Performance Monitoring of Bridges through Instrumentation

9th International Symposium on Advanced Science and Technology in Experimental Mechanics, 1-6 November, 2014, New Delhi, India

Performance Monitoring of Bridges through Instrumentation

Sahu G, Garg R, Goel R, Goyal J, Jangpangi L and Lakshmy P

CSIR-Central Road Research Institute, New Delhi-110025, India

Abstract Bridge engineers did not pay adequate attention to certain

very vital aspects adversely affecting the serviceability

condition and sometimes even jeopardize the safety of

bridges. Some examples of these aspects which might be

appropriately termed “Performance factors” are: the true

stress conditions in the bridge components, serviceability

factors such as deflection and cracking, actual stress and

deformation conditions in the foundations, corrosion of

reinforcing and pre-stressing steel, etc.

In recent years, the steadily increasing number of

bridges, severely distressed owing to one or more of these

factors acting over a period of time, has sharply brought

home to engineers throughout the world to consider these

factors in the design and construction of all major bridges.

In India too, afflictions caused by some of these

„performance factors‟ have severely affected the health of

several major bridges on our national and state highways.

However, while the importance of considering these

factors has now been adequately recognized by bridge

engineers, the task poses a major challenge for the

following reasons:

In general, such performance factors are not readily

amenable to straight forward design calculations. Their

influence can be studied only through performance

„measurements‟ and „monitoring‟.

Data relating to these factors which affect the

construction and in-service performance of bridges can

only be obtained from the „field‟ i.e. through

instrumentation of actual bridges.

Several of these factors relate to long-term „progressive‟

phenomena. The condition of potential distress would

thus have been progressing for quite some time before it

manifests itself in the form of some serious „distresses‟.

There is paucity of comprehensive data relating to bridge

performance worldwide.

The foregoing considerations bring out the imperative

need and urgency to instrument bridges and monitor their

performance through systematic field studies. The

scientific approach to tackle these effects would be by

monitoring of such bridges through instrumentation from

its construction stage to service life. Nowadays, bridges are

being constructed using several innovative techniques such

as pre cast segmental construction for large spans, use of

shock transmission units and use of jack-down process for

sinking of wells etc. The use of innovative techniques also

calls for long-term performance monitoring of structures

through instrumentation. Several performance parameters

such as strain, temperature gradient, tilt, and deflection etc.

have been monitored through instrumentation from its

inception. Further, the long-term collected response of

structure leads into adopting appropriate remedial

measures timely.

The paper describes a case study of Lok Nayak Setu,

New Delhi, India performance of which was monitored

through Instrumentation. The paper covers

instrumentation scheme, various parameters to be

monitored, the corresponding instrumentation techniques

adopted, analysis of data and the results & inferences

derived there from.

Key words

Performance factor, Global Structural Response Test,

Instrumentation technique, Diurnal variation, Temperature

Gradient, Diurnal Variation, Long-term Stability,

Automatic Data Logger, Vibrating Wire Sensors.

1. Introduction

Lok Nayak Setu, New Delhi is the first four lane single cell

simply supported box girder bridge built in India. In the

context of the need for the scientific monitoring of health

of the major bridges on our highways and for creating a

reliable data base for their efficient management, the

construction of the Lok Nayak Setu, New Delhi presented

an opportunity to instrument the bridge and monitor its

performance from inception. Due to this, Ministry of Road

Transport and Highways (MoRTH), Government of India

decided in 1996 for the instrumentation of the bridge and

study of its long term behaviour.

The broad aim of the study was to do instrumentation of

the superstructure for monitoring of the performance of the

bridge through measurement of several structural

parameters such as strain, temperature gradient, deflection

profile etc. from the construction stage itself. The study

involved planning of the instrumentation scheme,

extensive site work covering the installation of instruments

and related activities, continuous monitoring of instrument

data starting from the stage of concreting and finally,

studies related to the behavior and performance of the

bridge based on the field data.

Box girder No.3 of the bridge was extensive

instrumented in 1997. All other box girders were also

instrumented but with limited instruments. Global

Structural Response Test of the bridge was also carried out

in the year 1997. Thereafter, summer and winter data were

collected during 1997 to 2006 to study the effect of diurnal

and seasonal temperature variations on various important

performance parameters.

