Blast-Resistant Highway Bridges: Design and Detailing Guidelines (2010)

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134 9.1 Summary of Research Program Past terrorist events consisting primarily of explosive at- tacks, including 60% of those against highway infrastructure between 1920 and 2000 (Jenkins and Gersten, 2001), highlight the need for blast-resistant structures. Although the chances of a terrorist attack against most structures are typically as- sumed to be very small, the economic and socio-economic consequences can be extremely high. Therefore, the National Cooperative Highway Research Program funded NCHRP Project 12-72 (Blast-Resistant Highway Bridges: Design and Detailing Guidelines) to address this concern. The results of this research program are described in this report. Columns provide the main support for nearly all bridges independent of the type of superstructure, and as such they play a major role in the response of bridges to explosions. The loss of a column will most likely lead to the loss of a bridge’s integrity. Thus, to implement the design of bridges for security, there was a need for experimental and analyt- ical research on blast-loaded bridge columns to evaluate the effectiveness of current blast-resistant design guidelines for buildings applied to bridges and to assess whether or not bridge seismic detailing can provide adequate protection for blast-loaded bridges. Furthermore, there was a need to develop new design and detailing guidelines tailored specifically for bridges subjected to explosions, and the main goals of this research were as follows: • Investigate the response of concrete bridge columns sub- jected to blast loads, • Develop blast-resistant design and detailing guidelines for highway bridge columns, and • Develop analytical models of blast-load distribution and the resulting column response that are validated by exper- imental data. The research program included two different phases of testing. In Phase I, small-scale blast tests on square and round non-responding columns were carried out near Vicksburg, MS, with support provided by the Engineering Research and Development Center of the U.S. Army Corps of Engi- neers. These tests were critical for determining how loads interact with slender structural components, and they pro- vided valuable data on how blast pressures vary with time and position at various locations on the tested columns under a variety of blast scenarios. In Phase II, large-scale blast tests were performed at a remote testing site in central Texas by the University of Texas at Austin with the help of Protec- tion Engineering Consultants and the Southwest Research Institute. The Phase II test program included the fabrication of ten half-scale columns with five main test variables, designed to represent a national survey of current bridge column specifi- cations and the AASHTO LRFD Bridge Design Specifications (2007). Seismic design and detailing provisions were used in designing one column, while two columns included new blast-resistant design details. Ten half-scale, small standoff and six half-scale, local dam- age blast tests were completed in this phase of the experi- mental program (six columns were tested twice). The goal of the small standoff tests was to observe the mode of failure (i.e., flexure or shear) for eight different column designs. The objective of the local damage tests was to observe the spall and breach patterns of blast-loaded concrete columns. Each blast test evaluated the relative importance of the five main test vari- ables, including scaled standoff, column geometry, amount of transverse reinforcement, type of transverse reinforcement, and splice location. The columns were instrumented with six strain gauges that collected data during the small standoff tests. Strain data were used to verify boundary conditions and blast-load distribution for each small standoff test. The ex- perimental observations and strain data were used to develop design and detailing guidelines for blast-loaded reinforced concrete highway bridge columns. C H A P T E R 9 Summary, Conclusions, and Recommendations

