* Corresponding authors: Department of Civil Engineering, Rajshahi University of Engineering & Technology, Rajshahi-6204, Bangladesh E-mail address: [email protected] (Md. Kamruzzaman)
9
Remote Monitoring of Bridge Deflections using Total Station
Md. Kamruzzaman
1, Md. Rashedul Haque
21 Department of Civil Engineering, RUET, Rajshahi 6204, Bangladesh
2 Department of Civil Engineering, PUST, Pabna 6601, Bangladesh
ARTICLE INFORMATION ABSTRACT
Received date: 04 Feb. 2019 Revised date: 31 Aug. 2019 Accepted date: 29 Oct. 2019
Bridges are essential components of road network and deflection is an important index of bridge health. In Bangladesh, collapse of bailey bridge is frequent and monitoring of bridge health is seldom practiced. In this study, remote observation of bridge deflection is introduced using Refractorless Total Station (RTS). In addition, bridges were modelled with computer program to validate deflection results. A total of 15 highway bridge including bailey bridge and 3 railway bridge within Rajshahi division were studied. The least count of the RTS was 3 mm and monitored deflection ranges from 12 mm to 93 mm for all studied bridges. The results revealed that bailey bridges deflect more than the other type of bridges. Most of the bailey bridges exceed permissible deflection limit even due to self-weight. Again, railway bridges deflect less despite having long span and aged. Use of RTS to measure bridge deflection is accurate, simple and requires fewer work forces to operate.
The deflection results of bailey bridges convey the message that the structural health of bailey bridges is alarming and need immediate attention.
Keywords
Bailey Bridge Bridge Deflection Highway Bridge Railway Bridge SAP
Total Station
1. Introduction
Measurement of deflection of bridges is an important parameter for structural health assessment of in-service bridges, but until recently, it rather remains an unsolved problem [1, 2]. Moreover, deflection is an important index for safety evaluation of bridges [3]. A high rate of deflection indicates the materials are significantly displaced which may bend, warp or shift in response to the superimposed load. Lower rate of deflection indicates higher structural stiffness. Bridges whose deflections overpass the specified limit of design may increase damage accumulation and even collapse at any time, which pose a serious threat to people’s lives and assets.
In Bangladesh, there is absence of proper framework under which structural health of bridge is monitored. Vertical deflection of bridges can predict the structural health status of bridges and can provide the important reference for the structural performance and operational status of bridges [4]. According Roads and Highways Department of Bangladesh [5]; approximately 4,500 bridges have been found to be aging and deteriorated. In addition, another 1,000
Journal of Engineering and Applied Science
Contents are available at www.jeas-ruet.ac.bd
10 bridges are bailey bridges, which are prefabricated steel truss bridges for temporary purpose. Most of these bailey bridges are in unsafe condition; the bridges have already become deteriorated and damaged to great extent. Bailey bridges are initially installed for temporary purpose. Later, it continued to use until a major problem arises. Collapse of highway bridges is seldom while failure of bailey bridge is quite frequent [6]. Traditional methods or visual inspection could often overlook the actual flaws and that can enhance the likelihood of structural failure. Monitoring bridge deflection not only improve safety and ensure longer life spans for bridges but also it can greatly reduce long and short-term costs related to structural maintenance. Applying effective approaches to regular upkeep of bridges is essential. In addition to the need to keep bridges running smoothly, maintaining safe public movement is also of great importance. Thus, the goals are to reduce the possibility of structural failure, reduce unwanted repairs and costs related to inspection labour and improve public safety for all.
Bridges are often built over highways, water, mountains, valleys and these site restrictions hinder tradition deflection measurement of bridges. Conventional techniques and equipments for bridge deflection measurement in Bangladesh are limited to contact-based or visual inspection. To overcome the limitations of traditional contact systems, various non-contact displacement measurement systems using GPS technology [7, 8], laser Doppler vibrometry [9], radar interferometry [10], robotic total station [11], and vision-based (or image-based) optical techniques [12–20], have been developed and advocated for bridge deflection measurement with their claimed advantages. An alternate technique is the use of Refractorless Total Station (RTS), which has been successfully used for the measurement of semi-static [21] and dynamic deflections of relatively long-period bridges characterized by large deflections [22–25]. Despite the diversity of remote deflection measurement techniques, use of RTS have received increasing attention due to their outstanding advantages, such as low-cost, easy-to-use setup, require less manpower, real-time measurement and applicability. Thus, this study aims to introduce RTS for assessing vertical deflection of bridges as a remote or non-contact observation technique.
