1. Introduction
In medium- and high-rise buildings, elevators are frequently utilized as vertical transportation equipment. To reduce the cost of construction and the use cost, elevators are commonly not included in residences built in China in the last century that have fewer than seven stories. In addition, there are many buildings with elevators, but they do not have enough capacity to meet the demands of daily use. According to statistics from 2016, more than one million residential buildings in China require additional elevators [
1]. In 1958, the UK government proposed that additional elevators be installed in older buildings that have more than four stories and no elevator, and that two additional elevators be installed in structures that have more than six stories [
2]. In 1997, the United States government mandated that buildings with more than four stories must have additional elevators with an ability to accommodate ambulance beds [
3]. In the same year, Japan also introduced a code of practice for post-installed elevators in public residential buildings [
4]. The Singaporean government introduced the “Home Ownership Scheme” in 2004. Under this program, homeowners were only required to pay 5–12% of the cost of installed elevators in their homes, and the government covered the remaining cost [
5]. With the development of urbanization, China and other countries have experienced a huge demand for installing elevators in existing buildings, which has become one of the important development directions of housing transformation.
Typically, post-installed elevators are new elevators installed to the outside of existing buildings. Post-installed elevators have two schemes. The first scheme consists of post-installed elevators in an independent state, where the connection with the existing structure is relatively weak, and there are seismic joints and settlement joints between the two [
6]. In this scheme, the post-installed elevator is an independent structure with clear force transmission paths and has no impact on the existing building. However, the elevator shaft span is small, and the elevator shaft frame height is high, resulting in a large height to width ratio of the post-added elevator structure that makes it prone to overall instability. The second scheme involves the existence of a reliable connection between the post-installed elevator and the existing building, where the post-installed elevator becomes an accessory part of the existing building. The existing building can act as a restraint for the post-installed elevator, and the bearing capacity, deformation, overturning resistance, and seismic performance of the shaft can easily meet the code requirements. Compared with the initial elevator structure, the interaction between the post-installed elevator and the existing building is not considered in the early design stage. The post-installed elevator is connected to the existing building through the connecting beam, which has a low stiffness and is in a more complex state of stress under earthquake action. Therefore, there are certain safety risks regarding the post-installed elevator structure, and further research on it is necessary.
Some studies have been conducted by scholars in various countries on the influence of nonstructural components, such as elevators, on structural performance. Marko [
7] et al. discovered in their post-earthquake survey of high-rise concrete buildings that the usage of masonry infill walls in elevator shafts produced a good seismic performance. The findings revealed that the structural stiffness and seismic performance were significantly enhanced. Jankowski [
8] used finite element software to model the collision between the main Olive View Hospital building and the nearby stair tower caused by seismic activity. The results showed that collisions may lead to a significant increase in the response of the lighter stairway tower. The influence of nonstructural components on the performance of the main structure is not negligible. This is mainly related to the details and transmission paths within the structure, and thus, it is difficult to generalize the effect of nonstructural components [
9,
10,
11,
12].
Therefore, many scholars have studied the effect of elevators on the performance of the main structure. Zhou et al. [
13] found that the connection between a light wood structure and a masonry elevator shaft can significantly reduce the lateral drift and interstory displacement at the top of the building, which makes the building stronger and improves the seismic performance when studying the seismic performance of light wood structures and masonry structures. Jiang Lu et al. [
14] conducted a 1:4 scaled-down shaking table test of the addition of an elevator to a multistory masonry structure with additional reinforced concrete shear walls, and the results showed that the structural stiffness and seismic performance were significantly improved. Mazza et al. [
15,
16,
17] studied the collision between the elevator shaft and the main structure during seismic activity. They proposed a magnetically damped link between the elevator shaft and the building. In comparison with other connection methods, the magnetically damped connection was found to significantly reduce the structural pounding effects between the elevator shaft and the building, which reduced seismic damage. To ensure reliable connections between existing buildings and new elevators, Tan [
18] and Xiong [
19] proposed multiple connections and weak connections by welding, anchoring, and gluing, which can meet the requirements of various design codes and reduce engineering costs. There are also many studies on the reinforcement of existing buildings through externally attached substructures [
20,
21], since the external substructure is connected to the existing building and cooperates together to resist the seismic impact [
22]. Wu [
23] developed a novel external tube substructure, which simultaneously functioned as a newly added residential elevator shaft and as a lateral-force resistance system. It showed both a usage function and structural function, achieving the maximization of economic benefits.