135 Complementing the experimental testing program, ana- lytical research to model the response of blast-loaded bridge columns was carried out at various levels of fidelity through- out the research program. Models included simplified meth- ods for predicting loads, such as design charts and equations, as well as software provided by the U.S. Army Corps of Engi- neers (BEL). Simplified methods of response modeling con- sisted primarily of single-degree-of-freedom dynamic analyses. These simplified load and response models established the foundation for the recommended design procedure. Aside from these models, detailed finite element models were utilized to represent specific tests within the experimental program. Once these high-fidelity models were validated, parametric studies were carried out to extend the results beyond the range of parameters that could be tested physically. Finally, compil- ing the results of the analytical models and collected test data, design and detailing provisions were proposed. 9.2 Conclusions and Recommendations The conclusions and recommendations presented herein are based on the experimental evidence and numerical sim- ulation results gathered during the course of this research study. The primary conclusions leading to the proposed blast- resistant design and detailing guidelines for highway bridge columns are summarized in the subsections below. 9.2.1 Summary of Small Standoff Blast Tests The small standoff tests enabled the observation of the mode of failure for eight different concrete column designs for vari- ous blast-load scenarios. During the small standoff tests, three columns exhibited significant shear deformations at the base, including Columns 1A2, 3A, and 3-Blast. The other seven columns experienced a combination of shear and flexural cracking and exhibited less significant shear deformations at the base. The most common mode of failure was shear; how- ever, the majority of columns had adequate shear capacity, ex- perienced essentially no spall or breach, and were very robust. Column 1A2, an 18-in. circular gravity column, experi- enced a shear failure at the base; however, by increasing the standoff (1A1) or using continuous spiral reinforcement (1B), the column’s response to close-in blast loads improved. Col- umn 1A1, identical to Column 1A2, was tested at a larger stand- off distance than Column 1A2 and experienced only minor damage. Continuous spiral reinforcement also improved the column response to close-in blast loads as demonstrated by the performance of Column 1B. One finding from the exper- imental test program is that, in cases where spiral reinforce- ment is not used, discrete ties can be made to perform well if adequate anchorage is provided. The 30-in. diameter blast-detailed column (2-Blast) was exposed to a more intense loading than its less reinforced counterparts while experiencing a similar response. Thus, given the same blast loading scenario, the circular blast-detailed col- umn would be expected to perform better than the respective gravity or seismic columns. The 30-in. square blast-detailed column (3-Blast) experienced extensive shear deformation at the base; however, it required very large loads to achieve this level of damage. Therefore, the square column with blast- resistant reinforcing details would be expected to perform better than other columns that had less reinforcement when tested at similar loads. 9.2.2 Summary of Local Damage Tests The local damage tests allowed the observation of spall and breach patterns of blast-loaded concrete columns. Only two of the six columns (1A1 and 2A2) experienced a complete breach. Columns 2A1 and 2B experienced a significant loss of the concrete core, while the remaining two columns stayed intact. Column 2-Blast and 3A exhibited spalling of the side concrete cover, which was not initially expected. 9.2.3 Summary of Design Guidelines The intent of the design categories proposed under this re- search is to provide adequate detailing for bridge columns as the structural demand and design threat increase. All columns tested in this experimental program fell into Design Cate- gory C (i.e., significant threat scenario) and experienced a range of damage levels depending on the scaled standoff. Columns with a small scaled standoff were exposed to a severe blast load that resulted in the formation of plastic hinges, spalling of concrete cover, and in some cases, total breach of the column. Decreasing the design threat by providing sufficient standoff distance from bridge columns is a safe alternative to increasing the design category and detailing requirements. In general, a higher scaled standoff requires less stringent detailing requirements because of the lower inten- sity of the blast loading. One of the best ways to decrease the design loads (and hence the design category) is to increase the standoff distance with physical deterrents such as bollards, security fences, and vehi- cle barriers. Maximizing the standoff distance is the easiest and often the least costly method to achieve the appropri- ate level of protection for a bridge. Therefore, if access to the columns is sufficiently limited, the design standoff dis- tance can be increased, which will decrease the effects of blast loads on the columns and the associated design category. When standoff distance is not available to avoid Design Cate- gory C, the design and detailing provisions described below should be met.