In the following section of this paper, basic principles and procedures of dead load deflection measurement with RTS are briefly described. The studied bridges were modelled with computer software; i.e. Structural Analysis Program (SAP) for theoretical evaluation of dead load deflection. Then, the theoretical results and field measurement with RTS are compared for evaluation. Finally, field measurement of 15 highway and 3 railway bridges with the aid of RTS is recapitulated for structural health assessment.
2. Method of Study
Bridges often suffer structural deterioration due to aging and lack of proper maintenance. Among the many factors, which led to the unsatisfactory performance of bridges, irregular inspection and health monitoring of existing bridges is crucial. The most common objectives for monitoring bridge deflection is to obtain quantitative data about the structural behaviour and to provide a real-time feedback. The monitoring system is used to increase the safety of the structure and provide early warning of foreseeable failure.
It is well known that movements of structures can be monitored using a total station with good results [26-28].
Again, total station provides accuracies of better than 1 mm for bridge and tunnel surveys [28]. Reflectorless total station is also used to monitor the long-term deformation of bridges [27]. Principles and measuring techniques with RTS, used in the study, are briefly discussed in this section.
2.1 Concept of Dead Load Deflection Measurement by RTS
Deflection is the degree to which a structural element deforms under loading. For a weightless truss as in Fig. 1, the imaginary horizontal plane through the supports will be straight line as line AB. Alternately, considering the self- weight of truss, the elastic curve of the deflected bridge will appear like the curved dotted line as in Fig. 2. For a weightless truss bridge having span ‘L’ as in Fig. 1, the co-ordinates of supports A, B and mid-span C will be (0, 0), (L, 0) and (L/2, 0) respectively. Assuming, ‘-2’ unit dead load deflection for the truss bridge as in Fig. 2, the co- ordinate of ‘D’ will be (L/2, -2). Under this simple mechanism, mid-span deflection of bridges due to dead load is measured using RTS.
11 Target board is a paper having equally spaced grid lines in both direction having co-ordinates of origin at 0, 0 (Fig. 3b). For remote observation, a target board (Fig. 3b) is attached over the bottom chord of bridge girder at the mid-span. An imaginary horizontal plane (i.e line AB in Fig. 1) is assumed which is supposed to pass though the centreline of two supports or, the straight edge of the bottom chord of the truss bridge. The origin (0, 0) of the target board and the cross hair of RTS (Fig. 3a) should be kept over the mid-point (C) of imaginary horizontal plane (i.e.
line AB). For a deflected beam, the elastic curve is likely to appear as the dotted line in Fig. 2 and ‘point D’ of the sagged beam becomes visible over target board as ‘point D’ in Fig. 4. Then the vertical difference between points C
& D (in Fig. 2 & 4) on the target board i.e. vertical difference between the imaginary horizontal plane of the bottom chord and the bottom most point on elastic curve of bridge girder is counted using RTS. Vertical Distance (VD) between point C and D in Fig. 4 can be measured using Missing Line Measurement (MLM) command of RTS without applying target board. Results were accepted while RTS produces same result using target board and MLM.
RTS was calibrated before using it for actual deflection measurement at the bridge sites. Vertical distance between two points within the target board was measured by a steel tape and RTS simultaneously. RTS was kept at various distant places and measurements were taken. RTS produced same measurement with tape up to 122m (400 ft). Thus accuracy of RTS was checked before deploying it in remote deflection measurement of bridges.
Figure 1. Line AB and Coordinates of Point A, B, C of a Weightless Truss.
A C B
Imaginary Horizontal Plane
Mid-span
Span (L)
Figure 2. Line AB and Coordinates of Point A, B, C & D of a Deflected Truss.
D
Elastic Curve
Dead Load Deflection
A C B
Mid-span
Span (L) Imaginary Horizontal Plane
Figure 3. (a) Cross Hair of Total Station
VD = Vertical Deflection due to
Dead Load C
D C
Figure 3. (b) Cross Hair of Total Station over Target Board.
