Safety without a bunker-like feeling.

The concrete construction industry is full of tough talk when it comes to portraying reinforced concrete as a "must-have" for building safety. Weakening its position is the failure to recognize that other materials often serve better in this role. A striking example can be found in Oklahoma City's new Federal Building, a replacement for the structure virtually destroyed on April 19, 1995 when a massive bomb inside a rental truck exploded in front of the building. In the course of planning the replacement facility, designers switched from reinforced concrete to structural steel in devising a blast-resistant wall, because steel was better able to achieve their safety goals without creating a bunker-like feeling.

Ross Barney + Jankowski Architects in Chicago, selected by the General Services Administration to design the project, was challenged with creating a lobby for the new building that invited public access without jeopardizing the safety of the occupants. In order to shield office spaces adjacent to the lobby's security stations from a potential blast event within the security area, the designers chose initially to screen the stations with a massive reinforced concrete wall some 12 to 14 inches in thickness. To RBJ, a wall of this size in the space available for the lobby of the new building hardly made for an inviting entry, so they set themselves to the task of reviewing alternatives with their blast design consultants for the project, Weidlinger Associates of New York.

"There was so much steel reinforcing in the screening wall as originally designed," says Kevin O'Connor, Project Architect with RBJ, "that we asked Bob Smilowitz of Weidlinger whether it was the concrete or the steel that provided the blast protection." RBJ found that the reinforcement was extensive and necessarily closely spaced because tests of their undulating design had shown that a blast would punch out chunks of concrete from the back of the wall where gaps existed between steel reinforcing bars (similar to the cratering a BB shot leaves in window glass, although on a much larger scale). "If steel was doing so much of the work," O'Connor wondered, "what if steel was to do all the work?"

In response, Weidlinger Associates--well known for its expertise and innovation in this area--approved a redesign of the wall using steel plate as the only material. An immediate benefit was the ability to reduce the wall's thickness from 14 inches to less than two inches, allowing the architect to avoid the bunker-like feeling that the reinforced concrete threatened to create. As a result, the lobby more successfully achieved the original design intent established by widely regarded (and recently retired) GSA Chief Architect Ed Feiner, of safeguarding the public without making federal buildings bastions of freedom in the literal sense.

More importantly, constructing the wall in steel improved its expected performance under blast conditions. Instead of having a situation where concrete projectiles might explode from the wall on the side away from a potential blast, the building operators could count on the structural steel absorbing the blast energy without creating dangerous projectiles.

In this respect, the redesign spotlights one of the remarks made by NIST in its recently released report on the findings of its 9/11 investigation. NIST cautioned that its recommendations on protecting/hardening of structures were not a prescription for specific systems, materials, or technologies. Instead it encouraged competition among alternatives that can meet performance objectives.

It is clear that the designers--and occupants--of Oklahoma City's new federal building benefited from a search for alternatives, when it was found that steel met the project's performance and design objectives in a way that reinforced concrete could not.

Gary Higbee AIA, director of industry development, The Steel Institute of New York