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

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88 6.1 Overview The Phase I test program included eight small-scale blast tests at four sets of standoff. The experimental observations and ana- lytical models indicate how cross-sectional shape, standoff, and geometry between the charge and column positions influence blast pressures on the front, side, and back faces of bridge columns. Phase II of the experimental testing program included ten half-scale, small standoff and six half-scale, local damage blast tests on eight different column designs. Column specimens were constructed with consideration given to five main test variables, including scaled standoff, column geometry, amount of transverse reinforce- ment, type of transverse reinforcement, and splice location. The following design and detailing recommendations have been devel- oped from the experimental observations, test data, and corresponding analytical models. 6.2 Risk Assessment Guidelines for Bridges Section 2.7 of the AASHTO LRFD guidelines, given below, focuses on risk assessment and design demand. Type, geometry, and importance of a bridge should be considered when completing a vulnerability assessment. C H A P T E R 6 Design and Detailing Guidelines 2.7 BRIDGE SECURITY 2.7.1 General An assessment of the importance of a bridge should be conducted during the planning of new bridges and/or during rehabilitation of existing bridges. This should take into ac- count the social/economic impact of the loss of the bridge, the availability of alternate routes, and the effect of closing the bridge on the security/defense of the region. For bridges deemed important, a formal vulnerability study should be conducted, and measures to mitigate the vulnera- bilities should be considered for incorporation into the design. C2.7.1 At the time of this writing (Winter 2008), there are no uni- form procedures for assessing the importance of a bridge to the social/economic and defense/security of a region. Work is being done to produce a uniform procedure to prioritize bridges for security. In the absence of uniform procedures, some states have de- veloped procedures that incorporate their own security pri- oritization methods which, while similar, differ in details. In addition, procedures to assess bridge importance were devel- oped by departments of transportation in some states to assist in prioritizing seismic rehabilitation. The procedures established for assessing bridge importance may also be used in conjunction with security considerations. Guidance on security strategies and risk reduction may be found in the following documents: Science Applications In- ternational Corporation (2002), The Blue Ribbon Panel on Bridge and Tunnel Security (2003), Winget (2003), Jenkins (2001), Abramson (1999), and Williamson (2009).

89 6.3 Blast-Load Guidelines After the completion of a risk and vulnerability assessment, a bridge component should be designed for the appropriate blast load. The following section defines important blast load variables. C2.7.2 It is not possible to protect a bridge from every conceivable threat. The most likely threat scenarios should be determined based on the bridge structural system and geometry and the identified vulnerabilities. The most likely attack scenarios will minimize the attacker’s required time on target, possess sim- plicity in planning and execution, and have a high probabil- ity of achieving maximum damage. The level of acceptable damage should be proportionate to the size of the attack. For example, linear behavior and/or local damage should be expected under a small-size attack, while significant permanent deformations and significant damage and/or partial failure of some components should be accept- able under larger size attacks. The level of threat and the importance of the bridge should be taken into account when determining the level of analysis to be used in determining the demands. Approximate meth- ods may be used for low-force, low-importance bridges, while more sophisticated analyses should be used for high-force threats to important bridges. 2.7.2 Design Demand Bridge owners should establish criteria for the size and loca- tion of the threats to be considered in the analysis of bridges for security. These criteria should take into account the type, geometry, and importance of the structure being considered. The criteria should also consider multi-tier threat sizes and de- fine the associated level of structural performance for each tier. Design demands should be determined from analysis of a given size design threat, taking into account the associated per- formance levels. Given the demands, a design strategy should be developed and approved by the bridge owner. 3.15 BLAST LOADING 3.15.1 Introduction Where it has been determined that a bridge or a bridge component should be designed for intentional or uninten- tional blast force, the following should be considered: • Size of explosive charge, • Shape of explosive charge, • Type of explosive, • Standoff distance, • Location of the charge, • Possible modes of delivery and associated capacities (e.g., maximum charge weight will depend upon vehicle type and can include cars, trucks, ships, etc.), and • Fragmentation associated with vehicle-delivered explosives. C3.15.1 The size, shape, location, and type of an explosive charge determine the intensity of the blast force produced by an ex- plosion. For comparison purposes, all explosive charges are typically converted to their equivalent TNT charge weights. Standoff refers to the distance between the center of an ex- plosive charge and a target. Due to the dispersion of blast waves in the atmosphere, increasing standoff causes the peak pressure on a target to drop as a cubic function of the distance (i.e., for a given quantity of explosives, doubling the standoff distance causes the peak pressure to drop by a factor of eight). The location of the charge determines the amplifying effects of the blast wave reflecting from the ground surface or from the surfaces of surrounding structural elements. The location of the charge also determines the severity of damage caused by fragments from the components closest to the blast trav- eling away from the blast center. Information on the analysis of blast loads and their effects on structures may be found in Biggs (1964), Baker et al. (1983), Department of the Army (1990), Bulson (1997), and Depart- ment of the Army (1986). Section 3.4.1 of the AASHTO LRFD considers blast loading as an extreme event. Bridge scour associated with normal flow only needs to be considered in combination with blast loads.