Figure 4. Vertical Distance between Point C and D is the Dead Load Deflection.
D C
12 2.2 Deflection Estimation by SAP
Before starting the field survey, 3 railway bridges and 5 bailey bridges were modelled using SAP. At the beginning, line diagram or the edge view of the bridges were prepared. Geometric design, members’ connectivity, sectional properties, support conditions, material properties were collected from bridge sites. Using these data, computer models were developed and analysis was done for self-weight only. Three Railway bridges namely:
Hardinge bridge (span 105.18m), Gorai bridge (span 57m), Atari bridge (span 47.56m) were selected for study. Among these three bridges Hardinge bridge have largest span. Figure 5 & 6 represents the SAP model and deflection results of a typical girder of Hardinge bridge.
Five bailey bridges in Rajshahi division were selected for SAP analysis. Those are Mohadebpur bailey bridge, Naogaon (span 41.76m), Pontarjan bailey bridge, Bogura (span 39.63m), Bahuli bailey bridge, Sirajgonj (span 33.53m), Baiguni bailey bridge, Bogura (span 30.48m), Kholishagura bailey bridge, Bogura (span 24.39m). Among the five bridges Mohadebpur bridge has the largest span and mid-span deflection were found to be 28.45 mm by SAP analysis and the results are presented (Fig 7 & 8).
2.3 Bridge Deflection Survey using Total Station
During the field survey, 15 highway bridges and 3 railway bridges have been selected for the field survey. Out of these 15 bridges, 13 bridges were bailey bridges. Two bridges were not bailey bridge and referred as ‘other’ type highway bridge in the following sections. Name, location, type and span of the studied bridges are listed in Table 1.
Figure 5. SAP Model of Typical Girder of Hardinge Bridge
Deflection Curve of Hardinge Bridge Due to Dead Load by SAP -50 -40 -30 -20 -10 0
0 10 20 30 40 50 60 70 80 90 100 110
Span (m)
Deflection (mm) ii
Figure 6. Deflection Curve of a Typical Girder of Hardinge Bridge by SAP
Figure 7. SAP Model of Mohadebpur Bailey Bridge
Figure 8. Span Vs Deflection Curve of Mohadebpur Bailey Bridge by SAP
-30 -20 -10
0
0 5 10 15 20 25 30 35 40 45
Span (m)
Deflection (mm) ii
13 Table 1. List of Bridges for Deflection Study with RTS.
No. Name of Bridges Location Type Span (m) Figure No.
1. Hasildoho Bridge Sirajganj Bailey 36.59 Fig. 9. (a) 2. Boikunthopur Bridge Sirajganj Bailey 33.54 Fig. 9. (b)
3. Bahuli Bridge Sirajganj Bailey 33.53 Fig. 9. (c)
4. Hajibari Bridge Sirajganj Bailey 36.59 Fig. 9. (d) 5. Katakhali Bridge Sirajganj Bailey 43.29 Fig. 9. (e)
6. Buruj Bridge Rajshahi Bailey 42.07 Fig. 9. (f)
7. Mohadebpur Bridge Naogaon Bailey 41.76 Fig. 9. (g) 8. Kurir Chor Bridge Sirajganj Bailey 24.39 Fig. 9. (h) 9. Chondidas Bridge Sirajganj Bailey 29.27 Fig. 9. (i) .10. Pontarjan Bridge Bogura Bailey 39.63 Fig. 9. (j)
11. Baiguni Bridge Bogura Bailey 30.48 Fig. 9. (k)
12. Kholishagura Bridge Bogura Bailey 24.39 Fig. 9. (l)
13. Sonai Bridge Sirajganj Bailey 18.29 Fig. 9. (m)
14. Hardinge Bridge Pabna Railway 105.18 Fig. 9. (n) 15. Gorai Bridge Kushtia Railway 57.01 Fig. 9. (o) 16. Atari Bridge Noagoan Railway 47.56 Fig. 9. (p)
17. Gorai Bridge Kushtia Other 110.98 Fig. 9. (q)
18. Atrai Bridge Naogaon Other 29.27 Fig. 9. (r)
Figure 9. (b) Boikunthopur Bridge, Sirajganj.
Figure 9. (a) Hasildoho
Bridge, Sirajganj. Figure 9. (c) Bahuli
Bridge, Sirajganj.