136 To the extent practical, the cross-sectional shape of a blast- loaded column should be selected to minimize the intensity of the blast load. Cross-sectional shape affects how a blast load interacts with a column. The use of a circular column is an effective way of decreasing the blast pressure and im- pulse relative to a square or rectangular column of the same size, and the decrease in impulse can be up to 34% for small scaled standoffs. Therefore, the use of a circular column cross-section over a square cross-section is recommended. With proper detailing, however, columns with rectangular and square cross-sections can be made to be blast resistant. The cross-sectional dimensions of a column also have a major impact on column capacity in the case of a close-in blast, and this parameter controls the onset of a breaching failure. Consequently, a minimum column diameter of 30 in. is rec- ommended for columns subjected to close-in blast loads. Experimental observations show that continuous spiral re- inforcement performs better than discrete hoops with standard hooks for small standoff threats. To avoid anchorage pullouts and to improve the performance of blast-loaded columns with discrete hoops or ties, longer hook lengths than currently specified should be used. Also, to improve the energy absorp- tion and dissipation capacity of potential plastic hinges, the minimum amount of confinement reinforcement should be increased by 50% over that which is currently specified for seismic designs in the current AASHTO LRFD (2007). This increased level of detailing should extend over the entire col- umn height to help account for the variability associated with different threat scenarios. The splicing of longitudinal reinforcement should be avoided when feasible. Locating splices away from contact charges can help minimize localized blast damage. As stated previously in the report, it is not possible to design all bridge columns to resist all possible threats. An acceptable level of risk must be accepted for these extreme load cases. If a large enough quantity of explosive is placed close enough to a bridge column, failure is to be expected. 9.3 Recommendations for Future Work The recommendations for blast-resistant design and detail- ing of reinforced concrete highway bridge columns proposed in this research were based on experimental data gathered during a two-phase testing program and detailed analytical studies. It is desirable to obtain additional experimental data that can be used to further validate the proposed recommen- dations. Also, more research is needed to better understand the spall and breach patterns for concrete columns subjected to close-in blasts. Spalling on the column sides was noted in this experimental program with six half-scale blast tests. Prior to these tests, columns subjected to close-in blast loads were assumed to perform similarly to walls, with spalling on the front and back face. According to Winget et al. (2004), “Retrofit techniques should enhance concrete confinement, increase bending resis- tance and ductility, add protection against breaching, or a com- bination of these effects.” To improve a column’s response to blast loads, increasing the amount of transverse steel is a viable option that will increase a column’s ductility and con- finement. This added ductility and confinement allows for the concrete core to stay intact and continue providing sup- port to the superstructure once plastic hinging starts to occur. As the amount of transverse steel increases, however, the ease of construction decreases. Placement of reinforcing steel becomes problematic, which increases the probability of voids in the concrete. To avoid such problems, other blast-resistant design alternatives that should be considered include the use of concrete-filled tubes, fiber-reinforced concrete, mechani- cal couplers at splice locations, and external retrofits. Concrete-filled steel tubes would eliminate the difficulties in construction seen with regular reinforced concrete columns. More research is needed, however, to better understand connections of these members to the foundation and super- structure. Bruneau et al. (2006) successfully demonstrated the applicability of this design approach for blast-loaded columns tested at a small scale, but additional study is needed to deter- mine how feasible this approach is for large-diameter columns for which typical steel shapes are not available. A concrete- filled tube may be an economical solution if labor is expensive and steel prices are low. Fiber-reinforced concrete, in combi- nation with a typical gravity-reinforced column design, may be a viable alternative to a heavily reinforced concrete column. Fiber-reinforced concrete aides in the prevention of con- crete spall by reinforcing concrete away from the location of reinforcing bars. Additional research is needed, however, to validate the performance of fiber-reinforced concrete columns subjected to blast loads and to determine if they provide a cost- effective option for such load cases. Mechanical couplers at splice locations in reinforced con- crete columns subjected to close-in blasts may reduce the chances of column failure associated with discontinuous longitudinal reinforcement. According to Zehrt et al. (1998), “Mechanical splices must be capable of developing the ultimate dynamic strength of the reinforcement without reducing its ductility before they can be used in blast resistant concrete elements.” Therefore, additional blast testing of large-scale concrete columns with mechanical couplers is recommended. External retrofits can also be employed to improve the response of reinforced concrete columns to close-in blasts. The use of fiber-reinforced polymer wraps and steel jacketing are two

potential solutions to improve concrete confinement and pro- tection against spalling and breach. The research described in this report focused primarily on the response of reinforced concrete bridge columns, but additional research on blast-resistant design is needed for other types of bridge components. It should be noted that the field of blast-resistant bridge design is relatively new to the general bridge engineering community, and while the research pre- sented in this study is believed to advance the state of prac- tice considerably, much additional research is needed to mature this field to the current level of the design of bridges for other types of extreme loads such as earthquakes. 137