performance based seismic design framework for rc floor diaphragms in dual systems

Abstract Floor diaphragms play several roles in the seismic response of dual systems: support vertically spanning components, transfer lateral forces to walls and frames, provide restraint to columns and walls, tie the structure together, and enable redundant load paths for lateral forces. In many buildings after events as recent as the 2010-2011 Christchurch earthquakes, floor diaphragms were unable to perform one or more of these functions, leading to extensive damage and collapse. Research consistently highlights three underlying causes: a failure to ensure the integrity of the load path, underestimation of in-plane forces, and poorly understood interactions with walls, supporting beams and RC moment frames. Most recent and ongoing research has focused on modifying the prescriptive code provisions, and indeed many codes–but not all- have been consequently updated. Such prescriptive rules, however, are not helpful for assessing existing buildings, comparing alternate means and methods, or in displacement-based design (DBD) for which a performance-based design (PBD) framework is necessary. This paper proposes a new performance-based framework for the seismic design of reinforced-concrete (RC) floor diaphragms with or without precast elements. The floor diaphragm performance limit states (LS) are re-defined in terms of the observed failure modes (FM). Results from prior research on these failure modes are used to select damage measures (DM, e.g. 'crack width') for pairs of FM and LS. Expressions for DMs in terms of engineering demand parameters (EDP, e.g. , 'strain') are derived from experimental results or from first principles. EDPs are the basic output from numerical analysis in PBD; this paper comments on the suitability of different analytical approaches

Seismic assessment of rc structures with infill masonry panels in Nepal: sensitivity analysis

Reinforced concrete (RC) buildings in Nepal are constructed as RC frames with masonry infill panels. These structures exhibit a highly non-linear inelastic behaviour resulting from the interaction between the masonry infill panels and the surrounding frames. In this context, the paper presents an extensive case study of existing RC-framed buildings in a high seismic risk area in Nepal. A sensitivity analysis of the structures with masonry infill is performed. For this, the influence of different material properties is studied, namely diagonal compressive stress, modulus of elasticity and tensile stress of masonry infill panels. Result shows the influence on the structural behaviour particularly by variation of the diagonal compressive strength of infill masonry panels

Mohr circle-based graphical vibration analysis and earthquake response of asymmetric systems

The maximum seismic response of torsionally coupled plan asymmetric structures can be rationally visualized and computed through a Mohr Circle Response Spectrum Analysis (MCRSA). This is done combining the graphic modal properties of the torsional dynamic equations of motion with the structural earthquake demand in terms of a displacement spectrum as a function of the modal eigenvalues SD(ω2). A compact representation of the modal properties and of the response envelope is built and visualized in the Mohr plane. The maximum modal responses are then combined using a graphic adaptation of the SRSS and CCQ combination rules based on the elastic response spectrum. This Graphic Dynamic rule proves to be an effective response prediction tool, and is particularly suited to estimate the response of seismic base isolation systems

Seismic safety assessment of existing masonry infill structures in Nepal

Reinforced concrete (RC) buildings in Nepal are constructed with RC frames and masonry infi ll panels. Thesestructures exhibit a highly non-linear inelastic behavior resulting from the interaction between the panels and frames. Thispaper presents an extensive case study of existing RC buildings in Nepal. Non-linear analyses were performed on structuralmodels of the buildings considered as a bare frame and with masonry infi ll, in order to evaluate the infl uence of infi ll wallson the failure mechanisms. Five three-storey buildings with different structural confi gurations and detailing were selected.The effect of masonry infi ll panels on structural response was delineated by comparing the bare-framed response with theinfi ll response. Seismic performance is evaluated with regard to global strength, stiffness, energy dissipation, inter-storeydrift, and total defl ection of the structure. A parametric analysis of structures with masonry infi ll is also performed. Forthis, the infl uence of different material properties is studied, namely diagonal compressive stress, modulus of elasticity andtensile stress of masonry infi ll panels. Study results show that masonry infi ll increases the global strength and stiffness ofthe structures; it decreases the inter-storey drift and hence the total displacement of the structure. The results quantify theinfl uence of the infi ll panels on structural response and, in particular, the effect of the diagonal compressive strength of themasonry wall

Assessment of seismic strengthening solutions for existing low-rise RC buildings in Nepal

The main objective of this study is to analytically investigate the effectiveness of different strengthening solutions in upgrading the seismic performance of existing reinforced concrete (RC) buildings in Nepal. For this, four building models with different structural configurations and detailing were considered. Three possible rehabilitation solutions were studied, namely: (a) RC shear wall, (b) steel bracing, and (c) RC jacketing for all of the studied buildings. A numerical analysis was conducted with adaptive pushover and dynamic time history analysis. Seismic performance enhancement of the studied buildings was evaluated in terms of demand capacity ratio of the RC elements, capacity curve, inter-storey drift, energy dissipation capacity and moment curvature demand of the structures. Finally, the seismic safety assessment was performed based on standard drift limits, showing that retrofitting solutions significantly improved the seismic performance of existing buildings in Nepal

