Seismic retrofitting of a historic building by using an isolation system with a weak restoring force
Introduction
Historic buildings and cultural relics are extremely valuable treasures. The significance of the remaining historic structures lies in not only their usage functionalities but also their cultural value [1], which often makes such structures more valuable than general structures. Moreover, the seismic resistance of most historic buildings is inferior to that of modern structures because the mechanical performance of the construction materials deteriorates with time [2,3]. Consequently, historic structures can be easily destroyed when subjected to ground motion excitations. Therefore, it is of significance to protect historic buildings and the cultural relics housed in such buildings.
To improve the seismic performance of historic buildings, traditional retrofitting methods that involve enhancing the connection of structural elements or attaching shear walls and braces to the existing structure may be employed to improve the stiffness of the structure. Bento et al. [2,4] proposed the use of steel or reinforced concrete members to strengthen the connection of masonry walls, thereby improving the lateral resistance of the structure. However, in this case, the stiffness of the structure also increases, which increases the seismic force on the structure. Another retrofitting method involving the use of a timber bracing system was proposed to protect historic masonry buildings [5]. The seismic performance of structures with timber X-bracing and K-bracing was analyzed, and the X-bracing was noted to exhibit a better performance. Nevertheless, the seismic force on the structure increased. The structure performance can also be enhanced by improving the mechanical performance of the structural members [[6], [7], [8], [9], [10]]. In this regard, Bui et al. [7], who studied the shear behavior of masonry walls enhanced using a fiber reinforced polymer and textile reinforced concrete, indicated that the behavior of such walls was better than that of the original walls. Witzany et al. [9] experimentally examined and analyzed the failure mechanism and final compression bearing capacity of a masonry wall strengthened by carbon fiber reinforced polymer. Akcay et al. [10] performed experimental and numerical analyses regarding the strengthening of a historic masonry building by using different conventional techniques. All these methods can help improve the seismic performance of a structure; however, the originality of the structure cannot be maintained, indicating that the original structure and cultural relics housed within may be damaged during retrofitting. In addition to the traditional method, energy-dissipated devices [[11], [12], [13], [14], [15], [16]] can be used to improve the seismic energy dissipation effectiveness of structures, and the base isolation technology [[17], [18], [19]] can be used to reduce the seismic input of the structure, thereby mitigating the dynamic response of the structure. Another advantage of the base isolation technology is that the originality of the structure can be maintained after retrofitting because the complete retrofitting is performed in the isolation layer [20], which is beneficial for historic buildings. Branco and Guerreiro [21] compared three different retrofitting methods (traditional method and the use of viscous dampers and base isolation technology) for historic buildings and noted that the base technology could reduce the interstory drift of the structures and tensile stress of the elements more effectively. Pauletta et al. [22] studied the seismic mitigation effect of the base isolation technology and traditional retrofitting method for historic buildings and found that the isolated structure could attain the design target, in contrast to the structure reinforced using the traditional retrofitting method. Lupășteanu et al. [20] studied a historic masonry church strengthened using an isolation system including friction pendulum sliding isolators and noted that the interstory drift and shear force of the church was reduced when subjected to an earthquake. These studies proved the effectiveness of the base isolation technology in retrofitting the existing structures.
Different kinds of isolation technologies can be used to extend the natural period of a structure and reduce the dynamic response. The frequently used isolation devices include rubber bearings [[23], [24], [25], [26], [27]] and friction pendulum bearings [[28], [29], [30], [31], [32], [33]]. Rubber bearings represent one of the most popular isolation bearings in the base isolation design, including laminated rubber bearings [23,25,26], lead rubber bearings [24,30] and high damping rubber bearings [[34], [35], [36]]. Friction pendulum bearings include single and multiple curved surface friction pendulum bearings [[37], [38], [39]]. Sliding bearings can be regarded as a type of frictional pendulum bearings with an infinite radius [40]. In general, when using these bearings, the seismic response of the superstructure can be reduced; however, the displacement of the isolation layer may be increased. To solve this problem, a damper can be used to decrease the displacement of the isolation layer and further dissipate the seismic energy [29,34]. Different types of combined isolation systems have been proposed to satisfy different structural performance objectives and site conditions [[29], [30], [31],[41], [42], [43]]. Because historic and modern buildings are significantly different, the usual isolation measures cannot be applied to protect historic buildings. In the isolation retrofit design process, although the performance of the primary structure is significant, the performance of the nonstructural members, especially the cultural relics, cannot be ignored. According to an existing study, the damage of nonstructural members may lead to human injuries and fatalities during earthquakes, and the economic loss from the destruction of nonstructural members may exceed that due to the damage of the primary structure [44], especially in locations in which nonstructural are valuable, such as in museums [45]. Thus, several scholars have focused on the seismic performance of nonstructural members and their protection measures [[46], [47], [48], [49]]. Sousa and Monteiro [50] evaluated the earthquake loss caused by nonstructural partition walls and proposed several rehabilitation methods. Fiorino et al. [51] performed a shaking table test on nonstructural members and estimated the retrofitting cost. In general, nonstructural members are usually used for decoration, and they may have certain architectural or usage functionalities; however, the nonstructural members in historic structure are cultural relics, whose value supersedes financial evaluations. In such cases, the nonstructural members are as important as the primary structure, and thus, their performance must be considered during the seismic assessment of the structure [52].
