Collapse Capacity Research Articles

Multiaxial Fatigue Life Prediction on S355 Structural and Offshore Steel Using the SKS Critical Plane Model

This work analyses the prediction capabilities of a recently developed critical plane model, called the SKS method. The study uses multiaxial fatigue data for S355-J2G3 steel, with in-phase and 90° out-of-phase sinusoidal axial-torsional straining in both the low cycle fatigue and high cycle fatigue ranges. The SKS damage parameter includes the effect of hardening, mean shear stress and the interaction between shear and normal stress on the critical plane. The collapse and the prediction capabilities of the SKS critical plane damage parameter are compared to well-established critical plane models, namely Wang-Brown, Fatemi-Socie, Liu I and Liu II models. The differences between models are discussed in detail from the basis of the methodology and the life results. The collapse capacity of the SKS damage parameter presents the best results. The SKS model produced the second-best results for the different types of multiaxial loads studied.

Collapse analysis of wind turbine tower under the coupled effects of wind and near‐field earthquake

AbstractIn this paper, the pattern of wind turbine tower collapse as a result of the coupled effects of wind and an intense, near‐field earthquake is investigated. The constitutive relation of the tower cylinder steel is simulated via a nonlinear kinematic hardening model, and the specific value of each parameter in the constitutive model is provided. A precise model of the tower structure coupled with the blade is created using a nonlinear, finite element method. This method is compared with the results from a static pushover test of a small cylindrical tower to validate the finite element modeling method in this research. Two earthquake wave sets are selected as inputs. One contains 20 near‐field velocity pulse‐like ground motion waves with various pulse periods; the other contains 20 ordinary far‐field ground motion waves. A wind turbine tower with a hub height of 60 m is selected as an example for analysis. The dynamic response of this tower as a result of the coupled effects of the two ground motion wave sets and a transient wind load is calculated using nonlinear time‐history analysis. The calculation results shows that the average horizontal displacement of the tower top as a result of the near‐field velocity pulse‐like ground motion is 33% larger than the case with far‐field ground motion. Finally, the seismic collapse vulnerability curve of this wind turbine tower is calculated. The seismic collapse capacity of the tower is evaluated, and the seismic collapse pattern of the tower is analyzed.

Effect of beam-column connection fixity and gravity framing on the seismic collapse risk of special concentrically braced frames

In concentrically braced frames (CBFs), braces are typically connected at beam-column connections through gusset plates, which also increase the rotational stiffness and moment capacity of the beam-column connection. This fixity provides a reserve lateral force resisting capacity that may improve the seismic collapse capacity of the system, but that is not considered in design. Recently, a new brace connection type has been proposed that does not include a gusset plate that would stiffen and strengthen the beam-column connection. To address the implications of the range of possible connection design alternatives, this paper assesses the effects of the fixity of beam-column connections on the behaviour of three special concentrically braced frames of different heights. The results show that flexural strength and stiffness at the beam-column connections reduces the collapse probability when the gravity framing contribution is ignored, but this influence is minor for low-rise buildings and is typically much less significant than the influence of the gravity framing's stiffness and strength. Simple design recommendations are presented regarding the beam-column connection fixity within the braced bay.

Influence of earthquake duration on the response of steel moment frames

The influence of ground motion duration on the seismic response of steel moment frames is examined is this paper, with due consideration for cyclic degradation effects. A set of 77 spectrally equivalent pairs of short and long records is utilised in detailed nonlinear dynamic assessments in order to isolate the effects of ground motion duration. The influence of duration is firstly evaluated considering degrading and non-degrading idealised bilinear SDOF systems, for various levels of lateral strength representing practical ranges encountered in design. Subsequently, a sensitivity assessment focusing on the main parameters affecting the response of hysteretic degrading models is carried out through comparative incremental dynamic analysis. Whilst the effect of duration becomes more pronounced with the increase in lateral strength demands, particularly when approaching collapse, the cyclic degradation rate is shown to play a significant role even at lower levels typically associated with design. The performance of EC8-compliant frames indicates a higher probability of collapse when long-duration ground motion records are used, with a typical reduction of about 20% in the collapse capacity, in comparison with short-duration cases. The influence of duration is also examined through collapse capacity spectra, based on the seismic performance of 50 steel moment frames, which show that considerable reduction in the structural collapse capacity of structural systems occurs when relatively long duration records are adopted, for a wide range of dynamic characteristics. This becomes particularly evident in the case of buildings with relatively significant cyclic deterioration rates, where collapse capacity reductions up to 40% due to the influence of earthquake duration are obtained.