OPTIMUM DESIGN OF A HYBRID ISOLATION DEVICE FOR SERVER RACKS USING CONSTRAINED DIFFERENTIAL EVOLUTION ALGORITHM

Nonstructural elements and contents often constitute a large fraction of the economic investment in ordinary buildings. In case of seismic events, damage to nonstructural elements not only contributes to the overall direct material costs but can also significantly impact the indirect costs. The latter are especially affected by earthquake-induced damage if production and business flows depend on proper functioning of such nonstructural components, since consequent downtime costs turn out to be very high. Within this framework, server racks' performance under seismic loading is of interest in the present work. The economic relevance of these nonstructural components requires the implementation of proper design solutions so that their performance under earthquakes can fulfill specific requirements. In this perspective, including isolation devices between server racks and building floors is deemed effective for enhancing the stability of the protected equipment, preserving the computer components' integrity and, minimizing downtime losses. Hence, the present work is meant to optimize a hybrid isolation system for server racks. Specifically, the hybrid isolation device designed for such application combines at least two elastomeric isolators and three sliders, and it is intended for the seismic protection of server racks characterized by different configurations. The objective function is formulated to minimize the accelerations transmitted to server racks and manufacturing cost

Earthquake loss estimation for the Kathmandu Valley

The capital city, Kathmandu, is the most developed and populated place in Nepal. The majority of the administrative offices, headquarters, numerous historical monuments, and eight World Heritages sites are in the Kathmandu Valley. However, this region is geologically located on lacustrine sediment basin, characterized by a long history of destructive earthquakes. The past events resulted in great damage of structures, losses of human life’s and property, and interrupted the social development. Therefore, earthquake disaster management is one of the most serious issues in highly seismically active regions such as the Kathmandu Valley. In recent years, the earthquake risk in this area has significantly increased due to uncontrolled development, poor construction practices with no earthquake safety consideration, and lack of awareness amongst the general public and government authorities. In this context, this study explores the realistic situation of earthquake losses due to future earthquakes in Kathmandu Valley. To this end, three municipalities: (a) Kathmandu metropolitan city (KMC), (b) Lalitpur Sub-Metropolitan City (LSMC) and (c) Bhaktapur Municipality (BMC) are selected for study. The earthquake loss estimation in the selected municipalities is performed through the combination of seismic hazard, structural vulnerability, and exposure data. For what concerns the seismic input, various earthquake scenarios considering four seismic sources in Nepal were adopted. Regarding the exposure, data about the type of existing buildings, population, and ward level distribution of building typologies is estimated from the recent national census survey of 2011. The economic losses due to the scenario earthquakes are determined using fragility functions. The commonly used standard fragility curves are adopted for adobe, brick/stone with mud mortar buildings, and brick/stone with cement mortar buildings. For the reinforced concrete structures, a new fragility model was derived considering four construction typologies: i) current construction practices (CCP), ii) structures according to the Nepal buildings code (NBC), iii) structures according to the modified Nepal building code (NBC+) and iv) well designed structures (WDS). In this study, a set of fragility functions is converted into a vulnerability model through a consequences model. Finally, the ward level distribution of damage for each building typology, building losses and the corresponding economic loss for each scenario earthquake is obtained using the OpenQuake-engine. The distribution of damage within the Kathmandu Valley is currently being employing in the development of a shelter model for the region, involving various local authorities and decision makers

Micro-scale continuous and discrete numerical models for nonlinear analysis of masonry shear walls

A novel damage mechanics-based continuous micro-model for the analysis of masonry-walls is presented and compared with other two well-known discrete micro-models. The discrete micro-models discretize masonry micro-structure with nonlinear interfaces for mortar-joints, and continuum elements for units. The proposed continuous micro-model discretizes both units and mortar-joints with continuum elements, making use of a tension/compression damage model, here refined to properly reproduce the nonlinear response under shear and to control the dilatancy. The three investigated models are validated against experimental results. They all prove to be similarly effective, with the proposed model being less time-consuming, due to the efficient format of the damage model. Critical issues for these types of micro-models are analysed carefully, such as the accuracy in predicting the failure load and collapse mechanism, the computational efficiency and the level of approximation given by a 2D plane-stress assumption.Peer ReviewedPostprint (author's final draft

Nonlinear static analysis by finite elements of a Fujian Hakka Tulou

Hakka Tulous are massive circular earth constructions of the Fujian Province, China, included in the UNESCO World Heritage list. They are subjected to earthquakes of medium magnitude, but their response to the seismic action is not yet investigated in depth. The seismic response of Fujian Tulous was herein investigated through pushover analysis modelling the Tulou structure by finite elements. Although the Tulou is a big construction with a circular earth wall of about fifty meters in diameter, a micromechanical approach was used to model the earth nonlinear behaviour. Even if no binder is added to the earthen material, the Concrete Damaged Plasticity model can be adopted and has shown to be effective in modelling its nonlinear behaviour, as well as the nonlinear response of the Tulou earth wall. Performing pushover analysis of a big earth structure using a micromechanical approach seems to give reliable results, that must be proved by future research