Risk-Based Management Guidelines for Scour at Bridges with Unknown Foundations (2007)

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NCHRP 24-25 Page 5 Phase II Final Report 1. INTRODUCTION Bridges are skillfully designed to withstand the most common natural hazards (e.g. floods, erosion/scour, earthquakes), but the inherent uncertainty associated with natural phenomena requires bridge owners to regularly inspect bridges for signs of a problem (i.e. vulnerability to failure). While some aspects of a bridge’s vulnerability to failure are easy to inspect (e.g. visible cracks or corrosion), the condition of a bridge’s foundation is comparatively difficult and expensive to inspect. This difficulty is compounded if the buried portion of the bridge’s foundation is unknown. In other words, if the pertinent aspects of vulnerability – dimension, composition, or geologic context of the foundation, etc. – are unknown, it will be difficult to estimate the bridge’s vulnerability of failure during a hazard, even if the nature and severity of the stress is understood. Thus, the primary goal of this research is to develop guidelines that will help bridge owners manage bridges with unknown foundations with respect to their vulnerability to hazard-induced failure. While the general approach to risk management developed herein should be applicable to a variety to natural hazards, scour is the primary hazard used to evaluate the guidelines in this report. 1.1. Bridges with Unknown Foundations For context, the following two tables show the number of bridges with unknown foundations that were recorded in the National Bridge Inventory (NBI) at the end of 2005. The NBI coding guide published by FHWA (1) explains the various NBI items. Table 1 yields interesting information concerning the State and functional classification distribution of bridges with unknown foundations: „ Many States have less than a couple of dozen bridges with unknown foundations and seven States have no bridges with unknown foundations in the NBI

NCHRP 24-25 Page 6 Phase II Final Report database. This indicates that bridges with unknown foundations are not a significant issue for all States. „ Almost half the States report over 1,000 bridges with unknown foundations, indicating a potentially significant management issue. „ Along with the total number of bridges with unknown foundations, the functional classification of those bridges indicates the severity of the management issue. While Texas has the largest population of bridges with unknown foundations (9,113), only 33 of these are principal arterials (less than one half of one percent). Alaska on the other hand has only 151 bridges with unknown foundations, but a far higher percentage of these are principal arterials (37, or 25 percent). In other words, while Alaska has relatively few bridges with unknown foundations, it has more principal arterials with unknown foundations than Texas. Table 2 presents the temporal distribution of bridges with unknown foundations by functional classification. This table indicates the following temporal characteristics: „ Bridges built between 1950 and 1980 constitute a large proportion of the population of bridges with unknown foundations. This era also coincides with the construction of the interstate system. „ Sixty-nine principal arterials have been built between 2000 and 2005 that are identified as having unknown foundations. This is surprising given their functional importance and their very recent construction. This brief review of the NBI database shows that the scale of the problems that States face in managing bridges with unknown foundations vary significantly. Both the number and functional classification of these bridges contribute to the scale of the problem. It is equally troubling that this problem is still growing.

NCHRP 24-25 Page 7 Phase II Final Report Table 1 Numbers of Bridges with Unknown Foundations by State Rural Functional Classifications Urban Functional Classifications 01 02 06 07 08 09 11 12 14 16 17 19 State P r i n c i p a l A r t e r i a l - I n t e r s t a t e P r i n c i p a l A r t e r i a l - O t h e r M i n o r A r t e r i a l M a j o r C o l l e c t o r M i n o r C o l l e c t o r L o c a l P r i n c i p a l A r t e r i a l - I n t e r s t a t e P r i n c i p a l A r t e r i a l - O t h e r F r e e w a y s o r E x p r e s s w a y s O t h e r P r i n c i p a l A r t e r i a l M i n o r A r t e r i a l C o l l e c t o r L o c a l T o t a l s Alabama 4 70 79 503 843 1,662 4 5 25 39 55 164 3,453 Alaska 7 29 4 19 23 46 1 0 1 7 1 13 151 Arizona 0 0 0 1 0 33 0 0 1 3 5 25 68 Arkansas 0 1 11 48 4 10 0 0 1 2 0 2 79 California 4 23 112 318 305 993 3 9 71 84 60 126 2,108 Colorado 1 2 9 4 3 8 0 0 0 1 1 0 29 Connecticut 0 0 0 0 0 0 0 0 0 0 0 0 0 Delaware 0 0 0 0 0 0 0 0 0 0 0 0 0 DC 0 0 0 0 0 0 0 0 0 0 0 8 8 Florida 3 110 111 224 188 837 13 27 74 136 280 444 2,447 Georgia 3 346 434 1,227 565 1,780 0 32 178 288 188 406 5,447 Hawaii 0 0 0 0 0 0 0 0 0 2 0 8 10 Idaho 0 1 1 71 74 318 0 0 3 6 9 14 497 Illinois 0 0 0 1 0 1 0 0 0 0 0 0 2 Indiana 0 1 0 140 263 828 0 0 42 101 75 156 1,606 Iowa 0 1 3 92 256 1,371 0 0 0 11 6 30 1,770 Kansas 0 0 0 0 1 5 0 0 0 0 0 0 6 Kentucky 0 0 0 0 0 1 0 0 0 0 0 1 2 Louisiana 17 13 180 527 488 2,963 12 1 30 84 58 401 4,774 Maine 6 2 1 4 3 76 2 0 0 2 5 4 105 Maryland 0 0 0 0 0 4 0 0 0 0 0 1 5 Massachusetts 2 0 10 25 16 70 0 1 42 95 45 52 358 Michigan 3 36 43 157 13 360 2 2 9 10 11 11 657 Minnesota 0 0 2 16 24 161 0 0 0 4 2 7 216 Mississippi 0 16 11 1,205 187 4,790 0 0 32 54 101 137 6,533 Missouri 0 0 1 6 1 0 0 0 0 0 0 0 8 Montana 2 1 5 1 429 1,244 0 0 0 1 0 2 1,685 Nebraska 0 0 0 0 0 0 0 0 0 0 0 0 0 Nevada 0 0 2 1 3 24 0 0 1 10 1 3 45 New Hampshire 0 0 0 3 6 22 0 0 2 5 4 1 43 New Jersey 0 6 7 11 7 53 0 4 20 23 20 14 165 New Mexico 0 7 7 46 41 254 1 0 13 27 39 33 468 New York 0 0 0 1 1 13 0 2 7 9 4 12 49 North Carolina 0 29 95 464 700 3,949 0 2 30 81 77 379 5,806

