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Blast Resistance of Steel Orthotropic Bridge Decks J Son1and A Astaneh Asl M ASCE2 Abstract The main objectives of this study were to 1 study the response of bridge decks to blast effects resulting from an explosion on the deck and 2 develop efficient technologies and systems that can be used in design and construction of bridges to enable them to survive blast attacks without collapse To understand the actual behavior of orthotropic decks subjected to blast loads typical steel orthotropic decks used in long span cable supported bridges were modeled using MSC Dytran nonlinear finite element analysis software and the models were subjected to simulated explosions occurring on the deck The parameters that varied in the analyses were the size of the explosive device in terms of equivalent TNT the axial compressive force present in the deck and the high strain rate mechanical properties of the material of the steel used in the orthotropic decks Through dynamic analyses the behavior of the orthotropic decks under blast loads was established and their failure modes were identified Also implementation of a new and effective blast resistant technology the fuse system was simulated In this system to limit the effect of a blast to a localized area of the bridge and to prevent catastrophic progressive collapse of the span special fuses comparatively weaker structural elements are placed between two deck segments DOI 10 1061 ASCE BE 1943 5592 0000283 2012 American Society of Civil Engineers CE Database subject headings Bridge decks Steel Blasting Progressive collapse Author keywords Bridges Blast protection Blast resistance Steel orthotropic deck Progressive collapse Introduction The number of terrorist attacks on civilian structures using car bombs has increased significantly in recent years throughout the world Although to date most of these attacks have been on buildings research and development projects on other infra structure elements such as bridges and dams are needed because terrorist attacks on these targets cannot be ruled out Especially long span cable supported bridges which are generally main traf fic routes must be protected from progressive collapse Many of these structures are so called legacy critical structures and their col lapse can result in massive loss of life and injuries as well as social and economic catastrophe Studies of bridges subjected to blast loading are very rare compared with studies of buildings subjected to blast but the current codes have almost no specific provisions or guidelines for blast protection of bridges ASCE 1997 ASCE SEI 2002 CEN 2002 2006 FEMA 2003a b Mays and Smith 1995 DoD 2002 The current bridge design codes have only general and limited guidelines for progressive collapse prevention after a blast Typical structural analysis under blast loadings is based on the decoupling approach where structures are analyzed under given blast loading patterns Blast loading patterns are calculated using theories or experiments described in for example ASCE 1997 Aslanov 2006 Baker 1973 Baker et al 1983 Beshara 1994a b Brode 1955 Henrych 1979 Kinney and Graham 1985 Mays and Smith 1995 Tr lat et al 2007a b U S Army 1986 1990 and Zukas 2004 In several research papers Krauthammer et al 1986 Krauthammer et al 1993a b Lellep and Torn 2005 Stevens and Krauthammer 1991 Weidlinger and Hinman 1988 the decoupling approach was used to study the structure s performance against blast loading The studies using the decoupling approach were accomplished based on a simplified structural model or a single degree of freedom system using as sumed blast loading patterns The simplification assumed that a primary dynamic phenomenon was expected to govern structural performance This approach has some major limitations in evalu ating the structural performance under blast loading First the blast loading patterns are applicable only for typical geometries For ex ample the reflected pressure values described in the U S Army technical manuals U S Army 1986 1990 are valid only for rigid flat geometry with a large area such as a large wall and the pres sure direction is assumed to be normal to the structure Second the experimental data used in the derivation of the equations were not obtained at a close distance from the detonation point due to the difficulty of instrumenting for high blast pressures and capturing responses in a very short time Thus the pressure values calculated using predefined blast loading patterns are questionable when the detonation point is close to the structure and blast pressure propa gation is not normal to structural members In this study steel orthotropic decks were analyzed for close detonation events and accordingly a more advanced approach was necessary to better estimate the performance of the decks subjected to blast loads The approach used in this study adopts a fully coupled fluid structure interaction model through a general Eulerian Lagrangian coupling algorithm The algorithm is able to consider charge detonation distance shock wave propagation in air high strain effect on material strength and the 3D geometry effect of the structure which are the most important factors under blast loading This approach was used in previous studies Astaneh Asl et al 2005 Lu and Wang 2006 Rutner et al 2005 Son and Astaneh Asl 2008 Son et al 2005 Astaneh Asl et al Rutner et al and Son and Astaneh Asl successfully analyzed bridge piers and decks and suggested methods to enhance blast resistance behavior of bridge pier and deck systems Lu and Wang performed 1MSC Software Corp 840 West California Ave Suite 230 Sunnyvale CA 94086 corresponding author E mail Jin Son 2Dept of Civil and Environmental Engineering Univ of California Berkeley CA 94720 1710 Note This manuscript was submitted on November 9 2010 approved on June 30 2011 published online on July 2 2011 Discussion period open until December 1 2012 separate discussions must be submitted