Abstract

The strain hardening fiber reinforced concrete—generally known as the Engineered Cementitious Composite (ECC)—has rapidly gained the attention of researchers in recent years. However, most of the research on ECC is limited to material and member level, leaving a gap in the understanding of its behavior at the structural scale. Therefore, this study investigates the global seismic response of ECC structures and compares their performance with conventional reinforced concrete (RC) structures. For this purpose, a case study long span building structure (an aircraft hangar having a span length of 40 m) is selected. Under the design-level gravity and lateral loads, its members are separately designed using the conventional RC and ECC. It is observed that for ECC members, the requirement of longitudinal steel is reduced by 30% when compared with the conventional RC members. Similarly, owing to an improved tensile behavior, the ECC members also exhibited a higher shear capacity than RC members, resulting in a significant reduction in the requirement of transverse reinforcement. The detailed inelastic finite element models for both design cases (RC and ECC) were subjected to the pushover analysis and nonlinear response history analysis (NLRHA) to assess their seismic performance. It is observed that (in terms of local and global seismic demands, structural damage, and ductile behavior) the performance of the ECC structure is significantly improved when compared to the conventional RC structure. The comparative cost analysis showed a reduction of 11.9% in the overall material cost of the ECC structure as compared to RC. These results show that ECC can be effectively used at the full structural level as an economic solution to ensure the ductile response and superior seismic performance.

Introduction

The structural design of long-span concrete structures is extremely challenging for designers. This is mostly owing to high moment demands, which cause undesired high tensile stresses [1] and most of the construction materials except steel are weak in tension. Despite the complexity involved in their design, they are very popular commercially. Their popularity derives from their necessity in numerous special applications such as industrial sheds, aircraft hangars, halls, auditoriums, etc. Additionally, long-span structures may assist in cutting off the cost of intermediate supports, resulting in a more economical design. In the early 10th century, these high demands were resisted by the use of arches and domes, which reduced the flexural stresses and produced axial compressive stresses [2]. Later in the 1940 s, reinforced concrete (RC) was proposed for medium to long-span structures incorporating an excessive amount of steel reinforcement [3]. Furthermore, the advancements in construction materials helped structural engineers to design cost effective infrastructures. In this way, pre-stressed concrete was introduced for long-span structures, eliminating high reinforcement and cross-sectional requirements. However, prestressed concrete possesses a shortcoming related to its ductility [4], [5]. This limitation becomes a critical concern when the structure lies in a seismically active region, as seismic design codes recommend the structure and its elements to be ductile in high seismic zones [6]. Therefore, RC with an excessive amount of steel reinforcement becomes a compulsion in seismic regions to ensure adequate confinement for ductility [4], [5]. However, RC makes the design uneconomical due to high reinforcement requirements. So, there is a need for an innovative material that can provide ductility without any compromise on cost, to ensure resilient and cost effective infrastructures.

Several innovative solutions have been proposed to improve the seismic performance of structures by providing ductile elements and links [7], [8]. One of these is to introduce fibers in the cementitious matrix to make the composite ductile. These composites are termed as fiber reinforced concrete (FRC) [9], [10], [11], [12]. A special class of high performance FRC that exhibits strain-hardening behavior in uniaxial tension is classified as Engineered Cementitious Composite (ECC) [13], [14], [15] [16], [17], [18]. ECC can be a potential candidate to reduce cross sections and reinforcement requirements due to its enhanced tensile capacity as compared to conventional concrete[19]. Additionally, ECC is lightweight and highly ductile which may also help to improve the seismic performance of the structure [20], [21], [22], [23], [24], [25], [26]. However, its material processing and design is not as simple as that of conventional concrete [27], as it requires a micro-mechanical model to obtain a mix that will ensure strain hardening response. To study its structural behavior, its material properties are incorporated by the integrated structures and materials design (ISMD) approach proposed by Li [28]. ISMD specifies that material design should be carried out first to obtain the necessary constitutive model for structural analysis and design.

Conclusions and recommendations

This research was able to conduct a comprehensive seismic assessment of the long-span ECC structure. For the very first time, both linear and nonlinear FEA models of ECC on the structural scale were developed considering a material level constitutive model. A complete structural design and nonlinear analyses of ECC structures and their comparison with RC structures were performed, and the following conclusions were made:

• Due to the lower unit weight of ECC, the design actions for structural members were approximately reduced by 25% because of the reduction in dead and seismic loads.

• Due to the better tensile capacity of ECC, the cross sectional sizes of the structural members were reduced by 25%, making ECC a good alternative to RC in the case of long-span structures. This observation indicates the potential of ECC for long-span structural applications.

• The structural design performed using JSCE guidelines showed an over 30% reduction of longitudinal steel in flexural members and about 15% reduction in longitudinal steel in compression members. Alongside, theoretically, the shear reinforcement requirement throughout the structure was eliminated, due to additional shear capacity provided by the fibers.

• The nonlinear analyses clearly showed better seismic performance (in terms of local and global seismic demands, structural damage, and ductile behavior) of ECC due to its increased capacity and lower inertial forces being developed within the structure.

