Equivalent Lateral Force Procedure Research Articles

Seismic analysis of RC building with plan irregularity in Baghdad/Iraq to obtain the optimal behavior

Abstract A study was conducted for four-story reinforced concrete (RC) structure existing in Baghdad-Iraq (residential building) to evaluate the accuracy and reliability of the equivalent lateral force procedure (ELFP) analysis and the response spectrum model (RSA) according to Iraqi seismic code (ISC 2016) for designing and analyzing irregular buildings. The building was classified as an irregular in plan according to American Society of Civil Engineers (ASCE 7-16). The seismic behavior of the building was investigated by verifying many parameters: the ratio of shear demand to shear strength on the columns ( V V / V n {V}_{n} ), internal force (bending moment, shear force, and axial force) for beams and columns, and the inter-story drift ratio (IDR) obtained from the (ELFP and RSA); then, the results were compared with the nonlinear static procedure (NSP) and the nonlinear dynamic procedure (NDP) according to ASCE 7-16. Also, this study investigated the effects of the higher modes in the plan (roof plan displacement). From the results, it can be concluded that ELFP and RSA are not a suitable analysis for asymmetric structures and should use appropriate methods such as the NSP and NDP.

Seismic Behavior of an Inter-story Hinged Lateral Resistance Braced Frame

Based on the deflection behavior of structural components occurring at the lateral deformation of moment frames, a new lateral force-resisting system, the R-BRACE Frame (RBF) system, was proposed. This system is capable of effectively limiting the inter-story drift response of tall buildings. An evaluation was conducted using the finite element method program SAP2000 to simulate the internal force distribution and deformation of the system. The equivalent lateral force procedure (ELFP), the capacity spectrum method (CSM), a linear time-history analysis and the pushover method were applied to assess the yielding mechanism of the structure under different earthquake intensity levels. The findings revealed that the sub-unit, referred to as an R-BRACE, had a major impact on improving a structure's lateral stiffness. The placement of R-BRACE units could guarantee controllable stiffness degradation and enhanced seismic ductility, but would not alter the vertical load transfer path of the initial structure, which makes the RBF system an ideal option for seismic retrofitting. KEYWORDS: Lateral force-resisting system, R-BRACE frame, Enhanced lateral stiffness resistance, Seismic ductility

Influência de pilares-parede no comportamento estrutural de um edifício de múltiplos pavimentos de concreto de acordo com a norma técnica brasileira ABNT NBR 15421:2006

abstract: This paper presents a comparative study of the influence of shear walls on the behavior of a multi-storey concrete building when subjected to seismic actions. Two structures with different earthquake-resistant systems were evaluated. One is composed of concrete frames with usual detailing in both directions (x and y), and the other one is formed by a dual system, composed of frames and shear walls with usual detailing. The Equivalent Lateral Force procedure and the Modal Response Spectrum analysis presented by the Brazilian Technical Code ABNT NBR 15421:2006 were considered. The dual system building presented a greater stiffness with the shear walls arranged parallel to direction y. In the x direction, the responses are equivalent in both buildings. The horizontal forces obtained by the Equivalent Lateral Force method at the base of the buildings were greater than the Modal Response Spectrum method.

Developing a simplified method for analysis and design of isolated structures with the novel quintuple friction pendulum system under bidirectional near-field excitations

Simplified analysis methods for seismically isolated structures proposed in recent structural codes and specifications are frequently used to reduce the computational effort and to simplify the design procedure, either directly for special cases or for checking the results of nonlinear response history analysis. Of the approximate methods, the equivalent lateral force procedure using the effective stiffness and effective damping is one of the best known. In this study, the simplified method is developed by combining the equivalent lateral force procedure with the capacity spectrum method and evaluated in terms of maximum isolator displacements and base shears for isolated structures with recently invented quintuple friction pendulum isolators , with different geometrical and frictional properties, under two different response spectra with corresponding two different sets of bidirectional near-field ground motions for stiff and soft soils site classes. In order to assess the accuracy of the simplified method, the delivered results of the ELF procedure are compared to those of nonlinear response history analysis, by modelling the quintuple friction pendulum isolator 3D element in OpenSees. Eventually, comments on the accuracy of the simplified method are given to make its applications more appropriate in practical design of base isolation systems.

