Studies on Vertically Irregular RC Infilled Frame Buildings

Abstract

A regular building is defined as a building with uniformly distributed mass, stiffness, strength and structural form. When one or more of these properties is non-uniformly distributed, either individually or in combination with other properties in the vertical direction, the building is referred to as being vertically irregular. There are many examples of the failure of such buildings in past earthquakes due to non-uniform distribution of structural properties. Major international codes including ASCE/SEI 7 (2016) recognize five different classes of vertical irregularity in multi-storeyed buildings that need special design considerations. Most inter-national design codes either prohibit construction or recommend alternative seismic analysis and design of vertically irregular buildings depending on the degree of irregularity and site hazard. Force-based quantities such as mass, stiffness, and strength or geometrical quantities such as plan dimensions are used by design codes as measures (irregularity indicators) for assessing the degree of vertical irregularity present in buildings. Previous literature have proposed different methodologies to quantify the vertical irregularity of buildings in terms of their elastic mode properties. However, the definition of vertical irregularity of buildings mentioned in the codes and standards appears to be not supported by their associated seismic risk. Present study reviews the existing provisions of quantifying vertical irregularity in the context of seismic risk and found that all the vertically irregular buildings listed in the design codes do not pose higher seismic risk. Seismic risks of these buildings are evaluated in terms of fragility function, drift hazard, probability of failure and design confidence level. A concept of ‘vulnerability indicator’ in RC moment resisting vertically irregular framed buildings is proposed to replace the existing ‘irregularity indicator’. A good correlation between the proposed indicator and associated seismic risk is observed for different types of vertically irregular buildings.
The design codes recommend five different irregularity quantifier, one for each of the five categories of vertically irregular building. If there is no correlation between ‘irregularity measures’ and ‘seismic safety’ exists the purpose of estimating ‘irregularity measures’ is lost. Therefore, a direct performance indicator of seismic risk is essential for the design code to impose special design requirement in place of presently used indirect irregularity indicator. This study also concludes that vertical geometric irregular buildings exhibit seismic risks lower than even a reference regular building and can be excluded from the list of special design group of building codes.
Vertically irregular infill framed buildings are conventionally built with burnt clay brick masonry. However, with growing environmental concern for conservation of natural resources and disposal of waste, fly ash bricks, Autoclaved Aerated Concrete (AAC) and Cellular Lightweight Concrete (CLC) blocks are emerging as a substitute to burnt clay bricks for the construction of masonry infill. AAC and CLC blocks have been widely used as infilled masonry all over the world as a potential infill material due to various advantages. A study on the effect of such modern infill materials in the seismic performance of the vertically irregular building can be useful to ensure the safety of such buildings. However, the variability of mechanical properties related to the modern infill masonry materials are not readily available unlike the conventional building material like steel, concrete, clay and fly ash bricks. For this purpose, an extensive experimental programme was carried out to determine various physical and mechanical properties of AAC and CLC block masonry and best-fit probability distribution models are proposed. Higher order analyses such as XRD and field emission scanning electron microscope (FESEM) are conducted to understand the morphological and microstructural differences in block unit leading to variation in its properties. The proposed probability distributions are used to study performance of typical vertically irregular buildings made of modern infill masonry. The seismic risk of a vertically irregular building with AAC and CLC infill is found to be lower than that with conventional infill materials like clay and fly ash bricks. Although clay and fly ash brick masonry have higher strength and stiffness properties, the lightweight properties may be attributed to the lower seismic risk of buildings with AAC and CLC block masonry. This study concludes that the use of modern lightweight infill materials can improve the building performance in seismically active areas.