall bridge Condition Index (MOBI).
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Abstract:
deteriorating structural members. These 
variables change with time. (Lima et al., 2008)
Inspections were done on four selected steel 
bridges in Kaluthara district. Three of these 
bridges are truss bridges while the other one is 
a steel arch bridge. This study was carried out 
to evaluate the current condition of the bridge. 
This paper presents the inspection procedure, 
inspection data obtained by visual 
observations, defects found at all bridges and 
steel bridge condition rating methods. Two 
condition rating methods have been developed. 
The modified overall bridge condition index is 
employed to assess the bridge condition.
Description of bridges
Bridge No. 01 which is located between 
Narthupana and Kalutaraconsists of a single 
span 22.5 m steel trusssupported on abutments. 
The bridge is made of riveted iron members. 
The abutments have been made of reinforced 
concrete. Figure 1(a) shows the general view of 
bridge No. 01.
Bridge No. 02 which is located between Nagoda 
and Naboda is a single span 31.3 m steel truss. 
The bridge was constructed 50 years ago. The
Introduction1.
A bridge is a structure built to pass over a 
physical obstacle such as valley, road, river or 
railway. Bridges can be classified in several 
different ways which are type of structural 
elements used, whether bridges are fixed or 
movable, by what they carry and by what 
materials that are used. Bridges should be 
managed and maintained properly to 
that the road networks can safely and 
economically operate.
Most steel bridges in Sri Lanka have been built 
than 100 years ago. Most of these bridges 
have been subjected to change such as 
deterioration, change of 
mechanical damage. Most of these bridges 
in a deteriorated condition. Therefore it is 
necessary to assess the existing steel bridges. 
(Rathnayaka et al.,)
The aim of bridge condition assessment 
establish the structural reliability of a bridge 
including evaluation of strength, serviceability, 
stability and fatigue. The bridge performance 
depends on two variables which are the load 
applied and the residual resistance ot
ensure
more
2.repair oruse,
are
is to
69XJ
in the members. The bridge inspection guide 
prepared referring existing bridge 
inspection manuals. The inspection was started 
from superstructure. Every element of the 
bridge was inspected and defects were 
identified and recorded. Other than that the 
general details of bridges such as span, width, 
member sizes were also recorded.
4. Development of Condition 
Rating Method
There are five condition states to categorize the 
condition of element. These condition states are 
1 for very good, 2 for good, 3 for fair, 4 for poor 
and 5 for very poor. The descriptions of each 
condition states have been mentioned in the 
bridge inspection guide. The quantities within 
the bridge element can be in different condition 
states. The quantities (unit, length, area) of 
bridge element were estimated and recorded. 
Condition is estimated for each element 
independently. If numbers of deteriorated 
quantities are high, the condition state value 
will be increased. The following equation gives 
the condition state of an element.
bridge is made of riveted steel members. The 
substructure and foundation have been made of 
dressed stone. The general view of bridge No. 
02 is shown in Figure 1(b). (Bridge Inventory, 
RDA)
Bridge No. 03 is located between Lathpadura 
and Molkawa. It consists of a single span 35m 
steel truss supported on abutments. This bridge 
was completed with Austrian assistance in 
2007. Figure 1(c) shows the general view of 
bridge No. 03.
Bridge No. 04 consists of a single span 75 m 
steel arch supported on abutments. It is located 
between Matugama and Horana. It was 
constructed in 1999 over Kalu River. The 
abutments have been made of reinforced 
concrete. The general view of bridge No. 04 is 
shown in Figure 1(d).
Visual Inspection
was
3.
Visual inspection was carried out in order to 
evaluate the condition of bridges. The bridge 
inspection guide was used to inspect the 
elements of the bridges and defects such as 
corrosion of the members, joint defects, cracks
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USS. *G'"'"vlcw t,ld8“ w ”■« No. „2, (c) Brldgc No. 0i
1 70
The overall condition state of an element (Csl) 
Csl = ^ If all elements of a bridge are in very good 
condition (condition state of all elements is 
equal to one) the Overall Bridge Index will be 
5.467. Also if all element are very poor 
(condition state of all element is equal to five) 
the Overall Bridge Index will be 27.333. 
Therefore the OBI will lie between 5.467 and 
27.333. The OBI is divided by 5.467 to obtain the 
Overall Bridge Index from one to five.
(1)U
Where,
cs condition state of sub element 
cs e (1,2,3,4,5)
q- quantity of element reported in 
condition states.
The overall bridge condition index is calculated 
by combining the different element condition 
indices. Important factor has to be taken into 
account to combine the different element 
indices.
The overall bridge index is modified increasing 
the range from 10 to 100. The Modified Overall 
Bridge Index(MOBI) is given by equation(3).
MOBl = 22.5 x — - 12.5
5.467 (3)Important factor
5. Analytic Hierarchy ProcessTable 1 presents the important factor for 
different bridge element. Abu Dabous 
described the structural importance of various 
bridge elements.(Abu Dabous., 2010). The 
importance of bridge element is defined 
considering overall structural integrity and 
safety of the bridge in this research.