The effect of story drift in a multi-story building under the influence of an earthquake

This paper content is structure subjected sudden story drift as a result from earthquakes, forming hinges and eventually collapsing. The aim of this paper is to develop building thirty story building for seismic in Khartoum using finite element method (FEM) and the equivalent lateral force (ELF) procedure of American code ASCE 7-16. In current work the thirty-story reinforced concrete building was considered is to analyze the seismic behavior of the reinforced concrete structure to find the drift between the story by finding the maximum displacement from the program that causes the building to collapse, by choosing the shear wall as the support system to resist the lateral load and by looking to model the building inclined to the horizontal plane. Calculations were also made on the drift between the story to compare with the allowable drift. It is implemented in the Robot structural program - an ingenious program for designing and analyzing lateral (seismic) loads.

A comparison of structural analyses procedures for earthquake-resistant design of buildings

Several seismic analysis procedures in the latest standards have been developed for structural design and assessment. Since these methods have different advantages and limitations, a comprehensive comparison of these procedures is required to select the most effective one. The three most common methods are the Equivalent Lateral Force (ELF) method, Modal Response Spectrum (MRS) analysis, and Linear Response History (LRH) analysis. This research intends to present a comparative study of these methods, according to ASCE 7-16 standard by utilizing ETABSR software. They were examined in terms of base shear and distribution of story shear forces for a sixth-story reinforced concrete (RC) building, designed according to ACI 318-19 standard. Building code requirements for RC structures with the dual lateral force-resisting system in a high seismic zone are discussed. The results show that the ELF procedure's base shear for the building under consideration is conservative compared to the MRS or LRH analysis. The vertical distribution of the ELF procedure is just a function of the structure's fundamental period; however, the advantage of the MRS and LRH analysis is that they provide information as to how the distribution of mass and stiffness of a structure influences the member forces and displacements.

Performance of high-damping rubber bearings for seismic isolation of residential buildings in Turkey

The applicability of high-damping rubber bearings for seismic isolation of residential buildings in Turkey is studied using numerical and experimental approaches. Seismic isolation system composed of high-damping rubber bearings is designed according to the recently updated Turkish Building Seismic Code-2018 (TBSC2018). Three model buildings of different height at assumed seismically active area are chosen from an actual building database, on which, equivalent lateral force procedure, and time history analyses are carried out.The seismic responses of the buildings are evaluated and the seismic isolation system's efficiency is confirmed. It is pointed out that the requirements in the new code in terms of the maximum allowable shear strain of elastomeric isolators are excessively conservative for those isolators with much larger capacity which is verified by sufficient test data, and as a result, designed isolator size becomes larger than necessary from a practical aspect. In order to verify the isolator design without compliance of shear strain limitation in the code, full-scale prototypes of high-damping rubber bearings are specially designed, developed and are subjected to dynamic loading test under test protocol specified in the code. The specific values of test conditions, such as compressive force, shear displacement and frequency, are developed referring several projects in Turkey. The results are comprehensively discussed and the applicability of high-damping rubber bearing for seismic isolation of residential buildings in Turkey is concluded with numerical and experimental approaches and a possible modification of TBSC2018 regarding maximum shear strain is proposed.

