Marc O. Eberhard

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The assumed stiffnesses of the structural members of a building strongly influence the computed response of the building to ground shaking. For linear analysis, the member stiffnesses control predictions of the period of the structure, the distribution of loads within the structure, and the deformation demands. For nonlinear analysis, an accurate estimate of the member stiffness is required to reliably estimate the yield displacement, which in turn, affects the predicted displacement ductility demands. Practical, accurate procedures are needed to estimate the effective stiffness up to yielding of each structural component. This research digest compares the measured effective stiffnesses of reinforced concrete columns from the PEER Structural Performance Database (Berry et al. 2004) with stiffnesses calculated following the Federal Emergency Management Agency (FEMA) 356 seismic rehabilitation guidelines (ASCE 2000). The FEMA 356 procedure substantially overestimates the stiffness of columns with low axial loads, in which there can be significant bar slip in the beam-column joints or footings. The digest provides practical recommendations for improving estimates of effective stiffness. Effective Stiffness Model The yield displacement of a column can be considered as the sum of the displacements due to flexure, bar slip, and shear: y flex slip shear ∆ = ∆ + ∆ + ∆ [1] Assuming the column is fixed against rotation at both ends and assuming a linear variation in curvature over the height of the column, the contribution of flexural deformations to the displacement at yield can be estimated as follows:

The 12 January 2010 M w 7.0 earthquake in the Republic of Haiti caused an estimated 300,000 deaths, displaced more than a million people, and damaged nearly half of all structures in the epicentral area. We provide an overview of the historical, seismological, geotechnical, structural, lifeline‐related, and socioeconomic factors that contributed to the catastrophe. We also describe some of the many challenges that must be overcome to enable Haiti to recover from this event. Detailed analyses of these issues are presented in other papers in this volume.

A new concept has been developed for connecting spread footings and precast columns in bridges. The socket connection is constructed by precasting the column, erecting it, and casting the reinforced concrete footing around it. This system saves construction time on site because, in little more than the time needed to construct the footing, both the column and footing can be constructed. Site erection is facilitated by the fact that the field tolerances are essentially unlimited. The longitudinal column bars are straight and are terminated with mechanical anchors. This arrangement improves constructability, because no bars cross the interface between the column and footing, and it provides better transfer of forces in the connection region than is possible with conventional bent-out longitudinal bars. The surface of the column is roughened to improve adhesion to the surrounding cast-in-place concrete. Axial-load tests demonstrated that the connection can resist column axial loads far above those expected in practice. Cyclic, lateral-load tests demonstrated that the seismic performance of the connection is at least as good as, if not better than, that of a comparable cast-in-place system. The recent deployment of the new system in a highway overpass provided both field experience and initial estimates of the potential time savings.