90 2.6.4.4.2 Bridge Scour As required by Article 3.7.5, scour at bridge foundations is investigated for two conditions . . . When combined with blast loading, only the scour associ- ated with normal flow should be considered. C2.6.4.4.2 A majority of bridge failures in the United States and else- where are the result of scour . . . The probability of blast loading taking place at the time the scour associated with the design or check floods exists is assumed to be quite small. For this reason, only the scour associated with normal flow needs to be considered when investigating the effects of blast loading. 6.4 Design and Detailing Guidelines for Columns After the completion of a risk assessment, the design category for a blast-loaded, reinforced concrete bridge column can be established. Recommended guidelines for Section 4.7.6.2 of the AASHTO LRFD given below define each blast design category as a function of the scaled standoff, Z. Design and detailing guidelines for each category are further described below. 4.7.6.2 Substructure Blast Design Categories For the purpose of designing substructure components for blast loads, the substructures shall be classified as Blast Design Category A, B or C based on the value of the scaled standoff, Z, as follows: • For Blast Design Category A: Z > 3 • For Blast Design Category B: 3 ≥ Z > 1.5 • For Blast Design Category C: Z ≤ 1.5 in which: (4.7.6.2-1) where: Z = Scaled standoff (ft/lb1/3) R = Owner-specified standoff distance (ft) W = Owner-specified charge Weight (lbs TNT equivalent) Detailing of the transverse reinforcement for blast loading shall satisfy: • For Blast Design Category A: No additional requirements beyond those for other applicable loads. • For Blast Design Category B: All requirements for Seismic Zones 3 and 4 as specified in Articles 5.10.11.4.1c, 5.10.11.4.1d and 5.10.11.4.1e. The provisions of Article 5.10.2.3 shall also apply. • For Blast Design Category C: All requirements for Seis- mic Zones 3 and 4 as specified in Articles 5.10.11.4.1c, Z R W = 1 3/ C4.7.6.2 The value of the scaled standoff, Z, is an indication of the intensity of the blast loading. In general, the higher the value of this parameter, the lower the expected intensity of blast loading and the less stringent the detailing requirements need to be for concrete columns. Physical measures to increase the standoff distance are good design practice. Bridge owners should establish criteria for the size and location of the threats to be considered in the analysis of bridges for security as specified in Article 2.7.2. These criteria should take into account the type, geometry, and importance of the structure being considered. TNT equivalency is the ratio of the weight of an explosive to an equivalent weight of TNT. TNT equivalencies are used in the majority of research on blast effects to relate the energy output of common explosives to that of TNT. A table of av- eraged free-air equivalent weights for different explosives is provided in Tedesco (1999) or the U.S. Army’s Structures to Resist the Effects of Accidental Explosions (Department of the Army, 1990). Columns tested in Design Category C experienced a range of damage levels depending on the scaled standoff, Z (Williamson, 2009). Columns with a small scaled standoff were exposed to a severe blast load that resulted in the forma- tion of plastic hinges, spalling of concrete cover, and, in some cases, total breach of the column (breach is defined as com- plete loss of concrete through the depth of the cross-section). Typically for Z < 0.5, local damage failures such as breaching control (Williamson, 2009). Columns with a large scaled stand- off within Design Category C experienced a combination of shear and flexural cracking. Columns in Design Categories A

91 5.10.11.4.1d and 5.10.11.4.1e and as modified by Article 5.10.12.3. The provisions of Article 5.10.2.3 shall also apply. 4.7.6.3 Substructure Columns Blast Design Shear Force Where substructure columns satisfy the requirements for Blast Design Categories A and B, as specified in Article 4.7.6.2, specific blast loading magnitudes and distributions need not be considered in the design and detailing. Detailing require- ments of Article 5.10.12 shall apply. Where the transverse reinforcement of substructure columns is designed and detailed for Blast Design Category C, as specified in Article 4.7.6.2, they shall be designed and de- tailed to resist the shear force and moment resulting from a plastic analysis of the substructure column. Sufficient shear capacity shall be provided to assure that flexure controls. and B will be required to withstand less intense loadings than those in Category C, and the design requirements for blast are reduced accordingly. Under these conditions, less stringent detailing requirements are needed to achieve acceptable per- formance. C4.7.6.3 Substructure columns subjected to significant blast loads are expected to develop a plastic failure mechanism and, hence, plastic analysis of the substructure units is appropriate. Using the base shear force from the static application of the load, the required pitch of transverse reinforcement can be determined by modifying the shear design equations to ac- count for strain rate effects (ASCE, 1997), and solving for the spacing. The plastic moment, Mp, which is equal to the flex- ural resistance of the cross-section, Mn, should also account for the dynamic material strength, with dynamic increase factors for strain rate effects. Dynamic increase factors can be found in the U.S. Army’s Structures to Resist the Effects of Accidental Explosions (Department of the Army, 1990) and ASCE (1997). The maximum shear demand on a substructure column is a function of the boundary conditions and load distribu- tion. Boundary conditions should be determined to corre- spond to the geometry of the column in question and its connections to adjacent components. Blast load distribu- tion is a function of the standoff distance and the cross sec- tion of the column. The expected failure mechanism and the estimated load distribution shall be taken into account when determining the shear demand on substructure columns. The intent of the design categories is to provide adequate detailing for bridge columns as the structural demand and design threat increases. 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 detail- ing requirements because of the lower intensity of the blast loading. All columns tested during the experimental program fell into Design Category C 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. Columns with a large scaled standoff but still within Design Category C experienced a combination of shear and flexural cracking. Columns in Design Cat- egories A and B will be exposed to less intense loadings than those in Category C; thus, the design requirements for blast are reduced accordingly. 6.4.1 Design Category A Highway bridge columns in Design Category A do not require any design modifications for blast resistance and should follow the design and detailing provisions required by the AASHTO LRFD Bridge Design Specifications (2007) for the normally antici- pated loading conditions. Therefore, Category A columns should be designed ignoring blast loads.

