Lateral Seismic Loads Research Articles

Application of fuzzy logic approach in predicting the lateral confinement coefficient for RC columns wrapped with CFRP

Worldwide ageing infrastructures which are vulnerable to seismic lateral loads and located in high seismicity regions have arrested the interest of many researchers to find alternative materials and techniques to strengthen in bending and shear, for example reinforced concrete (RC) beams, slabs, columns, etc. There are several strengthening/repair techniques and materials in literature. Although the method of strengthening concrete structures with fiber reinforced polymers (FRP) is a relatively new technique, it has existed for more than two decades. In this context, several confinement models have been developed for FRP-confined concrete for the prediction of stress–strain response and several researchers have developed various constitutive models to measure the increase in the axial strength of concrete due to the confinement effect of FRP laminates. In this study, RC columns wrapped with carbon FRP (CFRP) considering some existing confinement models in the literature have been investigated. Moreover, based on the experimental data set in the literature, a new artificial intelligence-based algorithm (a Mamdani-type fuzzy inference system) was implemented to model the strength enhancement of CFRP confined RC columns using fuzzy logic methodology. Fuzzy logic predicted results were compared with the outputs of a non-linear regression analysis-based exponential model derived in the scope of the present work. The best predictive performances of the models were assessed by means of various descriptive statistical indicators. The comparison of the proposed prognostic approach with existing empirical and experimental data exhibits a very good precision of the developed artificial intelligence-based model in predicting the lateral confinement coefficient in CFRP wrapped RC columns.

多雪地域に建つ学校体育館の妻壁に起因する屋根型単層ラチスシェルの水平地震時鉛直応答の分析と保有耐力評価

The study deals with school gymnasium structures composed of EP roof type lattice shells, gable walls with a high rigidity and with truss arch frames. The collapse mechanisms of the structure are taken to be a horizontal structural collapse such as the member buckling collapse of the lattice shell and a vertical structural collapse such as the tensile member collapse of the gable wall. The purpose of this study is to investigate the static and the dynamic collapse by means of the both elasto-plastic analyses considering a geometric nonlinearity. The estimation method is also proposed to calculate practically the base-shear strength coefficient and the applied seismic lateral and vertical load distributions on the lattice shells, the gable walls and the truss arch frames. The study is practically useful to determine the collapse mechanism and calculate the ultimate lateral strength on the earthquake-proof design for the damage and safety limit earthquakes.

Effect of Inelastic Behavior on the Code-Based Seismic Lateral Force Pattern of Buckling Restrained Braced Frames

In recent earthquakes, the observed collapses have been mainly attributed to inappropriate distributions of stiffness and strength of structural members which result from using inefficient seismic lateral load distribution pattern in design phase. Using nonlinear time history analysis under 88 strong ground motions, this paper investigates the seismic lateral load distribution pattern of 3-, 6-, 12- and 16-story buckling restrained steel braced frames, which have been increasingly used during the past decade. Through comparing analytical results with those predicted by IBC-2009 lateral load distribution equation, it can be concluded that except for low-rise structures, code pattern lateral load is quite different from analytical lateral load pattern in particular for taller structures. Therefore, code pattern lateral load would not necessarily lead to efficient distribution of strength and stiffness within the structure; consequently, the structure does not response in a predictable and desirable manner.

