Higher Mode Effects Research Articles

Assessing earthquake resilience in RC buildings: The role of transfer beams and trusses on structural integrity

Buildings equipped with transfer systems, such as transfer beams or transfer trusses, generally possess structural walls, necessitating an assessment of their adequacy in higher earthquake zones. A parametric study is performed on 2D RC frame buildings containing structural walls, (i) utilizing a transfer beam system by altering the position and distribution of the structural walls; and (ii) employing a transfer truss system by altering its positions and increasing the number of bays. The mass participation of buildings using transfer beam systems (∼57 to 69 %) is lower than those of employing transfer truss systems (∼61 to 86 %); it is essential to consider higher mode effects. The deformation behaviour of buildings with transfer systems has transitioned from flexural to shear, or a combination of both, by terminating structural walls or altering the positions of transfer trusses. The shear forces at transfer storeys are markedly increased, leading to considerable stiffness irregularities in elevation. The ratio of the average stiffness of the transfer storeys to the average stiffness of the conventional stories has increased from around 6.9 to 40 due to the inclusion of the transfer truss. The outcome from nonlinear static and dynamic analyses based on the maximum considered earthquake reveal that (i) it is essential to incorporate structural walls throughout the height of buildings; (ii) the presence of transfer beams enhances the vulnerability of the columns above, which leads to major stress concentration at the junctions of transfer beams and structural walls; (iii) the overall enhancement in stiffness is around 75 %, while the reductions in strength and deformability are 28.6 % and 27.5 %, respectively, when transitioning from a transfer beam to a transfer truss system; thereby, buildings using a transfer truss system exhibit reduced energy dissipation capacity, with a reduction in the response reduction factor of ∼ 86.5 %, and (iv) transfer beam systems caused severe damage to beams (exceeding 50 %) and occasionally to columns above or below the transfer storey, complicating retrofitting activities and requiring demolition instead. In turn, it is preferable to entirely avoid buildings with transfer systems in high earthquake zones, regardless of the presence of structural walls.

Impact of Vertical Vibration on Group Piles During Earthquake Loading: Experimental Findings

This study presents novel research on the impact of vertical vibration on the dynamic response of pile groups embedded in stratified soil under seismic loading conditions. An experimental setup was employed wherein the piled machine foundation was subjected to vertical vibrations at three operating frequencies (10, 20, and 30 Hz) and subsequently exposed to four levels of seismic acceleration (0.1 g, 0.34 g, 0.77 g, and 0.82 g). Piles with a length-to-diameter ratio of 25 were embedded in a stratified soil profile, with an upper loose layer (30% relative density) and a lower dense layer (80% relative density) acting as end-bearing. Measurements and analyses of horizontal and vertical accelerations, and amplification factors were conducted using the Fast Fourier Transform (FFT), spectral acceleration (Sa), and variation of acceleration with depth. The results demonstrated a significant reduction in horizontal acceleration, with a peak ground acceleration (PGA) reduction of up to 64% in average, particularly at higher frequencies such as 30 Hz. The mitigation efficiency at 30Hz improved with increasing PGA, showing reductions of 42, 68, 75, and 63% for seismic accelerations of 0.1 g, 0.34 g, 0.77 g, and 0.82 g, respectively. The analysis further revealed harmonic resonance and higher mode effects at lower frequencies, with nonlinear soil behavior affecting the resonance and amplification patterns. Additionally, the results demonstrate that the far-field accelerations exceeded the near-field accelerations within the pile group, particularly in the surface layer. The results indicated that the initial vibration amplitudes exceeded the safe operating limits outlined in ACI 351.3R-18 under seismic loading, particularly at higher seismic acceleration levels and lower frequencies. Additional modified charts were presented to account for these conditions. The results presented promising evidence for using vertical vibrations as an earthquake-mitigation strategy. However, avoiding operating at frequencies less than 10 Hz is recommended because of the potential resonance and interaction with horizontal accelerations during earthquakes. Doi: 10.28991/CEJ-SP2024-010-010 Full Text: PDF

Comparative analysis of seismic response and vulnerability of laminated bamboo frame structure under near-field and far-field earthquake actions

