Research On The Design Theory Of Buckling-Restrained Braces And Buckling-Restrained Braced Frames

Lateral resistance system adoption plays an important role in the design ofstructure, especially for high-rise buildings. Diagonal and V-shape concentric braceswere ever popular in seismic regions, but the 1994 Northridge and 1995 Kobeearthquakes convinced that the degradation of such braces in stiffness and strengthcaused by buckling aggravated the damage to the steel frames under severeearthquakes.The Buckling-Restrained Brace (BRB) is a new brace focusing on the solution tothe problems associated with buckling. The concept of the BRB is the bucklingprevention of the central steel core by encasing it over its length in a steel tube filledwith concrete. The compression and tension loads are designed to be carried only bythe core by means of unbonded layer to allow the free slip between the core and thesurrounding concrete. The BRB can reach and retain fully sectional yielding of thecore under compression, and exhibits stable hysteretic behavior when cyclicallyloaded.Theoretical study is firstly executed to find the rational composition of BRB.Stability analysis is carried out to determine the necessary stiffness and momentresisting capacity of the outer tube, as well as other factors governing the mechanicalbehavior of the BRB.Next, nonlinearity is introduced in the contact analysis by finite element methodto precisely simulate the loading progress of the BRB. The buckling restraining ratio,defined as the ratio of Euler eigen load of the outer tube to the yielding load of thecore, is the major factor governing the behavior of BRB, though other factors such aswidth to thickness ratio of the core, initial imperfections, gap and connection stiffnesscan also affect the behavior of BRB to some extent. Rational values of the abovefactors are suggested. Finite element analysis is verified in comparison with results ofpublished tests.Non-braced, traditionally braced, and BRB braced one-bay, one-story steelframes are compared, both in monotonic and cyclic loading. Results indicate that thebehavior of the frame is enhanced by employing BRBs.Dynamic behaviors of a 7-story BRB braced steel frames are studied byelastic-plastic time history method. Different layouts of the BRBs and the lateralstiffness ratio, which is defined as the lateral stiffeness of the BRB to the frame, arecompared and analysed.Based on the current Chinese design code and the concept of the energy-basedand displacement-based design, a simplified aseismic design method is recommended.Finally, essential points concerning the design theory of BRB braced steel frame andBRB itself are proposed.