Equivalent linear and bounding analyses of bilinear hysteretic isolation systems

With verifications through many relevant researches in the past few decades, adopting the equivalent lateral force procedure for designing seismically isolated structures as a preliminary or even final design approach has become considerably mature and publicly acceptable, especially for seismic isolation systems that mechanically exhibit bilinear hysteretic behavior. During the design procedure, in addition to a given seismic demand, structural designers still need to previously determine three parameters, such as mechanical properties of seismic isolation systems or design parameters and performance indices of seismically isolated structures. However, an arbitrary or improper selection of given parameters might cause diverse or even unacceptable design results, thus troubling structural designers very much. In this study, first, based on the criterion that at least either two design parameters or two performance indices of seismically isolated structures are decided previously, the rationality and applicability of design results obtained from different conditions are examined. Moreover, to consider variations of design parameters of seismically isolated structures attributed to uncertainties of mechanical properties of seismic isolation systems, one of the conditions is adopted to perform bounding analysis for seismic isolation design. The analysis results indicate that with a reasonable equivalent damping ratio designed, considering a specific variation for two design parameters (the effective stiffness and equivalent damping ratio) could present more conservative bounding design results (in terms of isolation displacement and acceleration transmissibility) than considering the same variation but for two mechanical properties (the characteristic strength and post-yield stiffness).

Seismic economic losses in mid-rise steel buildings with conventional and emerging lateral force resisting systems

This paper investigates the seismic loss performances of mid-rise steel buildings adopting different lateral force resisting systems. The considered lateral force resisting systems include a moment resisting frame, a buckling restrained braced frame, and a frame consisting of re-centering non-buckling braces, which are denoted as MRF, BRBF, and RNBF, respectively. MRF and BRBF were designed following the equivalent lateral force procedure. The re-centering non-buckling braces in RNBF were intentionally designed to enable the system to have the same elastic inter-story strengths and stiffness values as BRBF. Incremental dynamic analyses (IDA) were performed for the three considered systems and the IDA result database were converted into their respective vulnerability functions. This research establishes economic loss performance pyramids based on the critical quantities identified from the vulnerability functions. Taking the volume of the economic loss performance pyramid as a measure, the authors compared the respective advantages and limitations of the considered systems in mitigating seismic economic losses.

Effects of design and seismic parameters on horizontal displacement responses of sloped rolling‐type seismic isolators

The influences of design parameters, including sloping angles of bearing plates and friction damping forces, together with seismic parameters, including corner periods and effective peak accelerations that are usually used for characterizing a design response spectrum, on the maximum horizontal displacement responses of sloped rolling-type seismic isolators are numerically discussed in this study. Because the seismic isolators feature the constant acceleration control (or zero postelastic stiffness) performance, the equivalent linear model conventionally adopted in the equivalent lateral force procedure might not be adequate for predicting their maximum horizontal displacement response under a given seismic demand. Therefore, considering a small number to a large number of coefficients, several statistics-based empirical formulas that are able to approximate their maximum horizontal displacement response are proposed. Not only the accuracy but also the conservative property of the proposed empirical formulas are discussed by comparing their predictions with the nonlinear response history analysis results. To efficiently determine the horizontal displacement capacity of sloped rolling-type seismic isolators during the preliminary design stage, the statistics-based empirical formula considering a reasonable number of coefficients is recommended.

Evaluation of Simplified Analysis Procedures for a High-Rise Reinforced Concrete Core Wall Structure

Nonlinear response history analysis (NLRHA) procedure is one of the most precise and accurate numerical method to compute the seismic demands of high-rise structures but is complex, rigorous, and time-consuming and requires a lot of expertise for nonlinear modelling and results interpretation. Therefore, practicing engineers in developing countries like Pakistan still use the simplified analysis procedures to compute the seismic demands. Among the simplified analysis procedures, equivalent lateral force and response spectrum analysis procedures are widely used for the design purpose. However, other procedures have also been proposed in the recent past to accurately capture the higher mode effects in mid-to-high-rise structures. In the current study, results of a forty-story core wall building are used to check the relative accuracy and ease of application of different simplified analysis procedures. Furthermore, a modal decomposition technique is used to separate the modal responses from the NLRHA results, and a mode wise comparison of different demand parameters for different simplified procedures is performed. The current study has been used to clearly identify the reasons of inaccuracies in different simplified procedures. Furthermore, a simplified analysis procedure is proposed to accurately estimate the seismic demands of high-rise buildings and the possible solutions to improve their predictions.

