Proposed Guideline for Reliability-Based Bridge Inspection Practices (2014) / Chapter Skim
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Final Research Report: Developing Reliability-Based Inspection Practices P a r t I I
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131 S U M M A R Y Introduction The National Bridge Inspection Standards (NBIS) mandate the frequency and methods used for the safety inspection of highway bridges.
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132 Findings Reliability theories and practices were applied through the research to develop a guideline for Risk-Based Inspection (RBI) that provides a new approach for bridge inspection.
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133 The methodology developed through the research was tested using two case studies in different states. During these case studies, the processes described in the Guideline for RBI analysis were implemented using state forces to develop RBI intervals for typical highway bridges with superstructures constructed of steel and prestressed members.
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134 The procedure, methods, and approach described herein can be applied for atypical bridges as well. For example, non-redundant bridge members can be assessed using this approach, as illustrated in previous research (60)
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135 Background The periodic inspection of highway bridges in the United States plays a critical role in ensuring the safety, serviceability, and reliability of bridges. Inspection processes have developed over time to meet the requirements of the National Bridge Inspections Standards (NBIS)
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136 As such, the goals of this project were to develop reliabilitybased inspection practices to meet the goals of: (1) Improving the safety and reliability of bridges and (2)
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137 Research Approach The Guideline were developed in consideration of modern industrial practices and is the result of an exhaustive review and analysis of current methodologies and practices for the inspection and management of structures and facilities, assessment of needs and capabilities, and the development of methodologies focused on the unique needs of highway bridges. The research was aimed at identifying the most effective strategy regarding development of a reliability-based bridge inspection practice.
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138 of damage to a bridge. The methodology is strongly grounded in existing industrial practice.
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139 Findings and Applications 3.1 Introduction The Guideline developed under this project describes the methodology for RBI practices for typical highway bridges. The goal of the methodology is to improve the safety and reliability of bridges by focusing inspection efforts where most needed and optimizing the use of resources.
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140 the complement of the reliability. Consequence is a measure of the impact of the event occurring, which may be measured in terms of economic, social, safety, or environmental impacts.
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141 tial outcomes or consequences of damage are also assessed. Based on the assessment of the OFs and CFs for the various elements of the bridge, an inspection procedure is established, including the interval and scope for the inspection.
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142 with applying theoretical structural reliability concepts, such as those used in modern design specifications, is that the envisioned damage mode (loss of load-carrying capacity) represents only a portion of the required information needed from a bridge inspection.
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143 In other words, the reliability is the probability that the item will not fail during the time period of interest. The challenge for RBI was to determine an appropriate and practical method of estimating the probability, or likelihood, of failure described by the function F(t)
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144 rating of 3, "serious condition," was chosen as a general, durable, and readily understood definition of failure. Bridge elements that have deteriorated to this extent may no longer be performing their intended function, and remedial actions are typically planned to address such conditions.
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145 will lead to a shorter than expected service life. Following the period in which infant mortality may occur, elements typically have long service lives and failures are rare.
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146 bridge design, operational environments, and construction practices. Verification of the assumption requires observation of bridge performance over its service life, and therefore, by definition, cannot be determined in time to be usefully applied.
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147 meaning and vary widely according to the assumption made in a particular analysis. Additionally, "failure" is not typically well defined as is the case, for example, with a pipe or valve.
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148 can be used to map such data or models to the qualitative scales used in the analysis. For example, data from PONTIS deterioration curves or from probabilistic analysis, or other deterioration models, could be incorporated directly into the assessment of the OF using these scales.
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149 joint may be a key attribute in determining the likelihood that corrosion will occur in the bearing area. 3.4.1.4 Screening Attributes Attributes can also be identified as screening criteria that identify certain characteristics that have a predominate effect on the reliability of an element.
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150 condition attributes can result in a change in the likelihood of a given damage mode resulting in failure (i.e., the OF) , reassessment of the inspection requirements is necessary.
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151 toward assessing and differentiating elements in terms of the consequences, assuming that failure of the element occurs. It should be noted that failure of an element is not an anticipated event when using an RBI approach, rather the process of assessing the consequences of a failure is merely a tool to rank the importance of a given element relative to other elements for the purpose of prioritizing inspection needs.
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152 that assessment, sufficient access to the superstructure of a bridge is required to determine if fatigue cracking is currently present, obviously, and the inspection procedure must include reporting the presence or absence of fatigue cracks. In some cases, NDE techniques may be required within the inspection procedure to allow for reliable detection of certain damage modes identified through the RBI analysis.
