...an engineer to follow prescriptive requirements to achieve a chosen performance level. In a separate effort, engineers are developing methodology to apply FEMA 356 to new buildings.Click here to view chart
In seismic zones, PBD is used relatively frequently for upgrades but not for new buildings. "It would be fabulous if we were able to design new buildings to specific performance levels," says Poland. "That will take time."Click here to view chart
The American Institute of Steel Construction addresses PBD in its 2005 specification. And the American Concrete Institute next year expects to publish a guide on PBD of reinforced concrete buildings under wind. "There are great discrepancies in design based on different assumptions about drift requirements," says Viral Patel, managing director of the research and development group at Walter P. Moore, Houston. "This is something most engineers would welcome for it will make the design process uniform."
In 2000, ICC put out its first model performance code. ICC chose to keep the code separate from its model International Building Code because, "the minute you say performance code, people, especially building officials, shut down," says Beth Tubbs, senior staff engineer for codes and standards development in ICCs Boston office. The document explains "what we want to achieve without saying exactly how to achieve it," says Tubbs.
Rival NFPA has issued its first Building Construction and Safety Code, NFPA 5000, and its long-standing Life Safety Code, with new chapters on PBD. The codes offer "a fork-in-the-road choice" between the prescriptive and performance routes, says Milosh Puchovsky, NFPAs principal fire protection engineer.
Developing NFPA performance language took seven years. "It was strenuous work to come to a consensus," says Puchovsky. "There was a significant amount of debate over how much should be regulated and how much should be left to the designer."
Jurisdictions are not rushing to adopt performance codes. They maintain that prescriptive codes work for all but a small percentage of buildings. Tubbs isnt concerned. She expects officials to use the ICC code as a guide, at least for the time being. "It provides a rigorous administrative process," she says.
In the absence of standards, engineers and building officials in certain jurisdictions, including San Francisco, Seattle and Clark County, Nev., have been working out ways to ease the stress associated with PBD reviews. All agree it is critical to contact the building official early in schematic design to get approval of the approach and begin discussions. "The number one way to get a performance-based design killed is to walk in, at the end of day, and say this is what we did," says Brian J. Meachim, principal risk and fire consultant in the Boston office of Arup. Up front, "you need to share your design brief, tools used and more," he says.
San Franciscos Kornfield says "unclear communication" is one of the weaknesses in submissions. "We cant review things on a conceptual or theoretical basis," he says. "This is not a think tank."Click here to view chart
The goal is to show officials "you are not out to cut corners but to design what the owner wants and do it safely," says Meacham. Arup has not had any outright rejections yet. The firm still has one project in review after a couple of years.
At times, "the only way to get a PB fire design approved is to change the code," says Baldassarra. In one case, that took a year. "It gave us all upset stomachs, weekly, over the uncertainty," he says.
When building officials dont feel adequately trained to assess a PBD, they go to peer review. A review adds three to four months to approval time, and can cost the developer $25,000 to $50,000.
San Francisco follows a codified process to select a "structural advisory committee." From a list of prequalified firms, the official chooses one member, the proposer selects one and the third is selected jointly.
Peer review has its pitfalls. The most difficult part of a PB project is "coming to an agreement, quantitatively, as to what the design criteria really are," says Ronald O. Hamburger, a principal with structural engineer Simpson Gumpertz & Heger Inc., San Francisco. Engineers will disagree on the intent of the code and types of damage permissible. "They can argue for hours on this," he says.
"Too often, reviewers come up with an objection to justify earning their keep," says Gregory R. Miller, president of fire protection engineer Code Consultants Inc., St. Louis.
For structural, Seattle has worked out criteria for reviewers to consider. These include dynamic response characteristics, lateral force resistance, overstrength and strain hardening or softening, strength and stiffness degradation, energy dissipation characteristics, system ductility and redundancy. The reviewers product is a letter saying the design meets the code, says Siu.
For PB fire engineering, approvals tend to be murkier. Officials are more skeptical of fire protection engineering, a relatively young discipline, than of structural engineering. Fire modeling is also considered more subjective than structural modeling. "Is the engineer using the correct assumptions in the model?" says Siu. "You dont know." A firms credibility is important. "Most of the time, you have to ask, Are they trying to snow you?"
In the face of myriad bottlenecks, there is cautious optimism that PBD could become mainstream, though not for a couple decades. "There have been changes in the profession, thanks to technology, education and training," says Bruce R. Ellingwood, a professor in Georgia Techs College of Engineering, Atlanta. "Thats why I think its going to go this time."