The study offers an evaluation of seismic response and vulnerability for a five-story laminated bamboo frame structure, assessing the effectiveness of seismic intensity measures across various damage thresholds and the modeling uncertainties involved. A structural finite element model was developed in OpenSees software, utilizing the Pinching4 model to accurately represent the non-linear characteristics of T-shaped steel connections in beam-column joints. Two collections of earthquake records, one consisting of pulse-like near-field and the other of far-field events, were utilized to examine the structure's nonlinear behavior. Additionally, vulnerability curves for various damage states were generated using the multiple stripe analysis technique, applied to both ground motion datasets. The applicability of 24 seismic intensity measures was also analyzed across various damage limit states. The modeling uncertainty under different seismic actions was further estimated using a first-order second-moment method. Ultimately, the annual collapse probability of the structure are elucidated. Research indicates that due to the influence of joint stiffness, the overall structure demonstrates dynamic characteristics that are predominantly of the first mode shape. The spectral acceleration associated with the fundamental period achieves a reduced logarithmic standard deviation in the vulnerability analysis across various limit states. Other IMs considering higher mode effects or softening period actions no longer predominate. In most cases, the modeling uncertainty under far-field seismic action is higher than that under near-field, especially at the severe damage limit states by using an efficient intensity measure. The laminated bamboo frame structure faces a much higher collapse risk in near-field than in far-field conditions.

Self-centering dual rocking core system with viscous dampers in additional rocking sections

To mitigate the high mode effects and reduce the force requirements of the structural members in the multi-story self-centering dual rocking core (SDRC) system, this paper proposes the multiple self-centering dual rocking core system with viscous dampers in additional rocking sections (MSDRCV system). The sketch and ideal working mechanisms of the MSDRCV system are interpreted. The additional rocking sections are set to decrease the force requirements of structural members and viscous dampers are included to mitigate high mode responses of the additional rocking sections, introducing seismic performance enhancement. Comparative analyses on a 9-story MSDRCV system with viscous dampers in rocking sections, a 9-story multiple self-centering dual rocking core system without viscous dampers in rocking sections (MSDRC system) and a 9-story SDRC system were carried out. The MSDRCV and MSDRC system can realize the reduction of the force demands in structural members in contrast to the SDRC system. Nevertheless, the high mode responses caused the uncontrollable displacement concentration in the MSDRC system and hence aggravated the seismic performance under rare seismic events. The analysis results in the frequency domain confirmed that the viscous dampers in the MSDRCV system can efficiently control the high-mode responses. The effects of the design parameters of the viscous dampers on the peak responses of the MSDRCV systems were comprehensively studied via parametric analyses. The results offer guidance for the viscous dampers design.

Self-centering multiple rocking core systems for mitigating higher mode effects

The structural member forces of mid-story and high-rise self-centering energy-absorbing dual rocking core (SEDRC) systems can be significantly aggravated by higher mode effects. This study aims to introduce the multiple rocking mechanisms into the SEDRC system and propose the self-centering multiple rocking core system (denoted as SMRC system) to reduce the structural member force demands resulting from the higher mode effects. The working principle of the SMRC system is discussed. The performance-based design (PBD) procedures for the SMRC system are presented. The comparative dynamic history analyses are conducted on a 9-story SMRC and two 9-story SEDRC systems. Incremental dynamic analyses (IDAs) are carried out to investigate the dynamic behaviors of the considered systems under increasing seismic intensities. The accuracy of the developed designed procedures is verified with the evidence that the developed SMRC system can achieve the design displacement under design earthquakes. The multiple rocking mechanism can efficiently mitigate structural member force demands caused by the effects of higher mode. The SMRC system can achieve better seismic performance with lower structural member force demands than the SEDRC systems when the earthquake intensity is not higher than the maximum considered earthquakes (MCE). Nevertheless, the SMRC system shows worse performance than the SEDRC systems in resisting structural collapse when the intensity is higher than MCE, which could be a research gap for further investigations.