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153 inspection intervals may be longer based on rational assessments of potential damage, inspection scopes may need to be appropriately adjusted. As a result, determination of the reliability of the inspection method becomes a factor in the overall approach to the inspection process.
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154 an inspection team leader or program manager that is familiar with the inspection procedures and practices as they are implemented for the inventory of bridges being analyzed. The team should include a structural engineer who is familiar with the common load paths and the overall structural behavior of bridges, and a materials engineer who is familiar with the behavior of materials in the particular environment of the state and has past experience with materials quality issues.
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155 narios and asking the RAP to assign relative likelihood to the outcome of the failure according to the CF scale. This is a useful tool for evaluating the appropriate CF for situations that are not well-matched to the examples and criteria provided in the Guideline, or to establish basic ground rules for the assessment of common situations.
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156 posed to the panel, "You are told a steel girder is condition rating 3, serious condition, according to the current NBIS rating scale. Based on your experience, what damage is likely to be present?
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157 on the most likely damage modes. This illustrates how RAPs in different operational environments may identify and prioritize damage modes differently, depending on their operational environment and experiences managing their bridge inventory.
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158 consequence scenarios. The purpose of this exercise was to determine if, given a certain damage scenario, there could be consensus on the most likely outcome of that damage, based on the defined consequence scenarios and applied to a specific bridge.
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159 be "high." These data, which illustrate the consensus formed from the independent assessments of individuals from a number of different states, would be refined when applied within a specific bridge inventory and operational environment. A series of criteria and requirements are provided in the Guideline to assist in this process, and in many cases the CFs may be governed simply by the criteria in the Guideline or owner policies regarding the treatment of redundancy or other factors.
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160 3.5.2 Data Needed for Assessment To perform a reliability-based assessment, the primary data required include data on bridge design characteristics and details, materials, environment, and current condition. Bridge inventory data describing the overall characteristics of a bridge, such as can be developed from existing NBI data tables, can provide some information.
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161 of 7, "Good Condition," as a surrogate for condition attributes associated with a certain bridge type. For example, Figure 8 shows the time-in-condition for prestressed bridge superstructures in the state of Oregon.
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162 are documented and well known. Because of the significant importance of corrosion-based deterioration modes to the degradation of bridges, there exists a significant foundation of knowledge regarding corrosion and its effects on bridges, which can be leveraged to develop estimates of future behavior based on the age, current condition, and design attributes of a bridge.
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163 initiation is not anticipated for almost 70 years. While this model does not consider localized effects, such as cracking of the concrete that can greatly increase the rate of chlorides intrusion into the concrete, it does illustrate that the time to corrosion for a generic, uncracked case varies significantly across geographic regions.
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164 3.6 Case Studies of the Methodology Two case studies were conducted to evaluate the effectiveness of the RBI method. The objectives of the case studies were as follows: • Demonstrate the implementation of the methodologies with state DOT personnel and • Verify the effectiveness of RBI analysis in determining suitable inspection intervals for typical highway bridges.
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165 approach of the research and the planned activities during the RAP meeting. Most of the individuals that participated in the RAP meeting also attended this webinar to be introduced to the technology and prepare themselves for participation.
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166 for this element. The members of the RAP recorded their responses on the bubble sheets and subsequently discussed the identified damage modes as a group.
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167 In this application, the user selects the attributes identified by the RAP for a particular damage mode, as shown in Figure 10. A check box is used to select screening, design, loading, and condition attributes as described in the Guideline.
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168 data model to be used to determine the appropriate maximum inspection interval for a specific bridge or family of bridges. To verify if the use of these models provided a suitable inspection interval that did not compromise the safety and serviceability of bridges, a back-casting procedure was used.
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169 3.6.4.1 Inspection Data for Back-Casting Inspection data from each state were reviewed in detail to implement the data models developed through the RAP process, i.e., evaluation of the attributes identified by the RAP. This included design and loading attributes, which typically do not change over the life of the bridge and condition attributes that change as the bridge ages or undergoes repair or rehabilitation.
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170 3.6.6 Bridge Inventories in Texas and Oregon The families of bridges selected for the two participating states were based on the bridge inventories in each state. It was as desirable to have a sufficient inventory as to have a large inventory from which to draw sample bridges, and a representative population of bridges in terms of age.
