Howard Levine

Response of AISC Steel Column Sections to Blast Loading

One dozen American Institute of Steel Construction (AISC) W14 steel columns were tested at the Energetic Materials Research and Testing Center (EMRTC), New Mexico Institute of Mining and Technology in Socorro, New Mexico with loading from typical size vehicle bomb threats at very close to moderately close standoffs. Pretest predictions of structural response were performed using standard SDOF methods and the Weidlinger Associates, Inc. (WAI) FLEX finite element code. Loads acting on the columns were determined from the U. S. Army developed CONWEP code using the Kingery-Bulmash equations for the pretest predictions. Seven tests included individual columns with axial loading and blast loading applied simulataneously. One test included 5 columns built into a frame with moment connections at the top of the columns and base plate connections at the base of the columns. The columns were instrumented with accelerometers and pressure transducers. The tests were designed to produce various levels of damage from mild to severe. This paper will compare the pretest and posttest predictions using both the SDOF and FLEX finite element methods with the actual test results. The comparison between actual loading and CONWEP loading will also be discussed. Conclusions will be drawn with regard to the use of CONWEP loading for this type of threat at various standoffs. Also, the use of SDOF and FLEX finite element methods to predict the response of AISC W14 steel columns will be compared.Copyright

Non-Linear Numerical Modelling of Aircraft Impact

Abstract Non-linear, explicit, finite element models were generated to predict the response of aircraft impacting into concrete runways and soil surfaces or reinforced concrete, steel lined shelters. Previously, simplified methods were used to represent the forces resulting from an aircraft crash. Models of C-130 and C-141 aircraft were developed for a Defence Threat Reduction Agency sponsored safety assessment. The purpose of the calculations was to senerate the internal aircraft and structural environments for the safety assessment as a result of the aircraft impacts into shelters and onto soil and runway surfaces. “Riera-type” loadings were also used to determine the general capacity and failure modes of the impacted structures. Finite element C-130 and C-141 models were then used to impact the shelters with the same aircraft weight and velocity. Three explicit calculations were run to compare the shelter responses. One case did not fail the shelter and two cases - one C-I30 and the C-141 did. Details of the difference in results using the simplified loading and explicit aircraft impact modelling are presented as well as crash results into runways and soils.

Computational Failure Analysis of Reinforced Concrete Structures Subjected to Blast Loading

High-fidelity, explicit finite element analysis has proven to be an effective tool in simulating airblast effects on reinforced concrete structures. The challenge is to make intelligent predictions regarding the stability or failure of real RC structures using the results of these numerical simulations. Code guidelines, such as the Army’s TM5-855, provide one aid in assessing RC member failure based on support rotation limits. Mechanics-based assessments using momentum and kinetic energy; fracture energy, volumetric, tensile, and plastic strains; and material damage levels provide additional information for predicting failure. Highly localized blast effects, such as cratering of the concrete, must also be taken into consideration. This paper will discuss such issues as it presents a general methodology for predicting states of damage and failure of RC structures due to blast loadings using high-end finite element analyses.

Anatomy of a Disaster: A Structural Investigation of the World Trade Center Collapses

The purpose of this study is to analyze the damage to the structure of each of the WTC Twin Towers due to the high speed impacts of the Boeing 767 airplanes and subsequent fires such as to elucidate why the Twin Towers stood for as long as they did, and why they ultimately collapsed. The Boeing 767 airplane attacks on WTC 1 and WTC 2 caused immediate and significant structural damage to the towers: In each case, exterior columns were severed and the floor system at the point of impact was damaged. The airplanes broke up during the impact and the resulting projectiles and fragments proceeded to inflict further damage to the core. Much of the impact damage to the exterior walls of the towers was evident. However, damage to the interior was not visible and cannot be quantified on the basis of the physical evidence. Dynamic nonlinear explicit finite element FLEX simulations coupled with independently validated airplane crash models were leveraged to understand and assess the structural states of damage to the tower interiors that could not be observed; this includes the degradation or loss of the load carrying capacity of columns and floor assemblies as well as the stripping of fireproofing from structural members. The impact damage to the structure was substantial but so were the reserve capacity and redundancy of the structure. Iterative analyses of the load redistribution in the impact damaged towers clearly indicate that the that the outer tube structure was very effective in developing Vierendeel action around the severed exterior columns and that the outrigger hat truss provided a substantial redundant load path away from the damaged core columns. Although not specifically designed for this purpose, the hat trusses served to delay the eventual collapse of the towers. These analyses also indicate that the damage to the corner of the core in WTC 2 left it in a state more vulnerable to subsequent thermal loads compared to WTC 1. This eccentric damage, more than the height of the airplane impact, resulted in a shorter time to collapse for WTC 2, considering that the fire environments in both towers were not meaningfully different. Further degradation or loss of the load carrying capacity of columns stripped of fireproofing by direct debris impact and heated by fire is shown to be the cause of each collapse. The examination of smoke flow from each building indicates that there were no floor collapses subsequent to the initial impact throughout the fire [1] and our 1 Chief Technology Officer, Weidlinger Associates Inc., 375 Hudson Street, New York, NY 10014. Phone: 212.367.3000. Email: abboud@wai.com. 2 Chairman, Weidlinger Associates Inc., 375 Hudson Street, New York, NY 10014. Phone: 212.367.3000. Email: levy@wai.com. 3 Weidlinger Associates, Inc., 4410 El Camino Real, Los Altos, CA 94022. Phone: 650.949.3010 4 Weidlinger Associates, Inc, 2525 Michigan Avenue, Santa Monica, CA 90404. Phone: 310.998.9154