Abstract

It is imperative for residents to have access to safe facilities after an earthquake. This ensures the safety and well-being of individuals while minimizing the risks posed by aftershocks. This research has used multi-criteria decision-making (MCDM) and network-based analysis to select and allocate emergency shelters (ESs). Several ESs are initially chosen as potential candidates. A weighting process is then used to evaluate various criteria, including proximity to the fault, fire stations, hospitals, main roads, the area of the ESs, and the population’s vulnerability. The centers are evaluated and ranked using the CRiteria Importance Through Intercriteria Correlation (CRITIC) and Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) methods. The Maximized Weighted Capacitated Coverage with a Radius (MWCCR) problem is used to address location and allocation issues for varying ESs using option rankings. The findings showed that increasing the number of centers does not always lead to a higher level of service delivery, assuming a consistent service delivery radius. The distribution of the centers is more crucial. Additionally, line density analysis is used to evaluate traffic conditions in the study area, assisting in finding areas with heavy traffic flow. When the radius of access for ESs is assumed to be small, the main roads bear less additional traffic, and with the increase of the radius, the amount of traffic on the main roads gradually increases. This is valuable information for emergency services following an earthquake.

Introduction

Earthquakes, as natural disasters, can cause extensive damage. Due to their potential to cause widespread damage to critical urban infrastructure, they are a significant challenge for large cities. Therefore, organizing crisis planning, including implementing policies that govern urban decisions, is very important to respond effectively to earthquakes [1], [2]. After an earthquake, one of the priorities is to quickly and safely evacuate citizens to protect them from potential hazards such as earthquakes, fires, and the spread of infectious diseases [3]. Emergency shelters (ESs) are safe places with necessary services for evacuating citizens during earthquakes [4]. Therefore, it is crucial to construct secure and appropriately located emergency evacuation facilities to mitigate risks following an earthquake. These facilities should have sufficient resources to support affected citizens [5]. Researchers face challenges in selecting the appropriate location for emergency evacuation due to multiple criteria. The city officials should allocate a budget to adapt the facilities and create a wide range of services in these centers. Therefore, it is crucial to determine the optimal location and quantity of ESs to minimize costs. Selecting these centers’ locations involves a location–allocation problem with two components. A positioning component determines the placement of ESs and outlines how these centers serve the population in the allocation section [6], [7]. Therefore, to determine appropriate locations for ESs, a solution must be selected to address the location allocation problem.

Various approaches have been used in studies on location allocation for ES planning. Some approaches have focused on allocating ESs to citizens in a single-objective manner, considering only one criterion, such as time or distance. Many studies commonly use single-objective techniques such as p-median, p-center, and maximum coverage. Multi-objective and hierarchical models have been used in some studies. Additionally, geographic information systems (GIS) methods, such as the Voronoi diagram, can be used [8], [9].

Conclusion

ESs are essential for promoting peace and ensuring the safety of individuals during earthquakes and potential aftershocks. These centers’ locating and coverage areas should be optimized to prioritize citizen safety and maximize accessibility. This study aimed to identify and allocate ESs in a part of Tehran’s 12th district. Potential ESs were initially selected based on specific criteria, including their proximity to fault lines, fire stations, hospitals, main roads, ES areas, and population vulnerability. The criteria were weighted, and the centers were ranked using the CRITIC and TOPSIS methods.

According to the results obtained using the CRITIC method, the criteria of distance from the hospital and fire department had the highest weight (0.19). In contrast, the area criteria had the lowest weight (0.12). The TOPSIS method has been used to rank the ESs by assigning weights to the criteria. The ranked centers were utilized to address the issue of location and allocation. In the location problem, priority was given to centers with higher ratings. The location and allocation issues have been resolved for different numbers of ESs. Based on the results, the service delivery radius is assumed to remain constant. Simply increasing the number of centers does not guarantee better service delivery. The distribution is crucial. When the coverage radius was set to 500 m, increasing the number of ESs decreased the occupancy percentage of the centers. This indicates that the centers were not fully utilized because they were more than 500 meters away from the residents. If the coverage radius is small, the distribution of centers becomes more significant. This requires considering multiple centers in different locations. In cases where the radius was set at 1000 and 1500 m, there was an increase in the percentage of service delivery and occupancy of ESs. The service coverage percentage within a 1500-meter radius was only about 60%, well below the desired value of 80%. It is possible to expand the distance to increase this case’s coverage. It is important to note that as the distance from centers increases, quick and easy access becomes more complex, which is crucial during times of crisis.

Abstract

In the emergency repair process of seismic-damaged structures, it is necessary to determine the optimal repair strategy according to damage assessment results, available repair resources and repair time. In this study, the applicable conditions of various member-level and structure-level damage indexes or models are compared. Then, considering the actual conditions of limited available repair resources and repair time in the emergency repair process, the determination method of the optimal imperfect repair strategy is established. On this basis, the determination method of the optimal repair sequence is proposed by taking seismic resilience as the evaluation index. Finally, a seismic fragility analysis method that considers uniform collapse risk and residual service life of structures is introduced, which can be used to analyze the seismic performance of seismic-damaged structures and to compare the repair effectiveness of different repair strategies. Taken a 5-story reinforced concrete structure as an example for analysis. The analysis results show the optimal imperfect repair strategy and the optimal repair sequence determined by the proposed methods can effectively improve the seismic performance of seismic-damaged structures. The fragility analysis results further confirm that the repair based on the optimal imperfect repair strategy can effectively reduce the collapse risk of seismic-damaged structures.