92 5.10.12.2 Blast Design Category A Blast loads should not be considered in the design and de- tailing of substructure columns designed for Blast Design Category A, as specified in Article 4.7.6.2. C5.10.12.2 Due to the low intensity of the blast loading on substruc- ture columns classified as Blast Design Category A, such columns are expected to perform satisfactorily when de- signed and detailed for other applicable loads without direct consideration of blast effects. 6.4.2 Design Category B The design of highway bridge columns in Category B is based on the seismic design and detailing provisions of the AASHTO LRFD Bridge Design Specifications (2007), though there are some notable differences. The Caltrans Seismic Design Criteria (Caltrans, 2006) and Bridge Design Specifications (Caltrans, 2003) are additional resources for design and detailing require- ments. There are only two exceptions to the above documents. First, a more stringent extension length of hooks when using discrete ties or hoops (see Chapter 5) is recommended. Hooks should consist of a 135° bend, plus an extension of not less than the larger of 15.0 db or 7.5 in. Second, transverse reinforcement detailing for the plastic hinge region should be applied over the entire column height. The additional transverse reinforcement over the entire column height results from the uncertainty associated with potential blast locations. Both of these exceptions are noted in Section 5.10.2.3 of the proposed design guidelines. The more stringent hook length requirement performed satisfactorily when tested for seismic loading by Bae and Bayrak (2008) on full-scale concrete columns, where the opening of seismic discrete ties using hooks with a 135° bend, plus an exten- sion of 8.0 db, was first demonstrated. Blast and seismic loads are both dynamic loads that induce dynamic structural responses and inelastic behavior. To allow the formation of plastic hinges and achieve a favorable mode of failure (flexure), adequate anchorage into the core concrete must be provided over the entire column height. 6.4.3 Design Category C Highway bridge columns in Category C should, as a minimum, meet the design requirements for Category B columns. The following requirements place more stringent design and detailing guidelines on blast-loaded columns to further improve col- umn survivability. The guidelines should be implemented in the following order of effectiveness: standoff, column geometry, amount of transverse reinforcement, type of transverse reinforcement and anchorage, and splice location. 6.4.3.1 Increase Standoff One of the best ways to improve the performance of blast-loaded, reinforced concrete highway bridge columns is to increase the standoff distance with physical deterrents such as bollards, security fences, and vehicle barriers. If access to the columns is sufficiently limited, the design standoff distance can be increased, which will decrease the effects of blast loads on columns and the associated design category. If only vehicle standoff is limited, small charges may still be placed in direct contact with a column, potentially causing local- ized damage or breaching of the concrete core. Results from local damage tests illustrate that increasing the standoff from the face of the structural column by as little as a few inches can increase a column’s chance of survival substantially in a situation in- volving close-in blast loads. Aside from physical barriers, standoff from a structural member can be increased by adding a sac- rificial cover or architectural feature around the structural column. 6.4.3.2 Column Geometry The following recommendations regarding cross-sectional shape and dimension can improve the response of reinforced con- crete columns subjected to blast loads. 6.4.3.2.1 Cross-Sectional Shape. To the extent practical, the cross-sectional shape of a blast-loaded column should be se- lected to minimize the intensity of the blast load. Cross-sectional shape affects how a blast load interacts with a column. The

