For structural engineers such as UNLV professor Ying Tian, the problem might be best described as an architectural tug-of-war between form and function.
To create high-rise buildings with light-filled, wide-open floor plans, architects and structural engineers often use large reinforced concrete slabs -- called "flat plates" -- supported directly on columns.
The spaciousness of these designs can come at a price, however: Buildings using flat plates are vulnerable to catastrophic collapse caused by "punching shear failure," which occurs when the flat plates break free, appearing to "punch" their way through the supporting columns. Earthquakes can cause punching failure, as can terror bombings and ensuing fires.
Tian, who began conducting research on this subject in 2007, is seeking to better understand how fires contribute to this phenomenon with the goal of saving lives.
"I was inspired to conduct research on the effects of fire damage on flat plate building structures because of the collapse of a flat plate underground parking garage in Switzerland in 2004 due to a 90-minute fire," Tian says. "The collapse, causing the death of seven firefighters, was triggered by a punching failure of a slab at one column that immediately propagated throughout the structure."
Tian says receiving a Faculty Opportunity Award helped him to obtain data necessary to seek additional funding. "Without the Faculty Opportunity Award, it would have been difficult for me to generate convincing preliminary data to request competitive grants from extramural agencies," Tian says.
The award also helped him establish fruitful collaborations with other campus experts, such as colleague Aly Said; working together, they designed a project that used UNLV's Structural Engineering Laboratory, a facility capable of large-scale testing, to learn more about how uncontrolled fires set off punching failures.
The results will help address a significant void in the study of this area; early findings were recently published in the International Journal of Concrete Structures and Materials.
"We need experimental data, analytical tools, and education to design resilient structures that can survive fire events such as those that precipitated the collapse of the World Trade Center towers," says Tian. "Our results so far have clearly indicated the high risk of progressive collapse in flat plate buildings experiencing uncontrolled fires."
Building safer buildings, in other words, demands designing slabs and connections with columns that can survive brief periods of intense heat without cutting loose and "pancaking" the floors below. A related improvement would involve boosting the slabs' "shear capacity" -- their ability to resist the stresses inflicted by what engineers call "extreme loading events." These improvements can't be developed, Tian says, until engineers have better data.
"I hope our tests will address whether elevated temperatures really cause decreased shear strength as predicted from some computer simulations," he says. "To date, knowledge regarding the resilience of flat plates to elevated temperature is extremely limited. Hopefully, more researchers will conduct studies like ours, since flat plate designs constitute a major class of structures widely used in the U.S. and other countries."