Scholars have also provided some solutions to the problems that may emerge in the actual engineering of the installation of post-installed elevators in existing buildings. Combining their findings with previous cases, Liu [
24] pointed out that the foundation’s form in the structural design should be determined according to the elevator installation location, foundation depth, and the specific situation of the original building. Moreover, high-rise steel-structure buildings’ column footings should not use exposed column bases in high-intensity areas. Zhang [
25] summarized the typical problems that occur in the structural design of elevator installation projects in existing buildings by examining actual cases of elevator installation and listing the code methods, precautions, and techniques of program selection that are used in the design of similar structures. Zheng [
26], based on the practical experience of adding elevators, pointed out that pile-raft foundations can be set to avoid foundation overturning, and additional diagonal bracing can be installed to reduce the torsional effect of elevator shafts when the lateral stiffness of the steel structure is weak.
In summary, the impact of nonstructural components such as elevators on structural performance cannot be ignored, and the existing research has mainly focused on the impact of the original elevator on structural performance, whereas most of the research on the installation of elevators in existing buildings has been on frame structures or masonry structures. Studies on the seismic performance of installation elevators in existing buildings are very finite and there are major shortcomings. Therefore, in this study, a frame shear wall structure is used as an example according to an actual project and its original structural model, where the structural model of the post-installed elevator and a variety of working condition models are established. Midas Gen and SATWE, which are finite element dynamic analysis software, are used to compare the seismic response of the structure under different cases and to explore the safety and feasibility of this kind of structure with elevators. The analysis of seismic performance of post-installed elevators in this study can effectively fill the shortages of the seismic design method of the post-installed elevator and understand the seismic response characteristics of the post-installed elevator buildings. The seismic optimization design method proposed in this study can also be used for reference in elevator installation engineering.
3. Impact of Post-Installed Elevators on the Seismic Performance of the Existing Building
The results from the modal analysis and response spectrum analysis are shown in
Table 3 and
Table 4. The results of the two software programs are within 5% of each other, which indicates that the model is accurate and the finite element software results are accurate. Taken as a whole, all indicators meet the requirements of the “Technical Regulations for Concrete Structures in TALL Buildings” [
31]. The horizontal vibration period of the structure decreased, indicating that the stiffness of the existing building increased after the addition of the elevators and that the increase is greater than the mass. In addition, the period ratio (T3/T1) is less than 0.85, indicating that the plan layout of the existing building after the addition of the elevator is reasonable and the structure will not experience excessive torsional effects. The two-way interstory displacements of the overall structure are reduced as compared to the existing building structure, indicating that the post-installed elevator structure will improve the seismic performance of the overall structure, both under standard values of wind loads and under two-way frequent earthquakes. Due to space limitations, only the results of the Midas calculations are presented in the following analysis.
According to the requirements of the “Seismic Design Code of Buildings”, the elastic dynamic time history of the structure should also be analyzed. Considering the seismic action of 7-degree frequent earthquakes, the model’s input seismic acceleration peaks is 35 cm/s
2 in the x-direction and 29.75 cm/s
2 in the y-direction. The peak interstory displacement angle and peak base shear of the structure obtained from the dynamic time-history analysis are shown in
Figure 5. The peak interstory displacement angles are all less than 1/800, all of which can meet the requirements of the “Seismic Design Code of Buildings” for the elastic interstory limits of this frame shear wall structure. Additionally, the interstory displacement angle of the overall structural is smaller in both the X and Y directions than that of the existing building. The peak base shear increased with the installation of the elevators, but the increased base shear is within acceptable limits.
4. Optimization Design for Post-Installed Elevators
Based on the response spectrum analysis and dynamic time-history analysis, it was found that the post-installed elevators have some impact on the seismic performance of the existing building. However, this impact is small and beneficial, indicating the feasibility of adding the elevator to the existing building. In order to further enhance the seismic performance of the post-installation elevator, the main design factors of the post-installation elevator were targeted for further design optimization in this study [
32,
33,
34].
