Durgesh C. Rai

Code Approaches to Seismic Design of Masonry-Infilled Reinforced Concrete Frames: A State-of-the-Art Review

Masonry infill (MI) walls are remarkable in increasing the initial stiffness of reinforced concrete (RC) frames, and being the stiffer component, attract most of the lateral seismic shear forces on buildings, thereby reducing the demand on the RC frame members. However, behavior of MI is difficult to predict because of significant variations in material properties and because of failure modes that are brittle in nature. As a result, MI walls have often been treated as nonstructural elements in buildings, and their effects are not included in the analysis and design procedure. However, experience shows that MI may have significant positive or negative effects on the global behavior of buildings and, therefore, should be addressed appropriately. Various national codes differ greatly in the manner effects of MI are to be considered in the design process from aseismic performance point of view. This paper reviews and compares analysis and design provisions related to MI-RC frames in seismic design codes of 16 countries and identifies important issues that should be addressed by a typical model code.

Seismic evaluation and upgrading of chevron braced frames

Abstract Many Chevron type “ordinary” steel concentric braced frame (OCBF) structures have suffered extensive damage in recent earthquakes which raises concerns about their performance in future earthquakes. A building in the North Hollywood area, which suffered major damage in the 1994 Northridge earthquake, was selected for detailed study. Response spectrum, nonlinear static (pushover), and nonlinear dynamic (time history) analyses for a ground motion recorded at a nearby site compared well with the observed damage. The state-of-health of the damaged structure was assessed to determine the need and extent of repair. The seismic performance of non-ductile CBFs can be improved by delaying the fracture of braces, e.g., in the case of the tubular braces by filling with plain concrete. Changing the bracing configuration from chevron to 2-story X configuration can avoid the instability and plastic hinging of floor beams. Further improvement can be achieved by redesigning the brace and floor beams to a weak brace and strong beam system, as in Special CBFs. This full upgrading to SCBFs results in excellent hysteretic response and, with inelastic actions confined to ductile braces, exhibits reasonable distribution of damage over the height of the building.

Review of Seismic Codes on Liquid- Containing Tanks

Liquid storage tanks generally possess lower energy-dissipating capacity than conventional buildings. During lateral seismic excitation, tanks are subjected to hydrodynamic forces. These two aspects are recognized by most seismic codes on liquid storage tanks and, accordingly, provisions specify higher seismic forces than buildings and require modeling of hydrodynamic forces in analysis. In this paper, provisions of ten seismic codes on tanks are reviewed and compared. This review has revealed that there are significant differences among these codes on design seismic forces for various types of tanks. Reasons for these differences are critically examined and the need for a unified approach for seismic design of tanks is highlighted.

Landscape Changes in the Andaman and Nicobar Islands (India) after the December 2004 Great Sumatra Earthquake and Indian Ocean Tsunami

Lifeline systems in the Andaman and Nicobar islands performed poorly during the December 2004 Great Sumatra earthquake and tsunami. Several power stations and transmission lines were damaged by the ground shaking, affecting the electric power supply to parts of the islands. Telecommunication services were severely affected because of destruction of several telephone exchanges. These services were restored quickly by government agencies. The dams and reservoirs, which supply potable water, sustained minor damage from ground shaking. However, segmented pipelines connecting the dams and reservoirs to various storage sites broke at several places, which significantly affected the water supply for a few days. Ground shaking damaged several elevated as well as ground-supported storage tanks. Damage related to tsunami waves was substantial in the 500–1,000-m strip immediately next to the coastline.

Seismic Behavior of Framed Masonry Panels with Prior Damage When Subjected to Out-of-Plane Loading

Framed masonry panels are subjected to both in-plane and out-of-plane loading during earthquakes and their load-carrying capacity in the out-of-plane direction after being damaged is crucial for overall stability and safety. To assess the effect of in-plane damage on their out-of-plane behavior, three half-scaled clay brick framed masonry panels were subjected to a sequence of slow cyclic in-plane drifts and shake table-generated out-of-plane ground motions. The framed panels maintained structural integrity and out-of-plane stability even when severely damaged. Also, failure of specimens was primarily due to excessive out-of-plane deflection, rather than amplified inertia forces. Weaker interior grid elements divided masonry in smaller subpanels, and helped delay failure by controlling out-of-plane deflection and significantly enhancing the in-plane response. This subpaneling also greatly improved the in-plane response and energy dissipation potential, and consequently, the out-of-plane failure of the masonry was delayed and large in-plane drifts of up to 2.2% could be safely sustained.