93 use of a circular column is an effective way of decreasing the blast pressure and impulse on a column 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 (see Chapter 5). Therefore, the use of a circular column cross-section over a square cross-section is recommended, as stated in Section 2.7.3 of the AASHTO LRFD recommended guidelines. Figure 84. Importance of cross-sectional dimension: a) 18-in. diameter (Column 1A2), b) 30-in. diameter (Column 2A2). 2.7.3 Selection of Component Geometry To the extent practical, the cross-sectional shape of the components subjected to blast loading should be selected to minimize the effects of the blast load. C2.7.3 For components of the same width exposed to the same blast conditions, i.e., same charge weight and standoff dis- tance, the intensity of blast loading may differ depending on the cross-sectional shape of the component (Williamson, 2009). For example, the blast pressure on square and rectan- gular columns is higher than on circular columns of the same width. The decrease in impulse on circular columns relative to square columns can be up to 34% for small scaled stand- offs. Selecting shapes that result in reduced blast load will minimize the damage. 6.4.3.2.2 Cross-Sectional Dimension. Cross-sectional dimension also affects blast-wave propagation and the resulting spall patterns. Figure 84 illustrates two identically detailed columns with the only variables being column diameter and standoff. Column 2A2 was tested with a similar charge weight at a smaller standoff distance than Column 1A2, and despite the smaller scaled standoff, it sustained less damage. Therefore, increasing the column diameter improves the shear capacity of the col- umn, minimizing the effects of detailing. A minimum cross-sectional dimension of 30 in. is recommended to improve the re- sponse of columns subjected to close-in blast loads. (a) (b)

94 6.4.3.3 Detailing and Design If the standoff distance cannot be increased to decrease the effects of blast loads on columns sufficiently, the following design and detailing provisions are recommended: increasing the amount of transverse reinforcement, requiring continuous spiral reinforcement or discrete hoops with sufficient anchorage, and avoiding splices. Additional details for these design provisions are given below. 6.4.3.3.1 Amount of Transverse Reinforcement. Experimental observations show that increasing the volumetric reinforce- ment ratio is beneficial to the response of blast-loaded columns because it increases the column ductility and shear capacity. Di- rect shear is a major concern for blast-loaded columns, and adequate shear capacity is needed to ensure that columns fail in a ductile manner (see Chapter 5). Accordingly, to meet the high shear demands placed on a blast-loaded column in Category C, more stringent transverse reinforcement requirements than those used for seismic design are needed (see Chapter 5). Equation 21 is recommended as the minimum transverse reinforcement ratio for all circular blast-designed columns, while Equation 22 is recommended as the minimum area of transverse reinforcement for all rectangular blast-designed columns. Columns meeting these minimums tested at a small standoff sustained minor and extensive damage; however, the core still remained intact and the column could still carry load. Essentially, 50% more confinement is recommended for blast-designed columns over current seismic provisions to improve the ductility and energy dissipation capacity of the cross-section. (21) (22) where: f ′c = specified compressive strength of concrete at 28 days (psi) fy = yield strength of reinforcing bars (psi) s = vertical spacing of hoops, not exceeding 4 in. (in.) hc = core dimension of column in the direction under consideration (in.) This new minimum amount of transverse reinforcement should be applied over the entire column height to account for the uncertainty associated with potential blast locations. The proposed Section 5.10.12.3 of the AASHTO LRFD specifies these new minimum transverse reinforcement ratios. A sh f f sh c c y ≥ ′0 18. ρs c y f f ≥ ′0 18. 