Accidental Eccentricity Research Articles

Comparative behavior of circular precast UHPC columns with hoops and spiral reinforcement

Ultra-high performance concrete (UHPC) offers outstanding mechanical properties that can greatly influence the future of our structures. Despite the remarkable durability and high tensile and compressive strength of UHPC, its widespread use in large structural applications remains limited. This limitation is attributed to several factors such as the relatively high cost of the material and the slow development of large-scale production technology and procedures. In light of these challenges, the objective of this research is two-fold: first, demonstrate the successful fabrication of four full precast UHPC columns using economic mixture and construction-scale production in a precast plant setting, and next, compare the axial behavior of the fabricated circular columns with different transverse reinforcement detailing. The study carefully considers the choice of lateral confinement and spacing along the height of the column (spiral and hoop reinforcement with 1.5 in and 3 in spacing). The paper presents the global and local behavior that include load capacity, stiffness, axial and reinforcement strains, and ductility. The spirally confined column with 1.5 in spacing showed 26 % higher ductility than the counter hoop reinforced column, while both configurations showed a significant increase in ductility of 34 % when the transverse reinforcement spacing is reduced from 3 to 1.5 in. The research shows that the ACI design reduction factor for conventional concrete (0.65) and the accidental eccentricity reduction factor (0.8) for hoops are unnecessarily conservative and might be increased to 0.75 and 0.85, respectively as for the spiral reinforcement in the case of UHPC columns.

Steel structure design and analysis of a modern extreme sports center

A modern extreme sports center consists of a three-story main structure and ancillary structures. The overall architectural shape is simple and elegant. In order to ensure that the center has the high strength, earthquake resistance, corrosion resistance and other characteristics required for extreme sports venues, while taking into account comfort and aesthetics, the building structure adopts a steel frame structure system. This article uses the PKPM2021-V2.1.1.1 version software to conduct an overall calculation and analysis of this steel structure project, and checks the floor displacement calculated by the elastic method under the action of wind load or frequent earthquake standard values. The results show that the vertical structure of this structure is The deformation of the axial members meets the requirements of the code; the displacement ratio and inter-story displacement ratio of the structure under the action of specified horizontal seismic forces considering the influence of accidental eccentricity are checked, and the results show that the structure does not belong to torsional irregularity; the overturning resistance of the structure is checked and The shear-to-weight ratio was calculated and the results showed that the stability of the structure met the specification requirements. It can be seen from the above calculation and analysis that this structure can meet the specification requirements under various working conditions, and the structural design is safe and reliable.

An Approach for Modelling Accidental Eccentricity Effects in Symmetrical Frame Buildings Exhibiting Semi-Rigid Diaphragm Behavior

The accidental eccentricity effect is specified, in earthquake codes, to account for the possible uncertainties in the mass and stiffness distribution of the structural system and the effect of the torsional component of the earthquake ground motion on the building. Türkiye Building Earthquake Code (TBEC-2018) considers the additional eccentricity effect only for the cases where rigid diaphragm behavior is provided in the slabs. However, in buildings with A2 and A3 irregularities or flexible diaphragms including insufficient strength and stiffness, the in-plane deformations and stresses on diaphragms may change the behavior of buildings under earthquake loads. In this study, a practical approach to be used in the equivalent earthquake load method was proposed to apply the accidental eccentricity effect on symmetrical frame buildings with semi-rigid diaphragms. The approach is based on distributing the total accidental torsion to the nodes as fictitious forces. Two numerical examples were presented. The first is a single-story precast industrial-type RC building, where the calculation steps of the procedure explained in detail. The building was modeled with both semi-rigid and rigid diaphragm assumptions, and a comparison of two modeling assumptions under accidental torsion was presented. Torsional irregularity factors obtained from building modeled by semi-rigid diaphragm assumption provided greater values with respect to those modeled by rigid diaphragm. This shows the significance of considering accidental eccentricity for semi-rigid diaphragms. The second numerical example, which is a RC concrete building, was used to validate the proposed methodology via finite element model (FEM) built-in algorithm. The obtained displacement demands by using the proposed methodology were very close to FEM results as a reference for the real solution. It is concluded from this study that the proposed methodology is reliable and can be used for modeling accidental eccentricity effects on symmetrical-plan buildings with semi-rigid diaphragms.

