Seismic Analysis & Ductile Design of a G+11 Multi-Storied RC Residential Building using ETABS and Cost Increase for R=1.5 (Near Elastic Behaviour & Immediate Occupancy Limit State)

Abstract

With the development and application of earthquake engineering design, the relationship between structural and nonstructural components' performance levels becomes increasingly important. Even if the structural performance level is achieved after an earthquake; failure of architectural, mechanical, electrical, or equipment components may cause considerable economic loss. Furthermore, nonstructural damage may jeopardize the functionality of key services such as in hospitals and laboratories. The cost of contents of a building is usually far more than the cost of the structure housing. It is, therefore, not surprising that in past earthquakes the loss of damage from nonstructural components far exceeded that from the damage to structural components. There is little consideration about nonstructural damages compared to structural damage in our current code practices. As a country prospers economically, the cost of non-structures increases. Therefore, the present seismic design philosophy of permitting a large amount of nonstructural and even structural damage in the form of ductility before collapse is no longer tenable. Almost all damage structural as well as nonstructural increase with inter-story drift. Surprisingly, the seismic codes penalize structural systems that offer higher lateral stiffness. Therefore, in this study, an RCC building is economically designed with various structural systems, and the resulting inter-story drifts are compared. This research focuses on reducing the inter-story drift and life time costs due to earthquakes for realistic buildings. The design follows the Indian Standard for seismic and RC design using various response reduction factor as per Table 9 and compare the total structural dead load, inter-story drift, actual inter-story drift, and total cost increase of the building. The results show that as R values are reduced from 5 to 1.5, the total structural dead load increased between 5.81% to 22.63% and total cost increased by 1.08% to 3.96%. By reducing R values used for design, the total structural and non-structural damage shall be much reduced and the lift-time cost shall reduce by at least 20% to 40% for replacing nonstructural elements, repair, maintenance, retrofitting, and others. IS 13920 considers that plastic hinges shall form at the ends of beams due to lateral seismic loads to create a collapse mechanism, and nowhere else along the length of the beams. Further, these plastic hinges shall be “reversible” or “bi-directional” meaning that they shall be formed for hogging as well as for sagging bending moments.However, it is found that such bi-directional plastic hinges are not formed. Instead, four “uni-directional” plastic hinges are formed in the iv beam. Two hogging plastic hinges are formed at the ends; and, two sagging plastic hinges are formed within the length of the beam. Being uni-directional, the plastic rotations shall keep accumulating in each cycle of earthquake loading. This can give rise to low-cycle fatigue failure. Thus, a structure designed as per the current seismic codes are not only highly uneconomical, but potentially unsafe. They are based on an outdated and demonstrably wrong behavioral assumption. The location of sagging plastic hinges depends on the amount of gravity load on the beam. When only lateral load is considered, then and then only “reversible/bidirectional” plastic hinges form at the beam ends. But, gravity loads are always acting and it is the lateral loads that may or may not be present. For seismic analysis, the lateral loads are a small fraction of gravity loads. When gravity and lateral loads together are considered as acting on a portal frame, then four unidirectional plastic hinges are formed in the beam, as already described. The behavior of unidirectional plastic hinges is much more fragile and low-cycle fatigue failure with degrading stiffness and strength shall occur. Indian Standard IS 13920-2016 mistakenly considers that “reversible/bidirectional” plastic hinges shall be formed. In this study, we considered the bending moment diagram formed in beams of a properly designed realistic building as per code IS: 13920 2016 clause 6.3.3. It is observed that for design level earthquake only two unidirectional plastic hinges can form at the beam-ends; one shall form for loading from left to right and the other shall form when loading is reversed. And, if the earthquake load is increased to create a collapse mechanism, then another sagging plastic hinge shall form within the length of the beam, usually between mid-span point and beam-end opposite the hogging plastic hinge. When the lateral loading direction is reversed, then two other unidirectional plastic hinges shall form.