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171 5 years or less of consecutive data in a condition rating at the beginning or the end of the available time interval. The trimming value of 5 years was selected based on study of different possible trimming values, ranging from 3 to 7 years, performed by the research team.
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172 the rating decreases, the time in a particular rating is reduced. For example, for the prestressed bridge superstructures illustrated in the figure, the average time period a super structure was rated 8 was almost 14 years (s = 4.9 years)
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173 inspection intervals based on the estimates of the OF and the CF from the RAP analysis. In the figure, the inspection intervals are I =12 months, II = 24 months, III = 48 months, IV = 72 months, and V = 96 months.
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174 Rutting was also identified as a credible damage mode for decks by the RAP in Oregon. This damage mode illustrates one benefit of a RAP consisting of bridge owners.
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175 essentially identical for these families of bridges, with the exception that adjacent box girder bridges had a shear key damage mode that would need to be assessed as a screening tool. These data are reflected in the summary of damage modes shown in Table 11.
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176 High and considering the CF as uniformly Moderate, or determining the CF based on structural redundancy and feature under the bridge. For the latter, the CF was based on the following criteria: The CF was Moderate for the superstructure if: • Superstructure consisted of more than four members AND • Beam spacing of 10 ft or less AND • Bridge not over a roadway.
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177 "High" consequence, considering all superstructure damage modes as "Moderate" consequence, and determining the CF based on the redundancy of the bridge, as described in Section 3.6.9. Additional analysis was done to test the effect of including, or not including, the screening criteria of elements with a condition state identified as CS 4 or 5.
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178 determined through the RBI analysis for each year there was an available element-level inspection report is shown enclosed in a box near the bottom of the figure. In a few isolated cases, there were not element-level reports available for every year, though NBI data was available.
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179 Bridge ID Year Built Facility Under Simple span (SS)
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180 constructed in 1987, less than 30 years ago, and the back-casting assessment for the bridge was initiated in 1998, when the bridge was only 11 years old. However, the RBI inspection interval was determined to be 24 months, due to damage modes related to corrosion susceptibility of the superstructure.
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181 total deck area) ; the soffit element was 95% in CS 1 and 5% in CS 2, and the deck cracking element was 100% in CS 1.
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182 identifier (D1, D2, etc.)
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183 sis, overall results, and specific examples selected to illustrate implementation of the technology. 3.8.1 Environments The environmental conditions considered in the analysis of bridges in Texas also differed depending on the damage mode being considered for the RAP.
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184 consequence, and determining the CF based on the redundancy of the bridge, as described in Section 3.6.9. Additional analysis was done to test the effect of including, or not including, the screening criteria for a bridge with a pin and hanger connection.
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185 a pin and hanger connection. In the back-casting analysis, the presence of a pin and hanger connection was used as a screening factor that made the OF high, regardless of other attributes of the bridge.
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186 element condition state and other factors. It should also be noted that the deck and substructure are generally in satisfactory condition, and the superstructure is in "Fair" condition due to the damage mode of section loss caused by corrosion, known to be a slow-acting deterioration mechanism.
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187 Figure 25. Risk matrices for steel bridges in Texas.
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188 for the extended inspection interval. These data were also analyzed without regard to the age criteria identified in Table 25.
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189 Conclusions, Recommendations, and Suggested Research This research developed inspection practices to meet the goals of (1) improving the safety and reliability of bridges and (2)
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190 non-redundant bridge members can be assessed using this approach, as illustrated in previous research (60)
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191 technology include conducting additional case studies in certain states to test and develop the technology further, developing training modules and software to support the technologies, and conducting a study focused on the economic and safety impacts of transitioning to the new inspection approach. To develop community support, the implementation plan proposes developing an oversight committee to monitor and develop the Guideline and address bridge-owner needs, and developing an effective communication strategy.
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192 involved in the process. The inspection planning process is more involved and complex under an RBI scheme relative to a calendar-based inspection planning process.
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193 4.2.1.5 Task 4. Develop a Communications Strategy As discussed, the proposed method is a significant change in paradigm for the bridge inspection community, and as such developing an effective communications strategy will be a key element of overall success.
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194 potential diminishment of safety or safety effects on a given bridge inventory associated with varying inspection intervals to match the needs for bridges. The study should also examine the safety effects of continuing the current status quo, addressing both the cost and safety implications of the "do-nothing" approach, including the effects of decreasing available resources for inspection.
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195 References 1. National Bridge Inspection Standards, in 23 CFR part 650.