Seismic response estimation of steel buildings with deep columns and PMRF

The responses of steel buildings with perimeter moment resisting frames (PMRF) with medium size columns (W14) are estimated and compared with those of buildings with deep columns (W27), which are selected according to two criteria: equivalent resistance and equivalent weight. It is shown that buildings with W27 columns have no problems of lateral torsional, local or shear buckling in panel zone. Whether the response is larger for W14 or W27 columns, depends on the level of deformation, the response parameter and the structural modeling under consideration. Modeling buildings as two-dimensional structures result in an overestimation of the response. For multiple response parameters, the W14 columns produce larger responses for elastic behavior. The axial load on columns may be significantly larger for the buildings with W14 columns. The interstory displacements are always larger for W14 columns, particularly for equivalent weight and plane models, implying that using deep columns helps to reduce interstory displacements. This is particularly important for tall buildings where the design is usually controlled by the drift limit state. The interstory shears in interior gravity frames (GF) are significantly reduced when deep columns are used. This helps to counteract the no conservative effect that results in design practice, when lateral seismic loads are not considered in GF of steel buildings with PMRF. Thus, the behavior of steel buildings with deep columns, in general, may be superior to that of buildings with medium columns, using less weight and representing, therefore, a lower cost.

Seismic response estimation of steel buildings with deep columns and PMRF

The responses of steel buildings with perimeter moment resisting frames (PMRF) with medium size columns (W14) are estimated and compared with those of buildings with deep columns (W27), which are selected according to two criteria: equivalent resistance and equivalent weight. It is shown that buildings with W27 columns have no problems of lateral torsional, local or shear buckling in panel zone. Whether the response is larger for W14 or W27 columns, depends on the level of deformation, the response parameter and the structural modeling under consideration. Modeling buildings as two-dimensional structures result in an overestimation of the response. For multiple response parameters, the W14 columns produce larger responses for elastic behavior. The axial load on columns may be significantly larger for the buildings with W14 columns. The interstory displacements are always larger for W14 columns, particularly for equivalent weight and plane models, implying that using deep columns helps to reduce interstory displacements. This is particularly important for tall buildings where the design is usually controlled by the drift limit state. The interstory shears in interior gravity frames (GF) are significantly reduced when deep columns are used. This helps to counteract the no conservative effect that results in design practice, when lateral seismic loads are not considered in GF of steel buildings with PMRF. Thus, the behavior of steel buildings with deep columns, in general, may be superior to that of buildings with medium columns, using less weight and representing, therefore, a lower cost.

Rocking Effect of a Mat Foundation on the Earthquake Response of Structures

AbstractTo evaluate the effect of soil-foundation interaction on the earthquake response of structures, centrifuge tests were performed using an in-flight earthquake simulator. The test specimen was composed of a single-degree-of-freedom structure model, a shallow foundation, and subsoil deposits in a centrifuge container. The test parameters were the dynamic period of the structure model, the centrifugal acceleration level, and the type and level of input earthquake accelerations. The test results showed that the lateral forces of the structures were limited by the soil-bearing strength (i.e., ultimate moment capacity of the foundation) and the damping effect owing to the rocking motion of the foundation. Thus, even when the periods of the structures were close to the site periods, the lateral forces did not significantly increase. However, it should be noted that, because of the damping effect of the foundation, the maximum seismic lateral load of the structures exceeded the load statically determined b...

Girder Load Distribution for Seismic Design of Integral Bridges

Current seismic design practice related to integral bridge girder-to-cap beam connections allows little or no lateral seismic load to be distributed beyond the girders immediately adjacent to the column. However, distribution results from several large-scale tests have shown that the distribution of column seismic moment typically engages all the girders. An approach utilizing simple stiffness models to predict distribution in integral bridge structures is presented in detail; distribution predictions based on grillage analyses also are compared. The experimental results and the analytical results from the stiffness and grillage models show that current design methods related to vertical load distribution are sufficiently accurate. However, when applied to the distribution of lateral load, similarly obtained results reveal that current design practice does not appropriately account for the amount of load that is distributed beyond the girders adjacent to the column to the nonadjacent girders. The current practice leads to excessive girder-to-cap connection reinforcement, increased girder depth, unnecessarily high seismic mass, and increased construction cost. Finally, this paper makes recommendations for more appropriate distribution of seismic lateral load in integral bridge superstructures.