Characteristics of torsional ground motions obtained for Turkey and their appraisal against Eurocode 8

In current engineering practice, the torsional effects of ground motions are only considered implicitly through “accidental eccentricity coefficients". In seismic design codes, these coefficients account for not only the unintended (or uncertain) asymmetric distributions of structural mass and stiffness in the plan but also the torsional components of ground motions. The torsional effects may develop due to wave passage effects or the incoherency of translational ground motions along the foundation systems. While it is possible to obtain torsional components of ground motions directly from seismic records with modern instruments, such sensors are uncommon. Therefore, several numerical procedures have been proposed in the literature for estimating torsional components of ground motions from translational measurements. In this study, the Single Station Procedure (SSP)—based on the deconstruction of translational ground motions into body waves and the subsequent reconstruction of rotational ground motions—is employed to obtain torsional ground motions for seven different seismic events in Turkey. Additionally, the corresponding accidental eccentricity coefficients that can cause the same response modification with torsional ground motions are determined. First, a conventional procedure is utilized to obtain the corresponding accidental eccentricity coefficients. Various drawbacks of the conventional procedure, such as negative eccentricity coefficients, are addressed. Then, a relatively obscure definition of accidental eccentricity is revived to propose a novel procedure for determining accidental eccentricity coefficients. It is observed that the proposed procedure yields higher values for the accidental eccentricity coefficients than the conventional procedure. It also observed that the torsional spectrum provided in Eurocode 8 provisions requires higher eccentricity values than those implied by the recorded ground motions from Turkey.

Reliability Assessment of Masonry Infilled RC Frame Building’s Earthquake Performance through Accidental Torsion Consideration

Accidental torsional behaviour induced by horizontal loading is difficult to predict, being a complex phenomenon governed by many variables. This problem gains an additional dimension of complexity when nonlinear responses with imperfections need to be considered. Therefore, evaluation and understanding the influence of accidental torsion are fundamental in seismic reliability estimation. This study offers vital insights based on the results of a 1/2.5 scale three-story masonry infilled reinforced concrete frame building’s test on a shaking table. The building was tested under ten consecutive ground motions with increasing ag/g, recorded at Herzeg Novi station during the 1979-M6.9 Montenegro earthquake. The accidental eccentricity, considered a random variable, resulted from unsymmetrical masonry infill wall damage in an otherwise regular building. Its effect, in relation to that of other random (design) variables, was evaluated utilising weight factors and, in addition, assessed through various building code provisions and state-of-the-art research findings. The analysis revealed that the accidental eccentricity, as compared to other random variables considered, could, under certain conditions, reach values higher than those prescribed by the building codes. This unacceptable seismic reliability clearly warns that accidental torsion of masonry-infilled reinforced concrete frames in in-situ conditions must be considered even in regular buildings. Doi: 10.28991/CEJ-2023-09-02-017 Full Text: PDF

Torsion in symmetric buildings due to soil‐structure interaction: A discussion on code values of accidental eccentricity factor in seismic design

AbstractSoil‐structure interaction (SSI) is often neglected in seismic design because of its inherent complexity. Therefore, SSI‐related research is crucial for ensuring that more significant issues reach engineering practice. This study focused on torsional motion to investigate the accidental eccentricity caused by SSI effect by comparing the proportional relationship between the torsional motion of an SSI system and the translational motion of a fixed‐base oscillator. The SSI model was created using a three‐dimensional rectangular foundation embedded in a layered half‐space with an oscillator mounted on the top. We employed an indirect boundary element method (IBEM) combined with non‐singular Green's functions of distributed loads to calculate the system responses of the SSI. Parametric analysis‐based results in the frequency domain revealed that the accidental eccentricity factor owing to SSI was typically in the range of 0.03–0.15. Additionally, a case study of four buildings subjected to 42 earthquakes revealed that the accidental eccentricity factor exceeded 0.05 in several cases, and 0.2 in some cases. Furthermore, it depended on the excitation spectral shape and dynamic characteristics of the SSI system. Therefore, the coded accidental eccentricity factor value of 0.05 is insufficient for ensuring the safety of buildings from the torsional effect of SSI, and a value of 0.1 is more reasonable.