2. Description of the Lok Nayak Setu, New Delhi, India

The Lok Nayak Setu comprises of 13 spans of simply

supported prestressed box girders (about 40 m each) and

two end spans of reinforced concrete of 20m each. The

bridge deck is single cell box girder with a 16.3m wide

deck slab and supporting 4-lane carriage way width 14.5m.

The general arrangement of Lok Nayak Setu is shown in


Fig.1. The cross section of box girder is shown in Fig.2

along with scheme of instrumentation for strain.

Fig.1 General arrangement of Lok Nayak Setu, New Delhi

Fig.2 Cross-section of the box girder along with scheme of

instrumentation for strain measurement

3. Instrumentation Scheme and Selection of Sensors

The total instrumentation scheme aims at the installation of

sensors and monitoring of various structural parameters in

the different components of the box girder. The choice of

the instruments and sensors are governed by several factors

such as:

Suitability for embedment in concrete

Ruggedness for onsite handling and installation

Long-term stability (i.e. absence of drift)

Degradation of the output signals over long cable

Sensors based on the Vibrating Wire (VW) principle

possess very desirable characteristics in respect of each of

the above factors. Such sensors are designed for direct

embedment in concrete during construction and such are

quite rugged. VW sensors use frequency, rather than

voltage, as the output signal. Consequently, signals can be

transmitted over long cable length without appreciable

degradation. They possess excellent long-term zero

stability. Vibrating wire sensors are therefore the

automatic choice in this study for the monitoring of all

parameters amenable to measurement through direct

embedment of sensors in concrete. Thus, in this study VW

sensors have been used to measure strains and

temperatures within the body of the structure.

Performance of the sensors was checked prior to

installation at site using the appropriate checking system /

calibration zigs. Depending upon the nature of

instrumentation and monitoring activity involved, the total

instrumentation scheme had been divided into four distinct

component activities:

Instrumentation for strain measurements

Instrumentation for temperature measurements

Instrumentation for deflection and tilt measurements

Automatic logging of data

The details of the instrumentation scheme relevant to

each activity are given below.

3.1 Instrumentation for strain measurements

Box girder No.3 of Lok Nayak Setu has been

comprehensively instrumented with VW strain sensors at

three sections along its length as shown in Fig.2.

Installation of embedded type VW strain sensor is shown

in Fig.3.

Fig.3 Pictorial view of installation of VW strain sensors in

deck slab

3.2 Instrumentation for temperature measurements

To obtain temperature gradient, VW temperature sensors

had been installed at the mid span section of box girder

no.3. The scheme of instrumentation adopted for

installation of VW embedded temperature sensors is shown

in Fig.4 and installation details are shown in Fig.5. In

addition to the above, VW strain sensors are also having

inbuilt thermistor for measurement of temperature.

Fig.4 Instrumentation scheme for VW temperature sensors

3.3 Instrumentation for deflection and tilt

measurement

In addition to the above sensors, level plates were also

installed on the soffit slab along the length of the girders at

five locations in all the girders and at the top of piers. To

measure tilts of pier, tilts plates had been installed at the

pier head. A pictorial view of deflection and tilt

measurement system is shown in Fig.6 and Fig.7

respectively.

40.04 20.5 40.90 41.70 40.30 40.90 40.90 20.0

LAXMI NAGARI.T.O.


Fig.5 Pictorial view of installation of VW temperature

sensors at soffit slab in box girder

Fig.6 Pictorial view of deflection measurement system

Fig.7 Pictorial view of tilt measurement system

3.4 Automatic logging of data

For automatic data logging of all the embedded VW strain

sensors and VW temperature sensors, a portable Automatic

Data Logger (ADL) was used. Such a system is

indispensable for recording the continuously varying

temperature and strain data at an interval of an hour during

the peak summer and winter monitoring. This data logging

system is having facility to be controlled and monitored

through radio modem from the host PC at the site office.

Thus, cables of all the sensors of each instrumented section

were brought to a common point and connected to ADL as

shown in Fig.8.

Fig.8 Data acquisition through automatic data logger

Table 1: Details of instrumentation scheme adopted in Lok

Nayak Setu

Sl.

No.

