Nonlinear Dynamic Time History Analyses Research Articles

Seismic fragility analysis of corroded reinforced concrete bridges in Bandar Abbas, Iran

The seismic performance of reinforced concrete (RC) bridges in the coastal environment of Bandar Abbas, Iran, was investigated. Probabilistic seismic hazard analysis results of the region, covering a radius of 150 km keeping Bandar Abbas as the centre, are first presented. To determine the dominant earthquake scenario characterised by magnitude and source-to-site distance, disaggregation of peak ground acceleration (PGA) hazards corresponding to mean return periods of 475 and 2475 years was carried out. Results of the disaggregation analysis were then used to select a ground motion set for non-linear dynamic time history analysis of a bridge. In the next step, the effects of chloride-induced corrosion of the bridge columns, including degradation of steel bars, cover concrete and core concrete properties, were considered at different time steps in the bridge's service life. Finally, seismic fragility curves of the corroded bridge at selected time steps were developed based on multiple-stripe analysis results and the maximum likelihood approach. Based on these analyses, to ensure the appropriate seismic performance of a RC bridge over its service life, it is proposed that the design PGA should be increased approximately by 37%.

Lifetime seismic performance assessment of high-rise steel-concrete composite frame with buckling-restrained braces under wind-induced fatigue

Under a severe environment of multiple hazards such as earthquakes and winds, the life-cycle performance of engineering structures may inevitably be deteriorated due to the fatigue effect caused by long-term exposure to wind loads, which would further increase the structural vulnerability to earthquakes. This paper presents a framework for evaluating the lifetime structural seismic performance under the effect of wind-induced fatigue considering different sources of uncertainties. The seismic behavior of a high-rise steel-concrete composite frame with buckling-restrained braces (FBRB) during its service life is systematically investigated using the proposed approach. Recorded field data for the wind hazard of Fuzhou, Fujian Province of China from Jan. 1, 1980 to Mar. 31, 2019 is collected, based on which the distribution of wind velocity is constructed by the Gumbel model after comparisons. The OpenSees platform is employed to establish the numerical model of the FBRB and conduct subsequent numerical computations. Allowed for the uncertainties caused by the wind generation and structural modeling, the final annual fatigue damage takes the average of 50 groups of simulations. The lifetime structural performance assessments, including static pushover analyses, nonlinear dynamic time history analyses and fragility analyses, are conducted on the time-dependent finite element (FE) models which are modified in lines with the material deterioration models. The results indicate that the structural performance tends to degrade over time under the effect of fatigue, while the influencing degree of fatigue varies with the duration time of fatigue process and seismic intensity. The impact of wind-induced fatigue on structural responses and fragilities are explicitly quantified and discussed in details.

SEISMIC INTENSITY MEASURES FOR THE DAMAGE EVALUATION OF CIRCULAR TUNNELS

The seismic intensity measure (IM) is a key parameter affecting the accuracy of seismic risk assessment, which plays an important role in the performance-based earthquake engineering framework. A reasonable IM can make the prediction of structural seismic responses more accurate. To study on seismic IMs which are suitable for the damage evaluation of circular tunnels, a two-dimensional finite element model was established for the nonlinear dynamic time history analyses of circular tunnel-surrounding rock interaction. The near-field ground motions without velocity pulses and far-field ground motions recommended by FEMA-P695 were used as the input in the numerical model. The overall lining damage indices in compression and in tension were used as the engineering damage measures to estimate the damage states of the lining. The applicability of 20 ground motion IMs commonly used in the engineering practice were investigated. The results show that based on the criteria of efficiency, practicality, proficiency and sufficiency, the best seismic IM for the damage evaluation of circular tunnels is the sustained maximum acceleration (SMA), and the second best is the acceleration spectrum intensity (ASI).