Determination of Mass Properties in Floor Slabs from the Dynamic Response Using Artificial Neural Networks

Most of the research on accidental eccentricity is directed at both the evaluation of accidental eccentricity design code recommendations and the study of building torsional response. In contrast, this paper addresses how the mass properties of each of the levels of a building could be determined from the dynamic response of a building. Using the dynamic response of buildings, this paper presents the application of multilayer feed forward artificial neural networks (ANNs) to determine the magnitude, the radial distance, and the polar moment of inertia of the mass for each level of reinforced concrete (RC) buildings. Analytical models were developed for three regular buildings. Live-load magnitude and mass position are considered as random variables. Seven load cases were generated for the 1, 2 and 4-story models using two excitations. As for the input parameters of the ANNs, three different choices of input data to the network were used. The developed ANN models are able to predict with adequate accuracy the radial position, magnitude, and polar moment of inertia of masses of each level. The implementation of this method based on ANNs would allow the monitoring, either permanently or temporarily, of changes in mass properties at each building floor slab. Doi: 10.28991/CEJ-2022-08-08-01 Full Text: PDF

Accidental torsion within the frame of nonlinear dynamic analysis using code accidental eccentricities and Monte Carlo simulations

A comparative analysis of responses obtained with typical design procedures that consider accidental eccentricities to provide torsional strength to buildings is presented. Design accidental eccentricities are assessed in terms of both ductility demands and lateral distortions, using dynamic nonlinear analyses and Monte Carlo simulations. Two models representing generic symmetrical steel buildings of four and eight stories are considered. Results suggest that designs based on floor torsion moments lead to similar values of both ductility demands and inter-story drifts, regardless of the fraction (0.05, 0.10 or variable) of the building plan dimension b used as accidental eccentricity ea. The procedures to provide buildings with accidental-torsion resistance based on either floor torsion moments or shifting the centers of mass are not equivalent; the former leads to larger ductility demands. For the dynamic step-by-step (nonlinear) analyses used by some codes for building revision, shifting the centers of mass a distance equal to 0.05b of their nominal locations is a conservative device to take into account the accidental torsion. Moreover, shifting the masses is a simpler method than applying floor torsion moments.

Tensile Behaviour of FRCM Composites for Strengthening of Masonry Structures-An Experimental Investigation.

This paper presents the results of direct tensile tests performed on six different FRCM (fabric reinforced cementitious matrix) strengthening systems used for masonry structures. The emphasis was placed on the determination of the mechanical parameters of each tested system and a comparison of their tensile behaviour in terms of first crack stress, ultimate stress, ultimate strain, cracking pattern, failure mode and idealised tensile stress-strain curve. In addition to the basic mechanical tensile parameters, accidental load eccentricities, matrix tensile strengths, and matrix modules of elasticity were estimated. The results of the tests showed that the tensile behaviour of FRCM composites strongly depends on the parameters of the constituent materials (matrix and fabric). In the tests, tensile failure of reinforcement and fibre slippage within the matrix were observed. The presented research showed that the accidental eccentricities did not substantially affect the obtained results and that the more slender the specimen used, the more consistent the obtained results. The analysis based on a rule of mixtures showed that the direct tensile to flexural tensile strength ratio of the matrixes used in the test was 0.2 to 0.4. Finally, the tensile stress–strain relationship for the tested FRCMs was idealised by a bi- or tri-linear curve.

