Designing roofs post 9/11

"Is your building’s design blast resistant—even if it’s not for a high-security client?" That question had almost never been asked of joist and structural steel fabricators of the average building. But in the post-9/11 era, changing building design requirements are migrating upward, from the walls to the roof, requiring joist engineers to add new considerations to their design checklists.

"It’s rare that joists have been required to have blast criteria," said Jim Lucas, P.E., engineer for Canam Steel Corporation, who designed the joists for the Unified Communications Center (UCC), a Southeast Washington, D.C., government building scheduled to open in January 2006. "This is an area of responsibility we’ve never been asked to take on before—and it’s not one we took on in this project."

The project

The project is a new, state-of-the-art public safety call center for 911 and 311 (non-emergency) telephone service for the District of Columbia. City officials built the new consolidated center to house police and fire/emergency medical services communications personnel, who now will be located in the same facility to more efficiently answer and dispatch calls for service.

The center is designed to GSA Security Level C security standards (blast- and bullet-resistant glass and walls and a 100-foot security set back) and when complete will have 72-hour emergency self-sustaining capabilities. In addition to the large call floor area, the facility will have a food service area, a gymnasium, a visitor/media access center, an administrative area, and a childcare development center. Construction includes 140,000 square feet of building and site work (three stories above ground and one story below ground). The site is located on 11.8 acres—6.8 acres for green space, 2 acres for the building, and 3 acres for the parking and roadway.

Blast resistance engineered into the building

The District of Columbia government hired regional engineering consultant Eve Hinman during the bid process to set the level of criteria for blast resistance design. The level set during the bid—there are three levels—depends upon the shape, the anticipated threat, and what is on top of the roof.

Superior Iron Works—an American Institute of Steel Construction specialty fabricator from Sterling, Va., who won the job from general contractor Jair- Lynch/Tompins Joint Venture—recruited Weidlinger Associates, a consulting engineering firm based in New York, to evaluate the joist fabricator’s design based on the blast criteria level specified in the bid. Specifically, the bid required the joists to be designed for the same dynamic rebound equal to the dynamic inward force. The threat level is not disclosed here for security reasons.

The novelty of the requirement necessitated an extra layer of back-andforth between fabricators and engineers.

It took some agreement to determine who could best oversee how the blast evaluation would be done. "You should see the correspondence file I have on this one," quipped Kathy Landmesser, project manager for Superior Iron Works, who eventually settled on Weidlinger as the experts in the field. "There are not many engineering companies specializing in this," she said.

Jerome Rasgus, S.E., AIA, of Weidlinger’s Washington, D.C., office, agreed that this is still an extremely new design area for subcontractors and suppliers. His firm has some years of background in designing blast-resistant buildings, gained from high-profile projects that were logical terrorism targets. "But nowadays, even private developers are starting to look at designing buildings for federal guidelines," he observed. "It may cost a little more, but it will make them more competitive." The resulting demand for an increasing number of blast-resistant buildings is having a trickle-down effect on the industries that supply them. Initially, for example, Rasgus remembers performing similar blast-resistance analysis for designs of curtainwalls, skylights, and windows, which were the first industries hit with post-911 code changes. "We used to do their resistance design, but now more often they’re doing it themselves, in-house, because the frequency of requests has justified it," he said.

Rasgus predicts that steel fabricators increasingly will see similar requirements in their specifications.

In the case of the Washington, D.C., project, Weidlinger used elasto-plastic shell analysis finite element software—Weidlinger’s proprietary software—to evaluate the nonlinear dynamic response of the roof system.

The barrel joists were 52 inches deep, spanning 5 feet-on-center, and were 90 feet long. For the analysis, Weidlinger looked at a two-joist model of 50 kipper- square-inch (ksi) steel, noting that the roof deck collects blast pressures and transfers them to the joists.Total anticipated dead load was estimated at 15 pounds per square foot (psf ) and live load at 30 psf.

For the blast cases—which are not outlined here for security reasons—the engineering evaluation looked at connection details at the most heavily loaded joists, which were the members at the supports and the mid-span splice. Peak demand in the diagonals at the high-end support were 15.5 kips, 6.5 kips, and 9.3 kips. The analysis showed the extent of rebound and reversal effect on the joist design, for both internal and external threats, on the bottom chord of the joists.

"I designed only for static loads, and things such as wind," said Canam’s Jim Lucas. Weidlinger’s dynamic analysis required no changes to Canam’s original joist design, but did specify changes in the connections, which required bolstering welds. Canam was required to increase the weld lengths at the high end seat and the diagonals at either side of the midspan splice above the minimum required by the static design, one from its original design of 4-3/8 inches to 7 inches, and two others from a 1-1/2-inch original size to 4-1/2 inches. The top chord condition is a flush end plate, while the bottom chord end plate is extended.

Implications

The UCC bid is a tip-of-the-iceberg situation that indicates what fabricators of structural steel and joists can expect.

More building types will be evaluated for their security response, and the decision as to who is responsible for their design will continue to resurface. Fabricators will need to consider what level of expertise and liability they are willing to bear as this frequency increases.

Linda Norris is the communications and marketing manager at Canam Steel Corporation/Hambro Structural Systems located in Point of Rocks, Md. She can be reached at linda.norris@canam.ws.