Towards estimating higher-mode effects on the seismic response of tall-buildings

Towards estimating higher-mode effects on the seismic response of tall-buildings

Seismic design and performance evaluation of steel braced frames with post-tensioned and disc-spring-based self-centering buckling-restrained braces

In this study, we focus on two types of self-centering buckling-restrained braces (SC-BRBs), which are post-tensioned (PT) SC-BRB and disc-spring-based SC-BRB, and develop a seismic design method for steel braced frames with these braces according to the displacement-based design theory. The whole independent parameters that describe the hysteretic behavior of the SC-BRBs are first determined. Then nonlinear time history analysis (NLTHA) is conducted to explore the effects of these hysteretic parameters on the ductility demands of the nonlinear single-degree-of-freedom (SDOF) system. Based on the analysis results, a ductility demand spectral model is developed, followed by a nonlinear displacement ratio spectral model. Subsequently, the seismic design procedure for SC-BRB frames is proposed based on the two spectral models. Finally, a total of 24 six-story steel frames with the two types of SC-BRBs, two target inter-story drifts, and different parameter combinations are designed at the maximum considered earthquake (MCE) level. The pushover analysis and NLTHA results indicate that a smaller strength ratio β and a larger initial stiffness ratio αs are beneficial for reducing the base shear and obtain an economical design when the designed frames achieve the same target inter-story drift. Considering the initial stiffness of the SC-BRBs is highly sensitive to machining errors, it is recommended that the value of β be taken within 0.25 to reduce the influence of the initial stiffness uncertainty of the braces on seismic design results. Moreover, controlling β within 0.25 also helps to reduce the higher-mode effect and peak floor acceleration responses of the designed structures effectively.

A damage-oriented ground motion intensity for degrading super high-rise buildings

To explicitly identify the influence of different seismic damages on the correlation between ground motion intensity measure (IM) and demand measure (DM), with a specific concern of super high-rise buildings, a novel damage-integrated spectral acceleration-based combination-type IM is developed for degrading systems. With the integration of seismic damage measure, inelastic displacement spectrum, and design acceleration spectrum, the prototype of the IM is theoretically derived for single-degree-of-freedom systems with a Q-type hysteretic model. The high-mode effect is reflected by an optimal number of lower modes combined with corresponding modal damage. To estimate the optimal number of modes while maximizing the case-independency of DM predictions, the dynamic properties of concerned damaged and undamaged structures are estimated with the collaboration of the modified flexure-shear model (FSM-MS), and further utilized to establish the regression expression with a large number of near-field and far-field ground motions. The performance of the proposed IM is systematically demonstrated via a case study on two super high-rise buildings, along with other selected IMs.

Hybrid-self-centring steel frames: Insights and probabilistic seismic assessment

This paper comprehensively discusses the seismic performance of steel moment-resisting frames with hybrid-self-centring connections (SMRF-HSCC) employing energy dissipation sequences by probabilistic seismic assessment. The hysteretic model of the HSCC was first developed based on experimental results, and a simplified nonlinear-spring-based model to simulate the behaviour of the connection was proposed and verified by physical test data. A series of codified prototype structures were developed, considering the fracture behaviour of post-tensioned (PT) steel strands in the connections. The seismic performance of prototype structures was assessed in terms of nonlinear static analyses and nonlinear dynamic analyses. The results indicated that the hysteretic response of SMRF-HSCC exhibited trilinear hysteretic features with the expected energy dissipation sequences. The influence of the high-mode effects on SMRF-HSCC was mitigated with the increase in energy dissipation and post-yield stiffness of the structure. Finally, the fragility analyses of the prototype structures were further performed, and the risk assessment was also made considering a 50-year service period. The collapse probability and the exceedance probability of target residual drift over a 50-year service period of SMRF with conventional self-centring connections showing flag-shaped hysteresis were higher than that of SMRF-HSCCs. The results also confirmed that the collapse probability and exceedance probability of target residual drift over a 50-year service period of SMRF-HSCC can be adjusted by reasonably modulating the arrangement of the HSCCs.