Experimental seismic behavior of a two-story CLT platform building

Cross-laminated timber (CLT) manufacturing and construction has been steadily growing since its inception in Europe in the 1990s. In the US, the growth of the CLT adoption is inhibited by the lack of codified design provisions for CLT in high seismic regions. This led to a multi-year study conducted by Colorado State University to investigate suitable seismic design parameters of CLT shear wall systems. This paper presents the results from a series of shake-table tests featuring a full-scale two-story mass-timber building utilizing CLT Seismic Force Resisting Systems (SFRS). The building was designed using an R- factor equal to 4.0 under the equivalent lateral force procedure specifications of the ASCE 7-16 Standard. The test program included three phases with different wall configurations, reflecting different wall panel aspect ratios and the existence of transverse CLT walls. Test results indicate that the code-level life safety objective was achieved in all test configurations. The addition of transverse walls did not affect the ability of the panels to rock, and improved the performance of the building structural system.

Evaluation of Seismic Analysis Procedures for Concrete Moment-Resistant Frames with Horizontal Re-entrant Corners Irregularity

Seismic design codes and standards such as the IBC 2012 and ASCE/SEI 7-10 allow using different seismic analysis procedures for buildings having horizontal re-entrant corner irregularity. However, such codes allow using Equivalent Lateral Force (ELF) procedure for buildings having re-entrant corner irregularity but not exceeding 50 m in height. The aim of this study is to quantify the limits on building height and on the percentage of re-entrant corner of building, where ELF procedure may not produce a reasonable results when compared to Response Spectrum (RS) and Response Time History (TH) procedures. Usually, these limits in seismic design standards are based on judgments and experience rather than quantification studies. Based on the results of this study, it is observed that using ELF method mostly overestimates maximum displacements and drift ratio. However, this method can be used safely up to 12-storey buildings, because the maximum underestimation of column shear forces for buildings with higher stories gives 30% less compared to other procedures. Moreover, it has been found that the maximum underestimation of column shear forces for buildings without re-entrant corner is 0% less than those with re-entrant corners. Furthermore, buildings analysed by ELF method are also exposed to have a higher percentage value of underestimation of column shear forces as the case of 12-story buildings with 40% less when compared to TH Procedure. In addition, the column at the re-entrant corner is not effectively affected until a re-entrant corner of 25% exists in buildings. Thus the ASCE/SEI 7-10’s limit for using ELF procedure for buildings with re-entrant corner irregularity may be increased from 15% to 25% with less conservative results.

Collapse Prevention seismic performance assessment of new buckling-restrained braced frames using ASCE 41

This paper presents the results of a seismic performance assessment using ASCE 41-13 for six buckling-restrained braced frames (BRBFs) designed in accordance with the 2012 International Building Code. The correlation between ASCE 7-10 and ASCE 41-13 is investigated to compare the seismic performance anticipated by the two standards. Three archetype buildings (4-, 8-, and 16-story) with BRBFs along one principal direction are designed for seismic effects: (1) once using the equivalent lateral force (ELF) procedure and (2) a second time using the response spectrum analysis (RSA) procedure. Performance assessments are conducted using four analysis procedures, static and dynamic analyses performed under both linear and nonlinear analysis regimes. Linear analysis results indicate minor performance deficiencies in the columns and the braces. Surprisingly, the nonlinear analysis results indicate more performance deficiencies in the braces, which is opposite of the general expectation that a more sophisticated analysis would yield a less conservative result. The contributing factors to the performance deficiencies are investigated. Recommendations are made on how to alter the performance outcome such as using alternative ground motion selection approaches (e.g., conditional mean spectrum) and having acceptance criteria based on cumulative ductility demands.