5.10.12.3 Blast Design Category B and C Where columns are designed and detailed for Blast Design Categories B and C, as specified in Article 4.7.6.2, transverse reinforcement shall be designed to satisfy all detailing re- quirements for Seismic Zones 3 and 4 as specified in Articles 5.10.11.4.1c, 5.10.11.4.1d and 5.10.11.4.1e except that: • the requirements of Article 5.10.2.3 shall also apply • the length of the intermediate plastic hinges shall be taken equal to twice the length of the end region specified in Ar- ticle 5.10.11.4.1c In addition, for substructure columns designed for Blast Design Category C: • the volumetric ratio of spiral or seismic hoop reinforce- ment, ρs, for circular columns specified in Equation 5.10.11.4.1d-1 shall be increased by 50%, and C5.10.12.3 In Seismic Zones 3 and 4, it is assumed that plastic hinges will form directly above the footing and, in multi column bents, directly below the cap beam. The length of the end re- gion in Article 5.10.11.4.1c is based on these assumed locations. For bridges subjected to blast loading, the column may de- velop intermediate plastic hinges. It is assumed that the plas- tic hinge region will extend for a distance equal to the length of the end region on either side of the intermediate plastic hinge location. This assumes that the length from the point of maximum moment to the end of the plastic hinge is inde- pendent of the type of loading and the location of the plastic hinge. The theoretical location of the intermediate plastic hinge can be computed from a plastic analysis. The determination of an intermediate plastic hinge location can be uncertain because the blast loads vary significantly with both time and position along the height of a column. Accord- ingly, in most cases, the increased transverse reinforcement

95 specified by Article 5.10.12.3 should be placed throughout the entire height of the column. The minimum amount of confinement reinforcement is increased to improve ductility and energy dissipation capac- ity of potential plastic hinges. • the total gross sectional area, Ash, of rectangular hoop rein- forcement specified for rectangular columns in Equations 5.10.11.4.1d-2 and 5.10.11.4.1d-3 shall be increased by 50% 6.4.3.3.2 Type of Transverse Reinforcement and Sufficient Discrete Tie Anchorage. Experimental observations show that continuous spiral reinforcement performs better than discrete hoops for small standoff tests. The continuous reinforcement bet- ter confines the core at the base where a shear failure is most likely to occur. Therefore, continuous spiral reinforcement is rec- ommended for blast-loaded columns. Performance of discrete hoops can be improved by providing adequate anchorage into the concrete core. To avoid anchorage pullouts and to improve the performance of blast-loaded (and seismically loaded) columns with discrete hoops or ties, longer hook lengths than currently specified are recommended. Properly anchored hooks for blast loads should consist of a 135° bend, plus an extension of not less than the larger of 20.0 db or 10 in. Figure 85 illustrates the proper anchorage for each design category. a) b) c) 20 d 13 5° b 6d 90° b 13 5° 15 d b Figure 85. Discrete tie anchorage: a) Design Category A, b) Design Category B, c) Design Category C. 5.10.2.3 Hooks for Blast-Resistant Columns Where columns are designed and detailed for Blast Design Categories B and C, as specified in Article 4.7.6.2, hooks in the transverse reinforcement specified in Articles 5.10.11.4.1c and 5.10.12.3 shall consist of a 135°-bend, plus an extension of not less than: • For columns designed and detailed for Design Category B: the larger of 15 db or 7.5 in. • For columns designed and detailed for Design Category C: the larger of 20 db or 10 in. C5.10.2.3 Half-scale models of bridge substructure columns subject to blast loading indicated that columns constructed using transverse reinforcement utilizing 90 degree standard hooks with an extension length equal to 6 db did not perform satis- factorily. The deformation of the hooks caused the loss of core confinement and resulted in severe damage to the columns as shown in Figure C1. Bae and Bayrak (2008) demonstrated the opening of seis- mic discrete ties using hooks with a 135°-bend, plus an exten- sion of 8.0 db on full-scale concrete columns. Blast and seismic 6db (a) Before Test (b) Anchorage Pullout Figure C1. Unacceptable failure of discrete hoops with Standard Hooks when subjected to severe blast loading.