Seismic optimal design of 3D steel frames using cuckoo search algorithm

Summary The present article is concerned with optimization of real size 3D steel structures under seismic loading based on response spectral and equivalent static analyses. The effect of lateral seismic loading distribution on the achieved optimum designs is investigated. An integrated optimization procedure with the objective of minimizing the self-weight of frame is simply performed interfacing SAP2000 and MATLAB® software in the form of parallel computing. The meta-heuristic algorithm chosen here is the cuckoo search (CS) algorithm recently developed as a type of population-based algorithm inspired by the behavior of some cuckoo species in combination with the Levy flight behavior. The CS algorithm performs suitable selection of sections from the American Institute of Steel Construction (AISC) wide-flange (W) shapes list. Strength constraints of the AISC load and resistance factor design specification, geometric limitations, and displacement constraints are imposed on the considered frames. Results show similar weights for optimum designs using spectral and equivalent static analyses; however, different material distribution and seismic behaviors are observed. Copyright © 2014 John Wiley & Sons, Ltd.

Application of shape memory alloy bars in self-centring precast segmental columns as seismic resistance

The objective of this study is to investigate analytically the performance of self-centring precast segmental bridge columns with shape memory alloy (SMA) starter bars under nonlinear static and lateral seismic loading. For this purpose, a 3D finite element model for hybrid post-tensioned bridge column has been developed. The precast post-tensioned segmental bridge columns possessing a central tendon and adequate transverse confinement provided by the steel tube jacketing as self-centring bridge columns have an undesirable high lateral seismic demand due to their low energy dissipation. In order to eliminate this deficiency while keeping the residual displacement small, SMA starter bars are applied in this system. The effect of post-tensioning (PT) forces of the central strands and SMA bar size are investigated. The results indicate that in high seismicity zones, bridge columns with SMA bars at a higher level of PT forces have a superior performance against earthquake loading.

Seismic response of 3D steel buildings considering the effect of PR connections and gravity frames.

The nonlinear seismic responses of 3D steel buildings with perimeter moment resisting frames (PMRF) and interior gravity frames (IGF) are studied explicitly considering the contribution of the IGF. The effect on the structural response of the stiffness of the beam-to-column connections of the IGF, which is usually neglected, is also studied. It is commonly believed that the flexibility of shear connections is negligible and that 2D models can be used to properly represent 3D real structures. The results of the study indicate, however, that the moments developed on columns of IGF can be considerable and that modeling buildings as plane frames may result in very conservative designs. The contribution of IGF to the lateral structural resistance may be significant. The contribution increases when their connections are assumed to be partially restrained (PR). The incremented participation of IGF when the stiffness of their connections is considered helps to counteract the no conservative effect that results in practice when lateral seismic loads are not considered in IGF while designing steel buildings with PMRF. Thus, if the structural system under consideration is used, the three-dimensional model should be used in seismic analysis and the IGF and the stiffness of their connections should be considered as part of the lateral resistance system.

Seismic response of single-storey buildings with flexible diaphragms

The roof framing in single-storey buildings with large foot prints, generally used for commercial, educational, or institutional purposes, often consists of a flexible steel deck or wood panel diaphragm. Resistance to seismic lateral loads is provided by steel bracings, masonry shear walls, concrete shear walls, wood panel shear walls, or cold formed wall systems. The response of such buildings to seismic loads is strongly affected by the flexibility of the roof diaphragm. Diaphragm flexibility alters the manner in which the inertia forces, shears, and bending moments are distributed along the length of the diaphragm. In addition, it causes a significant increase in the ductility demand on the lateral load resisting system that is expected to be strained into the inelastic range under the design earthquake. Results of a study on the linear and nonlinear seismic response of buildings with flexible diaphragms are presented.

