Investigation of effect bracing in the performance and progressive collapse of steel tall building including long-span entrance and removed column under lateral loading

One of the most important points of interest for designers in the construction of the tall building is to create a large entrance to the lower floors of the building. The need for such large openings is mainly due to architectural issues such as high traffic congestion, aesthetics, and parking. Creating these large openings, if accompanied by the removal of a column, connects the issue with other structural issues such as the effect of the Construction sequence, progressive collapse, loads during execution, and the presence or absence of auxiliary supports (temporary piles) and making the problem more complex. In this study, 36 regular structures of 10, 20, and 30 floors with a height of 40 to 120 meters were considered with the Moment resistance frame system in the ETABS software. The Iranian National Building Code part 6 has been used for load gravity of structures and the Iranian standard No. 2800 has been used for calculation and loading of earthquake lateral loads. After spectral dynamic analysis and Pushover analysis of the mentioned structures, their behavior was examined from the structural point of view and the effect of using bracing in one to four upper floors of the removed column. In the studied structures, the changes in the process of plastic hinges formation, structural performance level, Demand- Capacity ratio (DCR) of structural elements, period of the first mode, and drift in case of exterior (non-corner) column removal were evaluated. The effect of elimination on the exterior column to create a large entrance to the building on the probability of progressive collapse of the 10-story steel structure was also studied. The results showed that the use of braces to strengthen the large span beam is a convenient and economical solution. In particular, the V braces show better performance compared to the Chevron brace, if the number of braced floors above the desired span is before the inflection point of the building. Because when the braces enter the area adjacent to the inflection point of the building, due to the low ductility in the pressure, with failure in the pressure, they reduce the level of performance of the building. The first plastic hinge at any performance level starts from the inflection point and as a result, the use of low ductility elements in these areas reduces the ductility of the structure. Structures with V brace in most cases have smaller elements than structures with Chevron braces and are more economical.  Structures that are reinforced only by increasing the dimensions of the beam and column sections and without adding bracing have more strength than structures with bracing. However, in this case, the dimensions of a large number of sections compared to the braced structures in some cases increase several times, and therefore this increase in strength will be accompanied by a large increase in cost. The results of pushover analysis and performance-based design showed that if the structure is designed from the beginning according to the common code design, assuming the absence of columns, the structure does not experience a reduction in performance and is generally better than to use braces with higher ductility.