NCHRP Synthesis 396 â Monitoring Scour Critical Bridges - TSP2

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25 damage was often not anticipated by the bridge owner. This resulted in much higher maintenance and repair costs than were anticipated. One respondent indicated that repair costs were double what they had budgeted. Numerous cases were also reported where new replacement instruments had to be installed after high velocity flows, debris, and/or ice forces caused the existing instrument to separate from the structure. The materials used to produce the scour monitoring instrumentation need to be robust when there are extreme conditions. Many survey respondents indicated that this is an area of concern because some of the materials being used do not last long enough when severe conditions are present. Fixed monitors often need to be placed in hazardous locations to monitor bridge scour. Debris or ice flows can collide with the monitors that are mounted underwater on the substructure and damage or destroy the devices. Debris and ice flows generally float on the top of the waterway; therefore, locating the monitors closer to the streambed can help protect the instrument from collision. Fixed monitors are generally placed in the location of the potential maximum scour. Often this is considered to be the center of the pier on the upstream side of the bridge. Depending on the angle of flow, this can also be the position where the maximum debris and ice flows collide with the bridge. Placement of the scour instrument to the side of the pier can help to protect it. As discussed in the section on soil conditions, if the streambed material is cohesive, the maximum scour hole can be to the side or back of the pier. Alaska has developed a retractable arm for mounting their sonar scour monitors. The retractable bracket is mounted under the bridge deck and the arm periodically extends out, takes the readings, and retracts under the protection of the deck. Alaska has also mounted sonar monitors in the snow (Figure 22). In Maryland, protective stainless steel shields for the sonar transducer mountings were placed on the upstream side of the approach piers on the Woodrow Wilson Memorial Bridge to protect monitors from the floating debris and boat traffic (Hunt et al. 1998). Shields were not placed on the two bascule piers being monitored because they were protected by navigation fenders of the main channel. Corrosion and marine growth in the harsh tidal environments have led to the use of certain materials as well as more stringent maintenance procedures. The survey respondents reported the use of AISI 316 stainless steel, similar materials to avoid electrolysis, zinc anodes, and anti-fouling paint to help keep the underwater components of the monitoring installations operational. They noted that marine growth (Figure 23) needs to be periodically cleaned from the monitoring devices. This can be done during the underwater inspections of the bridge, but often needs to be done at shorter intervals if the bridges are on the National Bridge Inspection FIGURE 22 Sonar monitor mounted on pier in the snow in Alaska. Standards (NBIS) five-year underwater inspection cycles (Figure 24). Access and Vandalism Selection of the various components of the scour monitoring system requires a careful balance between access and prevention of vandalism. Complex access requirements can FIGURE 23 Underwater sonar bracket installation in the tidal environment showing the marine growth.
26 FIGURE 24 Underwater sonar bracket installation in the tidal environment showing corrosion. make it difficult to install, maintain, and repair a fixed scour monitoring system. Figure 25 shows a variety of locations where the master and remote stations have been placed to provide access for maintenance and to protect the stations from vandalism. Master and remote stations can be mounted on bridge abutments, piers, catwalks, sidewalks, or inside the towers on movable bridges. Master stations can also be mounted on buildings, bulkheads, or other structures in the vicinity of the bridge. Additional parties or equipment can be required, such as divers, boats, and barges, both to install, and later to access the system for maintenance and repairs. These items must be given serious consideration especially when planning a maintenance and repair program and budget. Examples of access limitations include security clearances, traffic lane closures, boats, barges, keys to doors or gates, and under bridge inspection trucks (Figures 26 and 27). Survey FIGURE 25 Locating the master and remote stations (clockwise from left to right). Master stations mounted inside a bascule pier machinery room and on a building near the bridge, remote stations on a pier stem, a catwalk under the bridge, and on a pier pile cap.
