Ground Storey Research Articles

Comparative study of strengthening strategies for reinforced concrete frame with soft ground story

One of the common forms of reinforced concrete (RC) framed building is to provide parking facility at ground level which is created by not providing any infill masonry at parking floor level. Due to the presence of infill walls in the entire upper story except for the ground story makes the upper stories much stiffer than the open ground story resulting in their poor performance during earthquakes. So strengthening of such reinforced concrete (RC) frame buildings with an open ground story is indispensable. In the present study several Strengthening options were evaluated for their effectiveness in improving the performance of such building without disturbing the parking facility of ground story based on linear and nonlinear analysis. The strengthening techniques studied were changing column dimension, providing diagonal bracing, lateral buttresses, shear wall, and providing chevron. The Strengthened building results were compared with the results of the original structure to deduce the structural performance improvement and cost associated to each solution were determined to develop cost efficiency relation for different strengthening technique. Providing lateral buttresses in the open first story was found to be more feasible in both case of increase ground story strength and economic point of view among all strengthening options.

Effect of Unreinforced Masonry Infill Walls on Seismic Performance of Reinforced Concrete Framed Structures

Introduction:Unreinforced Masonry infill walls (URM) are commonly used in the Reinforced Concrete (RC) framed structures as interiors and exteriors partition walls. Although they usually are not considered in the structural analysis and design, their influence on the seismic performance of the framed structures is significant. A common practice in the modern and old RC buildings is to remove the URM walls in the lower stories for commercial reasons; garages, storages, shopsetc.Methods:In the present work, the effect of the URM walls on seismic performance of the RC framed structure will be studied. For that, three groups of 2-D three-bay framed structures, which are fully and partially infilled with the URM walls, will be studied. These groups are classified as three stories, six stories, and nine stories RC framed structures representing low, medium and, high rise buildings; respectively. In each group, different infill panels' configuration will be studied in order to simulate the cases of ignoring or considering the stiffness and strength of the URM. Double-strut nonlinear cyclic model for masonry panels has been utilized in order to account for the structural action of the URM walls. Pushover analysis is adopted for the evaluation of the seismic response of the frames considering the material inelasticity and the geometric nonlinearity in the analysis.Results and Conclusion:Some selected numerical simulation results in terms of base shear forces, lateral deflections, and inter-story drift ratios are obtained for all the considered configurations and presented in comparative way. The regular distribution of the infill walls can improve the framed structure performance. However, omitting the infill from the ground story leads to soft story phenomena as the columns in this story are more vulnerable due to the shear forces acting on them.

Seismic fragility of open ground storey RC frames with wall openings for vulnerability assessment

Reinforced concrete (RC) buildings with open ground storey (OGS) are characterized by accumulation of large lateral displacement at the ground storey. This is due to drastic reduction in relative stiffness of the ground storey compared to the upper stories, a phenomenon known as soft storey effect. Openings present in the masonry infill walls reduce both the lateral strength and stiffness of the fully infill RC frames. It is a general perception about OGS buildings that openings present in the infill walls reduce the stiffness of upper storeys, and thus, offsets the soft storey effect. In the present study, this issue is investigated by carrying out a simplified performance assessment of low- to mid-rise masonry infill RC frames with different infill configurations followed by fragility analyses. It is observed from fragility analyses that there is practically no influence of openings in masonry infill walls of OGS frames on lateral load behavior of such frames. OGS frames with any bay and storey configuration, even with large openings in infill walls, remain highly vulnerable to earthquakes. A new representation of fragility, Fragility Flow Plot, is proposed, independent of discrete damage states, where results of the fragility analyses can be shown for different parameters, such as, natural period of vibration, number of bays and storeys, and openings. The present study contributes towards quantification of seismic fragility and vulnerability of OGS building frames and establishes an initiative for seismic fragility based design of OGS frames using the Fragility Flow Plots.

