Seismic Behavior of Column Base Connections in Braced Steel Structures

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The principal objectives of this research project are: (1) to determine the causes of damage, (2) to formulate rational and practical analysis and design model, (3) and to develop upgrading schemes, for column base connections in steel structures similar to those observed in Northridge earthquake.Column bases are an essential element of steel moment-resisting or braced frame structures. The designers face two critical issues: (1) How to design a column base for combined axial, moment and shear forces, and (2) What degree of fixity can be counted upon for a given column base design and detailing? Neither of these two issues is addressed in the current AISC LRFD Manual for Steel Construction.Numerous column base failures were observed in earthquakes.This problem is not addressed as the currently in progress SAC Program is primarily focusing on the problem of beam-to-column moment connections. The importance of the problem of column base connections is recognized but no study is either planned or under way.A comprehensive research effort is needed to understand the behavior of column base connections and formulate realistic design and analysis procedures. This can be best accomplished by focusing on the "load path" through the column base connections.1. The principal tasks of this research include: formulation and verification of a FEM-based analytical model of the column base under combined moment, shear and axial force, (2.) to pin-point the causes of poor column base performance, identify the failure modes, and characterize them in terms of the potential for brittle failure, (3) to generalize the model for commonly used column base connections and establish the dominant flow path of forces, to formulate a simplified base connection design/analysis model that captures the principal characteristics of the force flow, to formulate practical details and design procedures for seismic evaluation and upgrading of potentially deficient column bases in existing structures.