Quantification of seismic performance factors for ribbed bracing system

In this investigation, seismic behavior and design coefficients of the ribbed bracing system (RBS) are evaluated by considering various archetypes that reflect configuration variability. RBS is a lateral bracing system with a unique mechanics that prevents buckling-induced damage by allowing free deformation of brace under compression. After successful experimental validation and numerical evaluation of RBS specimens in previous studies, FEMA P695 methodology is adopted in this study to subject 48 archetype structures to incremental dynamic analysis (IDA) procedure. Evaluation of the probabilistic collapse capacity of the archetypes against the code requirements reveals that RBS system can be designed using R values typically larger than that of the suggested ordinary braced frames. In addition, the results show that the archetypes designed using smaller seismic tributary masses or a less intense seismic design category generally benefit from an increased over-strength that leads to reduction of seismic risk. The performance of various load-carrying mechanisms provided by RBS system is assessed in different archetype configurations.

Probabilistic mainshock-aftershock collapse risk assessment of buckling restrained braced frames

Buckling-Restrained Braced Frames (BRBFs) are among the common seismic resistant systems with many beneficial characteristics such as stable cyclic behavior and high energy dissipation. However, recent studies have shown that BRBFs are susceptible to residual deformations during earthquakes which makes them vulnerable to aftershock events. The aim of the current study is to investigate the aftershock collapse capacity of BRBFs. In the first part of the paper, simplified procedures including IDA and collapse fragility analyses are carried out to gain more insight regarding the residual drift and collapse capacity of the intact frames. Then, aftershock fragility assessment is conducted for several damage states, to highlight the influence of post-mainshock residual drifts on the collapse of the structures. As for the second part, a detailed probabilistic framework is introduced and utilized to include the effects of upcoming aftershocks on the annual collapse probability of the structures. Results show that aftershock can highly intensify the structural response especially when the structure tolerates large residual drifts during the mainshock.

Buckling and collapse capacity of prestressed steel tube stayed columns with one and two crossarms

Prestressed tube stayed columns with one and two crossarms are investigated. The analyses are performed using SCIA Engineer software and ANSYS package and validated by tests of four stayed columns. First, the investigations concern the ideal (perfect) columns and cover linear buckling analyses (LBA) and geometrically nonlinear analyses (GNIA) to obtain critical loads and buckling modes under various prestressing levels. Second, the collapse loadings of the imperfect columns with initial deflections required by Eurocode EN 1993-1-1 are analysed with respect to relevant initial deflection modes and material behaviour (elastic for mild steel, nonlinear for stainless steel). All the analyses are performed for geometrical characteristics of the tested columns and the results concerning both the column with one or two crossarms are set against each other to assess the respective effectivity. In addition, the geometrical analysis of the prestressed ideal column with two crossarms and respective formulas concerning optimal prestressing and maximal critical loading are presented. Finally some recommendations for practical use are suggested.

Energy-based sidesway collapse fragilities for ductile structural frames under earthquake loadings

In assessing the likelihood of structural collapse under strong earthquake motions, uncertainties in structural properties and ground motions can be incorporated by use of a probabilistic analysis framework in conjunction with analysis methods such as incremental dynamic analysis (IDA). Maximum inter-story drift ratio (IDR) is typically selected as the key descriptor to characterize the global behavior of structural system in such a probabilistic assessment. The structural collapse capacity is often defined in terms of a threshold value of IDR or a reduced slope of the IDA curve between a selected seismic intensity measure and the corresponding IDR. However, collapse assessment approaches based on IDR may not accurately represent the overall structural collapse behavior due to redistribution and variation of local damage within the structure. Moreover, results of collapse predictions are found to be sensitive to variability in such drift measures, and assumed threshold values used in the collapse criterion. Recently, an energy-based seismic collapse criterion has been developed to describe collapse in terms of dynamic instability of the whole structural system caused by gravity loads. Using the energy-based collapse criterion, this paper proposes a more effective sidesway collapse risk assessment approach of ductile planar frames subjected to horizontal seismic loadings based on a new key descriptor of structural performance. The key descriptor, designated as the equivalent-velocity ratio, is related to the ratio of the energy dissipated through structural degradation to the seismic input energy. Using the equivalent-velocity ratio, a probabilistic collapse assessment method is developed for systematic treatment of uncertainties in the ground motions.

