Blast-Resistant Highway Bridges: Design and Detailing Guidelines (2010) / Chapter Skim
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Pages 46-59
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46 4.1 Overview While bridge engineers can adapt many currently available methods to predict blast loads and the response of building components for use with bridges, there is a need for bridgespecific, simple models that provide quick and accurate results because such models facilitate an efficient design process. In addition, the most sophisticated techniques are not always necessary because these methods often require large computational demands and time commitments, while providing a level of accuracy that may not be warranted given the uncertainty associated with identifying threat scenarios and corresponding blast loads.
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From page 47... ...
47 the figure is the idealized spring-mass system with an equivalent mass, Me, that goes through a displacement, δ, against the resistance of an equivalent stiffness, ke, under the application of an equivalent force, Fe. An engineer can transform the real system into the idealized system and obtain equivalent system properties by applying work and energy principles to the real beam using an assumed normalized displaced shape.
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48 principles or use factors for known scenarios. Transformation factors for known scenarios can be found in Introduction to Structural Dynamics by Biggs (1964)
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information on computing the shear demand and capacity for structural components, and the ACI and AISC design manuals contain additional information needed to calculate the shear capacity of concrete and steel members, respectively. 4.3 Simplified Modeling and Software Blast design specialists have a large number of options when choosing a technique or software to compute blast loads and structural response, and these methods vary widely in both cost and accuracy.
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50 represent reflections off and interactions with structures, and any user must clearly understand the limitations of the selected method before applying it to a specific loading case. While simplified analyses using available equations and curves are useful for relatively large-standoff problems (i.e., where the center of detonation is greater than a scaled range of three or more from the structure under study)
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subjected to blast should be familiar with the advantages, uses, and inadequacies of simplified analysis methods. This section describes two levels of analyses for predicting structural response due to blast and includes examples of commonly accepted and used software.
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52 ods may be useful to size individual members before using a high-level model to verify the global response of the structural system. Practicing blast engineers sometimes use this type of procedure involving a progression of analyses (Hinman, 1998)
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members and local buckling and fracture for steel members. These failures often result from close-in or contact blasts, but they are not necessarily limited to such attacks.
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54 the superstructure is desired. In most cases, however, assumptions can be made for the end restraints in order to use SDOF analyses to obtain conservative results.
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member failure on response that MDOF frame analysis methods may fail to capture. Moreover, because MDOF analysis methods are not coupled (at least in the context in which they are being described in this section)
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56 this fact may make explicit solutions less efficient than implicit solutions. It is important to note that the solution technique employed by an analysis method varies with each individual program, even within the levels of analysis defined within this document.
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Despite the increase in cost, nonlinear general-purpose finite element methods can be valuable because they can provide detailed information regarding failure modes and the effects of localized failure. "Meshes," or grids containing the "finite elements," model the physical system and allow computation of localized effects.
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58 analyses (National Research Council, 1995)
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are appropriate for all cases, and similarly not all levels of load prediction techniques are compatible with all levels of response prediction methods. An engineer should take care to select analysis methods that are suitable for the resources available, the goals of the project team, and the specific scenario being considered.
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