چكيده به لاتين
Structures according to seismic designing philosophy in severe earthquake will be entered to non-linear range. In non-linear range structure shape capability or deformation has main and determinant role in seismic behavior. So in structures designing, we must be respect regulation requirements which they guarantee resistance and rigidity and also we must sufficient pay attention to their shape capability or deformation. Studies results show crossed bracing frames improper behavior in earthquake. These frames have reduced cyclic behavior (bad resistance) and high primitive rigidity and they cause to severe rigidity loss after entering to non-linear range.Therefore,progressive failur(destruction)of bracing members are important in mentioned structures. By inserting earthquake severe forces, lateral load members in bracing frame structure are entered to non-linear range frequently and this event cause to structure total instability. Also earthquake forces are inserted to structure harsh and it may generate local destruction in bracing members. At the result of local destruction and without entered external loads increasing ; additional forces may be generate more members in plastic hinges. Plastic hinges creating pattern in structure was speed up and it causes to reduce ultimate and resistance load through system and at last it causes to structure deformity reduction. In current study, at first we investigate and determine ABAQUS finite elements and pressure brace buckling member’s behavior in different states. By using push over non-linear static analysis with ETABS software , we investigated crossed bracing frames seismic behavior. Parameters effect such as brace members buckling, members removing from structure system, geometry imperfection, stories number, building frame system type and models seismic behavior were studied. Results showed that :
1. Results of profile buckling and post- buckling analysis in current study show that , reducing members effective length coefficient cause to significant increasing of buckling load, but closing to members slink in middle range , members negative post-buckling rigidity amount will increase and pressure members will have brittle behavior.
2.Increasing brace members geometry imperfection in simple frames cause to creating low the first and primitive shearing force in plastic hinge destruction for less load balance. In large geometry imperfection cases ; structure behavior is not effected from brace members.
3.In non-linear statics analysis, using the first type distribution cause to lower capacity estimation in structure. This distribution is non-steady and it effects on structure upper part. So, mentioned structure enters to non-linear region with non-steady form and it can’t use total capacity. A plan was estimated to capacity equal according to structure seismic requirement and more pull amount and it will cause to larger certain l=places change specially in tall buildings against the second distribution type. The second distribution type can estimate structure capacity with high amount and this subject is happened with the reason of load distribution steady increasing on structure. In the other word, more numbers of structure members enter plastics range and structure capacity is estimated more than the first second distribution type.
4.In models with lower amount of braced members buckling effective length coefficient; high pressure capacity of braced members causes to local destruction and it resistant member removing and also it reaches to more placement changing of failur. So, braced members buckling effective length coefficient reducing cause to increasing structure resistance against system members removing.
Keywords: Progressive failur (destruction), Concentrically braced frames, Local failure, Static nonlinear incremental analysis (push over), Lteral load distribution, Deformity, Geometry imperfection