Introduction

A strong earthquake can cause serious damage to engineering structures. The damage or collapse of engineering structures and secondary disasters caused by structural damage or collapse are the main causes of casualties and economic losses [1,2]. After a main shock, it is of great theoretical significance and engineering value to repair seismic-damaged structures according to feasible damage assessment methods and the ideas of seismic performance rapid improvement, so as to reduce the damage degree and collapse risk of seismic-damaged structures under aftershocks, and to reduce casualties and economic losses.

The whole process of performance restoration of seismic-damaged structures includes three important parts: damage assessment, structure repair and retrofit, and seismic performance analysis. In the research on damage assessment, the damage of members can be assessed by damage indexes or models such as maximum drift ratio [3,4], residual drift ratio [5], crack characteristics [[6], [7], [8]], and Park-Ang model [9], and the overall damage of an integral structure system can be comprehensively assessed by deformation-related [10,11] or energy-related [12,13] damage indexes or models. Liu et al. [14] proposed a rapid damage assessment method for reinforced concrete members based on the fractal dimension of cracks, considering the correlation between the fractal characteristics of cracks and the damage state. The reliability of this method was demonstrated through experiments. Tazarv et al. [15] established the relationship between the damage state and deformation of beam and column members, and the computer vision technology was used to quickly detect cracks and failure modes of members, thereby achieving rapid identification of damage state. Makhloof et al. [16] reviewed existing damage assessment methods and emphasized the overall damage assessment methods based on deformation, frequency or modal parameter.

Conclusions

The repair strategy formulated through the damage assessment results of seismic-damaged structures can effectively improve the seismic performance. In this study, the application scope and application condition of different damage indexes or models are compared, the methods for determining the optimal imperfect repair strategy and the optimal repair sequence for seismic-damaged structures under the condition of limited available repair resources is proposed, and a seismic fragility analysis method considering the uniform risk and the residual service life of structures is introduced. Taking a 5-story RC frame structure with a residual service life of 40 years as an example for analysis, the main conclusions obtained from this study are as follows:

Abstract

Two destructive earthquakes (Mw: 7.7 and Mw: 7.6) occurred in Kahramanmaraş, Türkiye, on February 6, 2023. The epicenters of these two earthquakes were very close to each other (approximately 90 km apart), and both took place on the same day. The first earthquake occured at 04:17 local time with a magnitude of 7.7 and a depth of 8.6 km, while the second earthquake occurred at 13:24 local time with a magnitude of 7.6 and a depth of 7.0 km. These two earthquakes caused extensive damage, particularly in 11 provinces, with Adıyaman being one of the hardest-hit areas. Most buildings in Adıyaman were severely affected or collapsed as a result of the earthquakes. Reports indicate that over 35,000 buildings collapsed in the aftermath. Furthermore, between February 6, 2023, and May 6, 2023, more than 30,000 aftershocks, ranging in magnitude from 0.2 to 6.6, were recorded. In this study, earthquake damages in reinforced concrete buildings in Adıyaman province were evaluated. The causes of damages in reinforced concrete buildings were evaluated in terms of material quality, design of building and reinforcement details. Field observations are also provided, along with accompanying pictures. The earthquake regulations in Türkiye, established in 1975, 1998, 2007, and 2018, which apply to a significant portion of the country’s buildings, are discussed. An evaluation was conducted to assess whether the damaged buildings complied with the earthquake regulations in Türkiye. Field observations revealed that the earthquake regulations, particularly in relation to reinforcement details, were overlooked. In addition, designers should avoid design mistakes such as to make discontinuous of frames, to place most of vertical element strong axis in one direction, insufficient support condition of stairs and adjacent building effects.

Introduction

Türkiye has active earthquake fault zones, including the North Anatolian Fault, East Anatolian Fault, and West Anatolian Fault zones, which have the potential to cause destructive earthquakes. Türkiye has often been exposed to destructive earthquakes in the past. After these earthquakes, many studies about structural failures have been conducted and documented in the literature [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. On 6 February 2023, two destructive earthquakes occurred in Kahramanmaraş provinces in Türkiye (Mw: 7.7 and Mw: 7.6). The first earthquake was occurred in Pazarcık (distinct of Kahramanmaraş) at 04:17 local time with a magnitude of Mw:7.7. The second earthquake was occurred in Elbistan (distinct of Kahramanmaraş) at 13:24 local time with a magnitude of Mw:7.6. These two earthquakes were occurred in same day (approximately nine hour apart) and there is approximately 90 km distance between epicenters of these earthquakes [13]. The pair of earthquakes resulted in significant structural damage, particularly impacting 11 provinces, including Kahramanmaraş, Hatay, Gaziantep, Malatya, Diyarbakır, Kilis, Şanlıurfa, Adıyaman, Osmaniye, Adana, and Elazığ, where many buildings either sustained severe damage or collapsed entirely.

