Seismic retrofitting of a historic building by using an isolation system with a weak restoring force

Highlights

• An isolation system with a weak restoring force was used to retrofit a historic building.

• The performance of the primary structure and cultural relics was evaluated.

• The originality of the historic structure was maintained after retrofitting.

• Jack reaction joints were installed to recenter the structure after a rare earthquake.

• The structure was isolated by using the underpinning system of translocation.

Abstract

Historic buildings and cultural relics are valuable and must be preserved carefully. However, such structures may be destroyed during earthquakes owing to the deterioration of their structural performance over time. Therefore, retrofitting protection measures must be adopted. The typical structural style of ancient buildings in China and East Asia involves timber structures. Due to the diversity of the construction environment, the site conditions of such buildings are often unfavorable. Moreover, several important cultural relics in historic buildings exhibit low integrity, and therefore the primary structure must not be influenced during retrofitting. To overcome these problems, this paper proposes the use of an isolation system with a weak restoring force and post-earthquake recentering capacity. The proposed isolation system consists of elastic sliding bearings, laminated rubber bearings, viscous dampers and jack reaction joints. To better isolate lightweight structures under soft soil conditions, the stiffness of the isolation layer must be designed to have a low value, and therefore, the self-recentering ability cannot be realized. Thus, in this work, jack reaction joints were used to recenter the structure after a rare earthquake. Moreover, a performance index was introduced that is suitable for the seismic isolation protection of typical East Asian historic buildings. The proposed isolation system was applied to a famous historic timber building in Shanghai, namely, the Yufo temple, to improve the seismic performance of the translocated structure and nonstructural members. The underpinning system of translocation facilitates the use of the isolation system. The analysis results indicate that the proposed isolation method can help improve the performance of the structure involving both historic buildings and cultural relics and that the superstructure remains unaffected by the reinforcement measures. In addition, this approach can provide guidance for the protection of other cultural historical buildings that have different styles.

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.