Evaluation of the Seismic Retrofitting of Mainshock-Damaged Reinforced Concrete Frame Structure Using Steel Braces with Soft Steel Dampers

Most reinforced concrete (RC) frames would exhibit different degrees of damage after mainshock excitations, and these mainshock-damaged RC (MD-RC) frames are highly vulnerable to severe damage or even complete collapse under aftershock excitations. In the present study, the effectiveness of utilizing soft steel damper (SSD) as a passive energy dissipation device for seismic retrofitting of MD-RC frame under aftershock actions was investigated. A common three-story RC frame in the rural area was employed and a numerical evaluation framework of retrofitting analysis of the MD-RC frame was also proposed. Based on proposed evaluation framework, nonlinear dynamic time history analysis of the MD-RC frame with and without retrofitting schemes was conducted to evaluate the retrofit effect of the retrofitting schemes on the MD-RC frame. The results revealed that the retrofitting schemes could effectively improve the natural vibration characteristics of the MD-RC frame, especially the first-order natural frequency with a maximum increase of nearly four times. The retrofit effect of the MD-RC frame under pulse-like aftershocks is better than non-pulse-like aftershocks and the retrofit effect of minor damage MD-RC frame is slightly better than that of severe damage. In addition, only retrofitting the bottom story of MD-RC frame might cause aggravate structural damage.

Rocking isolation of bridge pier using shape memory alloy

The conventional design philosophy of bridges allows damage in the pier through yielding. A fuse-like action is achieved if the bridge piers are designed to develop substantial inelastic deformations when subjected to earthquake excitations. Such a design can avoid collapse of the bridge but not damage. The damage is the plastic hinge formation formed at location of maximum moments and stresses that can lead to permanent lateral displacement which can impair traffic flow and cause time consuming repairs. Rocking can act as a form of isolation by means of foundation uplifting which act as a mechanical fuse, limiting the forces transferred to the base of the structure. In this context, this paper proposes a novel resilient controlled rocking bridge pier foundation, which uses elastomeric pads incorporated beneath the footing of the bridge piers and external restrainer in the form of shape memory alloy bar (SMA). The rocking mechanism is achieved by restricting the horizontal movement of footing by providing stoppers at all sides of footing. The pads are designed to remain elastic without allowing their shearing. The pier, the footing and the elastomeric pads are assumed to be supported on firm rigid concrete sub base resting on hard rock. By performing nonlinear dynamic time history analysis in the traffic direction of the bridge, the proposed pier with the novel resilient foundation is compared against a fixed-based pier and classical rocking pier (CC). The proposed pier rocking on elastomeric pads and external restrainer (CP+SMA) has good re-centering capability during earthquakes with negligible residual drift and footing uplift. In this new rocking isolation technique, the forces in the piers are also reduced and thus leading to reduced construction cost with enhanced post-earthquake serviceability.

Seismic response assessment of high-strength concrete frames strengthened with carbon fiber reinforced polymers

In recent years, the use of new materials and technologies with the aim of developing high-performing and costeffective structures has greatly increased. Application of high-strength concrete (HSC) has been found effective in reducing the dimensions of frame members; nonetheless, such reduction in dimensions of structural elements in the most cases may result in the lack of accountability in the tolerable drift capacity. On this basis, strengthening of frame members using fiber reinforced polymers (FRPs) may be deemed as an appropriate remedy to address this issue, which albeit requires comprehensive and systematic investigations. In this paper, the performance of properly-designed, two-dimensional frames made of high-strength concrete and strengthened with Carbon Fiber Reinforced Polymers (CFRPs) is investigated through detailed numerical simulation. To this end, nonlinear dynamic time history analyses have been performed using the Seismosoft software through application of five scaled earthquake ground motion records. Unstrengthened (bare) and strengthened frames have been analyzed under seismic loading for performance assessment and comparison purposes. The results and findings of this study show that use of CFRP can be quite effective in seismic response improvement of high-strength-concrete structures.