Numerical study on the effects of diaphragm stiffness and strength on the seismic response of multi-story modular buildings

Modular building construction relies on prefabricated modules which are assembled onsite to form complete buildings. The assembly requires modules to be connected at discrete locations and results in the formation of discontinuous diaphragms. Diaphragm discontinuities could potentially lead to structural instability or possible diaphragm failure if unaccounted for. Therefore, the primary objective of this study is to evaluate the influence of in-plane diaphragm stiffness and strength on the seismic performance of multi-story modular buildings. A simplified method is presented to establish diaphragm service stiffness considering shear deformation of individual module diaphragms as well as shear and axial deformation of diaphragm connections. This method is used to construct numerical models of a four-by-four bay four-story modular steel building. Three diaphragm stiffness levels, namely rigid, stiff and flexible, are considered for these models and were each subjected to 44 horizontal ground motions relating to seismic events having a return period of 500 years. The results show that increased diaphragm flexibility leads to inter-story drifts that are dramatically large and inertial forces that are considerably different from calculated values using the equivalent lateral force procedure described in current seismic codes. This study is extended further to evaluate performance targets for both elastic and inelastic diaphragm response under a seismic event having a return period of 2500 years. The results are used to propose new seismic design factors, which include force and ductility amplification, and could be implemented for the design of diaphragm connections in multi-story modular buildings.

Accurate Methods for Elastic Seismic Demand Analysis of Reinforced Concrete Walled Buildings

Concrete walls are used commonly as part of the lateral-load resisting system for buildings in regions of high seismicity. Damage to walled buildings in recent earthquakes has highlighted the seismic vulnerability of these systems. In recent years, most research addressing the seismic design of walls has employed experimental testing and focused on detailing of boundary element reinforcement to improve wall deformability. The research presented here employs numerical modeling and focuses on determining appropriate moment and shear demands for use in design. Previous research by the authors employed experimental data to develop a computationally efficient model that provides accurate simulation of flexural wall response, including compression- and tension-controlled flexural failure modes that have been observed in the laboratory and field. The research presented here used this model to develop validated expressions of the shear demand and the moment envelope. To do so, the earthquake response of idealized walled buildings, ranging in height from six to thirty stories, was numerically simulated. Initially, a series of walled buildings was designed using current U.S. code requirements, with moment and shear demands determined using both the equivalent lateral force (ELF) procedure and elastic modal response spectrum analysis (MRSA). Nonlinear dynamic analyses of these code-compliant buildings were conducted using a set of far-field ground motions scaled to various intensity levels. The results of the nonlinear analyses indicated the shear demands developed during earthquake loading exceed the design demands that were calculated using the elastic analysis methods. This could be expected to result in walls developing undesirable failure modes and exhibiting reduced deformation capacity. Because nonlinear analysis is not practical for design of many walled buildings, the nonlinear analysis results were used to (1) develop new procedures for determining the seismic shear demand to ensure flexure-controlled response and (2) identify moment envelopes for use in design that ensure flexural yielding is isolated to locations identified by the engineer. A suite of walled buildings was designed using the new recommendations. Analyses show that use of these new procedures results in a shear demand/capacity ratio less than 1 and controlled flexural hinging. In addition, the response modification coefficients (i.e., R-factors) were revisited; the results show that lower R-factors are needed for walled buildings with planar or asymmetric walls to achieve acceptable collapse risk.

Seismic Analysis of a Ten-Storey Reinforced Concrete Building in Jazan Area, KSA

Jazan area is located in the most active seismic zone region of the Kingdom of Saudi Arabia where there is a complicated geological structures and tectonics. This project reviews the seismic activities occurred in Jazan area together with reviewing the Saudi Building (Seismic) Code (SBC-301-2007) [1]. A multi-story reinforced concrete building, in Jazan city, was seismically analyzed using the Equivalent Lateral Force Procedure with the aid of STAAD PRO software. The building, which was Ordinary Reinforced Concrete Moment Resisting Frame (ORCMRF), was analyzed in compliance with the provisions of (SBC-301-2007) [1]. The most important parameters governing the analysis of this frame were dead load, live load and seismic loads. Seismic loads were computed as pairs of accelerations versus times. The damping ratio was taken as 0.05 (5% of the critical damping). The ground accelerations versus time periods were calculated using SBC-301-2007 together with parameters necessary to be used as input data for the program to calculate the seismic parameters, i.e., reactions, displacements, base shear, bending moments, shearing forces, drifts. The obtained results show effects of earthquake ground motions on building studied herein are so greater for the higher increases of the values of outputs resulting from seismic loads comparing to that due to static load only. Finally, the results obtained, clearly, show the importance of taking the Saudi seismic code provisions into account when analyzing and designing multi-story buildings in Jazan area.