Figure 9. (d) Hajibari
Bridge, Sirajganj. Figure 9. (e) Katakhali
Bridge, Sirajganj. Figure 9. (f) Buruj
Bridge, Rajshahi.
14
Figure 9. (k) Baiguni
Bridge, Bogura. Figure 9. (l) Kholishagura
Bridge, Bogura.
Figure 9. (j) Pontarjan Bridge, Bogura.
Figure 9. (m) Sonai
Bridge, Bogura. Figure 9. (n) Hardinge
Bridge, Pabna. Figure 9. (o) Gorai Railway
Bridge, Kushtia.
Figure 9. (g) Mohadebpur
Bridge, Naogaon. Figure 9. (h) Kurir Chor
Bridge, Sirajganj. Figure 9. (i) Chondi Das Bridge, Sirajganj.
Figure 9. (p) Atari Railway
Bridge, Naogaon. Figure 9. (r) Atrai
Bridge, Naogaon.
Figure 9. (q) Gorai Bridge, Kushtia.
15 The deflections of railway and highway bridges due to dead load resulting from SAP analysis are presented as Fig. 10. Deflections due to dead load of railway bridges are less than deflections of highway bridges. The deflection vs. span, (∆÷L) ratio is shown in Fig. 10 and it is evident from the figure that (∆÷L) ratio is lower in case of railway bridges despite railway bridges have longer span. On the other hand, highway bridges have higher (∆÷L) ratio despite highway bridges have shorter span. It comes out from the analysis that highway (bailey) bridges are less stiff than the railway bridges.
The RTS estimation of deflection due to dead load showed the railway bridges deflect less than the highway bridges (Fig. 11). So, it is evident that railway bridges are stiffer than highway bridges. As well, all railway bridges are safe as the deflection of railway bridges are less than (Fig. 11) the standard limit (L/800) set by American Association for State Highway and Transportation Officials (AASHTO).
Figure 11. Comparison of Railway and Bailey Bridge Deflection by RTS.
0 25 50 75 100 125 150 175
Hardinge Bridge Gorai Rail Bridge Atrai Rail Bridge Mohadebpur Bailey Bridge Pontarjan Bailey Bridge Bahuli Bailey Bridge Baiguni Bailey Bridge Kholishagura Bailey Bridge
Left: Deflection by RTS (mm) in
0 25 50 75 100 125 150 175
Right: Deflection Limit by AASHTO (mm) in i
Δ by RTS (mm) Δ Limit by AASHTO (mm) Highway Bridge Railway Bridge
Figure 12: Comparison of RTS and SAP Deflection of Bailey Bridges.
0 20 40 60 80
Mohadebpur Bailey Bridge
Pontarjan Bailey Bridge
Bahuli Bailey Bridge
Baiguni Bailey Bridge
Kholishagura Bailey Bridge
Left: Deflection by RTS &SAP (mm) in
0 20 40 60 80
Right: Deflection Limit by AASHTO (mm) i
Δ by RTS (mm) Δ by SAP (mm) Δ Limit by AASHTO (mm)
Figure 10. Comparison of Deflection Vs. Span Ratio for Railway and Highway Bridges by SAP.
57m 48m
41m
40m 34m
30m
24m 105m
0 5 10 15 20 25 30
Hardinge Bridge Gorai Rail Bridge Atrai Rail Bridge Mohadebpur Bridge Pontarjan Bridge Bahuli Bridge Baiguni Bridge Kholishagura Bridge
Span of Railway and Highway Bridges (m) Deflection Vs. Span Ratio ( 10 -4 ) oii
Railway Bridge
Highway Bridge
16 Comparison of the deflection measured by RTS and SAP is presented in Fig. 12. Deflections from SAP analysis is always less than the deflections measured by RTS. It means the physical deflection due to dead load is higher than the theoretical deflection. This is due to aging, lack of periodic maintenance and repair. The difference of deflection by RTS and SAP of Baiguni bailey bridge is maximum. The structural health of Baiguni bridge is critical. Except Mohadebpur bridge, all other bridges exceed the maximum permissible limit for bridge deflection as recommended by AASHTO (Fig. 12). Table 2 gives the deflection results by RTS where deflection due to dead load of Katakhali bridge in Sirajganj district is highest among all studied bridges and Sonai bridge in Bogura deflects minimum.
