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.

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.

Capacity design precludes brittle actions thereby maximising energy dissipation capacity of moment resisting frame buildings through flexural yielding in beams before possible yielding in columns during strong earthquake shaking. The flexural strength of columns is required to be more than that of beams framing into it. Seismic design codes stipulate guidelines for design and detailing of members to achieve desired ductile behaviour of buildings. This paper examines seismic behaviour of RC moment frame buildings in seismic Zone V and IV designed as per the column-to-beam strength ratio (CBSR) requirements of the draft IS 13920, and its adequacy along with the detailing requirements. The CBSR of 1.4 specified in the draft code is not sufficient for buildings in seismic Zone V.

This paper presents an externally reinforced I-beam-to-box-column seismic connection. An inclined rib-plated collar-plated configuration with web plates is used to ensure planar continuity between I-beam and box-column webs; the rib plates, inclined in plan between the beam web and the two column web planes, along with collar-plates encircling the box-column at beam flange levels and web plates in plane with the rib plates at the beam web level constitute the new configuration. This connection configuration relieves stresses on box-column flanges and helps in force transfer to the box-column webs. Performance evaluation of the proposed connection configuration shows that sufficient inelasticity is mobilized in the beam away from the column face with connection elements and welds remaining elastic. The seismic performance of the proposed connection is also found to be better than two state-of-the-art connection schemes in terms of higher strength, stiffness, and higher reserve strength of the welds under cyclic displacement loading. © 2010 ASCE.

In multistoried RC wall-frame buildings, properly designed and detailed RC slender structural walls significantly improve earthquake resistance. In walls on isolated spread footings with marginal taper, severe stress concentration is observed at the wall-footing junction during earthquake shaking. In this paper, new tapered configurations are proposed in the bottom portion of walls with and without enlarged boundary elements. Analytical correlations are derived among salient structural and soil parameters of the tapered wall-footing. An extensive parametric study is carried out through linear-elastic finite-element analysis of an isolated wall-footing system under estimated actual vertical and lateral forces. Under the estimated forces, significant loss of contact is observed at the bottom of the wall-footing; thus, soil or rock anchors need to be provided to ensure stability of the wall-footings during strong shaking. Force flow from wall to footing improves significantly in the proposed integrated wall-footing system. In the wall, the region of the inelastic response and possible seismic damage is expected to occur above the tapered region and away from the footing level. Permissible parametric limits are also proposed through the observed stress-deformationresponse. © 2014 American Society of Civil Engineers.