A comparative evaluation of design provisions for seismically isolated buildings

Seismic isolation method is an innovative mature performance enhancement strategy to mitigate the earthquake risk on structures. As a result of the targeted response modification, feasible engineering solutions can be achieved during the service life of structures. For both bridges and buildings simplified analysis procedure of seismic isolation systems are set forth in guide specifications and design provisions. Equivalent lateral force (ELF) method in buildings can be considered as the simplest method of analysis with high importance. This procedure can be directly used in the analysis and design of seismically isolated structures or it can be used in the establishment of lower-bound limits to nonlinear time history analysis. This paper focuses on the overview of design procedures of isolated buildings and comparison of the analysis results of ELF procedure based on selectively well-known codes and guidelines used in pioneering countries. The buildings were equipped with two commonly used isolation systems named as lead rubber bearing and curved surface friction sliders. A comparison of the 2016 Edition of the Turkish Seismic Design Code with the US(ASCE/SEI 7–10), the European (Italian version of the Eurocode (EC-8) application, NTC-08) and the Japanese (Building Standard Law-2013) codes is conducted on the implementation of the ELF Method for parametric study of structures located at sites of similar probabilistic earthquake hazard in respective countries. Recommendations and concerns associated with the current-state of practice and ongoing development new reference section of TSDC are highlighted by comparing code compliance approaches and practical applications.

Collapse prevention seismic performance assessment of new special concentrically braced frames using ASCE 41

This paper presents the results of a seismic performance assessment using ASCE 41-06 for six special concentrically braced frames (SCBFs) designed in accordance with the 2012 International Building Code. The correlation between ASCE 7-10 and ASCE 41-06 is investigated to compare the seismic performance anticipated by the two standards. Three archetype buildings (4-, 8-, and 16-story) with SCBFs along one principal direction are designed for seismic effects: (1) once using the equivalent lateral force (ELF) procedure and (2) a second time using the response spectrum analysis (RSA) procedure. Performance assessments are conducted using four analysis procedures, static and dynamic analyses performed under both linear and nonlinear analysis regimes. Linear analysis results indicate minor performance issues in the columns and no performance issues in the braces. In contrast, the nonlinear analysis results indicate that the braces consistently fail to meet the assessment criteria. The contributing factors to the noted performance issues with regards to ASCE 41 are investigated. Recommendations are made on how to alter the performance outcome such as using alternative ground motion selection approaches (e.g., conditional mean spectrum), using higher fidelity brace models, and having acceptance criteria based on cumulative ductility demand or energy dissipation.

Force reduction factors for steel buildings with welded and post-tensioned connections

The seismic responses of steel buildings with perimeter moment resisting frames (MRF) with welded connections (WC) are estimated and compared to those of similar buildings with semi-rigid post-tensioned connections (PC). The responses are estimated in terms of ductility reduction factors (R µ,), ductility demands (µ G ) and force reduction factors (R). Two steel model buildings, which were modeled as complex-3D-MDOF systems, were used in the study. Results indicate that the reduction magnitude of global response parameters is larger than that of local response parameters, contradicting the same reduction implicitly assumed in the static equivalent lateral force procedure, implying that non-conservative design may result. The value of 8 for R, suggested in many codes for ductile steel MRF, and the value of 1 suggested in the well known Newmark and Hall procedure for the ratio of R to µ G , cannot be justified. The reason for this is that SDOF systems were used to model actual structures, where higher mode effects, energy dissipation and structural overstrength weren’t explicitly considered. The codes should be more transparent regarding the magnitude and the components involved in the force reduction factors. The seismic performance of steel buildings with PC may be superior to that of the buildings with WC, since their force reduction factors are larger and their ductility demands smaller, implying that PC buildings could be designed for smaller lateral seismic forces. The conclusions of this paper are for the particular structural systems and models considered. Much more research is needed to reach more general conclusions.