Calculation Tools Coming to Help Rank, Retrofit "Killer" Concrete Frames
Two Applied Technology Council hazard-mitigation projects targeting nonductile concrete buildings—often referred to as “killer buildings” because many are at risk of collapse in an earthquake—are benefitting from information gathered during an Oct. 9-13 reconnaissance trip to Mexico City, less than a month after the Sept. 19 magnitude-7.1 Puebla-Morelos earthquake. The 14-person ATC trip aided in the refinement of a new building-specific methodology to identify killer buildings by predicting their collapse potential. The trip also provided two specimens for a project aimed at evaluating the accuracy of seismic retrofit standards for nonductile concrete structures.
“ATC-78 Project Series: Identification and Mitigation of Nonductile Concrete Buildings” has developed relatively low-cost and easily implementable methodologies for estimating the probability of collapse of frames with or without walls, says William T. Holmes, senior adviser for Rutherford + Chekene and ATC-78 technical director. Still to come are methods for frames with brick infill.
The probabilistic, rather than deterministic, basis of the methodology is “a new way of thinking and represents a significant advancement,” Holmes says.
The method, tested against real buildings, can be used to rank older concrete structures. Building officials can then set priorities for mandatory retrofits and owners can identify dangerous structures. “We are giving the engineer an efficient, standardized methodology, with minimal calculations,” to figure out, for about $15,000, the collapse hazard, Holmes says. A more complicated, nonlinear time-history analysis would cost about $50,000.
The method, which presupposes available drawings for a building, estimates the probability of story collapse based on story-drift demand, column gravity-load-carrying capacity and other factors, extrapolating a collapse probability. The goal is to develop a methodology that is comparable to the U.S. seismic retrofit standard (known as ASCE-41) in terms of ease of use, but one that yields gradations of risk, rather than pass-fail results.
ASCE-41 is known to be conservative. “If you use ASCE-41 as a criteria, way too many buildings would require retrofits and way too much money would be spent,” says Holmes.
The effort is funded by nearly $4 million from the Federal Emergency Management Agency, which next year will publish “FEMA P-1100: Seismic Evaluation of Older Concrete Frame, Frame-Wall, and Bearing Wall Buildings for Collapse Potential.”
The other ATC effort, called “ATC-134 Performance-Based Seismic Engineering: Benchmarking of Existing Building Evaluation Methodologies” and funded by some $1 million from the National Institute of Science and Technology, is scheduled for 2019 completion. Using ASCE-41-17 and seismic retrofit codes in Europe, New Zealand, Taiwan and Japan, the team expects to assess objectively the accuracy of the codes’ evaluation methods by modeling predicted damage—based on drawings of quake-damaged buildings and their nearby ground motions—and comparing the results to the actual damage, says Russell Berkowitz, senior associate at Forell/Elsesser Engineers Inc. and ATC-134 project director.
Three- to 20-story damaged buildings have been selected in Mexico City, Southern California, Taiwan and Japan.
“There is no sense ASCE-41 is not accurate, but there has been no systematic benchmarking to compare the [code] book versus the real world,” says Berkowitz.