Effect of low and high temperature ECAP modes on the microstructure, mechanical properties and functional fatigue behavior of Ti-Zr-Nb alloy for biomedical applications

A Ti-18Zr-15Nb (at%) shape memory alloy was subjected to a high-temperature thermomechanical treatment (HTMT) combining an equal channel angular pressing (ECAP) at 500 °C for n = 4–8 passes and a short-time post-deformation annealing (PDA) at 600 °C. The phase composition, microstructure, texture, mechanical and functional properties were studied. The functional fatigue behavior observed in this study was compared to that resulted from the reference low-temperature thermomechanical treatment (LTMT) by ECAP at 200 °C for 3 passes + PDA (600 °C for 5 min). The ECAP at 500 °C (n = 4) led to the formation of a highly deformed, dynamically polygonised substructure of β-phase with a crystallographic texture close to the [101] direction. In this state, the alloy exhibited an excellent combination of the static functional and mechanical properties: a relatively high strength (UTS = 670 MPa), a sufficient ductility (δ = 13.3 %), a low Young’s modulus (E < 40 GPa), and a high superelastic recovery strain (εrsemax= 3.1 %). An increase in the number of passes during ECAP to n = 8 led to a greater substructural hardening of the material, grain/subgrain refinement, and the release of α-phase. This significantly increased the alloy strength (UTS = 897 MPa), but reduced its ductility (δ = 5.9 %) and suppressed martensitic transformation. The alloy after both the HTMT (ECAP at 500 °C, n = 4) and TMT (ECAP at 200 °C, n = 3) processes exhibited an equally excellent functional fatigue resistance accompanied by a superior superelastic behavior with small accumulated strains. However, the HTMT process appears to be more technologically advanced, as it eliminates the need for PDA and reduces the risks of specimen cracking during processing.

A modal conditional mean spectrum for nonlinear structural response time-history analysis of tall buildings to consider higher mode effects

A modal conditional mean spectrum for nonlinear structural response time-history analysis of tall buildings to consider higher mode effects

Floor acceleration control of self-centering braced frames using viscous dampers

The self-centering braced frames (SCBFs) have been widely investigated for enhancing the building structures’ post-earthquake repairability by reducing or even eliminating the residual inter-story drifts. Nevertheless, it has been highlighted in recent research that the flag-shaped hysteretic behavior of SCBFs amplifies structural floor acceleration (FA) responses, which may lead to severe nonstructural damage. This paper intends to overcome this critical shortcoming of SCBFs by controlling FA responses using viscous dampers. This paper focuses on proposing a practical performance-based design strategy for designing viscous dampers to control the FA responses of the SCBF to the targeted level. To this end, the parametric dynamic analyses of a single-degree-of-freedom (SDOF) system were conducted to investigate the influence of the hysteretic parameters of self-centering braces (SCBs) and the contribution of viscous dampers (VDs) on the peak acceleration control in the SCBFs. Based on the results from the parametric dynamic analysis, the prediction models of inelastic displacement and acceleration ratios of the SCBF with VDs (denoted as the hybrid self-centering braced frame, HSCBF) were developed using the artificial neural network (ANN). The design steps included in the proposed method were presented, where a formula was proposed for predicting the absolute FAs of the HSCBF. Four demonstration buildings with 3, 6, 9, and 12 stories were designed and simulated to verify the developed performance-based design method. The analysis results show that the VDs can effectively control the FA responses of the SCBFs and the mean FAs of the designed systems can reach the desired performance level. Moreover, the reasons why VDs can reduce the accelerations of self-centering building structures are preliminarily discussed from the perspectives of frequency domain and higher mode effects. The analysis results indicate that compared to the original SCBFs, the HSCBF tends to vibrate with a lower frequency and show smaller high-mode responses.

Evaluating the impact of higher-mode and inelastic dynamic responses of concrete frames on the performance of seismic intensity measures

The accuracy of seismic risk-based evaluation depends on the selected seismic intensity measures (IMs). The performance of the selected IM is commonly assessed based on efficiency and sufficiency criteria, which reflect the ability of the IM to reduce epistemic uncertainty and avoid bias, respectively. Although various characteristics of structural dynamic response affect IM performance to different extents, the current literature determines IM suitability based on the overall response. This study examines the impact of two dynamic response characteristics, namely higher-mode effect and nonlinearity, on the efficiency and sufficiency of a comprehensive set of common and advanced IMs. Two correlation-based measures are then proposed to explicitly evaluate the effect of higher-mode effect and nonlinearity using statistical tests. The results show that the developed statistical measures can explain the inefficiency and insufficiency of an IM based on dynamic characteristics of response. In addition, while the IM’s efficiency and sufficiency changes for different structures and ground motion sets, vector-valued and energy-based scalar IMs provide more consistent performance. Lastly, the impact of higher-mode response and nonlinearity is more pronounced on IMs efficiency than IM sufficiency.