Dissipative Bracing Systems for Seismic Upgrading of New and Existing Timber Structures

ABSTRACT In view of the development of heavy timber seismic-resistant structures, in the context of modern seismic design approach, a possible solution is to allow the timber structure to dissipate a part of the seismic energy. Since timber is a material with an elastic-fragile behaviour, the dissipative function should be delegated to connections, through plastic deformation of steel connectors. However, joints are primary structural elements, with a crucial role in bearing the design loads. Therefore, the role of seismic energy dissipation should be conveniently assumed by ad hoc devices. In this context, the paper deals with the application of fluid viscous dampers (FVD) to timber frames: FVDs dissipate seismic energy, while timber elements and steel connections remain elastic. Specifically, 2D single-storey structures with dissipative bracing systems, equipped with FVDs, in different configurations, are studied, assuming several rates of possible dampings. Non-linear dynamic time history analyses are performed (SAP2000, v18). Results are discussed showing significant reduction of the structural mass compared to the non-dissipative ones, recentering capability of the structures, high dissipative capacity, simpler elastic connections. The reduction of production and maintenance costs follows. All these involve the efficiency of the structural performance and sustainability under earthquakes.

Analytical Investigation of the Differences between Cast-In-Situ and Precast Beam-Column Connections under Seismic Actions

At present, the engineering designers generally design and analyze the precast structural models according to the equivalent cast-in-situ principle, and have a vague understanding of non-identical problems. However, these issues cannot be ignored, especially for high-intensity areas. This paper considers the differences of the hysteretic relationship between two typical precast joints and cast-in-situ (RC) joints, and researches the influence of these differences on the seismic response of frame structures. For the monolithic precast joint, the force mechanism was analyzed based on its assembly form, and the differences with the RC joint in the testing phenomena were explained accordingly. The dimensionless hysteresis models of two types of joints were proposed, and the rationality of the monolithic precast joint model was verified according to the existing experimental results. Different performances of joints were realized by assigning the constitutive models calculated from sectional reinforcement to the spring elements of analysis models. Considering two possible performance deficiencies of each type of precast joint separately, a total of seven structural analysis models were formed. Nonlinear static analysis and dynamic time-history analysis methods were adopted to reveal the differences between precast frames and the RC frame in terms of structural capacity curve, displacement response, ductility demands of components and structural residual deformation. The results showed that under strong seismic excitation, the response differences between precast frames and the RC frame were significant, so it is worthwhile to establish nonlinear models suitable for precast frames in seismic analysis. This study is valuable for understanding and distinguishing the nonlinear response of precast frames and traditional RC frames.

Seismic fragility assessment of bridge piers incorporating high-strength steel reinforcement and concrete under near-fault ground motions

Design codes in the United States and Canada limit the use of high-strength steel (HSS) and high-strength concrete (HSC) to bridge components that are expected to remain elastic during a seismic event. Although HSS and HSC have higher tensile and compressive strengths, respectively, their lower inelastic strain capacities impose for such restrictions. To assess the seismic performance of HSS and HSC, the pier of an existing bridge is redesigned using concrete compressive strength of 50 and 80 MPa, and reinforcement yield strength of 420, 690, and 830 MPa. Static pushover and nonlinear dynamic time-history analyses were performed to generate force–deformation and seismic fragility curves. Bridge piers incorporating HSS and HSC attained the maximum load capacity yet were the least ductile. They were less seismically vulnerable than those incorporating conventional materials at minimal and repairable damage states, but not at extensive and probable replacement damage states.

Seismic design of a curved bridge as per performance based criteria

In the present study, seismic design of a bridge curved in plan has been done for various response reduction factors (R) and the performance has been investigated as per FEMA-356 performance based criteria. Both uni-directional and bi-directional nonlinear dynamic time history analyses have been performed for the bridge. The performance of the bridge in terms of pier ductility demand has been determined for Kobe earthquake ground motion, compatible for Design Basis Earthquake (DBE) and Maximum Considered Earthquake (MCE) response spectrums. For different values of R-factors, suitable value of safe pier diameter of the bridge has been determined. The strength reduction factor and over strength factor have also been determined for various cases considered. It has been obtained that the bi-directional effect increases the performance state and ductility demand of the bridge. The maximum R-factor of 2.5 can be recommended for the studied bridge.