English

English

Earthquake Response of Reinforced Concrete Building Retrofitted with Geopolymer Concrete and X-shaped Metallic Damper

A three-storey half scale reinforced concrete (RC) building is fixed with X-shaped metallic damper at the ground floor level, is designed and fabricated to study its seismic response characteristics. Experimental studies are carried out using the (4 m × 4 m) tri-axial shake-table facility to evaluate the seismic response of a retrofitted RC building with open ground storey (OGS) structure using yielding type X-shaped metallic dampers (also called as Added Damping and Stiffness-ADAS elements) and repairing the damaged ground storey columns using geopolymer concrete composites. This elasto-plastic device is normally incorporated within the frame structure between adjacent floors through chevron bracing, so that they efficiently enhance the overall energy dissipation ability of the seismically deficient frame structure under earthquake loading. Free vibration tests on RC building without and with yielding type X-shaped metallic damper is carried out. The natural frequencies and mode shapes of RC building without and with yielding type X-shaped metallic damper are determined. The retrofitted reinforced concrete building is subjected to earthquake excitations and the response from the structure is recorded. This work discusses the preparation of test specimen, experimental set-up, instrumentation, method of testing of RC building and the response of the structure. The metallic damper reduces the time period of the structure and displacement demands on the OGS columns of the structure. Nonlinear time history analysis is performed using structural analysis package, SAP2000.

Dynamic Testing of Open Ground Story Structure and In Situ Evaluation of Displacement Demand Magnifier

AbstractThe majority of housing tenements in developing countries constitute a class of structure termed open ground story (OGS) of medium rise and highly vulnerable to strong seismic motions. Eval...

Performance evaluation of setback buildings with open ground storey on plain and sloping ground under earthquake loadings and mitigation of failure

Setback structures are highly vulnerable during earthquakes due to its vertical geometrical and mass irregularity, but the vulnerability becomes higher if the structures also have stiffness irregularity in elevation. The risk factor of such structure may increase, if the structure rests on sloping ground. In this paper, an attempt has been made to evaluate the seismic performance of setback structures resting on plain ground as well as in the slope of a hill, with soft storey configuration. The analysis has been performed in three individual methods, equivalent static force method, response spectrum method and time history method and extreme responses have been recorded for open ground storeyed setback building. To mitigate this soft storey effect and the extreme responses, three individual mitigation techniques have been adopted and the best solution among these three techniques is presented.

Assessment of the potential of a range of microhabitats for use as seed production areas in moderately degraded rangelands in semiarid Australia

Previous studies in mulga country with hard-setting red earth soils in north-west New South Wales, Australia, have demonstrated that small fenced seed production areas on local high points can contribute to regeneration of native perennial vegetation, and increases in the soil seedbank, on surrounding slopes. In this study the potential of seven microhabitats in this environment, each replicated twice, for use as seed production areas was assessed indirectly by a study of their functional characteristics and vegetation dynamics. Indices of landscape function (stability, infiltration and nutrient cycling) were determined for each of the 14 sites at the time of fencing and at the end of the study, over 2 years later, by determining the extent and surface characteristics of 53 surface types (in total) along three parallel transects within each site. Variation in establishment and survival of ground storey species among surface types was examined by co-ordination of all plants within belts centred on the line transects. At the start of the study landscape function indices for the ‘rocky ridge with rock outcrop’ microhabitat were not significantly higher than all, or some, other microhabitats. By the end of the study two of the three indices for this microhabitat were significantly higher than for all other microhabitats. This microhabitat also favoured the establishment and survival of the pastorally preferred species Monachather paradoxus Steud and Thyridolepis mitchelliana (Nees) S.T. Blake but did not favour establishment and survival of the pastorally unpreferred species Aristida jerichoensis (Domin) Henrad and Austrostipa variabilis (Hughes) S.W.L. Jacobs and J. Everett. This microhabitat was characterised by extensive areas of ‘water catchment’ surface types associated with in-situ rock outcrop, which facilitated the observed vegetation dynamics. This ‘rocky ridge with rock outcrop’ microhabitat occurs on local high points in the landscape and is readily recognised. It is therefore ideally suited for use as fenced seed production areas to assist rangeland regeneration.