Probability of collapse evaluation for high-rise reinforced concrete buildings in the event of near-fault earthquakes and soil-structure interaction effects

This paper presents a probabilistic evaluation of the seismic performance of 10, 20, and 30-story reinforced concrete buildings with dual systems located in Tehran, Iran, considering the effects of Soil-Structure Interaction (SSI). The buildings are designed according to the Iranian seismic code, and their nonlinear planar models are developed in OpenSEES, accounting for two fixed and flexible-base conditions. The concentrated plasticity approach is employed to simulate the nonlinear response of frame elements, while the displacement-based fiber elements are utilized to model the shear walls. The Winkler springs and dashpot technique was used to capture the uplift of the foundation in flexible-based models. The planar models are then subjected to nonlinear static and dynamic analysis, as well as Incremental Dynamic Analysis (IDA), followed by fragility assessment and collapse risk evaluation using the risk integral method. A suite of pulse-like and non-pulse near-field earthquakes is employed to account for the effects of impulsive characteristics of the ground motions on the performance of buildings with rocking foundation. The probabilistic assessment of building models was conducted based on FEMA P695 methodology. The results indicate that the performance of flexible-base buildings is improved for the strong earthquakes and pulse-like records. However, the advantages of foundation flexibility are diminished in taller buildings due to their larger weight and higher mode effects. The foundation rotation is found to be lower than the performance limits of the seismic code. Finally, the SSI exhibits varying effects on the collapse risk of buildings of different heights, with a positive effect on shorter buildings, whereas its impact on taller buildings depends on the expected seismic hazard level at the site.

Seismic design and engineering practice of a 10‐story self‐centering precast concrete wall structure

SummaryThe increasing expectation of structures capable of fulfilling the requirements of minimizing post‐earthquake repair or re‐occupancy has led to the emergence of damage‐control technologies. In recent years, self‐centering precast concrete wall systems that are characterized by low damage as well as full prefabrication have become a popular topic. Previous research has shown that the system is not only capable to reduce the construction time but also has the characteristics of small residual displacement and quick restoration of normal service function after a major earthquake event. Nevertheless, there is still much to be studied for high‐rise buildings and practical engineering applications. This paper introduces the process of self‐centering precast concrete wall systems from conceptual design to detailing and construction aspects of a 10‐story case study building. Specifically, the hybrid type of unbonded post‐tensioned wall is adopted with mild steel functioning as the energy‐dissipating component. The design and construction of mild steel take into account the requirements of both building function and replaceability. In addition, the lateral resisting system is decoupled from the gravity system using isolated joints for wall‐to‐floor connection. Various factors such as higher mode effects, torsional effects, and wind loads are considered in the design process in order to achieve the overall high performance of the structure. Finally, the numerical model of the designed structure is established and analyzed under both static and dynamic loading. Results show that the self‐centering wall structure studied in this paper has satisfactory seismic performance, i.e., each component and joint can work to achieve the function as expected, and has broad engineering application prospects in the future.

Data set from shaking table tests of a slender MDOF structure with base-rocking mechanisms

Numerical models of seismic-resistant structures are typically validated by comparing numerically predicted key seismic response parameters to those obtained from benchmark shake table experiments. However, studies have shown that for structures rocking on rigid foundations, the seismic response can be unrepeatable in experiments and deterministically unpredictable by numerical models. Therefore, researchers have proposed to validate rigid rocking models using statistical approaches, yet no experimental study has examined whether the same observations apply to slender multi-degree-of-freedom (MDOF) free-rocking structures or controlled rocking structures with positive post-rocking stiffness and dedicated energy dissipation. This data paper describes the now publicly available data obtained from a series of 254 shaking table tests of a slender MDOF tower structure with two base-rocking mechanisms, which can be used to examine the predictability of the seismic response of an MDOF rocking structure and to statistically validate different approaches aiming to model tall and slender structures that utilize base-rocking mechanisms to mitigate the dynamic response due to higher-mode effects.

