«R. SHANKAR NAIR R. Shankar Nair R. Shankar Nair, Ph.D., P.E., S.E. is a principal and senior vice president of Teng & Associates, Inc. in Chicago. In ...»
That preliminary study produced findings that raised major concerns about the blast resistance of moment-resisting structural steel connections, and the safety of using TM 5-1300 for the design of structural steel connections. It was shown that moment-resisting structural steel welded connections subjected to ‘safe’ explosive loads may fail due to weld fracture. Furthermore, it was shown that the corresponding deformations of the designed structural elements may exceed the limits set in TM 5-1300. It was shown further that dead loads have an adverse effect on the behavior. This was due to the added bending and twisting of the beams once they were deformed by the blast effects. Since TM 5-1300 does not address such effects, current design procedures should be modified to reflect the structural damage caused by weak axis deformations. Although steel is not expected to be very sensitive to strain rate effects, as compared to concrete, serious strain rate effects were note in the structural steel connections. Nevertheless, it was observed that the current DIFs, as defined in TM 5-1300, need to be modified to address more detailed pressure levels and the differences between 2-D and 3-D behavior. It is recommended that more detailed DIF applications, associated with pressure levels, might be necessary to avoid possible overestimation of strain rate effects. This could be very important when 2-D analysis are used to design 3-D structures.
It was concluded that improved design approaches for structural steel welded connections in blast resistant buildings are urgently needed. Such design approaches should be derived based on additional studies that must be supported by combined theoretical, numerical, and experimental efforts. On going studies at Penn State are aimed at addressing the issues raised during this investigation, but they need to be expanded significantly, in cooperation with several DoD organizations.
The planned follow up studies will be aimed at addressing the following issues (Mosher et al., 2001, and Krauthammer
January and October 2003):
• A comprehensive assessment of existing and modified structural steel elements and connection under blast and impact loads.
• Understanding the physical phenomena that could cause progressive collapse and/or are associated with progressive collapse.
• Defining the relationships between localized damage in structural steel buildings and progressive collapse.
• Development of improved design guidelines to enhance the survivability of structural steel building under blast and impact loading environments.
The first two topics are currently under investigation at Penn State, as a follow up to previous activities described above.
In the study of structural steel connections, the same type of connections described above have been modeled in great detail to capture the role of various parameters in the behavior (e.g., columns, girders, continuity and/or cover plates, bolts, welds, etc.). One such numerical model is shown in Figure 11.
The study on progressive collapse is focused on the behavior of individual elements, their support conditions, and their performance in a multi story moment resisting frame (115'8" H X 175' W X 125' D), as shown in Figure 12.
Figure 12 Numerical model for Progressive Collapse Studies (Choi and Krauthammer, 2003) Preliminary findings from the on going studies on progressive collapse (Choi and Krauthammer 2003) highlight the need to use an external criteria screening (ECS) technique to analyze progressive collapse. Necessary definitions and approaches were developed for material and geometric nonlinearities. A matrix reformation and stiffness reduction technique was described to achieve element elimination effects in the proposed analysis procedure. M atrix partitioning and variable boundary conditions (VBC) techniques were developed to improve solution convergence and stability problem. Such problems might appear in applying a stiffness reduction factor technique that is more advantageous and that leads to relatively shorter computing time. The behaviors and time histories using stress/strain failure criteria for the selected model were compared with those obtained with a general finite element analysis. The behavior of the structural model with a linear material behaved differently than that with a nonlinear material. Structural behaviors with different nonlinear material models were similar. The structural responses obtained with a general finite element procedure were different from those obtained with the modified approach, as presented here. Behavior comparison between those obtained with the developed procedures and those derived with a general procedure is meaningless for the differences of structural system. Buckling was considered as a contributing failure criterion, together with a strain failure criterion. A new solution that can analyze local buckling failure was implemented and inserted as an external module. The collapse started much earlier when buckling was considered than if only a strain criterion was considered.
This behavioral difference can indicate that collapse might progress very differently when buckling is considered, and the issues associated with these phenomena require more careful attention.
Besides these on going studies, several proposals are aimed at addressing these issues experimentally in both laboratory impact and field explosive tests. In contrast to static and seismic loading, the loading of steel frames engendered by a terrorist attack will produce some behaviors uniquely related to this type of event. For blast loads, these unique behaviors are primarily due to the rapid load rate— the significant loading is over in less than 5 ms. Loading this rapid often produces direct shear responses that can cause the elements of a section, which are the relatively thin-walled, to shear as if cut by a knife or buckle even before any significant flexure has occurred. High load rates also increase the propensity for brittle cracking and decrease the toughness of welds. In addition, the often highly non-uniform nature of the load can excite twist modes and other higher modes of the section causing instabilities and the general loss of flexural and axial resistance. Besides, the shock flow around complicated geometries (e.g., around the I shape of a column or girder) is not well understood. Therefore, it is difficult to define the anticipated pressure-time histories that would be required for any type of computational effort (e.g., simple design calculations or advanced high-fidelity physics based simulations).
Events like the collapse of the W orld Trade Center (steel frame) and the Murrah Building (reinforced concrete frame) are challenges to assess and predict because they require the structural system to perform in ways it was never intended.
It also requires engineers to understand, quantify, and calculate behaviors that only a few could do even for individual components. W hen the components (e.g., columns and girders) are combined to form a structural system, its response is far more complicated, and closer to the real conditions. The peculiar nature of steel frames is much more complicated and it includes various interacting modes of response and failure (e.g., local and global buckling, fracture, loss of strength and ductility due to temperature and strain rate effects, etc.). Scaling full size steel building components is much more complicated, as compared to structural concrete systems. This places steel frame multistory buildings in a special class of structure when it comes to predicting their potential for progressive collapse due to a terrorist attack. The uncertainty related to the vulnerability of steel framing of multistory buildings to terrorist attacks is too great to be ignored. Hence, the research program proposed to DTRA (Mosher et al, 2001). Some of these ideas are illustrated in Figures 13 through 15.
Figure 13 Some Proposed 3-D Connection Tests (Mosher et al., 2001) Figure 14 Proposed Full Scale Steel Test Building in the US (Mosher et al, 2001).
Figure 15 Proposed Full Scale Steel Test Building in the UK (Mosher et al., 2001).
This paper presented an overview of background in blast resistant structural behavior, identified technical difficulties in design and application of such technologies for blast resistant structural steel buildings and systems. This study will be directly linked to other DoD-sponsored R&D that are aimed at enhancing civilian-type buildings’ resistance to blast effects, with special attention to preventing progressive collapse. The approach will include using advanced numerical simulations, precision impact tests in the laboratory, and field high explosive tests on components, assemblies, and small- and full-scale buildings. Current and anticipated DoD-sponsored R&D efforts (possibly augmented by other agencies) will be used to define the required test parameters and activities. AISC can facilitate and enable these studies by coordination between the research team, industry, and government agencies.
The author wishes to acknowledge the generous support by the US Army Engineer Research and Development Center (ERDC) at the W aterways Experiment Station and by the Defense Threat Reduction Agency (DTRA) for the research described, herein. Also, the author wishes to express his gratitude to Reed Mosher of ERDC and to John Crawford of K&C Structural Engineers for their cooperation on the development of future research plans in this area. Finally, the author wishes to express his appreciation to AISC for their cooperation and support related to the Blast and Impact Resistant Design Committee activities.
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