Postyield Cyclic Buckling Criteria for Aluminum Shear Panels

Aluminum shear panels can dissipate significant amount of energy through hysteresis provided strength deterioration due to buckling is avoided. A detailed experimental study of the inelastic behavior of the full-scale models of shear panels of 6063-O and 1100-O alloys of aluminum is conducted under slow cyclic loading of increasing displacement levels. The geometric parameters that determine buckling of the shear panels, such as web depth-to-thickness ratio, aspect ratio of panels, and number of panels, were varied among the specimens. Test results were used to predict the onset of buckling with proportionality factor f in Gerard’s formulation of inelastic buckling. Moreover, a logarithmic relationship between buckling stress and slenderness ratio of the panel was observed to predict experimental data closely. These relations can be further used to determine the geometry of shear panels, which will limit the inelastic web buckling at design shear strains. DOI: 10.1115/1.2793135

Seismic Retrofitting of R/C Shaft Support of Elevated Tanks

The circular, reinforced concrete (R/C) shaft-type support for elevated tanks lacks redundancy, damping and additional strength typically present in building framing systems and, therefore, should be designed for larger seismic resistance. However, the Indian seismic code IS:1893-1984 prescribes the same basic seismic force as that for the most ductile building framing system for which the design force is the least. Furthermore, the code-specified one-mass idealization of elevated water tanks is not appropriate for large (large width to depth ratio) and partially filled tanks. The low design forces lead to a weak and slender support—a very unfavorable feature in high seismic areas, as evidenced in the failure of two water tanks in the 1997 Jabalpur earthquake and a great many in the 2001 Bhuj earthquake. It is rather difficult to enhance the ductility and energy dissipation capacity of thin-walled, R/C shaft supports. Concrete jacketing is used as a retrofit measure to enhance the lateral strength and ductility by changing the failure mode of concrete crushing to a more ductile tension yielding. This scheme requires substantial strengthening of the existing foundation.

Reconnaissance of the effects of the M7.8 Gorkha (Nepal) earthquake of April 25, 2015

The M7.8 earthquake of 25th April, 2015 caused widespread damage in the Nepal region by destroying many residential, public, religious and cultural heritage buildings and roads due to intense shaking, surface fissures and landslides. This earthquake provided an opportunity to study the vulnerability of the built environment and reassessment of the risk exposure of the region. The reconnaissance trip was aimed at surveying the Kathmandu valley region in Nepal and adjoining districts of Bihar state in India due to their high population density and rapid urbanization. The observed damage in Kathmandu and the northern districts of Bihar were consistent with the intensity reported in these regions. Complete collapse was observed in RC buildings and old unreinforced masonry buildings due to inherent structural defects in regions of MM intensity VIII and IX. Significant number of cultural heritage structures suffered partial to complete collapse. These observations provide a perspective on the widespread lack of preparedness even when the seismic hazard of the Himalayan region is well established. This letter cites some of the poor construction practices that are followed in the Kathmandu valley region which make the built environment vulnerable to unacceptable levels of damage under expected design levels of shaking.

Seismic strengthening of unreinforced masonry piers with steel elements

The system of wall piers and spandrels, created by openings, largely controls the inplane lateral resistance of the wall. For the “rocking-critical” masonry wall piers, the overall hysteretic behavior can be significantly improved by installing a steel framing system consisting of vertical and horizontal elements around the wall — without any braces. Vertical elements provide the necessary hold-down forces to stabilize the rocking piers. The stabilized piers “rocked” through a number of cycles of large displacements (up to 2.5%) without crumbling or shattering, displaying a ductile response. The strengthened system has excellent strength, stiffness and ductility, despite the brittleness of the masonry because of considerable load sharing between the existing masonry and the added steel elements. FE analyses predicted the envelope response of the rocking piers accurately. A simple mechanics based model was developed to predict the load-deflection behavior of a stabilized rocking pier which can be used to design the strengthening system more rationally.

A Novel Technique of Seismic Strengthening of Nonductile RC Frame using Steel Caging and Aluminum Shear Yielding Damper

A novel strengthening scheme for seismically-weak RC frames is proposed which utilizes external steel caging to improve flexural/shear strength of columns and aluminum shear-yielding damper (Al-SYD) to further enhance lateral strength, stiffness and overall energy dissipation capacity of the frame. This paper describes the effectiveness of this scheme as evidenced in an experimental study on a reduced scale (1:2.5) single-story, single-bay, gravity-only designed reinforced concrete (RC) frame. The strengthened frame was simultaneously subjected to gravity loads and reversed cyclic lateral displacements as per ACI-374 loading protocol. An innovative connection scheme was designed to transfer a portion of frame lateral load to the energy dissipation device (Al-SYD). Besides the significant increase in lateral strength and stiffness of the strengthened frame, RC frame members did not suffer any major damage during the entire test protocol. This indicates significant reduction in force demand on existing RC members because of enhanced energy dissipation through hysteretic shear yielding of aluminum panels. Moreover, the simple connection scheme proposed in this study proved very efficient in transferring the frame lateral load to strengthening elements.