According to official announcements from the Disaster and Emergency Management Authority (DEMA) on 2 June 2023, it was reported that over 35,000 buildings had collapsed after the earthquakes. Furthermore, during the period from February 6, 2023, to May 6, 2023, more than 30,000 earthquakes with magnitudes ranging from 0.2 to 6.6 were documented [13]. The Kahramanmaraş earthquakes had a profound impact on Türkiye, causing severe damage not only during the initial earthquakes but also due to subsequent aftershocks.

Conclusions

Two destructive earthquakes occurred in Pazarcık (Mw: 7.7) and Elbistan (Mw: 7.6), Kahramanmaras¸, Türkiye. These earthquakes severely affected 11 provinces (Kahramanmaras¸, Hatay, Gaziantep, Malatya, Diyarbakır, Kilis, S¸ anlıurfa, Adıyaman, Osmaniye, Adana,Elazıg). Adıyaman is one of the provinces most affected by the earthquakes. Due to these two earthquakes occurring on the same day ˘ (nearly 9 h apart), damaged buildings were only examined after the second earthquake. This study focuses on examining the damaged reinforced concrete buildings in Adıyaman. Structural damages and the reasons for these damages are examined in comparison with Türkiye Earthquake Codes.

Abstract

Coupled vibration control (CVC) systems, which typically consist of two frames and connectors, such as stiffness and damping elements, can be used for the seismic protection of coupled or adjacent structures. This study aims to experimentally and numerically elucidate the response behavior and control effects of CVC structural systems comprising a mainframe and subframe incorporating passive negative stiffness (PNS) at the connection subjected to earthquakes. Shake table experiments were conducted using CVC system specimens incorporating a PNS device (PNSD) consisting of curved-leaf springs under dynamic excitations. A significant seismic response reduction for the mainframe of the CVC specimen under simulated waves was achieved by incorporating the PNSD at the connection. Moreover, a response simulation was performed using six-story CVC structure models connected by negative stiffness and viscous damping elements subjected to simulated and observed seismic wave inputs. The displacement and acceleration peak response of the mainframe using the numerical CVC models was lower than that of the uncontrolled model. Therefore, incorporating negative stiffness and viscous damping connectors into CVC systems can reduce the seismic response. This study contributes to the advancement of aseismic technologies for coupled or adjacent buildings utilizing the PNS.

Introduction

Coupled vibration control (CVC) structural systems typically consist of two frames and connectors, such as stiffness and damping elements. CVC systems with a proper connecting mechanism that depends on the structural properties of the two frames can be a solution for protecting coupled or adjacent structures against earthquakes [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. To date, various CVC structures incorporating various connecting devices, including steel hysteresis dampers [3], [4], fluid dampers [1], [14], magnetorheological (MR) dampers [7], [10], [11], [12], tuned mass damper inerters [13], [17], viscous inertial mass dampers [15], and friction dampers [16], have been proposed and investigated to mitigate the response when subjected to seismic input motions. Passive [2], [3], [4], [6], [16], semi-active [7], [10], [11], [12], and active [5], [8] control methods for CVC systems have been studied theoretically, numerically, and experimentally, and the results have proved a potential effectiveness for the seismic protection of buildings. However, employing only zero or positive stiffness at the connectors of CVC systems to achieve an optimal tuning and obtain a better control performance may be difficult depending on the properties of the two frames [18].

In contrast to positive stiffness, negative stiffness generally generates a decreased (negative) restoration force as the displacement increases. The use of negative stiffness can be considered a potential response control strategy for buildings against dynamic disturbances, such as earthquakes. During the last few decades, many studies have been conducted to explore negative stiffness devices (NSDs) and structural systems containing NSDs [19], [20], [21], [22], [23]. Iemura et al. [19] used the self-weight of the structure and developed a negative-stiffness damper based on a slide bearing with a convex curve. Nagarajaiah et al. [20] and Sarlis et al. [21] studied a passive NSD based on a vertical preloaded coil spring placed between two chevron braces and a horizontal-gap spring assembly. Wang et al. [22] investigated a structural system with a negative-stiffness amplifying damper consisting of a passive NSD [20], [21], positive-stiffness spring, and dashpot. Shirai et al. [23] proposed another passive NSD comprising curved-leaf springs. This passive NSD exhibited an initial negative stiffness created by the pre-compressed strain energy stored in the curved-leaf springs, followed by an increased second negative stiffness with the occurrence of snap-through buckling. However, these previous studies on NSDs explored the seismic response of individual, that is, nonadjacent or noncoupled, structural systems with NSDs.

Conclusions

This study experimentally and numerically investigated the response behavior and control effects of CVC structural systems with a PNS spring as a connector subjected to dynamic excitations. Shake table experiments were conducted using CVC system specimens incorporating PNSDs consisting of curved-leaf springs. In addition, time-history earthquake response simulations were performed using six-story CVC building models connected by negative stiffness and viscous damping elements. The following conclusions were drawn:

(1) In the shake table experiments, PNSD-1 and PNSD-2 exhibited an initial negative stiffness owing to the pre-compression effect, and the three PNSDs exhibited an increased second negative stiffness owing to the onset of snap-through buckling in the subtracted hysteresis loops of the PNSDs.

(2) An effective response control performance for the mainframe under seismic excitations was attained by incorporating a PNSD as the connector of the CVC specimen.