The Effects of Viscous Damping Modeling Methods on Seismic Performance of RC Moment Frames Using Different Nonlinear Formulations

Nonlinear response history analysis, contributing to seismic performance assessment, is a constructive tool for evaluating buildings' behavior and damages due to the earthquake. Applying an accurate viscous damping model constitutes a crucial part in this regard. The seismic response and performance of two reinforced concrete moment frames considering various damping modeling techniques and nonlinear elements are investigated. Two basic approaches to consider nonlinearity are selected: distributed and concentrated plasticity, using force and displacement-based fiber elements and elements that contain two end springs and elastic parts to which zero and modified initial stiffness proportional damping are allocated respectively. Rayleigh damping with mass and four different stiffness matrices are applied in fiber elements. The results of Incremental Dynamic Analysis and loss estimation, considering performance-based earthquake engineering methodology, indicate that Rayleigh damping with mass and initial stiffness overestimates collapse capacity and underestimates performance parameters, while, the model with mass and tangent stiffness with updated proportionality terms shows the opposite trend. In concentrated plasticity models, the more flexible the springs, the more conservative the evaluation of building performance.

Machine Learning–Based Backbone Curve Model of Reinforced Concrete Columns Subjected to Cyclic Loading Reversals

AbstractBackbone curves constructed from experimentally derived hysteresis envelopes are often used to evaluate the force-deformation behavior and, thus, seismic residual collapse capacity of struc...

Performance limit states for progressive collapse analysis of reinforced concrete framed buildings

The definition of performance limit states for progressive collapse design and assessment of civil structures is an open issue and deserves special attention in view of future building codes and standards. In this study, the main findings of a multilevel sensitivity analysis are presented to characterize the progressive collapse capacity of a selected class of modern European buildings having a reinforced concrete framed structure. After that a group of capacity model properties are assumed as random variables on the basis of earlier studies, the sensitivity of both ultimate load capacity and corresponding maximum and residual drifts to the ultimate steel strain and location of column‐removal scenario is assessed. Then, the influence of the capacity model properties on maximum drift demand corresponding to a code‐compliant design gravity load is evaluated. Finally, five performance limit states associated with increasing levels of damage are introduced and the corresponding load capacity is quantified under varying capacity model properties. Analysis results indicate a high sensitivity to the ultimate steel strain, column location in plan, beam span and yield steel strength.

Experimental study on axially preloaded circular steel tubes subjected to low-velocity transverse impact

Steel circular hollow section tubes are widely used as tubular structural members, and impact accidents usually occur while these tubes are in their working states. This paper presents experimental studies on axially preloaded circular steel tubes subjected to transverse low-velocity mass impact. A total of 32 impact tests, representing four types of scenarios, are conducted to investigate the influences of preloading and the boundary condition on the structural responses of tubes. An axial force loading system is designed, and a bladder accumulator is adopted in the system to compensate for the axial compressive loading. The test processes of the tubes under transverse impact are described in detail, and four failure modes are identified. All specimens of the four scenario types deformed according to the three-hinge mechanism. Based on the test results, the influence and mechanism of axial force are proposed. The effects of axial tension, axial compression and the boundary condition on the structural behaviour of tubes are discussed. The results show that the axial tension can enhance the bending resistance and lateral collapse capacity of tubes, while the axial compression reduces the collapse capacity significantly. By comparing the responses of tubes under different boundary conditions, the lateral resistance of tubes and the structural response are shown to have a strong correlation with the axial constraint.

Evaluation the P-Delta Effect on Collapse Capacity of Adjacent Structures Subjected to Far-field Ground Motions

In urban areas, adjacent structures can be seen in any insufficient distance from each other, because of economic and refusal of acquired minimum separation distance according to seismic previsions. Collapse capacity assessment of structures is one of the important objectives of performance-based seismic engineering. The purpose of this study is to consider the pounding phenomenon and P-Delta effect in seismic collapse capacity assessment of structures. For this purpose, 2-, 4-, 6- and 8-story adjacent structures with different conditions of separation distance among them, were modeled in the OpenSees software. Furthermore, Incremental Dynamic Analyses (IDAs) were performed using 78 far-field ground motion records to compute the collapse capacities of adjacent structures. The results obtained from IDAs for adjacent structures show that during pounding, taller structure reaches its collapse capacity earlier than shorter one. In addition, by considering the P-Delta effect and increasing the distance between adjacent structures, time of collapse and number of impacts increases. According to results, considering the P-Delta effect in modeling has significant influence in seismic collapse capacity assessment of pounding structures.