Impact of Aircraft Engines Into Reinforced Concrete Walls

Major components of storage facilities and nuclear power plants are designed using reinforced concrete walls. Accidental or intentional impact of these structures by aircraft is a concern. The potential for penetration of these facilities by the aircraft or its components and the subsequent damage to the contents and release of toxic substances is a major concern. This paper focuses on analyzing the impact of jet engines into heavily reinforced concrete walls. These engines are among the stiffest and most massive components of an aircraft and the most likely to seriously damage and penetrate the reinforced concrete. We model both the engine and the reinforced concrete deformations using failure models for reinforced concrete and metals. Unlike many projectile impact problems, the impacting engine cannot be considered to be rigid. A large amount of energy is consumed in the plastic deformation and fracture of the engine components. The reinforced concrete is modeled using hexahedral elements for the concrete and beam elements for the rebar reinforcement. An advanced three invariant viscoplastic softening cap constitutive model describes the ductile and brittle rate-dependent characteristics of concrete. The rebar is modeled using a rate dependent, strain hardening von Mises formulation with failure controlled by fracture energy dissipation. A similar constitutive model is employed for the shell elements used to represent the engine components. These failure models are included in the FLEX large deformation finite element code which uses an explicit, central difference solution procedure with subcycling to solve the equations of motion. Element erosion using different criteria for concrete and metals is used to remove severely distorted and failed elements. Procedures used to mitigate the deleterious and unrealistic effects of hourglass control and viscoplasticity in the softening and failure regimes are discussed. The results from the computations are compared with experimental data generated by impacting a TF-30 engine into a two foot thick concrete wall.Copyright

Simulating Explosive Detonations Within Multiroom Buildings

The detonation of an explosive charge within a building produces complex propagating blast pressures that are strongly influenced by the building’s room layout and construction of interior walls. This paper looks at the effects of internal blast on common, non-structural steel stud or wood stud walls and unreinforced CMU walls in various multi-room configurations. Their blast response is investigated through experimental and numerical models with the goal of quantifying the blast pressure propagation into rooms adjacent to the blast. Risk assessments to power generation facilities should consider the potential for an explosive event within control buildings or other support facilities. These events could be an accidental explosion or the result of a terrorist action. A better understanding of the failure mechanisms and pressure transmission characteristics of typical power generation facility structures will lead to improved vulnerability assessments of these types of structures and the critically important control equipment located within them.Copyright

Evaluation of Airblast Loads on Structures in Complex Configurations

A common terrorist threat worldwide is the use of large vehicle bombs to attack high value targets. Detonation of large yield devices can cause significant damage to nearby buildings, facilities and infrastructure with potentially high loss of life and large economic losses. Blast pressures can have major consequences on critical facilities such as nuclear power plants, causing economic loss, environmental damage and system failure. Closely spaced structures in a dense configuration provide a complicated setting for evaluating airblast pressures caused by explosive devices. The presence of multiple buildings can channel the airblast, resulting in significant effects on load magnitudes at range from the detonation. Buildings reflect propagating blast waves causing increased loading at some locations and reduced loads elsewhere due to shielding from direct blast waves. The complex interaction between structures, streets, alleys and geographical terrain can have a major impact on structural loads. Currently, the most common way to estimate airblast pressures resulting from above ground explosive detonations is to use fast running, approximate blast tools such as CONWEP. These simplified tools may not provide accurate guidance on airblast pressures in complex environments. The following paper illustrates the use of Computational Fluid Dynamics (CFD) calculations of complex building configurations to quantify the resulting blast environment. Comparisons with simplified methods are presented. An approach to using a database of CFD simulations, customized for a specific site, to provide a fast running blast assessment tool is described. This approach provides a convenient, fast running tool for designers and security planners to visualize and accurately quantify the hazard from any threat size and location within the area of interest.Copyright

Numerical Investigation Into End Condition Effects on the Response of Reinforced Concrete Columns to Airblast

Dynamic finite element analysis using explicit time integration is a useful tool for evaluating the response of reinforced concrete columns to both near-contact and offset charges. Typical analyses model a single column in the structure in order to decrease analysis times and isolate the target column response from the general structural response. The effects, if any, of the assumed boundary conditions at the isolated column ends on the column response to the airblast loads are not fully understood at this time. This paper attempts to provide a more complete understanding of such end condition effects by investigating the response of a single column model with a variety of end conditions and comparing these responses to those of a column in a much larger structural model.Copyright

Response of Reinforced Concrete Walls Loaded by an External Buried Detonation

This paper compares the full scale test response of buried reinforced concrete walls loaded by an externally buried, cylindrical cased explosive threat with FLEX/FUSE coupled nonlinear code computations. FLEX is a nonlinear, large deformation finite element structural code. FUSE is a total Lagrangian large deformation, explicit code that represents solid as well as fluid behavior. The threat was placed at various standoffs from the walls and two orientations to provide a wide range of damage. The walls were instrumented with accelerometers, displacement gages, and pressure transducers. Free field soil pressure transducers were also included. The FLEX/FUSE results and test gage data are compared.

Thermal analysis of Perforated Metal Air Transportable Package (PMATP) prototype.

Sandia National Laboratories (SNL) has designed a crash-resistant container, the Perforated Metal Air Transportable Package (PMATP), capable of surviving a worst-case plane crash, including both impact and subsequent fire, for the air transport of plutonium. This report presents thermal analyses of the full-scale PMATP in its undamaged (pre-test) condition and in bounding post-accident states. The goal of these thermal simulations was to evaluate the performance of the package in a worst-case post-crash fire. The full-scale package is approximately 1.6 m long by 0.8 m diameter. The thermal analyses were performed with the FLEX finite element code. This analysis clearly predicts that the PMATP provides acceptable thermal response characteristics, both for the post-accident fire of a one-hour duration and the after-fire heat-soak condition. All predicted temperatures for the primary containment vessel are well within design limits for safety.