Simplified Method of Determining Torsional Stability of the Multi-Storey Reinforced Concrete Buildings

This article proposes a simplified method for determining the elastic radius ratio of the multi-storey reinforced concrete building. The elastic radius ratio is the benchmark parameter of the buildings in determining torsional stability during an earthquake. When buildings are torsionally flexible, the torsional component of seismic response amplifies the overall response of the building. Because of the numbers of simplified assumptions such as the adoption of the single-storey model, much of the published articles have a very limited range of application. Quantifying the interaction of different forces in multi-story non-proportional buildings has been the main challenge of these studies. The proposed “shear and bending combination method” solves this by introducing parameters that can determine the relative influence of individual actions. Moreover, the proposed method applies to buildings with all type of structural systems, having asymmetry, and accidental eccentricity. The method is validated through a parametric study consisting of eighty-one building models and using computer analysis. The proposed method and the research findings of this study are useful in determining the torsional stability of the building, in verifying the results of the computer-based analysis, and in optimizing the structural system in the buildings.

Mass eccentricity effects on the torsional response of inelastic buildings – a case study

In this paper, the effects of accidental mass eccentricities on the inelastic rotational behaviour of multi-storey buildings subjected to unidirectional seismic excitations are studied. Design codes require the specification of structural detailing to preserve the post-elastic deformation capacity of building structures. Most of these code provisions are based on studies of planar structures, and it is recommended that any mass eccentricity be taken into account by performing three-dimensional static or dynamic analyses, which involve shifting the centres of the floor masses from their nominal positions to what is called an accidental eccentricity. This paper addresses the issue of accidental mass eccentricities on the torsional response of asymmetric multi-storey buildings, which are displaced beyond their elastic limit. It complements a recent study which investigates this issue in the linear phase of deformation and demonstrates that the effects of any height-wise variation of accidental eccentricities are not significantly different in the elastic and inelastic state of deformation, provided that (a) the structural configuration is designed to provide a predominantly translational response in the elastic state, and (b) the strength assigned to each lateral load-resisting element has been derived from a planar static analysis under a lateral loading simulating a translational mode of vibration.

Evaluation of an accidental-torsion design proposal considering firm-soil ground motions

Using non-linear analyses and Monte Carlo simulations, a simplified accidental-torsion design procedure is evaluated. The design procedure does not use an accidental eccentricity like the building codes do. For the evaluation, four reinforced concrete frame building models of four and seven stories are dynamically studied in the nonlinear range. The models are subjected to a set of five firm-soil, bidirectional seismic records. The design procedure is evaluated by comparing the ductility demands of both beams and columns for three conditions of each building model: a) the torsionally balanced model without accidental torsion (model TB), which establishes the reference values of ductility demands; b) the same nominal model but incorporating accidental torsion via the Monte Carlo method; and c) a model with amplified strength (model AS) according to the accidental-torsion design procedure to be evaluated. Results indicate that there is a probability smaller than 2.5% that accidental torsion can cause ductility demands approximately 20% to 25% larger than those of similar building models without accidental torsion. A comparison of ductility demands for the reference models without accidental torsion and those of models with accidental torsion and designed with the procedure that is evaluated, reveals that the design procedure is effective to control the effects of accidental torsion.

TORSION EFFECTS IN BUILDING SEISMIC PERFORMANCE

The paper considers torsion effects that occur during the building response to seismic action. Computation and parametric analysis are conducted for various values of building eccentricity induced by mass and stiffness variation. Such a model accounts for the actual dynamic effect of accidental eccentricity, usually considered in building design by quasi-static value of the torsion moment. Two types of models are employed to explore dynamic parameters of the building. The models are formed using Wolfram Mathematica software in which the mass and stiffness properties are parametrically related to the basic dynamic characteristics of the building. The commercial software package CSI ETABS ver.17 is used for validation of the model. Seismic performance of the building is evaluated, and the results of the parametric analysis are presented using the shear forces and torsion moment. The analysis showed that the nature of eccentricity has a major influence on distribution of seismic forces due to the torsion.