Page 1 and 2: NCHRP NATIONAL COOPERATIVE HIGHWAY
Page 3 and 4: NATIONAL COOPERATIVE HIGHWAY RESEAR
Page 5 and 6: The National Academy of Sciences is
Page 7 and 8: FOREWORD Highway administrators, en
Page 9 and 10: 53 CHAPTER EIGHT CONCLUSIONS Synthe
Page 11 and 12: 2 FIGURE 1 Causes of bridge failure
Page 13 and 14: 4 (TRIS), the U.S. Geological Surve
Page 15 and 16: 6 TABLE 1 TYPES OF FIXED SCOUR MONI
Page 17 and 18: 8 FIGURE 6 Detail of conduit to und
Page 19 and 20: 10 Sounding Rods, also known as BRI
Page 21 and 22: 12 80 70 Note: Number of bridge sit
Page 23 and 24: 14 FIGURE 17 States with fixed scou
Page 25 and 26: 16 TABLE 2 (continued) State Georgi
Page 27 and 28: 18 TABLE 3 (continued) State N ew M
Page 29 and 30: TABLE 4 (continued) State Arkansas
Page 31 and 32: 22 TABLE 5 BRIDGE SITES WITH FIXED
Page 33: 24 40 35 Number of Installations 30
Page 37 and 38: 28 can turn into rapidly flowing ri
Page 39 and 40: 30 TABLE 6 ESTIMATED COST INFORMATI
Page 41 and 42: 32 depths with existing scour formu
Page 43 and 44: 34 The states also found the need t
Page 45 and 46: 36 Appendix G contains sample progr
Page 47 and 48: 38 such as Microsoft Excel. Sample
Page 49 and 50: 40 FIGURE 32 Active streambed monit
Page 51 and 52: 42 including sending false alarms a
Page 53 and 54: 44 FIGURE 43 Remote station, solar
Page 55 and 56: 46 FIGURE 46 General elevation of K
Page 57 and 58: 48 Alaska The Knik River near Palme
Page 59 and 60: 50 Georgia The Georgia DOT report c
Page 61 and 62: 52 using thermocouples spaced along
Page 63 and 64: 54 TABLE 7 FIXED INSTRUMENTATION SU
Page 65 and 66: 56 future repair costs, especially
Page 67 and 68: 58 A clear chain of command of thos
Page 69 and 70: 60 FHWA LTBP The FHWA initiated a m
Page 71 and 72: 62 REFERENCES Avila, C.M.C., J.A. R
Page 73 and 74: 64 BIBLIOGRAPHY Blodgett, J.C., Mon
Page 75 and 76: 66 APPENDIX A Survey Appendix A con
Page 77 and 78: 68 Section 1 1.1 Agency/Organizatio
Page 79 and 80: 70 Section 5 Design and Installatio
Page 81 and 82: 72 6.18 Have analyses and/or tests
Page 83 and 84: APPENDIX B Survey Respondents State
Page 85 and 86:
76 APPENDIX C Summary of Survey Res
Page 87 and 88:
78 2.4 What innovations/advancement
Page 89 and 90:
80 NJ (1)—30,000 NJ (2)—70,000
Page 91 and 92:
82 FL—120 ft (District 2) HI (1)
Page 93 and 94:
84 3.17 What is the foundation type
Page 95 and 96:
86 HI (1)—2/1/04; Q = 930 cfs; 2-
Page 97 and 98:
88 AK (2.12)—$10,000 materials, $
Page 99 and 100:
90 5.9 Do the monitors have an auto
Page 101 and 102:
92 6.3 How often is the data collec
Page 103 and 104:
94 6.10 How is the data used, and w
Page 105 and 106:
96 AR—Sonar can be checked agains
Page 107 and 108:
98 7.7 Who repairs the scour monito
Page 109 and 110:
100 8.5 Were there any additional p
Page 111 and 112:
102 APPENDIX D Bridges with Fixed S
Page 113 and 114:
TABLE D3 MONITORED SCOUR CRITICAL B
Page 117 and 118:
10.00 0.00 -10.00 -20.00 -30.00 -40
Page 120 and 121:
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
Page 122 and 123:
-5 -10 -15 -20 -25 -30 -35 Scour De
Page 124 and 125:
-16 -17 -18 -19 -20 -21 -22 -23 -24
Page 126 and 127:
Sonar Scour Monitor Data for Wantag
Page 128 and 129:
119 APPENDIX F Case Studies Abbrevi
Page 130 and 131:
121 30 N. Abutment Pier Number S. A
Page 132 and 133:
123 APPENDIX G Available Plans, Pro
Page 134 and 135:
125 4. ALERT DEPTH If the “Alert
Page 136 and 137:
127 TABLE OF CONTENTS 1. Descriptio
Page 138 and 139:
129 3. Recording of a “no lock”
Page 140 and 141:
The Regional Structures Engineer sh
Page 142 and 143:
While continuing to encourage DOTs
Page 144 and 145:
A Scour Monitoring General Inspecti
Page 146 and 147:
137 16. CONTACTS Each bridge locati
Page 148 and 149:
139 Nevada Department of Transporta
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141 Location: Bridge B-420, US 95 a
Page 152 and 153:
143 Location: Bridge B-839S, I-15 a
Page 154 and 155:
145 Responses to Sonar Calls: First
Page 156 and 157:
147 4. RECOMMENDED ACTION(S) (see S
Page 158:
149 Emergency repair plans (include
Page 162 and 163:
153 Functional Applications Local S
Page 164 and 165:
Bridge owners to facilitate their e
Page 166 and 167:
157 DRAFT RESEARCH PROBLEM STATEMEN
Page 168:
Abbreviations used without definiti
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NCHRP Synthesis 396 â Monitoring Scour Critical Bridges - TSP2