C V R Murty

Soft or extreme soft storeys in multi-storied buildings cause localized damage (and even collapse) during strong earthquake shaking. The presence of such soft or extremely soft storey is identified through provisions of vertical stiffness irregularity in seismic design codes. Identification of the irregularity in a building requires estimation of lateral translational stiffness of each storey. Estimation of lateral translational stiffness can be an arduous task. A simple procedure is presented to estimate storey stiffness using only properties of fundamental lateral translational mode of oscillation (namely natural period and associated mode shape), which are readily available to designers at the end of analysis stage. In addition, simplified analytical expressions are provided towards identifying stiffness irregularity. Results of linear elastic time-history analyses indicate that the proposed procedure captures the irregularity in storey stiffness in both low- and mid-rise buildings.

Structural walls are important component of buildings, which have a high lateral load resisting capacity. These walls increase lateral stiffness of whole structure and reduce the lateral deflection. In low rise buildings, structural walls prevent the collapse of structure during strong earthquakes. Squat RC structural walls resist lateral forces through strut and tie action. An analytical study is carried out to understand lateral load behaviour of squat walls for various aspect ratios of squat walls and wall reinforcement ratios; the diagonal strut width and strut angle are assessed, which are crucial inputs in design of squat walls based on strut & tie approach. The results of nonlinear analyses suggest that strut & tie modelling is possible to design squat walls, because there is an orderly behaviour of walls even at large diagonal angle of wall and therefore of the strut angles. Existing code provisions are compared and new provisions to be included in the Indian codes are suggested.

A fresh damage approach is proposed for quantitatively estimating seismic damage in reinforced concrete (RC) frame structures. It considers the energy expended by the structure at each instant of lateral deformation to quantify damage in the structure through a simple damage index iE, which is the ratio of nonlinear energy expended at a deformation instant (on the pushover curve) to total nonlinear energy capacity of the structure. three definitions are considered to define damage index iE of the structure. these definitions are applied on pushover curves of two example 6 and 10 story RC bare frame buildings designed as per Indian seismic code. of the three definitions for the damage index iE, one with expended energy concept proved to be most appropriate, which reflects the true meaning of damage.

A simple hand calculation based method is presented to develop axial flexure design P-M interaction curves of rectangular reinforced concrete sections. The proposed method uses basic principles of mechanics satisfying compatibility of strains, equilibrium of forces and constitutive relations of constituent materials. Simple step-wise calculation is enough to develop the interaction curve; it does not require any iterations. Accuracy of the method is demonstrated by comparing the results of two RC sections with the interaction curves of the sections obtained using SP:16. Step-wise calculations are presented of the two RC sections to illustrate the use of this method for generating design P-M interaction curves.

Torsional response of buildings is attributed to poor structural configurations in plan, which arises due to two factors – torsional eccentricity and torsional flexibility. Usually, building codes address effects due to the former. This study examines both of these effects. Buildings with torsional eccentricity (e.g., those with large eccentricity) and with torsional flexibility (those with torsional mode as a fundamental mode) demand large deformations of vertical elements resisting lateral loads, especially those along the building perimeter in plan. Lateral-torsional responses are studied of unsymmetrical buildings through elastic and inelastic analyses using idealised single-storey building models (with two degrees of freedom). Displacement demands on vertical elements distributed in plan are non-uniform and sensitive to characteristics of both structure and earthquake ground motion. Limits are proposed to mitigate lateral-torsional effects, which guides in proportioning vertical elements and restricts amplification of lateral displacement in them and to avoid torsional mode as the first mode. Nonlinear static and dynamic analyses of multi-storey buildings are used to validate the limits proposed.

Seismic design codes permit the use of Equivalent Static Analysis of buildings considering torsional eccentricity e with dynamic amplification factors on structural eccentricity and some accidental eccentricity. Estimation of e in buildings is not addressed in codes. This paper presents a simple approximate method to estimate e in RC Moment Frame and RC Structural Wall buildings, which required no detailed structural analysis. The method is validated by 3D analysis (using commercial structural analysis software) of a spectrum of building. Results show that dynamic amplification factor should be applied on torsional eccentricity when performing Response Spectrum Analysis also. Also, irregular or mixed modes of oscillation arise in torsionally unsymmetrical buildings owing to poor geometric distribution of mass and stiffness in plan, which is captured by the mass participation ratio. These irregular modes can be avoided in buildings of any plan geometry by limiting the two critical parameters (normalised torsional eccentricity e/B and Natural Period Ratio τ =Tθ/T, where B is building lateral dimension, Tθ uncoupled torsional natural period and T uncoupled translational natural period). Suggestions are made for new building code provisions.

This article presents probabilistic seismic hazard analysis (PSHA) of India and adjoined region, carried out to develop a new national seismic hazard map for India. The hazard map is developed using fault oriented spatially smoothed seismicity approach. A catalog of earthquakes has been compiled for the region (Latitude 50N − 400 N and Longitude 650E − 1000E) from 2600BCE to 2019CE to estimate the seismicity parameters. Eighteen suitable ground motion prediction equations in the logic tree framework are used for the four major geological regions of the country. The hazard is estimated at rock sites (B-C boundary type) conditions in terms of peak ground acceleration (PGA), short-period (0.2 s), and long-period (1s) spectral acceleration maps and uniform hazard spectra, with 2% and 10% probabilities of exceedance in 50 years. Higher hazard values are observed in the Hindukush-Pamir regions and Northeast India, whereas central India and the southern peninsular regions are less prone to seismic threat. The proposed maps find their application in the seismic design of structures, risk assessment, and as an input for updating the existing code provisions.

In a companion paper, new tapered configurations are proposed of slender RC structural walls with and without enlarged boundary elements at the wall-footing junction region. On the basis of identified parametric limits and the wall response in linear-elastic finite-element analyses, a stepwise seismic design procedure is proposed of tapered integrated wall-footing systems with soil or rock anchors at the bottom. This incorporates a capacity design of the plastic hinge region above the tapered portion and an elastic design of the tapered portion. The location of the region of seismic damage and energy dissipation in the wall is controlled by proportioning of the tapered wall-footing as per the new design procedure. © 2014 American Society of Civil Engineers.

A quantitative approach is required to ensure desired global ductile behaviour with at least a guaranteed level of deformation capacity of buildings. This paper quantifies the required column-to-beam strength ratio, and insufficiency of the current code prescribed values of strength ratio to achieve a ductile mechanism. Column-to-beam strength ratio is varied by varying column size and reinforcement in the columns; beam size and reinforcement are kept constant. Nonlinear static pushover analyses of designed RC buildings with different column-to-beam strength ratios show different collapse mechanisms, lateral strengths and energy dissipation characteristics. Column-to-beam strength ratio alone does not result in the desired global ductile behaviour and do not guarantee the desired deformability. Further, the required column-to-beam strength ratio is same, depending on the design, for different hazard levels (reflected by the design seismic coefficient). An appropriate value of minimum column-to-beam strength ratio to achieve a ductile mechanism is presented along with pointers to ensure large deformability of buildings.

The paper reviews seismic behaviour and performance of reinforced concrete (RC) deep vertical members, particularly bridge piers and structural walls. The provisions of the relevant Indian codes of practice, concerning shear strength and shear demand in these members, are reviewed in light of the provisions of international codes of practice. The deficiencies are identified in the seismic shear design philosophy, prescribed by Indian codes.