Mitigating mass eccentricity effects on the rotational response of setbacks structures: An analytical solution for linear systems

Stiffness irregularities induced by the curtailment of some of the lateral load resisting bents introduce serious complexities in the response of multi-story setback buildings under seismic excitations. The effect of floor mass eccentricities may substantially increase the analysis requirements and it may be challenging for practicing engineers to gain an insight into the overall dynamic behavior of setback structures. This paper addresses this issue on a theoretical basis and investigates the torsional response of setback multistory buildings for an arbitrary spatial combination of mass eccentricities. The influence of height-wise mass eccentricities is compared with the nominal no eccentricity case and a simple analytical solution is proposed for minimizing the rotational response of setback structures. The accuracy of the analytical solution is verified using the results of a parametric study on 12-story setback buildings comprising mixed-type curtailed lateral load resisting elements (wall, frame) and representative height-wise variations of accidental eccentricities.

Assessment of Seismic Demand Due to Torsional Ground Motions on Symmetric Skew Bridges

ABSTRACT Potential effects of torsional components of ground motions (TGM) on the overall seismic response of symmetric skew highway bridges have been investigated. Analyses were conducted for various skew angles, plan dimensions, deck-abutment gap sizes, dynamic properties, and soil types (B and D). The models included nonlinear impact between deck-abutment explicitly. It was demonstrated that seismic response of symmetric highway bridges are amplified consistently due to TGM components for all considered parameters. It is further suggested that a systematic definition of “accidental eccentricity” in terms of system dynamics, geometry, and associated design spectral response may be used in lieu of analysis.

New Proposal to Incorporate Seismic Accidental Torsion in the Design of Buildings

This work studies a new design procedure to account for accidental torsion in buildings subjected to seismic ground motions. The procedure does not require a design accidental eccentricity and it is based on a simple amplification of design parameters obtained from the corresponding model without accidental torsion. The objectives of the study are to present and to evaluate the torsion design proposed procedure. Based on representative low-rise, reinforced-concrete building models subjected to a bidirectional earthquake ground motion, several scenarios of accidental torsion were simulated using the Monte Carlo method. In the first part of the study, elastic analyses of simulated models were carried out to obtain the amplification factor value (Fat). In the second part, nonlinear analyses were performed to evaluate the design proposal based on ductility demands. Main conclusions of the study are: mean values of ductility demands do not significantly change between the torsionally balanced (TB) system and the corresponding one with accidental torsion (TU with Fat = 1.0). However, these mean values decreased about 18% for beams and 16% for columns when the torsion amplification factor (Fat = 1.2) was used in the TU systems. As for peak ductility demands, the unbalance due to accidental torsion leads to increments close to 42% for beams and 54% for columns. On the other hand, the torsion amplification factor Fat = 1.2 reduces the peak values in 25%, approximately, for beams and columns of systems with accidental torsion. Main limitations of the study are: it is based on one bidirectional ground motion (with several incidence angles) and eight structural models.

The effect of accidental eccentricities on the inelastic torsional response of buildings

This paper investigates the influence of spatial varations of accidental mass eccentricities on the torsional response of inelastic multistorey reinforced concrete buildings. It complements recent studies on the elastic response of structural buildings and extends the investigation into the inelastic range, with the aim of providing guidelines for minimising the torsional response of structural buildings. Four spatial mass eccentricity configurations of common nine story buildings, along with their reversed mass eccentricities subjected to the Erzincan-1992 and Kobe-1995 ground motions were investigated, and the results are discussed in the context of the structural response of the no eccentricity models. It is demonstrated that when the initial linear response is practically translational, it is maintained into the inelastic phase of deformation as long as the strength assignment of the lateral resisting bents is based on a planar static analysis where the applied lateral loads simulate the first mode of vibration of the uncoupled structure.