Effects of unbonded steel layout on seismic behavior of post-tensioned precast concrete shear walls

Unbonded post-tensioned precast connections have been studied widely in various applications and shown to be promising seismic resisting systems with superior seismic performance including exceptional self-centering capability and minimized structural damage. In this paper, effects of unbonded post-tensioned steel layout of precast concrete shear walls on seismic performance of a typical five-story precast concrete parking garage structure with the walls being the only lateral load resisting system is studied. The seismic behavior of the walls is determined using static nonlinear push-over, cyclic, and nonlinear time-history dynamic analyses under both the design and survival level ground motions. Three prototype unbonded post-tensioned precast concrete shear walls are designed with different unbonded post-tensioning steel tendon layout. Roof drift, post-tensioning force, and base shear and normal forces at the wall-foundation horizontal joint are monitored, and the demand for the coefficient of shear friction along the horizontal joint is calculated. The results show that seismic behavior of unbonded post-tensioned precast shear walls, in particular, maximum building drift, permanent losses in post-tensioning steel force, and coefficient of shear friction, is significantly influenced by layout of unbonded post-tensioned steel. The results and findings presented are valuable to improve design guidelines and provide a useful reference for practical applications of unbonded post-tensioned precast concrete connections in seismic applications.

Earthquake resistant timber panelised structures with resilient connections

Timber panelised structures with platform style of construction are favoured by engineers and researchers due to their positive characteristics such as straightforward installation process, relatively high strength to mass ratio and architecturally favoured properties. In the platform style of construction, floors are stacked on top of each other so that each level is constructed on load-bearing walls, creating a platform for the next floor. Recent researches on timber platform buildings made with conventional connectors demonstrated that the connections might experience extensive damages under design level earthquakes. This paper presents a low damage concept for mass timber platform systems using resilient connections as replacements of conventional connectors. A numerical model for the presented system is developed for a Cross Laminated Timber (CLT) platform building. The methodology used was to investigate the seismic performance of the proposed concept through a case study structure modelled in SAP2000. Quasi-static pushover and nonlinear dynamic time-history analyses are performed on the structure. The results showed that the system maintained its performance through several cycles of deformation. Moreover, the system is capable of absorbing a notable amount of energy while exhibiting a recentring capacity. The results of this study demonstrated that the presented concept could be considered for further developments for the application in CLT panelised buildings in seismic prone regions.

A novel floor-isolated re-centering system for prefabricated modular mass timber construction – Concept development and preliminary evaluation

Prefabricated modular mass timber (MT) construction provides a resource-efficient, consistent-quality, and eco-friendly solution for the building industry. Connections that facilitate the assembly of prefabricated modules to form the final structure usually do not possess the characteristics needed for performance of these systems in high seismic zones. In this study, a novel seismic force resisting system called FIRMOCS that incorporates floor-isolation and re-centering techniques, is proposed for the prefabricated modular MT construction. Finite element models of the Floor Isolated Re-entering Modular Construction System (FIRMOCS) and of regular modular construction Seismic Force Resisting Systems (SFRS) were developed in ABAQUS, a general-purpose finite element program. The seismic performance of the regular prefabricated modular MT SFRS and that of FIRMOC systems was evaluated using nonlinear dynamic time-history analysis along the principles of the National Building Code of Canada. The preliminary results showed that FIRMOC Systems significantly reduced the seismic demand on the prefabricated modular MT construction, thus leading to improved seismic response. Using the proposed novel SFRS, an efficient and resilient prefabricated modular MT construction solutions can be achieved in high seismic zones.

