Published June 16, 2022
Graduate Student: Homero F. Carrion Cabrera
Principal Investigator: Michel Bruneau
Project Completion Date: December 2022
The concept uses inexpensive braces called Buckling Restrained Braces (BRBs) and sliding bearings that work together to protect the bridge columns and superstructure from damage, so that the bridge can withstand the three-dimensional ground motions caused by earthquakes.
The objective of this project was to make the bidirectional ductile diaphragm concept applicable to common multi-span bridges as a way to prevent damage in the substructure and superstructure due to earthquake excitation. The concept uses energy-dissipating buckling restrained braces (BRB) as “fuses” or sacrificial elements located at the end of the superstructure’s floating spans. The viability of the concept was initially demonstrated in a NCHRP-IDEA Type 1 project for single-span bridges. The current project aims to expand the concept to multi-span bridges through analytical and experimental research. Therefore, this project is a final step to expanding the use of ductile diaphragms (already implemented in the AASHTO Seismic Specification but limited only to transverse excitation applications), to provide resistance to bidirectional excitation, in multi-span bridge configurations. This innovative system using BRBs can provide seismic resilient and damage-free bridges applicable to different levels of seismic forces at low cost as a result of the availability of BRBs in different capacities ranging from 20 to 1,400 kips and already tested for building use purposes.
The experimental phase of this project was performed at SEESL. The specimen setup had four different BRBs per configuration and was subject to: 1) a sequence of thermal expansion demands to represent their respective life-cycle demands, 2) a sequence of four spectral matched earthquake displacement histories (with components in the longitudinal and transverse direction of the bridge) that represents the design level, 3) a sequence of five strong motions scaled to reach large displacement demands in the BRBs and, at the same time, to represent different types of motions (near field motions, far-field motions, motions with pulses, and motions in soft soils); and in case the bridge does not fail, 4) a sequence of two extreme motions that represents the case of a bridge with rigid piers that is represented with shake tables moving synchronized. The design of the specimen and the testing protocol made it possible: to provide a more stringent testing protocol than qualification hysteretic test protocol, even expanding it to 3-D; to test various types of connections and BRBs for all the applied earthquake histories; to validate the design for the combination of thermal and seismic life demands, and; to quantify the ultimate hysteretic energy capacity of BRBs in terms of low-cycle fatigue life.
This project was sponsored by the National Cooperative Highway Research Program (NCHRP) IDEA Program and the California Department of Transportation.