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196 33. Straub, D
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197 Abbreviations ADT Average Daily Traffic ADTT Average Daily Truck Traffic BME Bridge Management Element BMS Bridge Management Software CF Consequence Factor CFR Code of Federal Regulation CIF Constraint-Induced Fracture CS Condition State CVN Charpy V-Notch DOT Department of Transportation EMAT Electromagnetic-Acoustic Transducer GPR Ground Penetrating Radar HPC High Performance Concrete HPS High Performance Steel HS High Strength IE Impact Echo IPN Inspection Priority Number IR Infrared Thermography LFD Load Factor Design LIBS Laser-Induced Breakdown Spectroscopy MT Magnetic Particle Testing NBE National Bridge Elements NBI National Bridge Inventory NBIS National Bridge Inspection Standards NDE Nondestructive Evaluation NHI National Highway Institute OF Occurrence Factor PCI Precast/Prestressed Concrete Institute PDF Probability Density Function POD Probability of Detection POF Probability of Failure PT Dye Penetrant Testing QA Quality Assurance QC Quality Control RAP Reliability Assessment Panel RBI Risk-Based Inspection SIP Stay-in-Place SI&A Structural Inventory and Appraisal TRL Technical Readiness Level UPV Ultrasonic Pulse Velocity UT-T Ultrasonic Thickness Gauge
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198 Developing Reliability-Based Inspection Practices: Oregon Pre-Stressed Bridges 199 Bridge/Deck/Spalling 199 Bridge/Deck/Rutting 200 Bridge/Deck/Cracking (Non-Corrosion Induced) 200 Bridge/Superstructure/Cracking (Shear)
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199 Bridge/Deck/Spalling Similar Items in Guideline A ttr ib ut es Ty pe o f A ttr ib ut es High Medium Low Remote Sc re en in g D eg re e of Se ve ri ty M ax Sc or e Source of data C.9 and C.12 Cracking/Spalling Condition #358 CS 4 Or #359 CS 4 or CS 5 #358 CS 3 Or #359 CS 3 #358 CS 2 Or #359 CS 2 #358 CS 1 Or #359 CS 1 M 15 #358 Deck Cracking Smart Flag #359 Soffit Cracking Smart Flag (Oregon Coding Guide Pages 79 and 80) C.10 and C.11 Delamination/Patch Condition >25%CS 4 or CS 5 11%-24% CS 3 <10% CS 2 CS 1 H 20 Concrete Decks and Slabs without an Overlay : #12 - #26 -#27 -#38 #52 -#53 Concrete Decks or Slabs with a Thin or Rigid Overlay: #18 - #22#44 - #48.
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200 Bridge/Deck/Cracking (Non-Corrosion Induced) Similar Items in Guideline A ttr ib ut es Ty pe o f A ttr ib ut es High Medium Low Remote Sc re en in g D eg re e of Se ve ri ty M ax Sc or e Source of data C.9 Cracking Condition Unsealed cracks exist in the deck that are of severe size (>0.060 in.
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201 Bridge/Superstructure/Fire Damage Reason(s) for Attribute Incidences of fire on or below a highway bridge are not uncommon.