(3) The CVC specimen with PNSD-2 exhibited the most effective control performance among the three PNSDs for mitigating the response displacement and acceleration under the simulated wave input.

(4) In the numerical simulations, both the CVC-NV and CVC-V models exhibited a reduced mainframe peak response displacement and acceleration compared to that of the uncontrolled model (UC-MF).

Abstract

     This paper presents a rapid machine learning-based damage detection framework for identifying the damage extent of concrete shear wall buildings. For this purpose, a parametric study was carried out to determine the most efficient machine learning algorithm in classifying the damage states of the building. According to this parametric study, the K-Nearest Neighbor (KNN) learner was selected as the reference prediction model because of the higher accuracy achieved by this algorithm. Bayesian Optimization (BO) algorithm was used to tune the hyperparameters affecting the accuracy of the model. The most efficient attributes were selected from the set of damage indicators through the BO algorithm to train the model. Three different benchmark buildings, including 7-,9-, and 13-story concrete shear wall buildings, were used to evaluate the robustness of the proposed framework. A suite of 111 pair motions, originally developed for the SAC project, were employed to create a generalized dataset. These motions were uniformly scaled from 0.05 g to 1.5 g to expand the intensity range of the events. All the acceleration signals were polluted to 10% noise using white Gaussian signals to simulate the field condition. Results reveal the efficiency of the proposed framework in identifying the extent of damage in concrete shear wall elements of the building. In addition, a parametric study was conducted to illustrate the reliability of two commonly used features, called Cumulative Absolute Velocity (CAV) and the energy ratio between the acceleration response and the input excitation, in determining the damage states of the shear walls under seismic motions.

Introduction

     Assessment of structural safety is essential for post-earthquake restoration. Generally, to evaluate the post-earthquake vitality of the exposed structures, a complete visual inspection is required [1]. Coordination and implementation of the manual visual inspection needs several dedicated teams and monetary resources. In this regard, considerable efforts have been carried out to automate the visual inspection process, e.g., image-based visual inspection [2]. However, such an engineering visual inspection is only able to detect the visible defects that occurred in the structures [3]. This means that some serious invisible damages may be left latent during the visual inspection. The process of identifying and tracking the structural damage is known as the Structural Health Monitoring (SHM) [4,5]. In SHM, damage detection is related to the methods developed for identifying the probable existence, severity, and location of the structural damage. Model-based and data-driven methods are two of the most commonly used strategies proposed for damage detection. Model-based methods generally involve a system identification algorithm paired with a finite element analysis to update the structural model [6]. The performance of the model-based approaches directly depends on the accuracy of the information about the physical properties of the under-study structure. In addition, updating the finite element model based on the physical properties of the structure is computationally expensive for large-scale structures, and rapid condition monitoring could be challenging in this condition [7,8]. On the other hand, data-driven methods apply statistical learning algorithms to the vibration data captured from the structure. This method uses learning algorithms to construct a classification or regression learner for predicting structural damage [9, 10]. Tsou and Shen [11] proposed the use of Neural Networks (NNs) for predicting the severity and location of the structural damage. They used the variations in the modal properties of the structure as the damage feature to identify the damage. Worden et al.

Conclusion

     This paper presented a rapid algorithm for identifying the severity of local damage in the concrete shear wall buildings. A total number of 1884 nonlinear response history analyses were conducted for each building using the SAC motions. A suite of damage indicators was extracted from the acceleration signals to construct the prediction models. A parametric study was carried out to determine the most efficient learner for classifying the damage states of the buildings. The KNN classifier was selected to construct the predictive models because of its maximum accuracy compared to the other algorithms. The Bayesian optimization algorithm implemented to tune the hyperparameters of the classification learners. The main conclusions derived from the study are as below:

Abstract

     This paper proposes an innovative seismic design approach for shallow rectangular cut-and-cover underground subway or railway stations. The traditional approach is to design rigid frame-like structures by connecting rigidly the main horizontal and vertical structural elements (side walls, top, bottom and intermediate slabs, and central columns); on the contrary, the proposed strategy consists of joining them by means of hinged and sliding connections, in order to obtain structures whose lateral stiffness is almost zero. The objective of this approach is to be able to adapt to the transverse racking motion imposed by the seismic ground motion without significantly increasing the internal forces in the structural members. The aforementioned flexibility of the joints is achieved by interposing rubber bearings between the connected structural elements. As a case study, an existing 2-story 3-bay subway station located in Southwest China is redesigned with the proposed technology; its seismic performance is numerically investigated by performing nonlinear dynamic analyses for a number of horizontal transverse input ground motions (accelerograms) representing the site seismicity. Such inputs are scaled to fit PGAs ranging from 0.1 to 0.6 g. As expected, the results of the time-history analyses reveal that the seismic damage to the structural members is significantly alleviated in the sliding-hinged alternative solution. This conclusion can be understood as a preliminary confirmation of the satisfactory seismic performance of the proposed technology.