Parameters to

be monitored

Instrument/

Sensor

Location

1. Strain VW

Embedment

strain sensors

(48 Nos.)

3 sections in box

girder no. 3, each

section having 16

sensors

2. Temperature VW

Temperature

sensors

(21 Nos.)

Thermistors

(70 Nos.)

At mid-section in

deck slab

Attached with each

strain and

temperature sensor

in all the three

section

3. Deflection Through

Precision level

(80 locations)

At the soffit slab

and pier head

4. Tilt of pier Tilt meter

(15 locations)

At the pier heads

3.5 Brief summary of the installed sensors

Brief summary of the installed sensors i.e. VW strain

sensors and VW temperature sensors, deflection and tilt

measurements point is given in Table-1.

4. Sequence of Site Operations

The scheme of instrumentation described earlier required

various sensors to be embedded within the structure of

bridge at certain predefined sections and locations. The

box girder of bridge had been constructed in various stages

viz. bottom slab, webs and the deck slab. Accordingly, the

sensors had been installed as per the construction schedule

of different components at various sections as per the

adopted instrumentation scheme. Installation of embedded

sensors was carried out in the year 1997. Fig.3 and Fig.5

show the installation of various sensors before concreting.

The cables from all the sensors had been taken through

steel pipes so as to protect them from damage at the time

of concreting operations. Proper precautions had also been


taken to protect the sensors during compaction of wet

concrete and de-shuttering. Continuous monitoring after

concreting was done to study heat of hydration and

hardening strain.

To monitor shrinkage, a shrinkage specimen was

prepared during concreting of soffit slab of the box-girder.

A VW strain sensor was embedded centrally in it. The

specimen was kept inside the box over two roller support

for free movements. Readings of shrinkage stain were

taken whenever the strain sensor readings in the main

structure had been recorded.

5. Monitoring of Field Data

To monitor the performance of the bridge, data is collected

from the instruments twice in a year mainly in peak

summer and peak winter and are grouped into six types as

detailed below:

Monitoring of Deflection Profiles

Monitoring of Sinking of Piers

Monitoring of Temperature Gradients

Monitoring of Strain

Monitoring of Tilt

Monitoring of Shrinkage

Recording of data of all the embedment type sensors i.e.

VW strain sensors and VW temperature sensors was done

through data loggers which was controlled through a lap–

top computer at the site. However, slope and deflection

measurements were taken manually.

5.1 Monitoring of deflection profiles

An N-3 precision level was used to monitor the deflection

profile of the girders. The deflection profiles of each girder

were monitored with respect to the bench mark point fixed

at its pier head. Five stainless steel level plates were

installed on the soffit slab in all the girders and at pier head

to monitor their deflection profiles. To study the

temperature effect on deflection profile of girder, readings

of girder no.3 were taken during peak summer and peak

winter every year. A typical peak summer diurnal variation

of deflection profile is shown in Fig.9.

Fig. 9 Typical peak summer variation of deflection profile

of a simply supported box girder

To observe change in deflection profile of all the girders

with age, routine level readings were taken under self

weight.

5.2 Monitoring of sinking of piers and tilt of pier

To monitor sinking and tilt of pier, routine level and tilt

readings of all the pier heads were taken as shown in Fig.6

and Fig.7.

5.3 Monitoring of temperature gradients

To monitor temperature gradients of the box girder No.3,

twenty one VW temperature sensors were embedded

during concreting at the mid span section. Readings of

atmospheric temperature outside and inside the box-girder

were also recorded. Readings were taken during Peak

Summer and Peak Winter every year. To study the

temperature gradient across the depth of the girder, hourly

readings were taken in the month of June and December

every year. The diurnal variations in temperature at various

identified locations had been recorded and a typical

temperature variation in deck slab of box girder is shown

in Fig.10. Peak summer temperature variation along depth

of the box girder is shown in Fig.11.

Fig. 10 Typical peak summer temperature variation at deck

slab in a simply supported box girder

Fig. 11 Peak summer temperature variation in a simply

supported box girder

5.4 Monitoring of strain through embedded VW

strain sensors

To monitor strains in the box girder no.3, sixteen VW

strain sensors were embedded during concreting at mid–

span section and at both the quarter span sections.