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202 Bridge/Superstructure/Impact Similar Items in Guideline A ttr ib ut es Ty pe o f A ttr ib ut es High Medium Low Remote Sc re en in g D eg re e of Se ve ri ty M ax Sc or e Source of data D.3 Clearance Design <15' 15-16 >17' L 10 Item 10 NBI (Minimum vertical under clearance) L.8 High Water Screening Look at item 71 in NBI database- if the code is 3 the chance of over top is occasional Item 71 in NBI database (See also page 117 Oregon Coding Guide)
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203 Bridge/Substructure/Settlement Similar Items in Guideline A ttr ib ut es Ty pe o f A ttr ib ut es High Medium Low Remote Sc re en in g D eg re e of Se ve ri ty M ax Sc or e Source of data D.21 Footing Type Design Spread FTG on soil/unknown Foundation - Drill Shaft friction Pile /ETC If foundation was based on Rock/Piles we do not need to deal with other following attributes H 20 Bridge File D.22 Subsurface Condition Condition Slide zone, clay, silt, shale, gravel Limestone solid, Rock H 20 Bridge File C.3 Existing Settlement Condition Active (No monitor data) Occurred but arrested None H 20 Item #360 on page 81 Oregon Coding Guide S.10 Scour Rating Screening 4-6 (Oregon Scour Code)
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204 Texas Steel Bridge Attributes Summary 205 Bridge/Deck/Spalling 205 Bridge/Deck/Punch Through 206 Bridge/Deck/Cracking 206 Bridge/Deck/Delamination 207 Bridge/Superstructures/Sectionless 207 Bridge/Superstructures/Impact 208 Bridge/Superstructures/Fatigue Cracking 208 Bridge/Superstructures/Fire Damage 209 Bridge/Superstructures/Deflection Overload 209 Bridge/Substructures/Corrosion Damages (Spalling/Delamination/Cracking/Rust) 209 Bridge/Substructures/Settlement A P P E N D I X B
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205 Bridge/Deck/Spalling Similar Items in Guideline A ttr ib ut es Ty pe o f A ttr ib ut es High Medium Low R em ot e Sc re en in g D eg re e of Se ve ri ty M ax Sc or e Source of data D.11 Clear Cover Design <1" 1"-2" >2" H 20 Bridge file or Covermeter D.10 Overlay Design Yes No L 10 Item 108A NBI (See also pages 9 and 10 Texas Coding Guide) C.10 Delamination Condition Yes No H 20 Pages 5 and 8 Texas Coding Guide D.8 Mixed design (Water)
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206 Bridge/Deck/Cracking Similar Items in Guideline A ttr ib ut es Ty pe o f A ttr ib ut es High Medium Low R em ot e Sc re en in g D eg re e of Se ve ri ty M ax Sc or e Source of data C.9 Existing Cracking Condition Yes No H 20 Page 30 Texas Coding Guide(Deck Cracking) D.20 Construction Tech/Spec Design Bad All Other M 15 Bridge file L.3 Environment Loading Above I-20 All Else H 20 Bridge file D.18 and D.19 Design Details (Cold Joints, Skew)
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207 Bridge/Superstructures/Sectionless Similar Items in Guideline A ttr ib ut es Ty pe o f A ttr ib ut es High Medium Low R em ot e Sc re en in g D eg re e of Se ve ri ty M ax Sc or e Source of data L.3 Environment Loading Coast North of I-20 All Else H 20 Bridge file S.9 Existing SectionLoss Condition Yes No H 20 Pages 10, 11, and 15 Texas Coding Guide D.18 Deck Drainage onto Superstructure Design Yes No L 10 Bridge file C.22 Debris Condition Yes No L 10 Pages 23, 25, and 30 Texas Coding Guide (Pack Rust) C.4 Joint Leakage Condition Yes No L 10 Pages 23 and 24 Texas Coding Guide D.13 Built-Up Riveted Design Yes No H 20 Bridge file D.19 Deck Cold Joints Design Yes No M 15 Bridge file or Observation D.6 Age Exposure Design 50+ Other L 10 Item 27 NBI (Year Built)
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208 Bridge/Superstructures/Fire Damage Reason(s) for Attribute Incidences of fire on or below a highway bridge are not uncommon.
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209 Bridge/Superstructures/Deflection Overload Similar Items in Guideline A ttr ib ut es Ty pe o f A ttr ib ut es High Medium Low Remote Sc re en in g D eg re e of Se ve ri ty M ax Sc or e Source of data D.2 Load Posting Condition Cond Posting Des Post None H 20 Item 41 NBI -Previous* Overload Damage Condition Yes No H 20 Bridge file -Highway Ownership Condition Local State M 15 Item 22 NBI *
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210 Controlling Damage Modes for Sample Bridges 211 Table C1. Controlling damage modes for RBI analysis of bridges in Oregon (CF Case 4)
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211 Table C1. Controlling damage modes for RBI analysis of bridges in Oregon (CF Case 4)
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212 Table C2. Controlling damage modes for RBI analysis of bridges in Texas (CF Case 3)
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Abbreviations and acronyms used without definitions in TRB publications: A4A Airlines for America AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACI–NA Airports Council International–North America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers MAP-21 Moving Ahead for Progress in the 21st Century Act (2012) NASA National Aeronautics and Space Administration NASAO National Association of State Aviation Officials NCFRP National Cooperative Freight Research Program NCHRP National Cooperative Highway Research Program NHTSA National Highway Traffic Safety Administration NTSB National Transportation Safety Board PHMSA Pipeline and Hazardous Materials Safety Administration RITA Research and Innovative Technology Administration SAE Society of Automotive Engineers SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005)
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