VULNERABILITY OF BUILDINGS WITH IRREGULARITY DUE TO OUT- OF-PLANE OFFSETS IN MEXICO

In recent years, the structural system called "transfer-slabs" has been popularized; and it has been used in mid-rise buildings. We detected a significant increase in the construction of this type of structural system in Mexico. These structures are characterized by a floor system containing slabs with principal and secondary beams on the parking stories of the building, which supports a shear wall structure of four to six stories. Most of these shear walls are interrupted in the first level, and hence a discontinuity in elevation is observed, and therefore a significant Out-of-Plane Offsets irregularity is evident. This structural system requires a detailed seismic and structural research due to additional demands that may occur both in the transfer-floor, in the load-bearing walls, and in the connections between walls and slabs. The final goal of this paper is to assign the vulnerability in buildings with out of plane offsets irregularity; and aims to define recommendations for buildings with transfer slab system. We have plans of some actual buildings constructed recently, these structures are located in “Colonia Roma”, one of the most affected areas during the 1985, Michoacán Earthquake. The buildings were reviewed to verify the requirements vertical and horizontal irregularity such as stiffnesssoft story irregularity, Weak Story and Out-of-Plane Offsets, these last two were revised with new expressions proposed here. Nevertheless, in the case of the horizontal irregularity, out of plane offset, all buildings were classified with a high irregularity, because the total discontinuity in wall areas in each principal direction was ranging between 31% and 91%. This wall discontinuity generate that shear forces in bearing walls are increased considerately. Results in both directions of the buildings show that shear capacity is exceeded on a lot of discontinuous walls, and in many walls, shear force ratios are greater than 4. Besides, in order to characterizing buildings structured with this systems, several models with irregularities are studied with ETBAS software. In this parametric study the variables are the discontinuous area, the wall shear force, the stiffness and resistance of the story over the floor transfer, the slab type (solid, waffle, plane), and wall position on the floor. The results showed that increasing the area of interrupted walls, the shear and axial forces in the walls are increased too, reaching to values that exceed the design resistance up to 3 times; a vulnerability index is then proposed. The most relevant results of this parametric analysis showed that increasing the discontinuity, i.e., as there are more walls that are interrupted and supported on the transfer floor system, the shear and axial forces in load-bearing walls are increased too, reaching values that exceed the nominal resistance and design up to more than 3 times. In this research some of the recent findings obtained from analysis on irregular buildings are outlined. Both horizontal and vertical irregularities were considered. The studies involved developing simple equations that can be used by designers to estimate the likely increase in demand due to out of plane offsets irregularity. This study also provides a technical basis for revision of the Mexican Construction code.