Monitoring of strains during Peak Summer and Peak

Winter had been carried out every year after installation. A

typical diurnal peak summer variation and routine

variations in the strain of the individual sensor is shown in

Fig.12 and Fig.13 respectively. The routine variation in the

strain with age is due to cumulative effect of shrinkage,

Distance (m)

Dis

tance (

m)


creep, prestress losses, seasonal and diurnal temperature

variation in the girder etc.

Fig.12 Typical peak summer diurnal strain variation at the

junction of web and deck slab in a simply supported box

girder

Fig. 13 Routine strain variation at the junction of web and

deck slab in a simply supported box girder

5.5 Monitoring of shrinkage

The shrinkage strain variation over a period of four years is

shown in Fig.14.

Fig.14 Variation of shrinkage strain with time

6. Observations

6.1 Deflection profile of girders

The observed peak summer mid-span deflection of the box

girder is in the order of 5.6 mm due to diurnal rise in

temperature. It is also observed that the variation in the

elevation of mid-span of box girders due to temperature in

a year is in the order of 10 mm.

After study of levels data of about eight years during

peak summer, peak winter and seasonal variation, the

upper and lower limits of Reduced Level of mid span of

every girder have been fixed up with respect to pier head

for the long-term monitoring of the bridge. Thus, by taking

a set of level measurements, one can comment about the

health status of the structure.

6.2 Sinking of piers

From the routine level readings over the pier heads taken

in different seasons, it is observed that their difference in

level is of the order of ±5 mm i.e. there is no sinking of

piers.

6.3 Tilt of piers

To monitor the tilts of the piers, certain sets of readings

were taken during the period November 1997 to January

2004. No significant tilting of piers was observed during

this period.

6.4 Temperature study

The maximum temperature gradient observed in the box

girder was 15º C. The maximum temperature of deck

surface was 54.5º C at an ambient temperature of 40º C. It

is also observed that the maximum temperature in the deck

slab occurs from 15:00hrs to 17.00hrs and the minimum

temperature occurs from 06:00hrs to 09:00 hrs.

6.5 Study of strain variation

The maximum diurnal strain variations observed in the

eight years at middle of webs, junctions of webs and deck

slab and in soffit slab are of the order of 50, 30 and 20

micro-strains respectively.

6.6 Study of routine readings of strain

Strain variation due to cumulative effect of shrinkage,

creep, etc. from 1997 to 2005 in the box girder ranges from

430 to 580 micro-strains at the soffit slab, 300 to 400

micro-strains at the centre of web and 170 to 240 micro-

strains at the deck slab.

On the basis of the eight years strain data regarding peak

summer, peak winter and seasonal variations, the upper

and lower bounds for each sensor had been fixed up. Thus,

by taking a set of strain measurements, one can comment

about the health status of the super structure.

7. Conclusions

As a result of instrumentation and monitoring, it has now

become possible to assess some salient structural effects in

structure which are relevant in monitoring their long term

performance. The performance monitoring on long term

basis provides information in terms of structural

parameters regarding the effects of any distress, in the

structure during its service life. The data collected during

monitoring would help in identifying the causes of such a

distress and facilitate adoption of timely and appropriate

remedial measures to avoid aggravation of distress in the

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structure. Thus instrumentation provides a scientific tool in

the health assessment of structures.

Acknowledgements

Authors sincerely express their gratitude to the Director,

CSIR- Central Road Research Institute, New Delhi, India

(CRRI) for the permission and guidance to prepare and

publish this paper. Authors also acknowledge the

assistance received directly or indirectly from the

concerned officials of CRRI as well as Public Works

Department, Delhi Sarkar, New Delhi.

References [1] Tamhankar MG: Assessment of Instrumentation of

Concrete Bridges, Proc. International Seminar on

New Trends in Highway Construction (1997) IRC,

New Delhi, India.

[2] Instrumentation and monitoring of Lok Nayak Setu,

New Delhi, SERC Report (March, 1998), CSIR-

Structural Engineering Research Centre, Ghaziabad,

India.

[3] Instrumentation and monitoring of Lok Nayak Setu,

New Delhi, Report No. CRRI/BAS/CNP-1337,

(December 2006), CSIR-Central Road Research

Institute, New Delhi, India.

Documents

Performance Monitoring of Bridges through Instrumentation