Seismic assessment of rocking timber walls with energy dissipation devices

The use of rocking walls with seismic fuses is an efficient solution for mass timber structures. This paper presents and compares mass timber wall systems with different types of seismic fuses. These devices range from conventional pinching connectors to self-centring friction connections. Pros, cons and performance measures of each system are described and discussed. Moreover, a preliminary design procedure based on the Direct Displacement Based Design (DDBD) approach is used to specify and size the damping devices. This approach was chosen to assure that the lateral displacements of the systems are appropriately controlled respecting the considered damage limit states.Numerical models are accordingly developed and subjected to non-linear static pushover and dynamic time-history analyses. The results are discussed in relation with the performance objectives considered for different systems. The results confirmed that the use of low damage friction connections as a substitute for traditional pinching connectors could considerably improve the seismic performance of mass timber structures. Nevertheless, with some of these systems, there is potential for having residual displacements that could compromise the post-earthquake performance of the system.

Seismic Retrofit of an Existing RC Building With Isolation Devices Applied at Base

Nowadays, seismic retrofit through isolation strategy represents a consolidated technique of protection against design earthquakes. This technique is also widespread applied on existing structures, due to the fact that usually it does not require any interruption of the building use and occupants evacuation. If applicable, it rapidly allows of seismically retrofitting a building installing seismic devices with low horizontal stiffness between the structure and the foundation decoupling, in fact, the motion of the superstructure from the ground one. In this paper an application on an existing RC building of the seismic isolation is presented. The chosen building was built in the ‘90s only for vertical loads and realized without any detailing rule for structural ductility. The seismic retrofitting requirement stems from the fact that only recently, after the National seismic hazard maps update in 2003, the considered area has been upgraded to a medium-low seismic intensity zone, while at construction time no seismic classification was in existence by law. The case study peculiarity is that the seismic retrofitting has required in addition to seismic devices at the base with related interventions, also the application of a bracing system consisting of two elastic steel frames. This intervention is required for stiffening the superstructure and, therefore, for minimizing the higher vibration modes effects. The paper presents the main results obtained with a FEM model, implemented for simulating the initial and the design state when the interventions are considered. Finally, some results of non-linear dynamic time-history analyses are illustrated and commented for verifying superstructure elements and seismic devices.

Assessment of the Seismic Performance for a Low-Code RC Shear Walls Structure in Bucharest (Romania)

Background: Low-code RC shear walls structures house a significant proportion (over 20%) of the inhabitants of Bucharest. Objective: In this research, the seismic performance of a low-code reinforced concrete shear walls structure located in various sites in Bucharest (Romania) is assessed. Methods: The seismic performance is assessed using both static and dynamic nonlinear time-history analyses, as well as detailed sectional analysis of the RC shear walls. Results: The results show that the structure has significant capacity differences in the two principal directions. Moreover, it has been observed that the maximum top displacements obtained from nonlinear time history analyses are larger in the transversal direction. Conclusion: Considering the behaviour of such structures during the 1977 Vrancea earthquake, the fact that these structures are already 40 – 50 years old and given the results of some of the analyses shown in this study, it can be concluded that immediate seismic rehabilitation measures are needed in order to correct, at least partially the structural issues of this section type.

Assessment of the Seismic Behavior of Selective Storage Racks Subjected to Chilean Earthquakes

A seismic performance evaluation of selective storage racks subjected to Chilean Earthquakes was conducted using nonlinear pushover and nonlinear dynamic time-history analyses. Nine seismic records with two horizontal components and magnitude Mw > 7.7 were applied to numerical models of prototype rack structures. The prototype racks were designed considering two types of soil and two aspect ratios. The inelastic behavior of beam connections was included in the models. The results showed a predominantly elastic behavior, mainly in the cross-aisle direction, in comparison to the down-aisle direction. The inelastic action was concentrated in pallet beams and up-rigths. Higher values of base shear were reached, due to elevated rigidity in rack configurations, and an acceptable performance was obtained. A response reduction factor was reported in both directions, reaching values larger than the limit imposed by the Chilean standard. However, values below this limit were obtained in the cross-aisle direction, in some cases. Finally, in all cases, the calculated response modification factor is highly influenced by the overstrength obtained from seismic design.

