NATIONAL CONFERENCE ON EARTHQUAKE | Year:2015 | دوره:2 |Papers Count:3

STEEL BRACED FRAME IS ONE OF THE STRUCTURAL SYSTEMS USED TO RESIST EARTHQUAKE LOADS IN BUILDINGS. THE UNEXPECTED FAILURE OF STEEL STRUCTURES DURING PAST STRONG SEISMIC EXCITATION LED TO FULL FILL ADEQUATE STRENGTH FOR MODERN STRUCTURES IN SEISMIC AREAS. MANY EXISTING STEEL STRUCTURES WITH THIS SYSTEM NEED RETROFITTING TO OVERCOME DEFICIENCIES AND TO RESIST SEISMIC LOADS. IN THE PRESENT STUDY, THE SEISMIC BEHAVIOR OF CONCENTRICALLY BRACED FRAME (CBF) STRUCTURES WAS INVESTIGATED AND THE "ZIPPER COLUMN" IDEA FOR RETROFITTING WAS PROPOSED. TWO TYPES OF STRUCTURES, INCLUDING CBF AND CBF WITH ZIPPER COLUMNS FOR 5, 12 AND 20 STORIES WERE DESIGNED PER THE IRANIAN SEISMIC CODE, THE IRANIAN STEEL STRUCTURES DESIGN CODE AND THE SEISMIC PROVISIONS. DYNAMIC ANALYSIS USING 4 EARTHQUAKE RECORDS WERE CARRIED OUT TO OBTAIN DYNAMIC RESPONSES. ACCORDING TO THE ANALYSIS RESULTS, THE MAXIMUM DISPLACEMENT, BASE SHEAR AND THE INTER- STORY DRIFT RATIO IN CBF SYSTEMS ARE DECREASED AS ZIPPER COLUMNS ARE UTILIZED. ALSO THE FRAMES SHOWED GOOD DISSIPATING- ENERGY CAPACITY AND LARGE DEFORMATION DUCTILITY WITHOUT SIGNIFICANT STRENGTH LOSSES. MOREOVER THE SEISMIC BEHAVIOR OF CBF SYSTEM IS IMPROVED.

REINFORCED CONCRETE MOMENT RESISTANT FRAME IS ONE OF THE STRUCTURAL SYSTEMS USED TO RESIST EARTHQUAKE LOADS IN BUILDINGS. MANY EXISTING CONCRETE STRUCTURES WITH THIS SYSTEM NEED RETROFITTING TO OVERCOME DEFICIENCIES AND TO RESIST SEISMIC LOADS. IN THE PRESENT STUDY, THE SEISMIC BEHAVIOR OF RIGID FRAMES WITH KNEE- TYPE BRACES WAS INVESTIGATED AND THEIR RESPONSE PARAMETERS, SUCH AS MAXIMUM STRAIN IN CONCRETE COVER, REINFORCEMENT AND CONCRETE CORE WERE EVALUATED AND HYSTERETIC BEHAVIOR OF FRAMES WERE DISCUSSED. TWO TYPES OF STRUCTURES, INCLUDING MOMENT FRAMES AND BRACED FRAMES WITH KNEE- TYPE BRACES IN BEAM TO COLUMN JOINTS WERE DESIGNED PER THE IRANIAN SEISMIC DESIGN CODE AND THE IRANIAN CONCRETE STRUCTURES DESIGN CODE. ALSO KNEE ELEMENTS WERE DESIGNED PER A TRIAL AND ERROR METHOD DEPENDING ON COLUMNS MAXIMUM AXIAL FORCE. DYNAMIC ANALYSIS USING 5 EARTHQUAKE RECORDS FOR MOMENT FRAMES AND BRACED FRAMES WERE CARRIED OUT TO OBTAIN DYNAMIC RESPONSES AND COMPUTE SEISMIC PARAMETERS.ACCORDING TO THE ANALYSIS RESULTS, CONCRETE CORE AND COVER STRAIN IN BEAMS AND COLUMNS ARE DECREASED WHEN KNEE BRACES WITH DIFFERENT CROSS SECTIONAL AREA ARE USED. ALSO THE MAXIMUM REINFORCEMENT STRAIN IN BEAMS AND COLUMNS SECTIONS IS DECREASED AS KNEE BRACES ARE UTILIZED. AS BRACED FRAMES ENTERS IN PLASTIC RANGE WITH LARGER STORY SHEARS THEREFORE THE PLASTIC CAPACITY OF STRUCTURES INCREASES. ALSO IN BRACED FRAMES, THE MAXIMUM DISPLACEMENTS OF THE STRUCTURE INCREASES SO THE STIFFNESS OF THE STRUCTURE INCREASES AND THE SEISMIC BEHAVIOR OF MOMENT FRAMES IS IMPROVED.

RECENTLY THE USAGE OF CONCENTRICALLY BRACED FRAMES (CBFS) ARE COMMON FOR LATERAL- LOAD RESISTING SYSTEM IN SEISMIC DESIGN. IN CBFS THE BRACES ARE CONNECTED TO THE BEAM AND COLUMN BY GUSSET PLATE CONNECTION. RECENT RESEARCHES SHOW THAT THE SEISMIC BEHAVIOR OF CBFS CAN BE IMPROVED BY CONSIDERING YIELDING IN THE GUSSET PLATE IN THE PERFORMANCE LEVELS. THIS STUDY HAS INDICATED THAT BUCKLING CAPACITY OF CBFS UNDER A CYCLIC LOADING CAN BE PROMOTED BY CONSIDERING THE BEHAVIOR OF GUSSET PLATE THROUGH DESIGNING ITS THICKNESS AND OBSERVE THE FREE BENDING LINE AND THICKNESS OF EDGE STIFFENER BY THE COMMERCIAL FINITE ELEMENT SOFTWARE ABAQUS. AT FIRST THE FINITE ELEMENT AND EXPERIMENTAL RESULT HAVE BEEN VERIFIED, FINALLY IT WILL SHOW THE CBFS PERFORMANCE DEPENDS ON GUSSET PLATE CONNECTIONS. THE RESULTS INDICATED THAT INCREASING THE THICKNESS OF GUSSET PLATE HAS THE MOST EFFECT ON TOLERABLE LOAD OF FRAME AND PROMOTE STIFFNESS AND PERFORMANCE OF BRACED FRAME.