David Vaughan

Laser alignment modeling using rigorous numerical simulations

This paper describes a three-dimensional computer modeling technique for alignment system simulation, and some example calculations. The technique has been developed to address issues of alignment and overlay accuracy for future generation VLSI technology. The analytical basis is a general finite element electromagnetic wave propagation code, EMFlex, that rigorously simulates light scattering from the 3-D alignment mark. Using the Nikon Laser Step Alignment (LSA) system as a model instrument, the overlay error and signal shape are simulated. Examples of an idealized asymmetric metal mark are studied. Preliminary results suggest that the rigorous simulations are substantially different from the one-dimensional Fresnel approximations that have been used previously.

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

Large-scale, explicit wave simulations on the CRAY-2

Most time-domain, wave modeling problems in geophysics are intractable by classical analysis methods, due principally to nonseparability and to a lesser extent material nonlinearity. Therefore discrete numerical solutions are often necessary for the simulation of realistic models. Applications in 2-D and 3-D geophysical modeling are the subject of this paper, particularly as solved on a CRAY-2 supercomputer. Implementation and performance differences between earlier CRAYs and the CRAY-2 are described, including the discrepancy between scalar fetch and vector processing speeds. Explicit finite element solvers are applied to applications involving 2-D simulation of a seismic refraction experiment across the state of Maine, 3-D simulation of near-source scattering experiments, and both linear and nonlinear axisymmetric source simulation. Results show that the CRAY-2 allows cost-effective 2-D simulations of truly large-scale models, but only begins to be effective in 3-D for models of interest in geophysics. The large memory (256 megawords) is adequate but a speed increase of at least an order of magnitude is necessary for cost-effective 3-D. True multiprocessor capability (rather than ‘multicomputer’) provides an alternative to raw speed but involves another set of difficulties.

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

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.