Parameters affecting the comprehensive performance of the post-installed elevator include materials, standard shaft height, overall height, connection method, placement location, number of arrangements, cross-sectional dimensions of beams and columns, etc. [
35]. From the perspective of the structural analysis, three key indicators were selected: floor height, install position, and connection method.
The stiffness and strength of the post-installed elevator are weak relative to the main structure. In order to ensure the seismic safety of the post-installed elevator structure, this study calculated the seismic response of the post-installed elevator structure in the overall model under the action of frequent and rare earthquakes with a 7-degree seismic protection. The seismic load is still input into the seismic acceleration time curve in
Figure 3, and the PGA of the model input for frequent seismic loads is still the same as above. The peak value of the input seismic acceleration for the rare seismic time analysis is 220 cm/s
2 in the x-direction and 187 cm/s
2 in the y-direction, and the damping ratio of the structure is 0.05.
4.1. Structural Optimization of Post-Installed Elevator Shafts
4.1.1. Design Case
The standard shaft height (the distance between adjacent cross beams in the elevator shaft) of a steel-frame elevator shaft determines the safety of the elevator structure. A low standard shaft height of an elevator shaft leads to the high consumption of steel and increased costs, but a high standard shaft height of an elevator shaft leads to insufficient stiffness and a reduced load-bearing capacity of the elevator shaft. In practical engineering, not only does the safety of the structure need to be considered, but the construction cost also needs to be reduced as much as possible. Therefore, in this section, the working conditions of different standard shaft heights are studied. The standard shaft heights were selected based on the existing building’s floor height (3.6 m), where 0.5 times the floor height (initial scheme height), 0.7 times the floor height, and 0.85 times the floor height were selected. The standard shaft height of 1.8 m is called Case A, the standard shaft height of 2.5 m is called Case B, and the standard shaft height of 3 m is called Case C. The three cases were analyzed with a frequent- and rare-earthquake time-history analysis.
4.1.2. Results and Analysis
The results of the time-history analysis for frequent and rare earthquakes are shown in
Figure 6 and
Figure 7. The seismic response rules for the elevator structure are similar to the rules of action of frequent and rare earthquakes. As shown in
Figure 6a and
Figure 7a, the peak displacement of the post-installed elevator structure increases with the increase in the standard shaft height; in other words, the peak displacement shows a gradual increase from Case A to Case C. Moreover, the peak displacement in the y-direction is significantly greater than in the x-direction, and the new design solution is not effective in reducing the lateral displacement. As shown in
Figure 6b and
Figure 7b, the peak base shear tends to decrease from Case A to Case C. This indicates that the optimized design of the post-installed elevator structure reduces the seismic forces to some extent and improves the seismic performance of the elevator structure.
Figure 6c and
Figure 7c show the peak acceleration of the post-installed elevator structure for the three cases. It can be seen that the peak acceleration of the elevator structure shows a small increase along with the increase in the height of the elevator’s standard floor.
Figure 6d and
Figure 7d show the peak interstory displacement angle of the post-installed elevator structure for the three cases, and it can be seen that the interstory displacement angle increases along with an increase in the height of the post-installed elevator shaft. The maximum peak interstory displacement angle of the post-installed elevator structure for the three cases is 1/311, which is far less than the requirement of 1/50 in the “Technical Regulations for Concrete Structures in High-Rise Buildings”. Therefore, the structure meets the design requirements of the code, and the interstory displacement angle of Case C is closest to the limit value of the code compared with Cases A and B. The structural performance of Case C is fully utilized, indicating that the direction of structural optimization is reasonable.
The stress cloud charts of the three cases of post-installed elevator structures were also obtained during the rare earthquake time-history analysis, as shown in
Figure 8. The maximum stresses in the post-installed elevator structure are 178.06 MPa for Case A, 188.53 MPa for Case B, and 193.3 for Case C, which are all less than the design strength of the steel (235 MPa). Additionally, all three structural design cases can meet the requirements of strength and stability. In this study, the material utilization rate is defined as the ratio of the maximum stress in the member to the design strength of the material. Case A has a material utilization rate of 75.77%, Case B has a rate of 80.22%, and Case C has a rate of 82.25%, which demonstrates that Case C has the highest material utilization rate and can fully utilize its performance.