for indi vidual papers This paper is part of the Journal of Bridge Engineering Vol 17 No 4 July 1 2012 ASCE ISSN 1084 0702 2012 4 589 598 25 00 JOURNAL OF BRIDGE ENGINEERING ASCE JULY AUGUST 2012 589 J Bridge Eng 2012 17 589 598 Downloaded from ascelibrary org by WUHAN UNIVERSITY OF TECHNOLOGY on 03 11 14 Copyright ASCE For personal use only all rights reserved blast analysis of a generic multistory reinforced concrete frame and showed the damage pattern of the structure in a typical above ground explosion scenario The current study explored the response of long span cable stayed and suspension bridge decks Fig 1 subjected to blast loading A typical steel orthotropic deck was designed and ana lyzed with varying parameters including steel strength explosive sizes and axial loadings using the nonlinear explicit finite element analysis program MSC Dytran MSC 2008 Eulerian elements were used to model ambient air and explosive material and Lagrangian elements were used to model a bridge deck Fluid structure interaction between Euler elements and Lagrangian elements was considered using a general coupling method Proper mechanical properties of steel material under high strain rates corresponding to blast loading were considered in the analysis The main objectives of this study were to develop recommen dations for necessary and sufficient protection of major bridges that can be targets of terrorist car bomb attacks conduct more ad vanced 3D explicit dynamic analyses of steel orthotropic decks and propose a hardening measure that can enhance blast resistance behavior and prevent progressive collapse of a bridge Blast Effects on Structures Several researchers have described detonation as a rapid energy release process Baker 1973 Baker et al 1983 This energy release process generates a blast wave that is applied on the structure as pressure Generally the blast wave properties vary de pending on the explosive materials and the medium of transferring the blast waves e g air For example underwater explosions use water as the medium of transferring the blast waves whereas air blasts use air as the medium of transferring the blast waves Thus the characteristics of blast waves are different in underwater explo sions and air blasts In this study only air blasts are considered and the air properties do not change during the blast phenomenon However the blast wave properties vary depending on the type and weight of the explosive charge Once the type and weight of the explosive are set the blast wave properties also vary with the distance from the explosive source When an explosive is detonated a detonation wave is generated and changes into a blast wave The blast wave then generates an initial pressure pulse in the air Because properties such as speed energy and mass are different in different air zones the air pressure shape changes continuously If it is assumed that the air which is the transfer medium is homogeneous and the explosive shape is a sphere a blast wave generally has a typical profile regardless of the explosive type then the blast wave properties depend only on the distance of the target from the explosion source center and time The air blast wave shape in the air without a structure is shown as the incident air pressure in Fig 2 The ambient air pressure remains before blast wave arrives When the blast wave generated by a detonation wave arrives at time Ta the air pressure suddenly jumps from the ambient air pressure P0 to a peak value Pi referred to as incident overpressure This incident pressure wave then decreases to the ambient air pressure during time T1 The time zone from Tato T1is the positive phase The pressure after passing T1 drops to the amplitude P i and finally reverts to the ambient air pressure The time zone from T1to T2is the negative phase During the negative phase it is possible that the air zone can drop to a partial vacuum condition Fig 1 Three typical cable supported steel bridges Pressure Time P0 Pr Pi T1TaT2 PrPi Reflected Pressure Incident Pressure Fig 2 Blast profile 590 JOURNAL OF BRIDGE ENGINEERING ASCE JULY AUGUST 2012 J Bridge Eng 2012 17 589 598 Downloaded from ascelibrary org by WUHAN UNIVERSITY OF TECHNOLOGY on 03 11 14 Copyright ASCE For personal use only all rights reserved However the blast wave reflects from or diffracts around a barrier if the blast wave impacts a barrier such as a structure This is not easy to explain using the typical blast load pattern because the patterns depend on the geometric properties of the barrier such as size and shape If in one of the simplest cases the blast wave is reflected normally on a rigid large wall the reflected pressure wave can be described as in Fig 2 The incident wave with rapid speed in ambient air hits the rigid wall When the initial blast wave is initially reflected from the rigid wall the following blast wave joins with the initial blast wave at the same location During this reflec tion process the blast wave speed decreases but the other proper ties such as pressure density and energy increase During the reflection process the reflected overpressure Prand P r increases to approximately 2 times to 12 times larger than the incident over pressure as shown in Fig 2 The wave involved in the reflection process is the reflected pressure Analytic Model and Simulation In this study the explosive charge the structure and the air sur rounding the entire area affected by the blast were modeled by finite elements The Euler element type was used to model the explosion in the air while Lagrangian structural elements were used to model the structure In this analysis the air structure inter action was calculated by using the so called general coupling method Fluid Structure Interaction The uncoupled method using given blast pressure profiles is one of the most common blast event simulation methods and does not need the fluid structure interaction scheme Although this approach produces reasonable results the pressure profiles that are used for