Non linear time history analysis of a 28 story steel building

Performance assessment for the buildings under earthquake loads has been popular as a study case for years. Many of the buildings including different material types, structural types has been put through either linear or nonlinear analysis to evaluate the performance of the building. In this study, a 110 m tall, 28 story steel building with different structural framing types including Special Moment Resisting Frames (SMRF), Special Concentrically Braced Frames (SCBF) and Eccentrically Braced Frames (EBF)'s have been put through a nonlinear time history analysis via the ground motions obtained based on TEC2007 and evaluated the performance of the structural elements. The building which has been put through a performance assesment involves several types lateral resisting frame systems. One of them is Special Moment Resisting Frames (SMRF) and they have been used widely across the building. The frame system is the oldest and most basic structural frames than other type of lateral resisting frames. Moment frames have been picked and used by engineers throughout the history. They are almost never interferes with the architectural plans and able to dissipate high amount of plastic energy. However, while dissipating that much of plastic energy, it comes with also big demands in terms of story drifts. So they are often preferred to be used along with braced frames. Other lateral resisting frame system has been used for the study building is Special Concentrically Braced Frames (SCBF). They are the type of framing system supported with braces in frame plane and in theory, as old as the Special Moment Resisting Frames. Braced frames are in terms of controlling drifts, undoubtly gives better results than moment frames but not as better as dissipating plastic energy since the response is more rigid than moment frames. The usage of brace elements are also limited since it generally interferes with the architectural plan. Because of this reason, placement of the braced frames are generally depended on the architectural layout. One of the main complexities of the concentrically braced frames are, the capacity of the framing system is depended on the buckling of the compression brace. Last type of the lateral resisting frame system used for the building is Eccentrically Braced Frames (EBF). They are the type of braced frame systems where the braces are connected to the other structural elements with a pre-defined eccentricity. The beam segment in this eccentricity is also called as "link element". The behavior of the EBF's are mixture between moment frames and concentrically braced frames. Meaning, they are able to dissipate huge amount of energy, while controlling drifts better than concentrically braced frames. They also do not interfere architectural plans as much as the concentrically braced frames do. The behavior and mechanism of the EBF's are depended on the length of the link element. If the plastic moment capacity to shear capacity ratio is higher than a value that is specified by codes, the link is called as "long links" and expected to yield in flexure first, before shear. In reverse situation they are called "short links" where they are expected to yield in shear first rather than flexure. The values between these often causes a yielding in both shear and flexure at the same time, so that zone is called as "intermediate links". In this study, OpenSEES has been selected as the main analysis software. Because OpenSEES is open source, short in analysis time and many of the modifications to the analysis is possible. SMRF's has been modelled as beam-column elements, using fibers as plastic hinges throughout the elements and they are verified through a nonlinear time history analysis with SAP2000. The same procedure has been used for SCBF's including modelling the connection plates in agreement with ANSI, determining the length of the gusset plate for two times of the connection plate thickness in order to start out of plane buckling. Rest of the connection plates were modelled as rigid. The clean span between braces has been divided into two and a initial imperfection around a portion of the clear length of the brace has been applied to the perpendicular direction to the framing plane, in order for OpenSEES to be able to start buckling simulation. For the link elements, elements supporting lumped plastic hinges have been preferred. The link element behavior has been defined through a spring at the both ends of the link elements which have tri-linear properties as in shear force deformation, that have three different tangent modulus at pre defined plastic shear force values. After all the modelling and verifying has been carried out in frame base. The structure has been modelled as 6 story grids and they are also verified. Lastly, whole structure has been modelled and put through a modal analysis and compared the results with SAP2000 model. A nonlinear time history analysis has been done to the whole model for seven selected ground motions after all the verifications has been finished. The structure has been checked with drift ratios, column strain ratios, brace axial loads and strains and beam element internal forces. Inter story ratios of the buildings were below the limit of 0.02 which is defined through the codes and standards. Highest drifts occur at the floors where the only lateral resisting systems are EBF's which are between first and fourth floor. Lowest drifts occur at the floors where EBF's and SCBF's exists together, which is between fourth and sixth floor. This is also the most rigid zone of the structure. Columns were expected to remain elastic in order for them to pass the check. Check for columns also has been done via checking the edge fibers at the flanges of the columns. The results showed that all the columns remain elastic. Strain and axial load check for braces also showed that braces have not been yielded nor buckled. According to the calculations before the analysis, the link elements are found to be "long links" therefore they are expected to yield at moment first before shear. Analysis results showed that they are indeed yielded at moment first before shear. Also for the link elements at first four floors yielded at both moment and shear.