04/30/2008
Progressive-collapse-resistance design requirements out this fall are product of blast tests
from www.enr.com
By Nadine M. Post
Improved and unified requirements for progressive-collapse- resistance design from both the U.S. Dept. of Defense and the General Services Administration are likely to be released this fall, according to industry sources. The final draft document, currently under peer review, will make compliance with federal requirements easier and less costly, says Protection Engineering Consultants (PEC), the document’s primary author.
For structural steel frames, the combined document incorporates results of an unprecedented series of six bomb-blast and progressive collapse tests carried out for GSA and DOD from 2004 to 2007 on two types of moment connections. The findings, finalized in January, are expected to be formally released this month or next.
The high performance of the steel structure was a “little surprising,” says the structural engineer that managed the $2.5-million test program, considered the most comprehensive of its type to date. “The steel system performed well after being damaged in the blast,” says Jesse Karns, program manager with MHP Structural Engineers, Long Beach, Calif. With proper selection of connections, “significant enhancement to the physical protection of federal government steel-frame buildings to counteract potential threats of progressive collapse is achievable,” says MHP.
The program also validated analytical tools used to do predictive analysis. “We can [now] model with confidence,” says David Houghton, MHP’s research program project executive. “This is important” because it eliminates the expense of a blast test, he adds.
The original DOD “Unified Facilities Criteria (UFC) 4-023-03 Design of Buildings to Resist Progressive Collapse” was first published in January 2005. GSA “Progressive Collapse Analysis and Design Guidelines” were released in June 2003. The update to UFC 4-023-03 presented a “logical opportunity” to combine the requirements into one document for use by both agencies as well as voluntary adoption by other government agencies and civilian organizations, said David J. Stevens, senior principal of PEC, Spring Branch, Texas.
PEC is contractor for the updated document under a Naval Facilities Engineering Command contract managed by the National Institute of Building Sciences and sponsored by DOD and GSA. It presented the final draft at the 2008 Structures Congress of the Structural Engineering Institute of the American Society of Civil Engineers. The April 24-26 conference in Vancouver, B.C., drew a record 1,500 attendees.
During the GSA transition to UFC 4-023-03, a separate GSA document called “Progressive Collapse Analysis and Design Requirements for New Federal Office Buildings and Major Modernization Projects” will be released in conjunction with the new UFC 4-023-03, said Stevens. PEC worked on the project with MHP, the University of Texas at Austin and the Austin office of Walter P. Moore, the peer reviewer. The GSA document will provide some GSA-specific guidance but will rely primarily on UFC 4-023-03 for design requirements and procedures, said Stevens.
For DOD, the level of required progressive-collapse design will be based on occupancy categories found in UFC 3-310-01 Structural Loads Data, which is a modified version of ASCE’s Standard 7 occupancy categories. For GSA, the facility security levels found in the February 2008 “Facility Security Level Determinations for Federal Facilities, An Interagency Security Committee Standard,” are used.
In the update, the existing hierarchy of design requirements has been revised and the availability of new options should make the application of the requirements to a wide number of existing buildings much easier and less costly, said Stevens.
Prescriptive requirements for continuity and ductility (tie forces) have also been revised. In the update, the flexural structural members, including beams, girders and spandrels, no longer carry tie forces. Instead, the floor system does.
The alternate-path procedure has been significantly modified to closely follow ASCE Standard 41. In addition, new load-increase factors to account for nonlinear and dynamic effects have been developed. The acceptance criteria are also based primarily on ASCE 41 but have been modified where appropriate by recently developed data, said Stevens.
Threats
A non-threat-specific local-resistance method was developed and implemented in the updated UFC 4-023-03. The procedure provides a rational way to harden or toughen structural elements without requiring an explicit threat definition, said the engineer.
Design requirements for upward loads and doubled effective-column-height lengths have been removed. Also, the requirement for peer review of nonlinear analysis has also been eliminated. That change was called “a step back 20 years” by a structural engineer at the conference.
The new data for the performance of steel connections, developed by GSA’s Office of the Chief Architect and the Defense Threat Reduction Agency, is implemented in the updated UFC 4-023-03. In the program, pristine and blast-damaged steel connections were tested to failure under monotonic loading and full lateral restraint.
The program included three tests on two different moment connections carried out at facilities of the U.S. Defense Threat Reduction Agency at Kirtland Air Force Base, Albuquerque. For each, there were two blast tests, one with a specimen with a floor deck and another without. A load test was then carried out to test for post-blast capacity of the double span.
One connection tested and analyzed was a traditional welded unreinforced flange with a bolted web connection, known as a “dogbone.” The other was a proprietary moment connection called SidePlate. The connection includes two steel plates that act like slices of bread to sandwich the connection of the column to the beam.
Four more connection types were analyzed only through modeling: the reduced-beamsection moment connection; the bolted, double-split-T partially rigid moment connection; the bolted, single-shear-tab “simple” gravity load-carrying connection; and two variations of the bolted double-angle simple gravity connection.
The test frame included a 12-ft-tall column with a 20-ft bay to either side. The flanges were 16 in. deep. The beams were 18 in. deep. Each test column had a foundation. Each blast represented the force of a pickup truck filled with explosives. After the column failed in the blast, a ram force was applied to the remaining double span until something failed. That force was also applied to an undamaged column in a no-blast scenario.
“There were deformations and twisting but not a huge reduction in capacity when the column was hit with a blast load,” says Karns. “In looking at the blasts themselves, the steel performed very well.” For the SidePlate connection, there was only a 20,000-lb reduction in capacity. “This is reflective of the resilience of steel,” Karns says.
The SidePlate connection, designed to be more ductile, performed better than the dogbone, says MHP. “Those designed to be more ductile tend to carry more load,” says the engineer. “The more load you can resist, the smaller the member size,” says Houghton. That means less steel and less cost, he adds.
MHP concludes that “with sensible steel-frame design configurations and the careful selection of the connection to accommodate inelastic levels of moment demand and concurrent axial tension, steel-frame buildings and specialty structures can provide reliable and cost-effective protection against blast-induced progressive collapse.”
HDR, Inc. performed the structural engineering for this new Hospital. The structural engineering team faced two major challenges—the two extending wings on the main body of the hospital with very limited room for any lateral load resisting elements, and compliance with vertical tie force requirements for progressive collapse mitigation per the USACE’s Unified Facilities Criteria (UFC) Design of Buildings to Resist Progressive Collapse, UFC 4-023-03. While these building configuration challenges would clearly not be a ‘piece of cake’ when designing to typical building code criteria for natural hazards such as earthquake and extreme wind, they in fact would introduce even greater and unique complications to the design of a federal healthcare facility which must also address malevolent threats; in particular direct bomb blast attacks and subsequent progressive collapse load conditions.
The architects developed a column grid layout to optimize the functional design and future flexibility of the hospital. As such, it was determined early on that the lateral load resisting system would consist of a steel moment frame, as is commonly used in healthcare facilities. Because of the limited quantity of columns available in the extended wings of the main hospital, it became extremely difficult to provide enough lateral stiffness in the steel moment frame system using traditional steel beam-to-column welded moment connections to control the lateral story displacements induced by the IBC/ASCE 7 building code prescribed wind forces.