Shake Table Testing of Shear-Controlling Rocking Isolation Podium System for Mitigating Higher-Mode Effects in Tall Buildings

Shake Table Testing of Shear-Controlling Rocking Isolation Podium System for Mitigating Higher-Mode Effects in Tall Buildings

Direct displacement-based design of SEDRC systems considering higher mode effects

Self-centering energy-absorbing dual rocking core (SEDRC) system was recently proposed for obtaining damage-free performance under design earthquakes or low damage under extreme earthquakes in steel building structures. Although the SEDRC system can mainly deform based on the fundamental mode shape, the higher modes may significantly impact the structural element forces. This research intends to investigate the effects of higher mode on the seismic performance and structural member force demands of the SEDRC system under earthquakes and propose a performance-based design (PBD) method considering the higher mode effects. The formulation for calculating the design base shear pondering the influence of higher mode is deduced. The detailed design steps and equations for calculating the force demands of structural members are presented. Three archetypes, including nine-, fifteen-, and twenty-story SEDRC systems, respectively, are designed according to the PBD method. Moreover, the other six corresponding archetypes are designed according to the previous design process without consideration of higher mode effects as comparative buildings. Nonlinear dynamic analyses were performed to evaluate the seismic responses of the designed structures. The analysis results validate the high precision of the proposed methodology in designing multi-story to high-rise SEDRC systems.

Seismic evaluation of vertically irregular RC frames subjected to mainshock-aftershock sequences of near-fault and far-fault ground motions

Sequential ground motions can significantly affect the seismic demands of reinforced concrete (RC) frames with irregularity along the height. In this paper, the effect of sequential near-fault and far-fault ground motion records with strong and weak aftershocks on the seismic demands of vertically irregular 10-story RC building frames is investigated. The vertically irregular frames with stiffness and mass irregularities were created by applying a modification factor of MF = 2 in three different locations along the height of the reference regular frame. Eigenvalue analysis was performed to evaluate the dynamic characteristics of the structures (i.e. the period and the effective modal participating mass ratio for different modes of vibration). In order to study the seismic demands of these structures, the non-linear response history analysis (NL-RHA), which is the most accurate method of seismic evaluation of structures, was carried out. The floor displacements, story drifts, plastic hinge rotations and residual story drifts obtained from the NL-RHAs were exhaustively studied, and the effect of seismic sequences on the seismic responses was investigated. Furthermore, the enhanced pushover analyses, including the modal pushover analysis (MPA), consecutive modal pushover (CMP) procedure, extended N2 (EN2) method, single-run multi-mode pushover (SMP) method and non-adaptive displacement-based pushover (NADP) procedure were implemented to consider the effect of higher modes in computing the seismic demands of the structures subjected to sequential near-fault and far-fault records as well as mainshocks. The results accentuate that the maximum seismic demands and damage of the structure for sequential near-fault and far-fault records with strong aftershocks are much greater than those for the corresponding mainshocks. Moreover, the enhanced pushover analyses in the case of sequential records have usually less accuracy in estimating the seismic demands in comparison with mainshocks only.

Development of a New Response Spectrum Analysis Approach for Determining Elastic Shear Demands on Shear-Dominated Steel Building Frames

This research is focused on improving the conventional response spectrum analysis (CRSA) method for the elastic shear demands estimation of shear-dominated steel building frames. An alternative approach named the improved response spectrum analysis (IRSA) method is proposed and validated in this paper. A simplified procedure to capture the dynamic features of a continuous shear beam (CSB) with stepped stiffness is first presented, and then validated. The CSB is employed in IRSA to replace the original eigenvalue analysis in CRSA to provide the modal parameter estimation for the considered system. A modified SRSS (MSRSS) mode superposition based on a genetic algorithm is then proposed and employed in IRSA. Based on the analyses conducted in this research, it is found that using first three modes in MSRSS to execute mode superposition could provide a great estimation of the elastic shear demands distribution. The amplification of weighting coefficients for the second and third mode contribution indicates the underestimation of the high mode effect in CSRSS. Further, response history analyses (RHA) are performed on two demonstration building frames to evaluate the improvement of the IRSA. The results indicate that IRSA provides a more precise estimation on the elastic shear force demand distribution in shear-dominated steel building frames under seismic effects compared with that which was achieved by CRSA.