Table 2. Deflections of Bridges due to Dead Load by RTS.
No Bridge Name Bridge Type Span, L (m) Deflection, ∆DL (mm)
1. Hasildoho Bridge Bailey 36.59 54
2. Boikunthopur Bridge Bailey 33.54 51
3. Bahuli Bridge Bailey 33.53 51
4. Hajibari Bridge Bailey 36.59 84
5. Katakhali Bridge Bailey 43.29 93 (Maximum)
6. Buruj Bridge Bailey 42.07 36
7. Mohadebpur Bridge Bailey 41.76 42
8. Kurir Chor Bridge Bailey 24.39 81
9. Chondidas Bridge Bailey 29.27 60
10. Pontarjan Bridge Bailey 39.63 54
11. Baiguni Bridge Bailey 30.48 78
12. Kholishagura Bailey 24.39 36
13. Sonai Bridge Bailey 18.29 12 (Minimum)
14. Hardinge Bridge Railway 105.18 93
15. Gorai Bridge Railway 57.01 45
16. Atari Bridge Railway 47.56 15
17. Gorai Bridge Other 110.98 60
18. Atrai Bridge Other 29.27 72
Based on the AASHTO limit of maximum deflection, Table 3 provides the list of safe and unsafe bridges considering only dead load. Addition of live load will make the bridges more susceptible to fail. Out of 15 highway bridges, only 4 bridges were found to be within the permissible deflection limit.
Again, Table 4 provides the gradation of highway bridges based on ‘deflection to span ratio’. Short span bridges have higher ‘deflection to span ratio’ and more vulnerable. Based on the ‘deflection to span ratio’, all studied bridges are graded in terms risk susceptibility and presented in Table 4.
17
No. Bridge Name Span,
L (m) ∆DL,
(mm) AASHTO
Limit (mm) Remarks
1. Hasildoho Bridge 36.59 54 45.73 Unsafe
2. Boikunthopur Bridge 33.54 51 41.92 Unsafe
3. Bahuli Bridge 33.53 51 41.92 Unsafe
4. Hajibari Bridge 36.59 84 45.73 Unsafe
5. Katakhali Bridge 43.29 93 54.12 Unsafe
6. Atrai Highway Bridge 29.27 72 36.59 Unsafe
7. Kurir Chor Bridge 24.39 81 30.49 Unsafe
8. Chondidas Bridge 29.27 60 36.59 Unsafe
9. Pontarjan Bridge 39.63 54 49.54 Unsafe
10. Baiguni Bridge 30.48 78 38.11 Unsafe
11. Kholishagura Bridge 24.39 36 30.49 Unsafe
12. Sonai Bridge 18.29 12 22.87 Safe
13. Mohadebpur Bridge 41.76 42 51.07 Safe
14. Buruj Bridge 42.07 36 52.59 Safe
15. Gorai Highway Bridge 110.98 60 138.72 Safe 16. Hardinge Bridge (Railway) 105.18 93 131.47 Safe 17. Gorai Bridge (Railway) 57.01 45 71.37 Safe 18. Atari Bridge (Railway) 47.56 15 59.45 Safe
Table 4. Gradation of Bridges According to Span Vs. Deflection Ratio.