Robustness of simplified analysis methods for rocking structures on compliant soil

SummaryRecognizing the beneficial effect of nonlinear soil–foundation response has led to a novel design concept, termed ‘rocking isolation’. The analysis and design of such rocking structures require nonlinear dynamic time history analyses. Analyzing the entire soil–foundation–structure system is computationally demanding, impeding the application of rocking isolation in practice. Therefore, there is an urgent need to develop efficient simplified analysis methods. This paper assesses the robustness of two simplified analysis methods, using (i) a nonlinear and (ii) a bilinear rocking stiffness combined with linear viscous damping. The robustness of the simplified methods is assessed by (i) one‐to‐one comparison with a benchmark finite element (FE) analysis using a selection of ground motions and (ii) statistical comparison of probability distributions of response quantities, which characterize the time history response of rocking systems. A bridge pier (assumed rigid) supported on a square foundation, lying on a stiff clay stratum, is used as an illustrative example. Nonlinear dynamic FE time history analysis serves as a benchmark. Both methods yield reasonably accurate predictions of the maximum rotation θmax. Their stochastic comparison with respect to the empirical cumulative distribution function of θmax reveals that the nonlinear and the bilinear methods are not biased. Thus, both can be used to estimate probabilities of exceeding a certain threshold value of θ. Developed in this paper, the bilinear method is much easier to calibrate than the nonlinear, offering similar performance.

Evaluation of uniaxial contact models for moat wall pounding simulations

SummaryMoat wall pounding occurs when a base‐isolated building displaces beyond the provided clearance and collides with the surrounding retaining wall, inducing very high floor accelerations and interstory drifts. Previous studies on moat wall pounding typically employ simplified models of the superstructure, with a uniaxial contact spring used to model the entire moat wall. Consequently, researchers have developed sophisticated contact models to estimate the normal‐direction contact force that is generated during seismic pounding. This study examines how the choice in contact model affects the seismic response of a base‐isolated building subjected to impact‐inducing ground excitation. Five widely used state‐of‐the‐art contact models are summarized and implemented into an experimentally‐calibrated numerical model of a base‐isolated moment frame. Results of nonlinear dynamic time history analyses are shown in detail for one ground motion, followed by a larger parametric study across 28 near‐fault ground motions. This work shows that peak impact force and base acceleration are moderately sensitive to the choice in contact model, while upper floor accelerations and interstory drifts are practically not affected.

Relationship of strength reduction factor and maximum ductility factor for seismic design of one-storey industrial steel frames

In the earthquake-resistant design, it requires a check of the adequacy of the maximum inelastic deformation of structural members designed corresponding to the value of strength reduction factor, R, used for determining the design base shear and consequently equivalent static seismic forces applied on the structure. This paper presents evaluation of a relationship of the factor R and maximum ductility factor, D, for irregular building structures, particularly considered in this study for the case of one-storey industrial steel building frames with cranes by considering the effects of structural irregularities, seismic intensities and simultaneously horizontal and vertical seismic actions. In the evaluation, approximate equations were presented to determine the relationship which was characterized by the factor F. The relationship was evaluated by conducting a number of rigorous nonlinear inelastic dynamic time-history analyses of various one- and two-span analytical frames designed with various values of R = 2, 3 and 4 and maximum ground acceleration a = 0.25g, 0.5g and 0.75g. Based on the analytical results, the means and standard deviations of the relationship factor F for the analytical frames considered were presented by using approximation equations as well as Monte Carlo simulations.