Introduction

     The nowadays rapid development of buried structures, such as tunnels, underground parking lots, subway and railway stations, etc. provides alternatives to the insufficient ground space. Taking subway stations as an example, according to the International Association of Public Transport [1], there were 11,084 stations spreading over 13,903 km of subway operating lines at the end of 2017 in the world, and in the next few years another 1700 km were to be built. However, recent and past investigation reveals that earthquakes can damage underground stations [[2], [3], [4], [5], [6], [7], [8]]; therefore, further research is required on their seismic design.

     The traditional seismic design of underground stations is based on providing sufficient stiffness and strength to absorb the strains imposed by the seismic ground motion without excessive damage. This approach is basically correct and leads to safe constructions, but omits that earthquakes are indirect actions (i.e. imposed displacements): designing stiffer structures generates an increase of the soil-structure interaction force, thus providing little benefit (if any). Another smarter strategy is to try to reduce that force, either by lowering the lateral stiffness of the soil (locally) or the station. The first objective is achieved with seismic isolation [9,10], and the second with structural flexibilization measures [9,10]. Both approaches are briefed in the next two paragraphs, respectively.

Conclusions

     This paper proposes a new alternative seismic design solution for rectangular cut-and-cover underground stations; it consists of replacing the traditional rigid connections between the structural elements with hinged and sliding joints, as to obtain near-zero lateral stiffness. The required flexibility of the joints is achieved through the use of rubber bearings. The ensuing structures are expected to be able to accommodate the imposed seismic racking without relevant strains in the structure. As a case study, an existing 2-story 3-bay subway station located in Southwest China is redesigned with the proposed technology; its seismic performance is investigated numerically by time-history analyses for a set of input seismic accelerograms. These inputs are scaled to fit PGAs ranging from 0.1 to 0.6 g.

      The main output of this study is that the damage to the structural elements is considerably lessened in the sliding-hinged alternative solution; this trend is particularly intense for not only central columns but also structural connections. On the other hand, the acceleration and drift do not alter much; therefore, no higher damage to the acceleration and drift-sensitive nonstructural components is to be expected in the proposed alternative solution. These conclusions hold for all the considered levels of PGA. The remarks obtained can be read as a preliminary validation of the proposed seismic design approach.

Highlights

Abstract

Keywords

1. Introduction

2. Research significance and objectives

3. Experimental program

3.1. Concrete specimens

3.2. Test setup and instrumentation

3.3. Concrete properties and test parameters

4. Results and discussion

4.1. Concrete deformations: Strains and deflections

4.2. Recovery strains

4.3. Deflection recovery

4.4. Analysis of uncracked and cracked concrete beams

5. Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgments

References

Abstract

The purpose of this research is to analyse the long-term deformations of reinforced concrete beams made of recycled coarse aggregates (RCA), using four different replacement ratios, 0%, 20%, 50% and 100%. For said purpose, three different loading stages were carried out: firstly, loading and sustained load for 1300 days, secondly, unloading and recovery where the deformations were registered over a one year period, and lastly, testing the concrete beams up to failure in order to analyse the effect of recycled aggregates on pre-cracked concrete members. The results led to the analysis of deformations under sustained load, as well as those produced after removing the load and the performance of pre-cracked members, while identifying the effect of recycled coarse aggregates on concrete performance in terms of plastic deformation, recoverability and concrete stiffness, and also on structural design.

1. Introduction

In recent years, the use of construction and demolition waste as aggregates for structural concrete has been widely studied. Numerous authors have contributed to the increasing knowledge in this field, analysing the structural performance of recycled concrete and its properties, such as mechanical strength, modulus of elasticity, creep and shrinkage or durability [1], [2], [3]. As a result, several expressions and methods have been assessed for the design of structural concrete using recycled aggregates [4], [5], [6], [7], [8], [9], [10], [11], [12]. Most authors deal with the short and long-term behaviour of concrete while considering only the influence of creep and shrinkage [13], [14], [15] and very few include the ageing effect of concrete, its different cracking behaviour or its ability to recover from deformations after a load is varied or removed [16].

As common concrete structures are usually submitted to load variations during their service life, it is important to consider the effect of creep over time, including not only loading periods but also unloading the stages and the recoverability of concrete. As mentioned by Rossi et al. [17] understanding the behaviour of concrete structures is greatly simplified by the numerical modelling of their mechanical behaviour. However, progress needs to be made to predict the long-term behaviour of concrete with sufficient accuracy. In order to determine the deformational mechanism, it is also important to analyse the effect of cracking and residual strain i.e. the irreversible part of strain. Many factors influence deformation and the ability of concrete to recover after unloading. Once the hysteretic recovery has finished, the residual deformation can be defined as plastic deformation. Additionally, it can be assumed that concrete deformations under loading are mainly due to creep, especially for long periods of loading and therefore, the ability of deformations to recover after unloading is closely linked to creep recovery.