Effect of strong-column weak-beam design provision on the seismic fragility of RC frame buildings

Incremental dynamic analyses are conducted for a suite of low- and mid-rise reinforced-concrete special moment-resisting frame buildings. Buildings non-conforming and conforming to the strong-column weak-beam (SCWB) design criterion are considered. These buildings are designed for the two most severe seismic zones in India (i.e., zone IV and zone V) following the provisions of Indian Standards. It is observed that buildings non-conforming to the SCWB design criterion lead to an undesirable column failure collapse mechanism. Although yielding of columns cannot be avoided, even for buildings conforming to a SCWB ratio of 1.4, the observed collapse mechanism changes to a beam failure mechanism. This change in collapse mechanism leads to a significant increase in the building’s global ductility capacity, and thereby in collapse capacity. The fragility analysis study of the considered buildings suggests that considering the SCWB design criterion leads to a significant reduction in collapse probability, particularly in the case of mid-rise buildings.

Aftershock fragility assessment of steel moment frames with self-centering dampers

This study explores the aftershock collapse performance of steel buildings designed with superelastic viscous dampers (SVDs) under seismic sequences. The SVD strategically combines shape memory alloy (SMA) cables and a viscoelastic compound to provide good self-centering and damping capabilities. A nine-story steel special moment resisting frame (SMRF) is first designed with or without SVDs to satisfy modern seismic design requirements. A mainshock incremental dynamic analysis (IDA) is conducted for the SMRF and SVD frames using a total of ten as-recorded seismic sequences. The specific levels of post-mainshock interstory drift ratios are then induced in both frames and an aftershock IDA analysis is conducted for the mainshock-damaged buildings. The maximum interstory drift and residual drift IDA curves are developed and compared for both frames at different mainshock damage levels. The results are analyzed in terms of aftershock collapse capacity, collapse fragility, and collapse capacity at demolition. The effect of aftershock ground motion polarity on the performance of both frames is also explored. The study reveals that the SMRF has increased vulnerability to aftershocks when higher damages are induced during mainshock, while the aftershock collapse performance of the SVD frame is not affected from the intensity of mainshock event. It is also shown that the SVD frame significantly improves the aftershock capacity associated to a residual story drift that leads to major alignment or demolition.

Quantification of the seismic performance factors for steel diagrid structures

There are a number of studies on seismic performance of a diagrid structure indicating that it is an effective choice for constructing tall buildings. This research investigates the seismic behavior of diagrid structures and quantifies its seismic performance factors including the response modification coefficient (R-factor), the over-strength factor (Ω0) and the displacement amplification factor (Cd) based on the FEMA P695 methodology. In that regard, a group of 3-D steel diagrid archetype models with different number of floors and various diagonal angles are designed using different R-factors. Then, in the first step, the over-strength factors of these models are determined by performing nonlinear static analyses. Then, utilizing the incremental dynamic analysis (IDA), the median collapse capacity and collapse margin ratio (CMR) of these models are calculated and their Cd factors are estimated using the computed R-factors. The obtained results indicate that the R-factor for steel diagrid systems depend on the diagonal angles. For diagrids with angles of 45°, 63.4° and 71.5°, amounts of R-factor were determined as 1.5, 2, and 3, respectively. Moreover, the pin and rigid types of end-connection of diagrid perimeter beams was found to have no effects on stiffness of the diagrid models. However, the IDA results indicate that the diagrids with pin-ended beams tolerate larger collapse displacements especially in shorter models. Furthermore, replacing the rigid-ended beams by pin-ended beams improves the seismic performance of diagrids, particularly for models with larger diagonal angles (63.4° and 71.5°). The OpenSees program was used for modeling and numerical analyses.

