Darren Tennant

Development of An Analytical Database to Support a Fast Running Progressive Collapse Assessment Tool

Rapid assessment of structures for vulnerability to progressive collapse has become a major concern in today’s world environment. Homeland Security and Overseas U. S. government agencies need the ability to quickly, and realistically, evaluate buildings to determine the risk of progressive collapse and understand how proposed retrofits would improve the building risk. To support the development of a fast running assessment tool, a series of simplified finite element simulations were conducted to generate a response data base. Models of simplified concrete and steel frames were run under gravity collapse conditions to determine displacements, forces and moments transmitted to adjacent structural members. The simulations were performed using Weidlinger Associates’ FLEX finite element code. This paper will discuss the salient issues involved in the nonlinear, dynamic modeling of progressive collapse. The FLEX modeling used to develop the data base and confirm expected response is also described in detail. Finally, the simplified analytical tool that was developed from the simulation data base development effort will be described.

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

Ship Impact Study: Analytical Approaches and Finite Element Modeling

The current paper presents the results of a ship impact study conducted using various analytical approaches available in the literature with the results obtained from detailed finite element analysis. Considering a typical container vessel impacting a rigid wall with an initial speed of 10 knots, the study investigates the forces imparted on the struck obstacle, the energy dissipated through inelastic deformation, penetration, local deformation patterns, and local failure of the ship elements. The main objective of the paper is to study the accuracy and generality of the predictions of the vessel collision forces, obtained by means of analytical closed-form solutions, in reference to detailed finite element analyses. The results show that significant discrepancies between simplified analytical approaches and detailed finite element analyses can occur, depending on the specific impact scenarios under consideration.

INVESTIGATION OF SHIP IMPACT SCENARIOS AND MITIGATION MEASURES

Ship impact is an important loading scenario for analysis and design of bridges, oil platforms, and other marine structures. Ships collision is also a very important design consideration for ship hulls. Designing structures to resist both accidental and intentional ship impact requires characterization of the impact loading history. Standard design practice relies on simplified methods to determine the impact loads, which typically consider only speed and mass of the vessel. However, ship impact is a complicated non-linear structural dynamic event that depends not just on the size and mass of the vessel, but also local stiffening pattern, location and function of the bulkheads, possible ice-strengthening classification, draft, presence of the bulbous bow, and many other factors. Neglecting these factors can lead to overestimation or underestimation of the loads, depending on a specific scenario. The discrepancies between simplified load estimates and detailed finite element analyses are investigated in this paper.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

Blast Resistant Design Technology and Its Use in Antiterrorism Protection of Structures

The purpose of this paper is to discuss current blast resistant design technology and demonstrate how it has been used in the protection of structures against future terrorist attacks. The paper will discuss historical and current approaches used to determine structural response to blast loading. The paper will demonstrate the impact of advanced finite element methods in the design of structures to resist blast loading with emphasis on economical construction and better understanding of complex response modes. Discussion of the critical need for test validation of advanced methods will be included.

Impact Mitigation for Buried Structures: Demolition of the New Haven Veterans Memorial Coliseum

We present an overview of the analysis and design of mitigation schemes for buried structures subjected to impact loading, with a focus on the hazard evaluation to underground utilities from the demolition by implosion of the Veterans Memorial Coliseum in New Haven, CT. We discuss the analytical and numerical investigations validated by field testing conducted prior to the implosion and leading to the design of the mitigation schemes aimed at protecting the utilities buried under ground. All the designed and constructed mitigation schemes proved successful during the January 2007 implosion of the Veterans Memorial Coliseum.