David J. Stevens

DoD research and Criteria for the Design of Buildings to Resist Progressive Collapse

The collapse of conventional/nonhardened structures was a concern of the U.S. Department of Defense (DoD) for years before the collapse of the World Trade Center (WTC) towers during the terrorist attacks on September 11, 2011 (9-11), owing to the bombings of the Murrah Federal Building in Oklahoma City, the U.S. embassies in Africa, and the U.S. Marine barracks in Lebanon. Since 9-11, motivated by the lack of any meaningful U.S. progressive collapse design requirements, DoD has worked with the civilian community on a number of significant efforts to improve the design of buildings to resist disproportionate collapse. The DoD efforts have included laboratory and field experiments, numerical simulations, and development of design requirements. Synergy and coordination with the civilian community resulted in combined programs with the General Services Administration, guidance and feedback provided by the ASCE Structural Engineering Institute (SEI) Committee on Disproportionate Collapse Standards and Guidance...

Development and Application of Linear and Non-Linear Static Approaches in UFC 4-023-03

Over the last 10 years, two United States government agencies have developed guidelines for the design of their structures to resist progressive collapse: 1. The General Services Administration, Progressive Collapse Analysis and Design Guidelines, (GSA Guidelines) and 2. The Department of Defense Unified Facilities Criteria 4-023-03 Design of Buildings to Resist Progressive Collapse (UFC 4-023-03). Although both documents incorporate some of the same approaches, there are notable differences in the application of these procedures. Within both approaches, the main direct design procedure is the Alternate Path (AP) method, in which a structure is analyzed for collapse potential after the removal of a column or section of wall. Different analytical procedures may be used, including Linear Static (LS), Nonlinear Static (NLS), and Nonlinear Dynamic (NLD). Typically, NLD procedures give better and more accurate results, but are more complicated and expensive. As a result, designers often choose static procedures which tend to be simpler, requiring less labor. As progressive collapse is a dynamic and nonlinear event, the load cases for the static procedures require the use of factors to account for inertial and nonlinear effects, similar to the approach used in ASCE Standard 41 Seismic Rehabilitation of Existing Buildings (ASCE 41). It is important that the design requirements incorporate appropriate dynamic and nonlinear factors such that the linear static and nonlinear static designs are more representative of the actual nonlinear and dynamic response of the structure.

Unified Progressive Collapse Design Requirements for DOD and GSA

As demonstrated in this paper, the updated UFC 4-023-03 has been significantly revised and improved relative to the initial version, to satisfy both DoD and GSA requirements. The modifications address shortcomings in the Tie Force and Alternate Path methods and result in a document that is more technically sound and rigorous, based on analysis and experimental data. The use of Occupancy Categories to determine applicability and level of design requirements should open the combined GSA and DoD progressive collapse design requirements to a wider audience. Procedures outlined in the updated UFC 4-023-03 will also provide a substantial basis for prescriptive methods planned as a part of progressive collapse mitigation approaches being developed for consensus-based, civilian building design codes.

Modeling structural collapse including floor slab contributions

When designing for the limit state of collapse prevention, engineers must be able to compute the ways in which structural components fail and how the failure of one or several members can affect the response of an entire structural system. Past research on the response of structures under extreme loading conditions, including progressive collapse, has focused on structural frames with only limited attention given to other structural components. Though often not modeled explicitly, concrete floor slabs can contribute significantly to overall structural performance during extreme loading events, and the ability to efficiently include nonlinear slab behavior in structural models is important for assessing the collapse potential of a structure. In this paper, the authors address the importance of accounting for floor slab contributions in carrying out collapse simulations of structures. Modeling approaches include simplified equivalent frame models and high-fidelity finite element models. Preliminary findings suggest that floor slab contributions to collapse resistance are significant and should be accounted for when assessing the performance of structures.

Revision of the Tie Force and Alternate Path Approaches in the DOD Progressive Collapse Design Requirements

The Department of Defense (DoD) Unified Facilities Criteria (UFC) 4-023-03 Design of Buildings to Resist Progressive Collapse was recently revised and a number of significant improvements were implemented, particularly in regards to the direct and indirect design approaches. Direct design explicitly considers progressive collapse during the design process and includes the Alternate Path method, in which the building bridges over a missing structural element, and, the Specific Local Resistance method, in which the building, or parts of the building, are designed for a specific load or threat. In indirect design, resistance to progressive collapse is incorporated implicitly through prescriptive requirements for strength and continuity, typically in the form of Tie Forces, which insure a minimum tensile strength in horizontal and vertical structural members. During the revision of UFC 4-023-03, the effectiveness of the indirect and direct design methods used in existing design requirements was evaluated and research was performed to improve these approaches. For indirect methods, tension membrane and catenary behaviors were used to develop improved Tie Force requirements. For the Alternate Path method, the linear and nonlinear analysis procedures were improved, through adaptation of the overall approach provided in ASCE 41-06 Seismic Rehabilitation of Existing Buildings . In addition, the load and dynamic increase factors were revised, to better account for inertial and nonlinear effects in linear static and nonlinear static models. The research, analyses, and improvements for the indirect and direct methods are reported in this paper.

Progressive Collapse Criteria and Design Approaches Improvement

AbstractMethods for the assessment of structural designs or existing structural systems for susceptibility to disproportionate (progressive) collapse and prescriptive methods intended to mitigate o...

Overview of the Revised DOD Progressive Collapse Design Requirements

In the three years since Unified Facilities Criteria (UFC) 4-023-03 Design of Buildings to Resist Progressive Collapse was first published in January of 2005, various omissions, ambiguities, and opportunities for improvement were identified by civilian and government designers and engineers. A significant revision to the Progressive Collapse UFC was initiated in the Fall of 2006 and was recently completed, during which a number of significant improvements were made. Occupancy Categories (OCs) similar to those in ASCE 7 Minimum Design Loads for Structures are used to define a buildings progressive collapse design requirements; previously, military definitions of levels of protection were used. Indirect design methods for enhancing load redistribution capacity with tie forces and direct design methods using alternate path and specific local resistance continue to be employed but with revisions. The alternate path method now includes dynamic and load increase factors that are based on careful analysis of the inertial and nonlinear aspects of load redistribution. Structural response criteria are specified in terms of force- and deformation-controlled actions, similar to ASCE 41-06 Seismic Rehabilitation of Existing Buildings . A non-threat specific, local hardening procedure was developed and implemented, to insure ductile behavior of critical elements without significant additional cost, for new construction. Finally, a brief overview of three example problems is provided.

Effects of Close-In Charges on Pipeline Components

Pipelines are an extensive and critical part of the nations infrastructure. Nationwide, there are 320,500 miles of natural gas transmission line and 168,900 miles of hazardous liquid line. Lines for local distribution of natural gas total 2.2 million miles. Nearly all natural gas and 65% of hazardous liquids are transported by pipelines. Natural gas provides over 25% of residential and industrial energy needs, while oil products provide 97% of the energy used for transportation. In total, 62% of the energy used in the US is derived from these two sources.