the applied forces on the structures are only applicable for typical cases as described in the previous section In other words the uncoupled method is not useful to simulate more sophisticated phenomena of structures subjected to blast loads The three types of commonly used fluid structure interaction FSI schemes are smoothed particle hydraulics SPH arbitrary Lagrange Euler ALE and general coupling methods The SPH method is based on the contact between discrete fluid particles and structures In general it produces reliable results for high speed impact behavior but not for continuous flow behavior Many finite element softwares use the ALE method which effectively combines a fluid solver and a structural solver The ALE method shares common nodes between fluid Eulerian elements and struc tural Lagrangian elements When structural Lagranigan elements deform fluid Eulerian elements deform to share the common nodes If Eulerian elements deform severely this method needs a rezoning process to restore the original Eulerian mesh geom etry Because the ALE method might cause geometric instability and energy leakage during the rezoning process it affects the accuracy of the simulation with rapid severe deformation of struc tures In the general coupling method MSC 2008 Century Dynamics 1998 two different element types are coupled without sharing nodes This method guarantees more reliable results be cause it does not generate geometric instability and energy leakage problems and it can simulate continuous flow Fig 3 explains how to calculate the properties of Lagrangian and Eulerian elements using information transferred by the general coupling algorithm The structural elements provide a barrier to Eulerian material flow and Eulerian material cannot penetrate the structural elements The Euler element is intersected by the surface of the structural elements which are coupling surfaces Using the original element faces and intersection faces the solver calculates the Eulerian element volume which is the effective vol ume The Euler element properties are updated by the properties at a previous step and the effective volume at a current step The gen eral coupling algorithm changes the fluid pressure of an Eulerian element into the nodal forces of a Lagrangian element and transfers the nodal forces to a Lagrangian solver Although the nodal forces are generally calculated in the normal direction of a coupling sur face they can be updated in the transverse direction under special conditions such that surface friction or viscosity is applied The structural Lagrangian solver applies the nodal forces on structural Lagrangian elements and calculates the behavior of Lagrangian elements After a Lagrangian element is deformed the general coupling algorithm changes the deformation into the new nodal positions and transfers the new nodal positions to an Eulerian solver The fluid Eulerian solver updates a new effective volume of an Euler element and calculates the Euler element properties Although the scheme is explained sequentially here up dating the Euler and Lagrangian element properties occurs simul taneously and transferring information using the general coupling algorithm to both solvers is also accomplished simultaneously The explicit method calculates the next step element properties using the current step condition whereas the implicit method does so using the nextstep condition The node and element results at the current step are transferred through thegeneral coupling scheme for the calculation of the next step results The scheme introduced is the unique scheme only for the explicit method Modeling Material Properties of Steel The response of steel under dynamic loading at high strain rates is significantly different from the response under static loading Table 1 gives the strain rate comparison regarding dynamic loading phenomena Because blasts create very high strain rates in the range of 101 5to 104 the high strain rate effects must be included in these types of studies The strain rate generated by blast event is 10 to 10 000 times larger than that generated by an earthquake or wind In general steel responds to dynamic loadings with in creased yield stress and ultimate strength whereas the elastic modulus and the ultimate strain remain the same under normal and high strain rates Consideration of strain rate effects in an analytical model is a complex undertaking Although for simplicity one can apply dy namic amplification factors to the material of all the finite elements such an approach may not be realistic because not all elements of the structure are exposed to the same strain rate Naturally elements Normal direction Structural surface Effective volume Euler element Lagrangian element Fig 3 Intersection between Euler and Lagrangian elements JOURNAL OF BRIDGE ENGINEERING ASCE JULY AUGUST 2012 591 J Bridge Eng 2012 17 589 598 Downloaded from ascelibrary org by WUHAN UNIVERSITY OF TECHNOLOGY on 03 11 14 Copyright ASCE For personal use only all rights reserved close to the explosion experience much a higher strain rate than those away from the explosion area Cowper and Symonds 1957 and Paik and Thayamballi 2003 suggested Eq 1 to describe the dynamic amplification factor for high strain rates In this equation the values of C and q are suggested to be 40 4 and 5 respectively for low strength steel Paik et al 1999 modified Eq 1 and suggested values of C and q to 3 200 and 5 respectively for high strength steel Using the dynamic amplification factor obtained from the equation yield and ultimate stress increase the ratio yd y 1 0 C 1 q 1 In this study two types of steel materials were used ASTM A709 Grade 345 steel and ASTM A514 Grade 690 steel which are widely used in steel bridge design ASTM A514 Grade 690 steel generally shows more brittle behavior than ASTM A709 Grade 345 steel Table 2 summarizes the applied steel material properties For each type of steel material the bilinear stress strain relation of steel was modeled an