NCHRP 24-25 Page 8 Phase II Final Report Rural Functional Classifications Urban Functional Classifications 01 02 06 07 08 09 11 12 14 16 17 19 State P r i n c i p a l A r t e r i a l - I n t e r s t a t e P r i n c i p a l A r t e r i a l - O t h e r M i n o r A r t e r i a l M a j o r C o l l e c t o r M i n o r C o l l e c t o r L o c a l P r i n c i p a l A r t e r i a l - I n t e r s t a t e P r i n c i p a l A r t e r i a l - O t h e r F r e e w a y s o r E x p r e s s w a y s O t h e r P r i n c i p a l A r t e r i a l M i n o r A r t e r i a l C o l l e c t o r L o c a l T o t a l s North Dakota 0 0 3 210 0 1,780 0 0 0 5 3 7 2,008 Ohio 0 2 1 13 23 222 0 0 2 1 5 12 281 Oklahoma 0 0 9 1 1 9 1 2 5 0 0 0 28 Oregon 5 58 90 425 235 801 4 2 18 50 51 56 1,795 Pennsylvania 0 0 0 0 1 7 0 0 0 0 0 0 8 Rhode Island 0 0 0 0 0 0 0 0 0 0 0 0 0 South Carolina 21 49 125 592 443 1,904 6 0 20 49 96 144 3,449 South Dakota 0 0 0 1 0 0 0 0 0 0 0 0 1 Tennessee 6 8 32 74 252 654 0 0 8 27 24 73 1,158 Texas 9 18 40 199 190 6,524 2 4 205 463 319 1,140 9,113 Utah 0 0 0 1 0 4 0 0 1 0 0 2 8 Vermont 0 2 5 29 26 155 0 0 2 4 9 6 238 Virginia 0 0 0 0 2 16 0 0 0 0 0 0 18 Washington 0 0 1 47 39 102 0 0 5 4 3 5 206 West Virginia 0 0 0 0 0 0 0 0 0 0 0 0 0 Wisconsin 0 0 0 0 0 0 0 0 0 0 0 0 0 Wyoming 0 0 1 0 43 347 1 0 0 3 7 13 415 Puerto Rico 0 0 21 70 40 77 0 0 9 23 36 36 312 Totals 93 831 1,456 6,777 5,739 34,478 52 93 857 1,714 1,600 3,948 57,638

NCHRP 24-25 Page 9 Phase II Final Report Table 2 Numbers of Bridges with Unknown Foundations by Age Rural Functional Classifications Urban Functional Classifications 01 02 06 07 08 09 11 12 14 16 17 19 Year Built P r i n c i p a l A r t e r i a l - I n t e r s t a t e P r i n c i p a l A r t e r i a l - O t h e r M i n o r A r t e r i a l M a j o r C o l l e c t o r M i n o r C o l l e c t o r L o c a l P r i n c i p a l A r t e r i a l - I n t e r s t a t e P r i n c i p a l A r t e r i a l - O t h e r F r e e w a y s o r E x p r e s s w a y s O t h e r P r i n c i p a l A r t e r i a l M i n o r A r t e r i a l C o l l e c t o r L o c a l T o t a l s 1900-1904 0 0 1 20 65 534 0 0 14 14 10 32 690 1905-1909 0 0 5 8 15 96 0 0 3 10 5 18 160 1910-1914 0 0 3 17 42 240 0 0 9 21 20 30 382 1915-1919 1 1 7 36 40 324 0 1 14 24 9 39 496 1920-1924 0 31 59 148 92 679 0 0 42 42 32 67 1,192 1925-1929 1 36 91 154 112 532 0 3 47 55 46 82 1,159 1930-1934 0 51 103 256 180 1,131 0 6 42 72 50 109 2,000 1935-1939 0 50 106 290 209 1,347 1 1 62 85 69 105 2,325 1940-1944 1 41 98 256 268 1,270 0 0 31 31 49 103 2,148 1945-1949 5 44 96 275 207 898 1 3 31 50 42 66 1,718 1950-1954 2 95 125 591 623 2,470 0 4 67 104 95 219 4,395 1955-1959 8 97 186 946 630 2,534 5 4 49 149 132 260 5,000 1960-1964 23 51 102 797 707 3,824 4 10 56 150 192 499 6,415 1965-1969 24 44 86 736 650 3,357 14 12 73 125 155 400 5,676 1970-1974 11 74 60 606 535 3,159 5 12 60 167 159 453 5,301 1975-1979 7 27 53 532 432 3,038 4 10 31 145 115 420 4,814 1980-1984 1 35 45 365 359 2,596 7 2 51 109 108 273 3,951 1985-1989 1 36 41 258 236 2,473 7 4 67 161 123 306 3,713 1990-1994 0 51 77 268 244 2,353 2 14 52 74 76 210 3,421 1995-1999 4 26 70 181 169 1,701 0 2 21 55 57 146 2,432 2000-2004 4 35 28 81 90 1,101 2 4 20 21 31 89 1,506 Totals 93 831 1,456 6,777 5,739 34,478 52 93 857 1,714 1,600 3,948 57,638

