Earthquakes and tall buildings mix about as well as running and scissors.
Engineers are working on ways to make the combination less destructive. But before they can develop viable, cost-effective methods to reduce the effects of earthquakes on tall buildings, they have to understand the methods currently used to improve them.
Earthquakes, as you probably know, cause vibrations in the earth as two masses of the earth’s crust move against each other. This movement releases energy in the form of seismic waves. We experience these waves as an earthquake. As these waves interact with the built environment, their energy can cause disruption in buildings and other infrastructure.
In a tall building, that disruptive energy acts on vertical elements such as walls, but has a particular effect on the heavier elements: the floors, which can’t directly transfer the energy down to the foundation. In floors, this energy or force transfer is assisted by structural elements called collectors that are added to the floors. These special elements, either through reinforcement in concrete floor slabs or in special beams below the floor, transfer forces to the vertical, force-resisting elements such as walls, frames, and braces, so forces can be moved down to the earth and away from the structure.
Collectors serve an important role in earthquake mitigation, but there is surprisingly little research on these structural elements, even among civil engineers. One researcher who has identified this gap in research and is concerned with studying collectors is Robert B. Fleischman, Professor of Civil Engineering and Engineering Mechanics at the University of Arizona. He uses Lehigh University’s Natural Hazards Engineering Research Infrastructure Experimental Facilities (NHERI) facilities in research supported by his National Science Foundation (NSF) award titled “Advancing Knowledge on the Performance of Seismic Collectors in Steel Building Structures.”
His research, both the experimental data he collects at Lehigh’s NHERI facility, and the analysis he performs, will contribute to the revision of building codes that will increase the safety of buildings. These safety improvements will apply to newly constructed buildings and can enable older, vulnerable buildings, especially critical ones such as hospitals, to be retrofitted according to building codes to make them safer and more resilient.
Fleischman has been collecting data on building-earthquake interactions using NHERI resources at the University of California San Diego (UCSD) and Lehigh University.
At Lehigh University, Fleischman conducted large-scale testing of the connector elements, using a methodology that involves quasi-static loading (slowly increasing load in small increments). The research team braced a collector against the NHERI Lehigh strong floor in a large-scale, 50-foot-long setup, with a pair of loading actuators supplying as much as 1 million pounds of collector axial force, and a pair of reaction actuators to simulate the rotation of a column at the collector connection, similar to inter-story drift.
Lehigh’s NHERI RTMD Experimental Facility Operations Manager Joseph Saunders says, “The NHERI Lehigh Facility is uniquely positioned to assist researchers with advanced multi-mode large-scale testing to help them solve complex problems and provide answers for practicing engineers. The University of Arizona team, led by Dr. Fleischman, have been our long-time collaborators, having worked with the NHERI Lehigh facility on multiple projects over the last 15 years. The team’s recent collector testing in the NHERI Lehigh facility exemplifies their commitment to understanding engineering problems that practitioners are concerned with.”
At UCSD’s NHERI facility, Fleischman used the high-performance shake table (which makes it a suitable complement to Lehigh’s structures lab), to test a 0.4-scale, single-story building, with steel composite floor systems known as LHPOST6. The facility has extensive instrumentation to handle large- or full-scale structural, geotechnical, and soil-foundation-structural systems tested under extreme earthquake loads, to produce the experimental data essential to advance predictive seismic performance tools.
Data collected and developed under this project will be available via publications and hosted on the NHERI DesignSafe platform for future researchers to use.