Lateral Load Behavior of an Open-Ground-Story RC Building with AAC Infills in Upper Stories

Autoclaved aerated concrete (AAC) masonry infills in upper stories can be beneficial for improving the seismic response of open-ground-story (OGS), reinforced concrete (RC)–frame buildings. Two reduced 1:2.5-scale models of single-story, single-bay RC frames with and without AAC infill masonry were tested for resistance properties and hysteretic behavior. Low strength and stiffness of AAC masonry, about half of the conventional brick masonry, led to improved load sharing between the infill and the frame, which helped an early development of frame yield mechanism for enhanced energy dissipation. Test results were used to evaluate the reliability of using existing strength and stiffness relations of conventional masonry infilled RC frames for AAC infilled frames. Analytical models were developed to predict the observed hysteretic behavior of tested specimens. Nonlinear analyses of a five-story, four-bay OGS-RC frame were performed for conventional brick masonry infills and relatively softer and weaker AAC infills in upper stories. The results indicated that the undesirable effect of weak/soft ground story mechanism of OGS-RC frames can be reduced to an acceptable level by using AAC infills in upper stories.

Effect of Bracings on the Seismic Response of a Soft-Storey Structure

A majority of the urban areas are growing vertically due to the crunch of horizontal space. This has led to the construction of numerous multi-storied buildings with an open ground storey for the provision of parking space for vehicles. This makes the first storey relatively less stiff compared to the upper stories which are stiffened by infill walls, thereby causing a sudden decrease in lateral stiffness. Recent and past earthquakes all over the globe have shown large number of such reinforced concrete buildings vulnerable to damage or even collapse during an earthquake. In this study, an existing soft storey reinforced concrete building located in Hyderabad was analysed using linear time history analysis considering the ground motion from Bhuj Earthquake (Mw=7.7). Bracings are incorporated at selected locations to stiffen the ground storey. The response of the structure was studied for different cases viz., (a) Neglecting infill strength and stiffness (b) Considering infill strength and stiffness and (c) structure with bracings in the ground storey. A comparative study of these cases is carried concerning storey displacements, storey drifts, the response of building roof and peak base shear. The results depict that the bracings significantly help in reducing the global response of the structure.

Multiplication factor for open ground storey buildings–a reliability based evaluation

Open Ground Storey (OGS) framed buildings where the ground storey is kept open without infill walls, mainly to facilitate parking, is increasing commonly in urban areas. However, vulnerability of this type of buildings has been exposed in past earthquakes. OGS buildings are conventionally designed by a bare frame analysis that ignores the stiffness of the infill walls present in the upper storeys, but doing so underestimates the inter-storey drift (ISD) and thereby the force demand in the ground storey columns. Therefore, a multiplication factor (MF) is introduced in various international codes to estimate the design forces (bending moments and shear forces) in the ground storey columns. This study focuses on the seismic performance of typical OGS buildings designed by means of MFs. The probabilistic seismic demand models, fragility curves, reliability and cost indices for various frame models including bare frames and fully infilled frames are developed. It is found that the MF scheme suggested by the Israel code is better than other international codes in terms of reliability and cost.

Vertical building collapse triggered by loss of all columns in the ground story−Last line of defense