96 loads are both dynamic loads that induce dynamic structural responses and inelastic behavior. To allow the formation of plastic hinges and achieve a favorable mode of failure (flex- ure), adequate anchorage into the core concrete must be pro- vided. Full-scale columns constructed using the 135°-bend with an extension equal to the larger of 15 db or 7.5 in. per- formed satisfactorily when tested for seismic loading (Bae and Bayrak, 2008). Such hooks are specified herein for columns designed and detailed for Design Category B. Square columns constructed using the 135°-bend with an extension equal to the larger of 20 db or 10 in. performed sat- isfactorily during severe blast testing (Williamson, 2009). The core of such columns remained intact, and the column could still carry load even after sustaining extensive blast damage. This article extends the use of the longer extensions to all columns in Design Category C. For all columns in Design Categories B and C, the in- creased level of detailing should be provided over the entire column height as recommended in Article C5.10.12.3 Further research is needed to evaluate the performance of welded hoops and other transverse reinforcement layouts. Where columns are designed and detailed for Blast Design Categories B and C, hooks in the transverse reinforcement and their required locations shall be detailed in the contract documents. 6.4.3.3.3 Location of Longitudinal Splices. If the splice location is at or near the blast location, there is a possibility of breaching at the splice. Breach is defined as the complete loss of concrete through the depth of a cross-section. Local damage tests demonstrated that columns with a splice can experience significant damage over the majority of the column height and col- umn failure due to lack of member continuity (concrete and steel) if breach occurs in the splice region. Columns without a splice contained the localized damage to about one column diameter above and below the blast location. Therefore, locating splices away from contact charges helps minimize localized blast damage. However, column survivability also depends on the amount and type of transverse reinforcement, cover depth, and cross-section size. Experimental data are lacking to determine column capacity in a blast-damaged state. To improve the blast performance of columns that fall into Design Category C, splicing of longitudinal reinforcement should be avoided when feasible. If the use of a splice is necessary, the splice location should follow Section 5.12.13.4 from the AASHTO LRFD proposed guidelines. 5.12.13.4 Splices in longitudinal Reinforcement of columns in Design Category B and C The provisions of Article 5.10.11.4.1f shall apply. Where columns are designed and detailed for Blast Design Categories B and C, as specified in Article 4.7.6.2, the entire length of splices in longitudinal bars of substructure columns subject to blast loading shall be located: • outside the end plastic hinge region specified in Article 5.10.11.4.1c, • outside the column region that extends for a distance equal to the length of the end region, as specified in Article 5.10.11.4.1c, on either side of the expected location of in- termediate plastic hinges, and; • no less than 12 ft above the ground surface (or the lower deck in the case of a double-deck bridge) in the vicinity of the column. C5.12.13.4 There is a possibility of breaching at the splice, if the splice location is at or near the blast location. The local damage tests on half-scale bridge columns (Williamson, 2009) illustrated that a column with a splice at a quarter of the column height from the base (0.25L) can experience significant local damage over the majority of the column height and column failure due to lack of member continuity (concrete and steel) in the splice region. Columns without a splice contained the local- ized damage to about one column diameter above and below the blast location. The intent of the requirements of this arti- cle is to locate splices away from plastic hinge locations and away from contact charges to help minimize localized blast damage. Locating the entire length of the splices at least 12 ft above the ground surface (or lower deck in the case of a double- deck bridge) is based on considering truck bombs to be located 6 ft above the ground (deck) surface.