To protect the cultural relics of a typical historic building, the primary structure and nonstructural members should not be affected in the retrofitting process [20]. The structure of historic buildings are often diverse, involving timber [53,54], masonry [55,56] and rammed earth structures [57]. In China and East Asia, the historic buildings with the most historic and cultural value are mainly built of wood along with brick masonry. In such structures, the function of the load bearing members and enclosure members is well defined: the weight of the roof is borne by the wooden frame, and the wall divides the space without any bearing function. In this regard, the base isolation technology can be applied to retrofit historic timber buildings. However, such structures usually sit low and are light weight. When such structures are located on soft soil sites, which predominantly experience the long period part of ground motion excitations, the isolation provided by a common isolation system may be insufficient. Therefore, it is necessary to explore more suitable isolation methods for such historic structures.
In this work, to prevent the damage of a historic structure and the associated cultural relics against the action of earthquakes, the performance targets for a retrofitting structure were defined based on the performance levels of the primary structure and nonstructural members (cultural relics). Next, considering the site condition (soft soil site) and specific requirements (to maintain originality) of the historic structure, a combined base isolation system including elastic sliding bearings, laminated rubber bearings, viscous dampers, jack reaction joints and an underpinning layer was developed. The mechanical model of the proposed isolation system was established, and the system was applied practically to the Yufo temple in Shanghai. Numerical models with and without the isolation layer were established, and a time history analysis was performed to calculate the dynamic response of the two models. Finally, the mitigation of the seismic response by using the proposed isolation scheme for historic buildings was evaluated.
Section snippets
Classification of objects to be protected
The objective of retrofitting historic structures is not only to protect the primary structure from destruction but also to ensure the safety of the cultural relics under ground motion excitation. Thus, the objects to be protected in the whole structure must be divided into two categories when considering different performance requirements, namely, the primary structure and the nonstructural members (Fig. 1). For a historic timber structure built in a typical Chinese or East Asian style, the
Isolation system
Although the existing isolation method can meet the retrofitting demand of existing low rise common buildings, these schemes involve certain limitations owing to which it is difficult to meet the high performance target of the isolated structures involving a low deadweight and unfavorable site conditions. The seismic response mitigation principle of base isolation technology is to extend the natural period of a structure, thereby reducing the dynamic response. Thus, the period of isolated
Case analysis
With rapid urbanization, the difference between the protection requirements of historic buildings and of urban construction is becoming increasing prominent. Translocation and retrofitting of historic buildings represent a suitable approach to protect ancient buildings. Several economically developed areas correspond to coastal soft soil sites. As described in this section, the proposed isolation scheme was applied in a practical engineering case.
Conclusions
A combined isolation system including elastic sliding bearings, laminated rubber bearings, viscous dampers and jack reaction joints was proposed to retrofit historic structures and provide translocation protection. This method is suitable to be applied to historic buildings that have unfavorable site conditions and require translocation. Considering the cultural value of the relics in the structure, the performance targets for both the primary structure and nonstructural members were defined.
CRediT authorship contribution statement
Ruifu Zhang: Methodology, Investigation, Software, Formal analysis, Funding acquisition, Writing – original draft, Validation, Writing – review & editing. Minjun Wu: Methodology, Investigation, Writing – original draft, Writing – review & editing, Validation, Visualization. Wensheng Lu: Conceptualization, Writing – original draft, Investigation, Validation, Visualization. Xuan Li: Writing – review & editing, Validation, Visualization. Xilin Lu: Writing – original draft, Validation,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was supported by the National Natural Science Foundation of China (grant no. 51978525) and the Science and Technology Commission of Shanghai Municipality (grant no. 19DZ1202500). The photos of this study were taken with assistance of Shanghai Research Institution of Materials and Shanghai Tianyan Translocation Engineering Corporation.
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