In multi-story buildings, one of the worst collapse types in terms of structural damage and loss of life is the pancake-type collapse, where some or all floors end up lying on top of each other like the layers of a pancake, with the floor contents crushed between them. Mostly observed after strong earthquakes, such collapses are triggered by loss of some or all vertical load bearing elements in some story−often the ground story. Once this occurs, the building part above the lost vertical elements−still intact−starts gaining downward velocity until it meets resistance from below. The ensuing impact forces often lead to collapse progression ending in total collapse. However, there are some examples of buildings in which the columns of an entire story failed and the collapse remained arrested after the subsequent impact. Such cases were observed in the 1995 Kobe earthquake and in the 1985 Mexico City earthquake. There have also been some failed controlled demolition attempts in which the intended total vertical collapse did not occur after letting a building collide with the ground by explosive removal of the vertical load bearing elements in the lowest stories. In an attempt to determine the factors which play the main role in arresting vertical collapse once initiated at the ground level, this paper studies the behavior of vertically falling multi-story building structures impacting a rigid surface representing the ground. A simplified analytical model of the problem is presented. Depending on the structural properties, several possible energy dissipation mechanisms, and−in case the collapse cannot be arrested at impact−collapse modes, are identified.

Seismic Evaluation and Preliminary Design of Regular Setback Masonry Infilled Open Ground Storey RC Frame

Current seismic code presents certain stringent factors for defining frame as regular and irregular. Thereby these stringent factors only decide the type of analysis (i.e. equivalent static analysis or dynamic analysis) to be done. On the contrary, development of new simplified methods such as pushover analysis can give lateral load capacity of any structure (e.g. regular or irregular frame etc.) easily. Design by iterative procedure with the help of pushover analysis for serviceability requirement (i.e. inter storey drift limitation) provided by present seismic code, can provide an alternative to present practicing procedure. Present paper deals with regular setback frame in combination with vulnerable layout of masonry infill walls over the frame elevation (i.e. probable case of “Vertical Stiffness Irregularities”). Nonlinear time history analysis and Capacity Spectrum Method have been implemented to investigate the seismic performance of these frames. Finally, recently developed preliminary design procedure satisfying the serviceability criterion of inter storey drift limitation has been employed for the preliminary design of these frames.

SEISMIC BEHAVIOUR OF OPEN GROUND STOREY REINFORCED CONCRETE BUILDINGS

Open ground storey (OGS) buildings are the most common type of building configurations witnessed particularly in the urban areas due to their inherent functional advantages (Viz., basement/underground parking facilities in Apartment and Commercial complexes). These categories of buildings usually referred as soft storey buildings. These configurations have shown poor seismic performance across the world due to sudden drop in stiffness and strength in open ground storey. Several open ground storey buildings have collapsed even during Bhuj earthquake in 2001 emphasising the need to understand its seismic behaviour. This led to special considerations of specification of large design forces in IS1893 part1 for the soft storey compared to the rest of the structure. These considerations increased the shear capacity of weak storey columns to prevent collapse and subsequent loss of life. Hence, the present study is focussed on simulating the open ground storey buildings. Non-linear static analysis (Pushover) is performed using the response spectrum specified in the IS code. Further, the importance of masonry infill wall and its contribution for strengthening the open ground storey is also discussed.

Seismic Vulnerability of Reinforced Concrete Frame with Unreinforced Masonry Infill Due to Main Shock–Aftershock Earthquake Sequences

This paper presents a parametric study on the inelastic response of six-story reinforced concrete (RC) frames subjected to main shock–aftershock (MS-AS) ground motions. For this purpose, one bare frame (BF) and four masonry RC frames with two infill thicknesses (75 mm or 125 mm) and two infill patterns (open ground story or fully infilled) are considered. They are situated at site class C in Vancouver, Canada, and are designed for office occupancy according to the 2005 National Building Code of Canada. The five frames are subjected to 100 ensembles of MS-AS ground motions scaled to seismic hazard level corresponding to the return period of 2,500 years. For each sequence of earthquakes, change in the fundamental period ( T1) and inter-story drift (ISD) for both MS-AS sequences are quantified. The analysis results show that the period change and ISD were significant for BF, whereas the infilled frames sustained small damage with negligible change in T1.