97 At the time of this writing (winter 2008), experimental data to determine the column load resistance in the damaged state are not available. Locating the entire length of the splices at least 12 ft above the ground surface (or the lower deck surface in the case of a double- deck bridge) is based on considering truck bombs to be located 6 ft above the ground (deck) surface. The intent of the above guidelines is to locate splices away from contact charges to help minimize localized blast damage. Additional research is needed to fully characterize splice behavior at locations very close to applied blasts. 6.5 Analysis Guidelines for Columns To design for blast loads in Design Category C, a dynamic analysis should be completed. 4.7.6 Analysis of Blast Effects 4.7.6.1 General As a minimum, bridge components analyzed for blast forces should be designed for the dynamic effects resulting from the blast pressure on the structure. The results of an equivalent static analysis shall not be used for this purpose. C4.7.6.1 Localized spall and breach damage should be accounted for when designing bridge components for blast forces. Data avail- able at the time these provisions were developed (winter 2008) are not sufficient to develop expressions for estimating the ex- tent of spall/breach in concrete columns; however, spall and breach damage can be estimated for other types of components using guidelines found in Department of the Army (1990). Due to the uncertainties that exist when considering likely attack scenarios and associated blast loads, an appropriate equivalent static load cannot be used for design. Moreover, the highly impulsive nature of blast loads warrants the considera- tion of inertial effects during the analysis of a structural compo- nent. Therefore, an equivalent static analysis is not acceptable for the design of any structural member subjected to blast loads. Information on designing structures to resist blast loads may be found in ASCE (1997), Department of the Army (1990), Conrath et al. (1999), Biggs (1964), and Bounds (1998). Section 4.7.6.3 outlines a procedure that checks the flexural capacity of a column exposed to a close-in blast load using a single- degree-of-freedom analysis. 4.7.6.3 Blast Analysis Procedure for Highway Bridge Columns To evaluate the capacity of a blast-loaded column, a flex- ural analysis that calculates rotation and flexural ductility shall be completed. In lieu of a refined analysis of blast loading, an equivalent blast load based on the scaled standoff is adequate. Software such as BEL or BlastX can be used to determine the equiva- lent blast load in terms of uniform pressure and impulse. A single-degree-of-freedom analysis using the equivalent loads shall be completed to calculate rotation and flexural ductility. The design limits are specified as follows: θ ≤ 1.0° μ ≤ 15 C4.7.6.3 This analysis uses flexural response as an indicator for shear response, and shear is not directly calculated. Direct shear capacity, per UFC 3-340-01, is not necessarily indicative of shear performance (Williamson, 2009). Shear checks are di- rectly built into the design limits of the flexural analysis. The design limits, rotation and flexural ductility, are based on large-scale test data (Williamson, 2009). Columns tested with slight to moderate damage without significant shear dam- age were used to select these limits. These limits help ensure that a shear mechanism does not form as a result of the large shear demand caused by close-in blast loads (Williamson, 2009). The flexural performance of a column can be improved by increasing the column cross-sectional size and the amount of longitudinal reinforcement.