Collapse Capacity of Inelastic Single-Degree-of-Freedom Systems Subjected to Mainshock-Aftershock Earthquake Sequences

ABSTRACTThis study investigated the collapse capacity of inelastic single-degree-of-freedom (SDOF) systems subjected to mainshock-aftershock earthquake sequences. A hysteretic model incorporating strength and stiffness deterioration was used to describe the inelastic behaviors of SDOF systems under seismic excitations. An ATC63 far-field record set was adopted as seed mainshock records to synthesize earthquake sequences. An extended IDA method was employed to calculate the collapse capacity of inelastic SDOF systems subjected to mainshock-aftershock earthquake sequences by scaling the earthquake sequence as a whole. An extensive parametric study was conducted for examining the effects of hysteretic model parameters and artificial earthquake sequences synthesized by different methods and with near-field and far-field mainshock earthquakes on system collapse capacity. Through comparing the collapse capacity for mainshock-aftershock earthquake sequences and that for mainshock earthquakes only, it is found that aftershocks can yield a significant reduction on the median collapse capacity for all systems and lead to an important increase on the dispersion of collapse capacity for the systems at most periods. Based on the parametric study results, empirical expressions were proposed to predict the aftershock-induced reduction and increase on the median and dispersion of collapse capacity, respectively.

Advanced scalar intensity measures for collapse capacity prediction of steel moment resisting frames with fluid viscous dampers

Nowadays, passive energy dissipation systems are used in the seismic design of new structures and the retrofit of existing structures. Fluid Viscous Dampers (FVDs) are one of the important types of passive energy dissipation systems. Using FVDs can considerably decrease the seismic demands on structures. In this study, seismic collapse behavior of steel Special Moment Resisting Frames (SMRFs) equipped with FVDs is investigated using different scalar Intensity Measures (IMs). Incremental Dynamic Analysis (IDA) method is applied to determine the collapse capacity, IMcol, values for low- to mid-rise steel SMRFs equipped with FVDs. After determining the collapse capacity, IMcol, values by using each of the IMs, the efficiency and sufficiency of the IMs for predicting the seismic collapse capacity of the structures are investigated. Then, advanced scalar IMs, including the effects of spectral shape and ground motion duration, are proposed to reliably predict the collapse capacity of steel SMRFs equipped with FVDs. The results indicate that the proposed IMs possess high efficiency and sufficiency for collapse capacity prediction of steel SMRFs equipped with FVDs.

A Simplified Modal Pushover Analysis-based Method for Incremental Dynamic Analysis of Regular RC Moment-resisting Frames

Incremental Dynamic Analysis (IDA) procedure is now considered as a robust tool for estimating the seismic sidesway collapse capacity of structures. However, the procedure is time-consuming and requires numerous nonlinear response-history analyses. This paper proposes a simplified Modal Pushover Analysis (MPA) procedure for IDA of RC moment-resisting frames. The proposed method uses the dynamic response of an equivalent single-degree-of-freedom (SDOF) system, characterized by a bilinear relationship between the lateral force (F) and roof-displacement (D). The F-D relationship is determined by the ‘first-mode’ pushover analysis of the structure. Four regular RC moment-resisting frames designed based on the current US building codes are selected and subjected to the proposed method. The analysis results obtained from the original MPA-based IDA method, SPO2IDA and the method proposed by Shafei et al are also presented for comparison. The performance of the proposed method is then evaluated through comparisons with the results obtained from IDAs. The results show that the proposed method is able to efficiently estimate the dynamic capacity of the example buildings for different seismic performance levels. Nonetheless like to MPA-based IDA and SPO2IDA methods less accueate results are obtained by the proposed procedure for 16% and 84% IDA fractiles in most case studies.

Evaluation of seismic collapse capacity of regular RC frames using nonlinear static procedure

The Incremental Dynamic Analysis (IDA) procedure is currently known as a robust tool for estimation of seismic collapse capacity. However, the procedure is time-consuming and requires significant computational efforts. Recently some simplified methods have been developed for rapid estimation of seismic collapse capacity using pushover analysis. However, a comparative review and assessment of these methods is necessary to point out their relative advantages and shortcomings, and to pave the way for their practical use. In this paper, four simplified pushover analysis-based methods are selected and applied on four regular RC intermediate moment-resisting frames with 3, 6, 9 and 12 stories. The accuracy and performance of the different simplified methods in estimating the median seismic collapse capacity are evaluated through comparisons with the results obtained from IDAs. The results show that reliable estimations of the summarized 50% fractile IDA curve are produced using SPO2IDA and MPA-based IDA methods; however, the accuracy of the results for 16% and 84% fractiles is relatively low. The method proposed by Shafei et al. appears to be the most simple and straightforward method which gives rise to good estimates of the median sidesway collapse capacity with minimum computational efforts.