Abstract

     The paper studies the performance of Offshore Wind Turbines (OWTs) founded on Suction Bucket Jackets (SBJs) in clay under combined wind and seismic loading. A detailed 3D FE model of the soil–foundation–structure (SFS) system is developed and used as a benchmark to assess the efficiency of an enhanced Winkler-based “Caisson-on-Winkler-Soil” (CWS) model, where the soil is replaced by nonlinear hysteretic elements. The proposed CWS model captures residual deformations and hysteresis and offers physical coupling between vertical and moment loading. It allows excellent prediction of the  failure envelope in the most relevant first quadrant of  space, where the loads act in the same direction. Despite its successful application for the simulation of inertial loading, the CWS model fails to reproduce the dual shearing mechanism that develops at the caisson shaft during shaking, stemming from the combination of kinematic soil shearing due to the vertically propagating shear waves, and shearing due to the superstructure inertial response, thus underpredicting the co-seismic caisson settlements. For the prediction of the latter, the research utilizes spectrum compatible input motions and 3D FE models of varying geometric and material properties to derive linear regression equations that correlate the co-seismic dimensionless settlement of caissons ) with characteristic dimensional variables of the problem under investigation and the Arias Intensity  of the surface ground motion. As a final step, the paper proposes a hybrid method for performance-based assessment of SBJ OWTs. The proposed method employs the simplified CWS model to calculate the VHM loads and approximately estimate horizontal displacements and rotations at the jacket legs, followed by a preliminary assessment of caisson settlements using the correlations of  with  on the basis of spectrum-compatible input motions.

Introduction

     Following the ambitious energy targets set by countries worldwide, the offshore wind sector has seen impressive growth over the last decade, transforming from niche technology to a global industry. As of 2019, Europe alone has 22 GW of installed offshore wind capacity – enough to cover 2.3% of its electricity consumption [1], while similar or even more potent growth is observed in many countries around the world, such as China, the USA, and India. Part of this energy transition is led by technology-driven innovation, with offshore wind turbines reaching new heights and larger capacities, while wind farms are moving deeper into the sea to harness the increased energy potential. Giventhis new “deep-water” environment, traditional OWT support structures are gradually being replaced by more cost-effective and agile foundation systems in a quest for reduction of investment costs.

Conclusions

     The paper has developed a simplified performance-based assessment technique for SBJ OWTs founded in clay under the combined action of wind () and earthquake () loading. Using an example of an 8 MW jacket-supported OWT, installed at 60 m depth in the Adriatic Sea, system performance was assessed employing a detailed 3D FE model of the soil–foundation–structure (SFS) system (global 3D FE model). After deriving insights on system performance, the global 3D FE model was used as a benchmark to assess the efficiency of an enhanced Winkler-based “Caisson-on-Winkler-Soil” (CWS) model. Soil–suction caisson interaction is represented by nonlinear hysteretic elements, capturing residual deformations and hysteresis. The proposed CWS model offers physical coupling between vertical and moment loading by introducing distributed vertical hysteretic elements along the caisson shaft, simultaneously contributing to vertical and moment shaft resistance.

Abstract

     An increasing number of offshore wind farms are being constructed in seismic regions over liquefaction susceptible soils. This paper presents a methodology for the analysis and design of monopiles in seismically liquefiable soils by extending the established “10-step methodology” with an additional 7 steps. These additional steps include assimilation of seismic data, site response analysis, stability check of the structure (ULS check through the concept of load-utilization ratio), input motion selection, prediction of permanent tilt/rotation, and ground settlement post liquefaction. A flow chart, which shows the interdependence of the different disciplines, is presented and can be extended to routine design. This proposed method is validated using the observed performance of an offshore and nearshore turbine from the Kamisu wind farm during the 2011 Great East Japan earthquake. Predicted results based on the proposed methodology compare well with the field observation and demarcate the (i) good overall performance of the offshore turbines and (ii) limit state exceedance of the nearshore turbine. It is envisaged that the proposed method will be useful towards the design of monopiles-supported wind turbines in seismic areas.

Introduction

     Towards the end of 2020, the global wind energy generation capacity amounted to 733 GW, with new installations accounting for 111 GW, almost doubling that of 2019 [1]. Due to higher efficiencies and more stable wind conditions offshore than onshore sites, an increasing proportion of wind power is produced through large offshore wind farms. This demand for cost-efficient wind energy production has facilitated the industry to develop larger turbines with higher capacities. Fig. 1 presents the evolution of turbine capacity over the last 16 years, color-coded with rotor diameter. However, due to the high capital investments involved, profitability of these wind farms requires continuous operation and immediate functionality post-natural hazards. Therefore, the seismic resilience of OWTs is an important consideration in their engineering design and is, therefore, an area of active research. Readers are referred to recent publications [[2], [3], [4], [5], [6], [7], [8], [9]] on various aspects of seismic design.

Discussion and conclusions

     An existing framework for the foundation design of monopile-supported wind turbines was extended to include seismic load considerations in liquefiable and non-liquefiable soil. The framework was validated using a case study from Wind Power Kamisu, a near-shore farm that performed well during the 2011 Tohoku earthquake and subsequent tsunami.

The discussion and conclusions are presented succinctly below:

     1. A quantitative appraisal of appropriate damping, as recommended by Lombardi and Bhattacharya [58] and Adhikari and Bhattacharya [57], could vary the structural demands and therefore needs proper consideration. Back analysis of a case study from the Kamisu wind farm further indicated that structural demands could vary if suitable damping models are not selected.