98 where: θ = Rotation (degrees) μ = Flexural Ductility These limits ensure that column damage is limited to allow continued service following an extreme event. In general, the blast analysis procedure for highway bridge columns includes the following steps. 1) First, determine the design threat in terms of standoff and charge weight (reference Articles 2.7.2 and 3.15.1) to cal- culate the scaled standoff and design category (reference Article 4.7.6.2). 2) Detail the column according to the design category required by the design charge weight and standoff distance specified in step 1), and conduct a dynamic analysis using steps 3)-5) if required. Article 4.7.6.2 provides references to the detail- ing and design guidelines required for each design category. 3) Using the charge weight and standoff distance specified in step 1), use an acceptable load prediction method to deter- mine an equivalent uniform pressure and impulse. At a minimum, the load prediction technique employed for this purpose should account for the increase in pressure and im- pulse due to reflections off the front face of the column and ground/deck surface. BEL and BlastX are two examples of load prediction software that are acceptable for this purpose. 4) Determine the mass of the column, and using a uniform load shape, calculate the maximum flexural resistance and stiffness for each stage of response of the column to create a resistance diagram. The boundary conditions used to de- termine the resistance diagram should correspond to those in the actual structure. If the exact boundary conditions in the actual structure are uncertain (i.e., if a boundary is neither pinned nor fixed but has some unknown restraint), two analyses assuming an upper and lower bound for the unknown boundary conditions will provide an adequate range of response prediction for design purposes. 5) Using appropriate load-mass factors to compute the equiv- alent load, mass, and stiffness of the column, employ a single-degree-of-freedom analysis method of choice to determine the peak displacement and support rotation of the column The load-mass factors for a uniformly-loaded column are as follows: Simply-Supported KLM.elastic = 0.78 KLM.plastic = 0.66 Propped-Cantilever KLM.elastic = 0.77 KLM.elastic-plastic = 0.79 KLM.plastic = 0.67 Fixed-Fixed KLM.elastic = 0.77 KLM.elastic-plastic = 0.79 KLM.plastic = 0.66

99 The time varying load should be a triangular load with a magnitude equal to the peak pressure calculated in step 3), and the triangular load curve should preserve the total im- pulse. Therefore, the duration of the triangular load should be equal to 0.5 × impulse / peak pressure. 6) Compare the computed peak displacement and support rotations to the allowable limits for member ductility and support rotation specified in section 4.7.6.3, and redesign if necessary Further details of the single-degree-of-freedom analysis procedure can be found in Bigg’s Introduction to Structural Dynamics (1964), Tedesco’s Structural Dynamics: Theory and Application (1999), and the Department of the Army’s Struc- tures to Resist the Effects of Accidental Explosions (TM 5-1300) (1990). Note that load factors are not specified in blast-resistant design due to the inherent uncertainty associated with blast loads; however, dynamic increase factors and strength increase factors are used to better estimate actual design strength and reduce design conservatism for an extreme load event. Dy- namic and strength increase factors can be found in ASCE (1997) and Department of the Army (1990). Additionally, the single-degree-of-freedom procedure does not necessarily re- quire the use of an equivalent uniform load. If the results of an acceptable load prediction technique produce the data necessary to develop and apply a non-uniform load distribu- tion to the column, a designer may elect to use this more refined load distribution in lieu of an equivalent uniform load. If this option is selected, however, the designer is required to use this more refined load distribution to calculate the appropri- ate stiffness, maximum resistance, and load-mass factors for each stage of response. The three references cited above pro- vide the information necessary to compute these values using the applied load distribution. Design examples in Chapter 8 provide detailed calculations required to evaluate performance of bridge columns subjected to blast. 6.6 Summary Blast-resistant design and detailing guidelines for reinforced concrete highway bridge columns, based on experimental obser- vations and data, were proposed in this chapter. This information was provided in a manner that is consistent with the AASHTO LRFD Specifications (2007) so that bridge engineers can readily use the results of this research. The following chapter summa- rizes guidelines for analytical modeling of blast-loaded bridge columns.