Table 1: Important factors for different bridge 
elements
(AHP)
The Analytic Hierarchy Process (AHP) is 
popular and widely used in decision making in 
a wide range of applications. The AHP 
evaluates decision alternatives by pairwise 
comparison and allows more accurate 
judgments than simple weighted product 
model. Each of this judgment is assigned a 
number on a scale. One common scale is shown 
in Figure 2.Important 
factor (IF)Element
1Guard rail Vgfy V»y
5*0-0 SlrcwjJy Otf Shtf-Sir Sivrsft £*ar3
Uver* V'j U*or& fav<ys favurs favors favorsWing wall, Bridge surface, 
Foot path, road drainage
3
-44Approach road
5Stems, Expansion Joint 97 531 3579
7Deck, Bearings
8Top members, Web members 
Girders, Cross girders, 
Connections _____
Figure 2: One common scale
There is no exact definition for structural 
importance of bridge element. The bridge 
elements are compared in pairs with respect to 
the structural integrity and safety of the bridge. 
The results of pairwise comparison are entered 
in a reciprocal matrix.
The priority vector is obtained by totaling the 
numbers in each column. Each entry in the 
column is then divided by the column total. The 
principal Eigen vector can be obtained by 
averaging the rows.
The Priority Vector is calculated as follows. The 
range of priority vector is 1-5.
9
Overall bridge condition index(OBl)
The calculated bridge element condition indices 
quired to be combined to form the overall 
bridge index. The bridge condition index 
represents the overall material and structural 
condition of the bridge.
ECsix/F
arc re
(2)OBI =
n = number of element.
IF = Important Factor.
Csl = condition state of an element.
71
= °',73c + °' SSS ^wm?.+o«tpS++o.o“ar : Srd+ °°33BS
^ss,Vtv mL4 s:
Approach Road; RD-Road Drainage)
+
assessment of6. Condition 
selected bridges
Case studies are used to verify the proposed 
mathematical model and assess the condition of
The priority vector shows the relative weights 
among the different bridge elements. The A 
value is 16.88 and n is 15.
max
CI _ ;WIn = (U34 (4)
bridge according to proposed methodology. 
The data obtained from bridge inspection 
report have been used feed the mathematical 
models. Table 3 shows the condition rating for 
selected bridges.
Table 3: Condition rating for selected bridges
n-1
The Random consistency Index for n- 15 is 1.59 
(Saaty, 1980).
CI
CR = — = 0.084 < 0.1 (acceptable) .... (5)
RI
The Consistency Ratio is less than 0.1. Therefore 
above assigned preference is consistent. Reciprocal
Matrix
Method
(AITP)
Important
Factor
Method
Bridge
The Overall Bridge Condition Index
66.2963.20Bridge No. 01The overall condition states of elements which 
were discussed in previous rating system are 
used to obtain the overall bridge condition 
index. The overall bridge condition index is 
obtained by substituting overall condition 
states of elements to the priority vector.
Then the priority vector value is modified as 
follows.
83.1982.32Bridge No.02
13.5916.31Bridge No. 03
32.5232.31Bridge No. 04
According to results obtained from both 
methods, Bridge No. 02 is in the worst 
condition. In comparison to the MOBI of the 
other bridges, Bridge No. 02 has highest value 
and therefore should be targeted as a top 
priority for rehabilitation.
Bridge No. 01 is also in a poor condition. Bridge 
No. 01 and Bridge No. 02 have been 
constructed more than 50 years ago. According 
to above results, Bridge No. 01 and Bridge No.
02 have higher MOBI values than other two 
bridges. Therefore it is 
immediate remedial action for Bridge No. 01 
and Bridge No. 02.
Bridge No. 03 is in good condition. Bridge No.
03 was constructed 5 years ago. Bridge No. 04 is 
reaching afair condition. That bridge was 
constructed in 1999. It has however not been 
subjected to any repair work after the 
construction. That is the reason for bridge has 
slightly higher MOBI value. Therefore 
necessary remedial actions should be taken for 
bridge No. 04.
The Modified Overall Bridge Index 
= 22.5 xPV- 12.5.......... (6)
The range of the Modified Overall Bridge Tndex 
(MOBI) is 10-100. The priority for remedial 
actions increases as the number rises. The 
MOBI range can be divided into five condition 
states as shown Table 2.
necessary to takeTable 2: MOBI ranges for different condition 
states
Range Condition State
MOBI = 10 Very good
10.0 < MOBI < 32.5 Good
32.5 < MOBI < 55.0 Fair
55.0 < MOBI < 77.5 Poor
77.5 < MOBI < 100 Very poor
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Conclusion7.
In this study the important factor method and 
reciprocal comparison matrix method are used 
to evaluate the condition of steel bridges. In 
important factor method, the importance of 
bridge elementsare defined considering overall 
structural integrity and safety of the bridge. In 
reciprocal comparison matrix method, the 
weightage of different bridge elements have 
evaluated through AHP. This methodology is 
adopted for four selected bridges in Kaluthara 
district in order to assess the condition of the 
bridge. The MOBI value of bridge should be 
maintained as less than 21.25 for better 
condition. Reciprocal matrix method is better 
than important factor method.
References
Abu Dabous., 2010. A Unified condition index 
for existing concrete bridges in Canada
Bridge Inventory, Planning Division, Road 
Development Authority, Executive Engineer's 
office, Kaluthara.
K.Lima, N. Robson, S.Oosterhof, S. Kanji, 
CJ.Montgomery., 2008. Rehabilitation of a 100 
year old steel truss bridge. CSCE 200S Annual 
Conference.
R.M.S.U.Rathnayaka and P.B.R.Dissanayake. 
Methodology for condition assessment and 
retrofitting of railway bridges.
Thomas L. Saaty., 1980. Decision making with 
the AHP: Why is the principal eigenvector 
European Journal of Operationalnecessary.
Research, 145, pp.85-91.
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