This project is developing bi-directional ductile diaphragms for implementation in straight and skewed bridge superstructures to provide resistance to bidirectional earthquake excitations. Non-linear time–history dynamic analyses were conducted to investigate the seismic performance of bridges with the proposed bi-directional ductile diaphragm concept, taking into consideration variations in skew, span length, reactive mass, fundamental period of vibration, and earthquake excitation characteristics. The proposed ductile end diaphragm systems (EDS) were designed for benchmark skew and non-skew bridges and analyzed using nonlinear time history analysis to examine their seismic performance. These dynamic analyses allowed investigating the impact of these parameters on the global behavior, as well as understanding the magnitude of local demands and the extent of bi-directional displacements that the hysteretic devices [Buckling Restrained Braces (BRB) in this case] must be able to accommodate while delivering their ductile response. A design procedure for the EDSs in skew bridges was developed based on the analysis results. The long-term service life of EDSs installed across expansion joints and subjected to bridge thermal expansion histories was also investigated and a minimum ratio of the BRB length over the whole bridge length was recommended. Work was also conducted to identify preliminary effective and practical details for hysteretic energy dissipating devices. Quasi-static experiments were then conducted to subject BRB to a regime of relative end-displacements representative of the results predicted from parametric analytical studies. A test set-up were developed, which consisted of connecting the BRB from the strong floor to a shake table in the SEESL. One end of the BRB was connected to a reaction block tied down to holes in the strong floor and the other to the shake table. The table was then used to apply horizontal bi-directional end-displacement demands to the BRB. The loading protocols included the bi-directional displacement histories to be applied to the specimens for the cyclic inelastic test, and the uniaxial displacement histories for the low-cycle fatigue test due to temperature change. Two types of BRBs with flat end plates and unidirectional pinholes (namely BRB-1 and BRB-2) were designed. The end plates of BRB-1 are designed to bend laterally to accommodate the required lateral displacement. The end plates of BRB-2 are connected to a spherical bearing working as bi-directional hinges, itself kept in place in a pre-drilled hole in the gusset plate in the reaction block. Four specimens of each type of BRB were tested, and different combinations of displacement protocols were applied to them. The ultimate behavior of a BRB is typically quantified in terms of the cumulative inelastic displacements that the BRB’s core plate experiences during the tests. All the BRB specimens tested developed a cumulative inelastic displacement of more than 200 times the BRB’s axial yield displacement, which is the threshold of inelastic performance specified by the AISC Prequalification and Cyclic Qualification Testing Provisions as part of its acceptance criteria for BRBs. More significantly, the specimens were able to sustain multiple years of severe temperature cycles in addition to meeting the prequalification criterion. Ultimately, as expected, all the BRBs failed in tension after extensive cycles of inelastic deformation. No end-plate failure or instability was observed (which would have been undesirable failure modes). Following the tests, some BRBs were opened, and fracture was found to typically occur where the BRB’s core plate locally buckled the most. BRB’s hysteretic behaviors under different displacement protocols were studied and compared. Detailed analyses of cumulative inelastic deformations and low-cycle fatigue life of all BRBs using experimental data were performed. A recommended design procedure for BRBs’ applications in bidirectional-ductile end diaphragms in both non-skew and skew bridges was developed based on the parametric analyses and experimental results. To transition this technology to field applications, recommendations are provided for a Type 2 NCHRP IDEA project at the system level. Results from both the Type 1 and Type 2 research will then be used to formulate design guidelines of bidirectional ductile diaphragms to resist earthquakes from any directions regardless of skewness. These could then be formulated in a language ready for implementation by AASHTO (via T3 Seismic and T14 Steel Design) and by departments of transportation.
The final report is available