4.1.3. Cost Analysis
The main cost of the project is the cost of steel. Therefore, the three structural design cases were compared in terms of the cost of steel. In this calculation, the unit price of steel is based on CNY5200 per ton of the current market price.
Table 5 shows the cost of steel for the post-installed elevator in the three structural design cases. The post-installed elevator shaft uses less steel as the standard shaft height increases, lowering the cost of structural Case C by CNY 57,408, making it 8.2% less than the original design’s structural cost. The optimized design of structural Case C can not only take into account the seismic performance to achieve the safe use performance of the building, but also significantly reduce the cost of construction, improving the economy of the construction projects.
4.2. The Installation Location of the Elevators
4.2.1. Design Case
Existing buildings have a very important role in the safety of elevator structures. The existing building can act as a restraint for elevators and influence the load-bearing capacity, deformation capacity, overturning resistance, and seismic performance of post-installed elevators. In order to further optimize the design case, the location of the post-installed elevator installation was analyzed. The post-installed elevators should be placed in such a way that is appropriate and does not damage the existing building’s structural form. Therefore, in this study, the elevators were installed in a location that has a non-shear wall and is convenient for employees to use. This study proposes a total of four types of design solutions for elevator installation, as shown in
Figure 9.
4.2.2. Results and Analysis
Figure 10 and
Figure 11 show the results from the time-history analysis of the seismic response of the post-installed elevator structure under the action of frequent and rare earthquakes, respectively. By comparing the seismic response of the elevator structure under different cases, it can be found that the peak displacement, peak base shear, peak acceleration, and peak interstory displacement angle of the elevator structure in Case 1 are better than the other conditions for both frequent and rare earthquakes. The post-installed elevator in Case 1 has the smallest seismic response, whereas the elevator structure in Case 2 has the largest seismic response. It is worth noting that the response of the y-directional displacements of the post-installed elevator structure is greater than the response of the x-directional displacements under horizontal, bidirectional seismic action. Taking the Chi-Chi wave of rare earthquakes as an example, the displacement in the y-direction is more than 50% larger than the displacement in the x-direction. Moreover, the interstory displacement angles of the post-installed elevator structure are at a low level, which is far less than the value specified to meet the requirements of seismic design in the “Seismic Design Code of High-rise Building Structures”.
The results of this study show that the seismic response of the elevator is smaller when the elevator is installed at the location where the structural shear wall is more concentrated (the original structural elevator room). Additionally, installing the additional elevator at the location of the structural elevator room results in a smaller amount of work needed for the renovation project. In summary, this study selects the working conditions of Case 1 as the optimal design case.
4.3. Formation of Node Connections between Post-Installed Elevator and Existing Building
4.3.1. Design Case
The connection between the post-installed elevator and the existing building is usually made by means of a hinged or rigid connection. The above-mentioned Case 1 model is utilized as the research object to compare the seismic response of elevator structures that have various connection methods.
4.3.2. Results of Time-History Analysis
Table 6,
Table 7,
Table 8,
Table 9,
Table 10,
Table 11,
Table 12 and
Table 13 show the post-installed elevator’s seismic responses under frequent and rare earthquakes with different connection modes, respectively. It can be seen that the results for frequent and rare earthquakes are similar. The peak displacement, peak interstory displacement angle, and peak base shear of the post-installed elevator structure in the model with rigid connections are smaller under frequent-earthquake effects. However, the base shear in the y-direction and the peak acceleration in the x-direction are lower for the elevator structure of the hinged model with partial seismic events under the action of rare earthquakes. The use of rigid connections significantly reduces the peak base shear in the y-direction during frequent earthquakes, but the peak base shear in the y-direction increases significantly during rare earthquakes. This is because the structure may enter a plastic state during rare earthquakes, and the deformation of the nodes of the main structure increases; thus, the hinge connection between the post-installed elevator and the existing building conforms more to the seismic requirements at this time.