Plastic Hinge Model and Displacement-Based Seismic Design Parameter Quantifications for Reinforced Concrete Block Structural Walls

AbstractA practical alternative to the traditional rectangular cross sections of reinforced masonry structural wall systems is to alter the wall ends to allow for smaller compression zone depths, and thus higher curvatures to develop under increased seismic lateral loads. Despite the significantly enhanced seismic performance of flanged and end-confined masonry structural walls compared with their rectangular counterparts, seismic design parameters related to the former two types of walls have not been widely investigated. In addition, prescriptive design requirements for rectangular walls are under continuous development to meet the ongoing research findings in this area. The focus of the current study is to extract specific seismic design parameters of these three types of masonry walls having different end configurations for different aspect ratios when tested under reversed cyclic loads. The parameters investigated include the equivalent plastic hinge lengths, lp, the hysteretic damping levels and the...

Nonlinear dynamically automated excursions for rubber-steel bearing isolation in multi-storey construction

Conventional building design includes the concept of providing more stiff structural solution to withstand the lateral seismic loading. Introduction of flexible elements at the base of a structure and at the same time ensuring enough damping is the evocative alternate option to mitigate seismic hazard. The device that is capable to meet such criteria is known as isolator. This paper covers design of base isolators for multi-storey buildings in medium risk seismicity region and the structural response evaluation. Automated nonlinear models for dynamic response investigation have been configured. Finite element method (FEM) has been incorporated to envisage the structural response behaviors. Lead rubber bearing (LRB) and high damping rubber bearing (HDRB) have been chosen for inserting isolator link element in structural base. The nonlinearities of rubber-steel bearing have been duly considered. Linear static, linear dynamic and nonlinear dynamic analyses due to site-specific earthquake accelerogram are performed for both fixed based (FB) and base isolated (BI) buildings. Both time domain and frequency domain approaches have been carried out for dynamic solution. It was found that for multi-storey buildings, base isolation diminishes muscular amount of structural responses compared to the conventional fixed base (non-isolated) structures. Allowable higher horizontal displacement induces structural flexibility. The suggested isolation technique is competent to mitigate the structural hazard even under strong seismic vulnerability in optimum manner.

屋根型単層ラチスシェルの地震荷重伝達能力に限界づけられるトラスアーチ架構の動的挙動と保有耐力評価

The study deals with EP roof type lattice shells with ceiling panels supported by truss arch frames on the both sides. The estimation method based on the elasto-plastic dynamic buckling analyses considering a geometric nonlinearity is proposed to calculate practically the base-shear strength coefficient and the applied seismic lateral and vertical load distributions on the lattice shell and the truss arch frame. As far as the collapsed mechanism is concerned, the member buckling occurs in the both structural elements. The proposed method is practically useful to determine the collapse mechanism and calculate the ultimate lateral strength on the earthquake-proof design for the damage and safety limit earthquakes.

Seismic Response of Asymmetrical Infilled Concrete Frames

In this paper, the nonlinear seismic behavior of intermediate moment-resisting reinforced concrete (RC) space frames with unsymmetrical plan in three, four and five stories are evaluated. The plan configurations of these space frames contain reentrant corners. Analyses of these buildings are made with and without considering the masonry infill (MI). For infills, three types of arrangements and two material types (strong and weak) have been considered. For lateral seismic loads, two types of lateral loads distributions have been assumed. The results revealed that the existence of infill increases the stiffness and decreases the drifts. However, by omitting infills from the ground floor (the soft story arrangement), the beams and the columns of the ground floor show inferior performance.

Pushover and Seismic Response of Foundations on Stiff Clay: Analysis with P-Delta Effects

Finite-element analyses are performed for the response to lateral monotonic, slow-cyclic, and seismic loading of rigid footings carrying tall slender structures and supported on stiff clay. The response involves mainly footing rotation under the action of overturning moments from the horizontal external force on—or the developing inertia at—the mass of the structure, as well as from the aggravating contribution of its weight (P-delta effect). Emphasis is given to the conditions for collapse of the soil-foundation-structure system. Two interconnected mechanisms of nonlinearity are considered: detachment from the soil with subsequent uplifting of the foundation (geometric nonlinearity) and formation of bearing-capacity failure surfaces (material inelasticity). The relation between monotonic behavior (static “pushover”), slow-cyclic behavior, and seismic response is explored parametrically. We show that with “light” structures uplifting is the dominant mechanism that may lead to collapse by dynamic instability (overturning), whereas “very heavy” structures mobilize soil failure mechanisms, leading to accumulation of settlement, residual rotation, and ultimately collapse.

