A new effort under way at Purdue University and the National Institute of Standards and Technology focuses on new buildings designed to withstand natural and terror-related disasters.
When the nine-story Alfred P. Murrah Federal Building in Oklahoma City was attacked by domestic terrorists in 1995, half of the reinforced-concrete structure collapsed when just four of its 33 columns failed. Three of the four columns are believed to have failed because of the loss of lateral support provided by beams and floor diaphragms that were destroyed by the blast.
Based on observation of debris, collapse patterns, damage patterns and thousands of photographs taken during search and rescue operations, engineers now believe that possibly only one column was destroyed by direct blast effects.
The distinction may be subtle, but it has significant implications for the design of safer buildings.
“The current focus is on preventing collapses of buildings in a chain-reaction manner that may result from loss of a critical supporting column,” said H.S. Lew, a senior research engineer in the NIST Building and Fire Research Laboratory.
Purdue engineers are conducting full-scale tests to learn precisely what happens to a building structure when a single column is removed. The tests simulate the extreme condition of a column being blown out of a reinforced-concrete frame.
Data collected with sensors will then be used by NIST to fine-tune computational models to improve building code requirements, standards and practices. In the end, the researchers hope to prevent devastating damage caused after the loss of merely a single column.
“These tests demonstrate the types of details one should use in a girder to avoid disproportionate collapse, so failure of one critical element does not lead to a cascading failure,” said Purdue structural engineer Mete Sozen.
In designing modern buildings using advanced computational tools, engineers tend to shave away as much material as possible to make them as economically attractive as they can.
“But the problem then is, there isn’t much reserve capacity to sustain the loss of structural elements such as columns,” Lew said. (Ed’s note: The fire service has encountered a similar situation in the form of “engineered” roof truss assemblies in commercial and multifamily construction over the past 15 years or so)
The Purdue researchers have tested a 40-foot-long full-scale assembly of reinforced-concrete beams and columns and are starting the construction and instrumentation of a second one. The assembly will be tested several times, with data collected from electronic and optical displacement sensors and a high-speed camera operated at 10,000 frames per second to track minute movements as the beam collapses.
The researchers believe this work lays the foundation for new building design and construction codes.
“Results of these tests can be used to develop new standards for reinforced-concrete construction not vulnerable to explosion or impact,” Sozen said.