Peak seismic response of a symmetric base-isolated steel building: near vs. far fault excitations and varying incident angle

Since the peak seismic response of a base-isolated building strongly depends on the characteristics of the imposed seismic ground motion, the behavior of a base-isolated building under different seismic ground motions is studied, in order to better assess their effects on its peak seismic response. Specifically, the behavior of a typical steel building is examined as base-isolated with elastomeric bearings, while the effect of near-fault ground motions is studied by imposing 7 pairs of near- and 7 pairs of far-fault seismic records, from the same 7 earthquake events, to the building, under 3 different loading combinations, through three-dimensional (3D) nonlinear dynamic analyses, conducted with SAP2000. The results indicate that near-fault seismic components are more likely to increase the building\'s peak seismic response than the corresponding far-fault components. Furthermore, the direction of the imposed earthquake excitations is also varied by rotating the imposed pairs of seismic records from 0° to 360°,with respect to the major construction axes. It is observed that the peak seismic responses along the critical incident angles, which in general differ from the major horizontal construction axes of the building, are significantly higher. Moreover, the influence of 5% and 10% accidental mass eccentricities is also studied, revealing that when considering accidental mass eccentricities the peak relative displacements of the base isolated building at the isolation level are substantially increased, while the peak floor accelerations and interstory drifts of its superstructure are only slightly affected.

A study on the effects of torsional component of ground motions on seismic response of low‐ and mid‐rise buildings

SummaryThe torsional component of ground motion is a potential factor to excite the torsional response of buildings during earthquake, which is not explicitly considered in seismic design codes. Building codes have proposed accidental eccentricity to consider the effect of torsional component and other unpredicted factors, which may contribute to torsion in buildings. This study investigated the effects of torsional component on the buildings' responses and the adequacy of the accidental eccentricity. For this purpose, the torsional component of some selected ground motions was generated using single‐station procedure. Subsequently, 5, 10, and 15‐story buildings with different ratios of rotational to translational frequencies were analyzed; first, by translational components only, and second, by simultaneous application of translational and torsional components. Also, the role of mass eccentricity in the effects of torsional component was studied. Furthermore, all models were reanalyzed by applying the 5% accidental eccentricity, and the effects of torsional component and accidental eccentricity were compared accordingly. Results indicated that torsional component has significant impact on the buildings' responses and can increase the displacement and drift ratio up to 36% and 41%, respectively. However, the 5% accidental eccentricity is not sufficient to take account the torsional component effects, and leads to unreliable responses.

Assessment of ASCE 7–16 Seismic Isolation Bearing Torsional Displacement

Base isolation provisions in ASCE 7 Standard have been historically shown to be conservative in estimating seismic demands. New torsional bearing base isolation displacement expressions have been proposed recently by the latest ASCE 7–16 Standard. This study compares the accuracy of the new ASCE 7–16 static-based isolation expressions for additional bearing displacement due to plane torsion to the response obtained using simplified structural dynamics’ plane torsion theory expressions. In addition, it conducts a parametric study to assess the effect of accidental eccentricity, damping ratio, and plan aspect ratio on the accuracy of ASCE 7–16 bearing displacement expression. The results showed that the ASCE 7–16 undamped displacement estimations improved compared to the significant conservatism of ASCE 7–10 by 7–33% depending on the eccentricity condition and ratio, which may further promote the use of base isolation in the US as a seismic hazard mitigation solution. However, the study also revealed that a considerable conservatism of the new base isolation bearing displacement provisions of ASCE 7–16 still exists in some cases, which ranged from 52 to 105% in the case of three equal fundamental frequencies and 5–20% in the case of equal lateral frequency and distinct torsional frequency. The study also showed that the ASCE 7–16 conservatism is proportional to the eccentricity and is more pronounced with biaxial eccentricity compared to single eccentricity. Furthermore, the results show that ASCE 7–16 expression accuracy significantly declines with damped systems with a possibility of un-conservative estimation of bearing displacements that can reach 25–40% with larger eccentricities. In addition, the ASCE 7–16 torsional displacement was shown to be inversely proportional to the plan aspect ratio with a discrepancy that can reach ± 20%. The study also exhibited that the ASCE 7–16 torsional displacement conservatism is slightly affected by increasing damping ratio above 4%.