Application of Energy Dissipation Devices for Seismic Protection of Soft-Story Wood-Frame Buildings in Accordance with FEMA Guidelines

AbstractRecent earthquakes have demonstrated the vulnerability of soft-story wood buildings. A performance-based retrofit option focusing on stiffening/strengthening only the soft ground story, as regulated by FEMA guidelines, was proposed in an attempt to achieve collapse prevention of the ground story while at the same time ensuring damage control of the upper stories. This paper focuses on an alternative retrofit approach that focuses on energy dissipation, in order to achieve a performance that is essentially comparable to the inherent performance target defined in FEMA guidelines. A viscous damper retrofit is designed based on conclusions drawn from parametric studies. Then, the proposed retrofit is validated via slow pseudodynamic hybrid testing of a full-scale three-story building. The test data are presented, interpreted, and compared against both pretest numerical predictions and posttest simulation results after model calibration. It is demonstrated that a damping retrofit, as compared to a stif...

REVIEW ON SEISMIC PERFORMANCE OF MULTI-STORIED RC BUILDING WITH SOFT STOREY

Soft storey is a storey in which the stiffness is less than 70% of the storey above or less than 80% of the combined stiffness of three storeys above. In a multi-storied building, soft storey is adopted to accommodate parking which is an unavoidable feature. This open ground storey is vulnerable to collapse during earthquake. Soft storey in a building causes stiffness irregularity in a structure. Due to this the structures undergoes unequal storey drift, formation of plastic hinges and finally collapse. The presence of infill wall improves the performance of building under the lateral forces. This paper deals with the study of literature of previous researches. These researches focus on the combination of measures adopted on the structure to reduce the effect of soft storey through static and dynamic analysis. The parameters studied in these researches are storey drift, axial and shear forces bending moment, displacement, time period, base shear. Also, it focuses on the equivalent strut approach to consider the effect of infill wall on the performance on building. From these researches, the interest arises to perform static and dynamic analysis to reduce the stiffness irregularity which is the main reason behind the poor performance of the building with soft storey.

FRAGILITY ANALYSIS OF OPEN GROUND STOREY RC BUILDING DESIGNED USING VARIOUS MULTIPLICATION FACTORS

The vulnerability of an element is defined as the probability that the said element will sustain a specified degree of structural damage given a certain level of ground motion severity. Significantly low stiffness and strength in any storeys compared to adjacent storeys is generally referred to as soft ground storey. As the columns of this Open ground storey are weakest element, ground storey is most vulnerable. Open ground storey framed buildings are generally analyzed in practice ignoring infill wall stiffness (linear bare frame analysis). Design codes impose a multiplication factor on the design forces in the columns of ground storey. The present study attempts to estimate and compare performance of open ground storey building designed with three different multiplication factors given by Indian code and Israel code. Thus fragility curves are derived using nonlinear dynamic time history analysis carried on a (G+9) OGS building by using method suggested by Cornell. Probabilistic seismic demand models are developed by using power law model. Results show that performance of upper storeys while applying multiplication factor only to the ground storey needs to be checked. The first storey is more vulnerable than the ground storey except for Israel code.

Displacement-based seismic design of open ground storey buildings

Displacement-based seismic design of open ground storey buildings

EVALUATION OF SIESMIC RESPONSE OF SYMMETRIC AND ASYMMETRIC MULTISTOREYED BUILDINGS

A building on hill slope differs in different way from other buildings. The various floors of such buildings steps back towards the hill slope and at the same time building may have setbacks also. Due to varied configurations of buildings in hill areas, these buildings become highly irregular and asymmetric. Buildings constructed in hill areas are much more vulnerable to seismic environment. In this study, 3D analytical model of four and nine storied buildings have been generated for symmetric and asymmetric building models and analyzed using structural analysis tool “ETABS Nonlinear”. To study the effect of varying height of columns in ground storey due to sloping ground, the plan layout is kept similar for both buildings on plane and sloping ground. The analytical model of the building includes all important components that influence the mass, strength, stiffness and deformability of the structure. To study the effect of infill during earthquake, seismic analysis using both linear dynamics (response spectrum method) as well as nonlinear static procedure (pushover) has been performed.