NCHRP 24-25 Page 10 Phase II Final Report 1.2. Performance-Based versus Traditional Design Practice This report focuses on estimating the vulnerability of bridges with unknown foundations to hazards that might cause these bridges to fail unexpectedly, which is obviously an important aspect of performance. The main problem in estimating a bridge’s vulnerability to failure, however, is the inherent uncertainty about its performance concerning infrequent or unobserved hazards – hence the notion of probability and risk. The literature review phase of this study highlighted several risk-based methodologies (see Appendix A), some of which were useful in developing guidelines for managing bridges with unknown foundations. The literature review for this report also catalogued a number of concerns about establishing performance standards (see Appendix B) that might affect bridge management. Since bridge performance obviously relates to the design of bridges, it is interesting to note that design professionals are increasingly implementing performance-based design approaches to design. These applications include structural and seismic engineering, fire protection, etc. The purpose of performance-based design is to provide methods for siting, designing, constructing and maintaining facilities, such that they are capable of predictable performance levels with a specified (minimum) reliability. Performance may be measured in terms of the amount of damage (e.g., displacement), which if realized compromises stated functional or life-safety goals. A fundamental foundation of performance-based design is the notion that engineering tools can be used to analytically evaluate the performance of a structural system and consider the uncertainties in loading and response (e.g., performance) such that a performance goal can be achieved. As part of a performance-based design process, performance goals are established in conjunction with the facility owner. A performance goal is a statement about a performance level (i.e., a functional objective) the owner of a facility wants to achieve and the likelihood

NCHRP 24-25 Page 11 Phase II Final Report that unsatisfactory performance (e.g., the functional objective is not met) will occur. Typically, multiple performance levels are defined that represent alternative levels of response/damage. For instance, a functional objective might be stated as; only minor damage occurs (under the load conditions of interest), with the facility retaining its strength and configuration and is available for normal use. The risk of extended closure should be negligible, following post-event inspection. The ability to achieve a performance goal requires a number of steps. Initially, a performance goal must be translated into terms of physical response/performance for the structural system (e.g., displacement or offset) that can be assessed by engineering methods. In addition, evaluation methods (engineering tools and design methods) must account for the uncertainties associated with the occurrence of load events and estimating facility performance, such that desired levels of reliability can be achieved. From an engineering design perspective, procedures are required to systematically and explicitly incorporate the desired levels of reliability dictated by different performance goals. In contrast, traditional methods of engineering design have been based on an assessment of building performance at code allowable limits (e.g., stress levels), and not the response/performance that would be expected. Whereas factors of safety (or safety margins) are incorporated in traditional design methods, they have not been established on the basis of actual material properties or on the expected performance. As a result, traditional design procedures do not provide the designer or the facility owner with insight to the performance (physical or functional) that would occur (under a given load condition), or a sense of the risk that is being implicitly accepted. Further, traditional methods of design tend to be more prescriptive and thus do not readily accommodate facility-specific factors that influence performance and reliability. Alternatively, performance-based methods offer

NCHRP 24-25 Page 12 Phase II Final Report increased flexibility in the design process and contribute to cost-efficiencies in a design that meet facility specific requirements and desired performance goals. 1.3. Report Overview This introduction primarily underscores the extent of the problem regarding bridges with unknown foundations and how it relates to bridge design. Section 2 describes how this uncertainty limits the ability to predict hazard-induced bridge failures, and outlines a general approach for using risk to select a pertinent, cost-effective management plan for bridges with unknown foundations. The remaining sections (3 through 6) describe how this approach applies to a common water-related bridge hazard known as scour. Note also that section 2 also serves as a summary of the research approach used to develop the “Scour Risk Management Guidelines.” For the sake of conciseness all of the original literature reviews and stakeholder interviews that are mentioned in this report appear in the auxiliary appendices.