No. Bridge Name Span, L (m) ∆, (mm) ÷ L Vulnerability Position
1 Kurir Chor Bridge 24.39 81 0.0033 1st
2 Baiguni Bridge 30.48 78 0.0026 2nd
3 Atrai Highway Bridge 29.27 72 0.0025 3rd
4 Hajibari Bridge 36.59 84 0.0023 4th
5 Katakhali Bridge 43.29 93 0.0021 5th
6 Chondidas Bridge 29.27 60 0.0020 6th
7 Bahuli Bridge 33.53 51 0.0015 7th
8 Boikunthopur Bridge 33.54 51 0.0015 8th
9 Hasildoho Bridge 36.59 54 0.0015 9th
10 Kholishagura Bridge 24.39 36 0.0015 10th
11 Pontarjan Bridge 39.63 54 0.0014 11th
12 Mohadebpur Bridge 41.76 42 0.0010 12th
13 Buruj Bridge 42.07 36 0.0009 13th
14 Sonai Bridge 18.29 12 0.0007 16th
15 Gorai Highway Bridge 110.98 60 0.0005 17th 16 Hardinge Bridge (Railway) 105.18 93 0.0009 14th 17 Gorai Bridge (Railway) 57.01 45 0.0008 15th 18 Atari Bridge (Railway) 47.56 15 0.0003 18th
18 4. Results
In this study, dead load is used to understand self-weight only. From the field survey and computer analysis, it is evident that most of the bailey bridges exceeds maximum permissible deflection limit. Hence live load analysis is skipped during the study as the bridges already become unsafe due to dead load. The results can be summarized as:
o Deflection Results from SAP Analysis
From SAP analysis, it was clear that railway bridges deflects less despite they have longer span. Alternately, bailey bridges deflects more even they have shorter span. For example, maximum dead load deflection of Hardinge bridge (railway bridge) was 42 mm having typical girder span of 105.18 m (Fig. 6). Again, maximum dead load deflection of Mohadevpur bridge (bailey) was 28 mm having a span of 41.16 m (Fig.
8).
o Deflection Results from RTS Survey
As like the SAP analysis, RTS results make it evident that railway bridges have less and bailey bridges have large deflection. For example, observed maximum dead load deflection of Hardinge bridge (railway bridge) was 93 mm having typical girder span of 105.18 m (Table 2). Again, maximum dead load deflection of Katakhali bailey bridge was 93 mm having a span of 43.29 m (Table 2).
o Trend of Deflection Results by SAP and RTS
Both railway and highway (bailey) bridges produces same trend of result either by RTS or SAP analysis.
Deflection vs. span ratio is more for bailey bridges and less in railway bridges. But the value of dead load deflection found in RTS results are more than SAP result. This is due to aging, deterioration and maintenance problems.
o Magnitude of Deflection for Bailey and Other Bridges
The maximum bailey bridge deflection is 93 mm for a span of 43.29m for Katakhali bridge. It forms the
‘deflection to span’ ratio as 0.00215. On the other hand, maximum highway bridge deflection is 59 mm for a span of 110.98m for Gorai highway bridge. So, the ‘deflection to span’ ratio becomes 0.0005. Thus the bailey bridges have more than four times ‘deflection to span’ ratio than the other type highway bridges.
o Structural Health of Bailey Bridges:
Most of the bridges exceed the maximum permissible deflection limit for only dead load. Addition of live load will put extra stress and make bridges more vulnerable. Observed maximum deflection for Katakhali bailey bridge (span 43.29 m) is 93 mm where as the AASHTO maximum deflection is of 54.12 mm. Out of 13 bailey bridges, 11 bridges have more deflection than the permissible limit. Therefore bailey bridges are unsafe although these bridges have shorter span.
o Structural Health of Railway and Other Type of Highway Bridges:
Deflection of railway and other type of highway bridges does not exceed AASHTO limit for maximum permissible deflection. Thus these bridges are stiff and safe.
o Use of RTS for Deflection Study
RTS can be used effectively to measure bridge deflection. For accuracy; up to 3 mm, RTS data for bridge deflection can be attained accurately, quickly and easily.
5. Conclusion
Bridge is a large infrastructure with huge investment and a long period of use, and its security has an important impact on the national economy [29]. In recent years, the recurrent occurrence of bridge collapse threatens people's life and property. Bridge as a public transport infrastructure, its health is about people's lives and national economic security [30]. Most of the bridges in Bangladesh have lack of adequate monitoring system to ensure the safety and durability. The current detection technology cannot make accurate and objective assessment of bridge deflection. At present, the detection of the bridge deflection is still based on the traditional artificial detection method. This method cannot make a reasonable and scientific evaluation of the overall safety of bridges. In this situation, it is very important
19 frequent failure of bailey bridges in our country. Majority of bailey bridges deflects more than the permissible limit even due to self weight. These slackly bailey bridges caved in when loaded vehicles passes over it. The relevant authorities need to strengthen the bridge health status attention to overcome the dilemma. Results obtained from RTS and SAP analysis conversed and it indicates that RTS can be effectively used for monitoring bridge deflection. It can be a useful tool for structural health monitoring of bridges to reduce bridge failure. As a smart, simple, reliable and economic method, use of RTS for health monitoring bridges, has the potential for promising results.
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