Condition Assessment of Railway Bridges
.
Condition Assessment of Railway Bridges
Investigators:
J.M. Chandra Kishen, Ananth Ramaswamy,
C.S. Manohar and D. Roy
Most of the railway bridges in the Indian Railway (IR) system that have been built several decades ago
have deteriorated both in terms of strength and stiffness due to a variety of reasons. These bridges
have been designed for live loads and service conditions that have changed drastically with time.
Increased axle loads and traffic density have necessitated bridge owners to get their condition
assessed in order to determine their residual structural strength and identify strengthening measures
to be taken for safe performance.
Condition assessment provides information regarding the intensity and extent of observed defects, the
cause for these defects and possible deterioration processes that have strong impact on the safety
and service life of structures. Further, these information forms the basis for estimating the residual
structural capacity and possible remedial works that need to be undertaken.
Five bridges out of which two were steel plate girder bridges, one steel truss bridge and remaining two
were arch bridges made of brick masonry and concrete have been examined.
The bridges were instrumented with LVDT (spring and capacitance type), Electrical resistance strain
gages, Vibrating wire strain gages, Uni-axial and tri-axial accelerometers, Acoustic emission sensors,
Tiltmeter and Temperature sensor.
The following four loading tests were carried out:
1.
Static Load Test: Two BRN wagons loaded with 200 (approx. 20.75 T axle load), 184, 168 and 152
prestressed concrete sleepers were placed on the bridge structure at a fixed position. The two wagons
could apply a static load on both the spans of the bridge. Displacements and strains were measured at
various locations on the bridge for each load level of this test.
2.
Test to obtain Influence Line diagram: A train formation consisting of two locomotive engines (WDG4),
two BRN wagons with 152 sleepers in each, two goods wagon filled with 25 tonnes axle load with four
axles each (BOXNEL), followed by two locomotive engines (WDG3A), was positioned on the bridge at
different pre-determined location and measurements were taken. At the start of the test, the first wheel of
WDG4 wagon was placed on the left springing position of span 1. The test progressed with subsequent
wheels of the formation occupying this first reference point. Since the formation had forty wheels, the
same number of measurements was taken. This test was performed to obtain the variations in
deflections and strains for different positions of the load to obtain some kind of an influence line diagram,
although strictly not.
3.
Speed Test: In this test, the same formation as used in the test for influence line was made to run at
speeds of 5, 10, 20, 30 and 40 kmph in both the directions. The objectives of this test were to obtain the
dynamic amplifications, natural frequencies and mode shapes of the structure.
4.
Longitudinal Load Test: For this test a formation consisting of 54 goods wagons (BOXNEL), each
loaded with 25 tonne/axle load and five locomotive engines (WDG4) two at one end and three at the
other was used.
19.4
18.5
18.4
19.2
18.3
18.2
Displacement
Displacement
19
18.8
18.6
18.1
18
17.9
17.8
18.4
17.7
18.2
0
20
40
60
80
100
Time s
(a)
120
140
160
180
200
17.6
0
10
20
30
40
50
60
70
Time s
(b)
Typical time histories of displacement measured at the crown of the masonry arch bridge (in mm) are
presented for two load cases; (a) data from passage of goods train; (b) data from passage of
passenger train. The initial and final larger spikes correspond to passage of locomotives and the
peaks in the middle regions correspond to passage of wagons/compartments;