      2. Definition of mechanism-specific p-y spring models for liquefiable and non-liquefiable soil is necessary as their selection could underpredict or overpredict structural response (deformation, tilt, RNA acceleration). As near-level ground conditions prevailed in this study, the authors employed hyperelastic (strain hardening) p-y curves for liquefiable soil to account for the increased resistance attributed to shear strain-induced soil dilation. During shear strain excursions, such dilative tendencies can transfer high acceleration pulses to the RNA and therefore need to be accounted for in the design stage.

Abstract

     The purpose of this paper is to study the behavior of pile-supported structures in liquefiable soils, specifically when the soil surrounding the pile transits from no-liquefaction to full-liquefaction. A series of shaking table tests were performed on four pile-supported structures subjected to different input motions and, as a result, with different times to reach full-liquefaction. The bending moment of the piles during the transient phase is compared with those predicted in pre- and post-liquefaction stages. The experimental results showed that the maximum bending moment may occur during the transient phase (i.e. during the development of excess pore pressure before the soil is fully liquefied). Arguably, the observed amplification in bending moment is caused by the the tuning effect between the predominant frequency of the input motion and the frequency of the pile-supported structure, which is progressively decreasing during the liquefaction process. Results are presented using a non-dimensional framework whose parameters are derived from the governing mechanics. A new parameter TAF (Transient Amplification Factor) is defined to predict the design bending moment during the transient phase. It is shown that the transient bending moment can be obtained from the newly introduced parameter TAF, and the values of maximum bending moments in the pre- and post-liquefaction stages. It is found that TAF is a function of two easily obtainable parameters: (a) time taken to reach full liquefaction, this can be obtained through site response analysis; (b) elongation of natural period of vibration, expressed as ratio of the time period of the structure at full liquefaction to the time period at zero-liquefaction. Finally, practical implications of the main findings are discussed.

Introduction

     The dynamic behaviour of pile-supported structures founded in liquefiable deposits is still an area of active research [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]] due to the poor performance of piled foundations as observed in most of the recent earthquakes [9,31]. The current methods of pile design – see for Example codes of practices such as JRA [32,33] NEHRP [34], IS-1893 [35], focus on avoiding bending, buckling and settlement failures. The effect of dynamic soil-structure-interaction (SSI) effects is taken into account by means of empirical correlations.

     Fig. 1 shows the two main stages of loading in a typical pile-supported structure during an earthquake. Fig. 1 (a) shows the stage at the start of the shaking, before any build-up of the excess pore water pressure takes place. In this stage, piles are mostly subjected to inertia loads induced by the ground shaking and oscillation of the superstructure. It is known that inertia loads tend to generate high bending moments at relatively shallow depths along the pile. Fig. 2 (a) shows the “Beam on Non-Linear Winkler Foundation” model, in which the effect due to soil-structure interaction is modelled by a set of springs distributed at discrete locations along the length of the pile. Each spring is defined by a relationship between the soil pressure (p) and pile deflection (y) referred to as p-y curve. This method is conventionally used to compute the internal forces (i.e. bending moment and shear force) and pile’s deflection. The typical shape of p-y curves for sand in non-liquefied condition is shown in Fig. 2 (b), and further details can be found in API [36]. However, in saturated loose to medium dense sand,the ground shaking induce a gradual increase in pore water pressure, resulting in the soil to progressively lose its strength and stiffness. When the excess pore pressure equalises the overburden pressure, the soil is said to be in a full-liquefaction condition; at full liquefaction the foundation loses the support from its sorrounding soil and the piles tend to act as an unsupported column over the liquefied layer, see Refs. [37,38]. Fig. 1c shows the time history of a real earthquake together with excess pore water pressure profile, which can be either measured in experiments or computed by means of site response analyses. From the figure it can be noted that the onset of liquefaction occurs in a finite amount of time, whose duration depends mostly on the input motion characteristics and the density of the soil; the latter is typically expressed in terms of relative density. Typical values of time to reach liquefaction range between 6 and 15s, however, the actual time can be estimated by means of nonlinear finite element analysis.

Experimental results

3.1. Frequency domain analysis

     To quantify the change in vibration characteristics of the models, the experimental data was first analysed in frequency domain. Before the start of each test, the natural frequencies of the models were estimated from free vibration tests performed by exciting the structure with an impact hammer. The frequency response functions FRFs obtained from the free vibration tests are depicted in darker lines (denoted by before shaking) in Fig. 4, Fig. 5, Fig. 6. In the same figures, the power spectral density (PSD) of the input motion and FRF estimated after liquefaction is shown for comparison. It may be observed that after liquefaction the natural frequency of the models reduced significantly, which may be attributed to the development of excess pore pressure, and in some cases formation of plastic hinges in the piles. From Table 3 it is interesting to note that the four models failed in tests CH1 to CH4. (see Fig. 4 for frequency response). Other tests listed in Table 3 were carried out to investigate the sole effect of liquefaction on the variation in natural frequency of the models and transient bending moments. In these tests, formation of plastic hinges was deliberately avoided with lower pile-head mass. The results shown in Fig. 5, Fig. 6 indicate that the natural frequency of the models reduced to about half due to subsurface liquefaction. As the model transited from higher to lower frequencies it was likely that the seismic response amplified due to a temporarily matching between natural frequency of the models and predominant frequency of the input motion. This aspect is further investigated in the next section by considering the bending moment time histories along the piles.