Seismic Performance Evaluation on Eccentrically Brace Steel Frame by Capacity Spectrum Method

According to the seismic demand spectra form the code for seismic design of buildings, seismic performances of eccentrically braced steel frames (EBF) in frequently, fortification and seldom occurred different intensity earthquakes were evaluated by the Capacity Spectrum Method (CSM). The capacity curves derived from base force-top displacement curves of structures subjected to the lateral seismic load of seismic codes were obtained from pushover analysis by applying sap2000 software and transformed to the spectrum curves, seismic performance were evaluated according to the intersection between the seismic demand spectra and the capacity spectrum. Seismic performances of 15 EBFs designed according to the Chinese design codes were evaluated by CSM. The certain ductility and displacement capacity of EBF in accordance with the China corresponding design codes are shown and some reference is suggested.

Enhancement of Seismic Performance Using Shear Link Braces in a Building Designed Only for Gravity Loads

The present work attempts to study the behaviour of building designed for gravity loads only under the effect of lateral seismic load. Such a building is generally deficient against lateral forces and need to be retrofitted against lateral earthquake forces. A retrofitting scheme by providing aluminium shear link with chevron braces is suggested to improve its performance. Past earthquakes have shown a great deal of damages to the deficient RC frame buildings designed without any consideration to the lateral earthquake forces. Chevron braces with the aluminium shear link can be implemented as an effective retrofit measure. A comparison of the performance of building initially designed for gravity load only with the retrofitted building using chevron braces with the aluminium shear link is presented in this paper. The behaviour of building is worked out by performing nonlinear static pushover analysis and nonlinear time history analyses. A parametric study has also been carried out to study the effect of shear link and braces on the retrofitted building. The performance of RC building designed for gravity loads only as evaluated from the nonlinear static pushover analysis lies in life safety and collapse prevention range for DBE and MCE level of earthquakes respectively. The same building when retrofitted by using chevron braces with aluminium shear link show improved performance. This device is very simple, economic, effective and can be placed in a building very easily. The dissipation of damaging energy/damage is localised in shear link which can be replaced after a major earthquake.

Convex Model for a New Lateral Load Pattern of Pushover Analysis

The uncertainties of earthquake currently were not considered with the various lateral load patterns of pushover. The convex set theory, which requires much less information, is employed to model the uncertainties of the seismic influence coefficient maximum and the characteristic period of response spectrum. Then the convex analysis method is integrated into the fundamental equation of pushover, and the analytic relationship of lateral seismic load and top displacement of buildings is derived. The results of numerical example shows that the new lateral load pattern of pushover proposed in this research may effective simulate the uncertainties of strong ground motion.

Seismic Base Isolation for Buildings in Regions of Low to Moderate Seismicity: Practical Alternative Design

Although Dhaka city has experienced any moderate to large earthquakes in the past, some recent ground shakings are certainly indications of its earthquake source and vulnerability. In addition, microseismicity data also supports the existence of at least four earthquake source points in and around Dhaka. However, it is important to appropriately consider the seismic lateral load effect in structural design. A newly adopted technology of seismic design is to isolate the superstructure from the substructure with the use of a base isolator. This paper covers the design of base isolators for a building located in Dhaka, Bangladesh, along with its structural and economic feasibility. A time history is generated for Dhaka, adjusting peak ground acceleration as per seismic region from a nearby recorded earthquake. The response spectrum curve based on the site geology of Dhaka is also generated from this time history. Linear static as well as dynamic (time history and response spectrum) analyses have been carried...