Rate-Dependence in High Performance Fiber-Reinforced Cementitious Composites for Seismic Retrofits

The Northridge earthquake in January of 1994 had a dramatic impact on the structural requirements of hospitals in California. The earthquake caused 23 hospitals to suspend some or all of their services, and caused more than $3 billion in hospitals related-damages. Hospitals that had become unusable were forced to transfer their inpatient population to nearby hospitals which were still operational, increasing the burden on their facilities.

In response to the Northridge earthquake, The California Legislature passed the Hospital Facilities Seismic Safety Act (California Senate Bill 1953) in November of 1994. This bill established that by 2008, all general acute-care inpatient hospitals must remain standing following a major earthquake. Those in danger of collapsing must be rebuilt, retrofitted, or closed. Retrofitting a structure such as a hospital poses a considerable challenge in that the facility must be allowed to function while the retrofit strategy is being implemented. This need for continuing functionality motivates the necessity for a versatile and portable retrofit system that can be put in place with minimal disturbance to the facility.

This research focuses on the continuing development of a retrofit strategy for steel framed structures that can not only accommodate floor plans and secondary system layouts of the existing facility, but also possibly provide minimal disturbance to the function of the building during installation. The retrofit strategy consists of a series of precast infill panels within the frame that act as deep beams under lateral load. The panels are composed of a high performance, fiber-reinforced cementitious composite (HPFRCC) that does not spall, strain hardens in tension, and exhibits fine multiple cracking, leading to energy dissipation under cycling loading.

Of particular interest in this work is the response of the steel frame structure to an earthquake with the retrofit system in place. Simulating the system response requires a reliable model for the infill panels, which in turn requires a thorough understanding of the rate-dependent of the HPFRCC materials. This research focuses on determining the rate-dependence of HPFRCC materials under tension, compression, and cycling loading conditions. The research objects are:

(1) Determine the rate-dependence of selected HPFRCC materials in monotonic tension, monotonic compression, and cyclic loading up to seismic-level strain rates.
(2) Develop a reliable model for the hysteretic behavior of the infill panels.
(3) Simulate the response of a structure to a series of earthquakes with the retrofit system in place.

It was found that the strength, stiffness, and tensile strain capacity of the selected HPFRCC materials were significantly affected by increasing strain rate. This in turn was found to have an important influence on the lateral load capacity, drift at which softening occurred, and post-peak softening slope of the infill panels. The retrofit system was shown to decrease beam-column joint rotations, the probability of beam-column connection failure associated with these rotations, and roof and interstory drifts by up to 50%, making it a viable solution for retrofitting steel frame buildings and critical facilities.