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1、Tests on a Half-Scale Two-Story Seismic-Resisting Precast Concrete BuildingThis paper describes experimental studies on the seismic behavior and design of precast concrete buildings. A half-scale two-story precast concrete building incorporating a dual system and representing a parking structure in
2、Mexico City was investigated. The structure was tested up to failure in a laboratory under simulated seismic loading. In some of the beam-to-column joints, the bottom longitudinal bars of the beam were purposely undeveloped due to dimensional constraints.Emphasis is given in the study on the evaluat
3、ion of the observed global behavior of the test structure. This behavior showed that the walls of the test structure controlled the force path mechanism and significantly reduced the lateral deformation demands in the precast frames. Seismic design criteria and code implications for precast concrete
4、 structures resulting from this research are discussed. The end result of this research is that a better understanding of the structural behavior of this type of building has been gained results of simulated seismic load tests of a two story precast concrete building constructed with precast concret
5、e elements that are used in Mexico are described herein. The structural system chosen in the test structure is the so called dual type, defined as the combination of structural walls and beam-to-column frames. Connections between precast beams and columns in the test structure are of the window type
6、. This type of construction is typically used in low- and medium rise buildings in which columns are connected with windows at each story level. These windows contain the top and bottom reinforcement. Fig. 1 shows this type of construction for a commercial building in Mexico City. In most precast co
7、ncrete frames such as those shown in Fig, 1, longitudinal beam bottom bars are not fully developed due to constraints imposed by the dimensions of file columns in beam-to-column joints. In an effort to overcome this deficiency, and as described later, some practicing engineers in Mexico design these
8、 joints by providing hoops around the hooks of that reinforcement in order to achieve its required continuity. However, this practice is not covered in the ACI Building Code (ACI318-02), nor in the Mexico City Building Code (MCBC, 1993). Part of this research was done to address this issue. The obje
9、ctives of this research were Io evaluate the observed behavior of a precast concrete structures in the laboratory and to propose the use of precast structural elements or precast structures with both an acceptable level of expected seismic performance and appealing features from the viewpoint of con
10、struction Emphasis is given in this paper on the global behavior of the test structure. In the second part of this research which gill be presented in a companion paper, the observed behavior of connections between precast elements in the test structure, as well as the behavior of the precast floor
11、system will be discussed in detail. Structural and non structural damages observed in buildings during past earthquakes throughout the world have shown the importance of controlling lateral displacement in structures to reduce building damage during earth- quakes. It is also relevant to mention that
12、 there are several cases of structures in moderate earthquakes in which the observed damage in non-structural elements in buildings was considerable even though the structural elements showed little or no damage. This behavior is also related Io excessive lateral displacement demands in the structur
13、e. To minimize seismic damage during earthquakes, the above discussion suggests the convenience of using a structural system capable of controlling lateral displacements in structures. A solution of this type is the so-called dual system. Studies by Paulay and Priestley4 on the seismic response of d
14、ual systems have shown that the presence of walls reduce the dynamic moment demands in structural elements in the frame subsystem. Also in conjunction with shake table tests conducted on a cast-in-place reinforced concrete dual system. Bertero5 has shown the potential of the dual system, in achievin
15、g excellent seismic behavior n this investigation, the dual system is applied to the case of precast concrete structures.DUCTILITY DEMAND IN DUAL SYSTEMSIn order to develop a base for a later analysis of the observed seismic response of the test structure studied in this project a simple analytical
16、model is used to evaluate the main features of ductility demands in dual systems. Fig 2 shows the results of a simple approach to analyze the lateral load response iii a dual system. The lateral load has been normalized in such a manner that the combination of maximum lateral resistance in both subs
17、ystern i.e. walls and frames-leads to a lateral resistance of the global system equal to unity b is also assumed that both subsystems have the same maximum lateral resistance. In the first case (Fig 2a), it is assumed that the wall and frame subsystems have global displacement ductility capacities e
18、qual to 4 and 2 respectively. In the second case (Fig. 2b), the frame subsystem response is assumed to be elastic, and the lateral stiffness of the wall subsystem is taken to be 4 times that of the frame subsystem.As shown in Fig 2, the lateral deformation compatibility of the combined system is con
19、trolled by the lateral deformation capacity of the wall subsystem. In the first case Fig 2ak an elastic-plastic envelope for the lateral global response of the dual system is assumed, and the corresponding displacement ductility (u) is equal to 33.For the second case (Fig. 2b) with an elastic behavi
20、or of the frame subsystem, this ductility is equal to 25. These simple examples illustrate that in the analyzed cases, due to the higher flexibility in the frame subsystems as compared to those of the wall subsystern, in a dual system, the ductility demands in the frame subsystem result in smaller d
21、uctility values than those of the wall subsystem. This analytical finding was verified in this study from the experimental studies conducted on the test structure. This verification is later discussed in the paper It is of interest to note that results of the type shown in Fig. 2 have been also foun
22、d by Bertero in shake table tests of a dual system. DESCRIPTION OF TEST STRUCTUREThe test structure used in this investigation is a two-story precast concrete building, representative of a low-rise parking structure located in the highest seismic zone of Mexico City. The prototype was constructed at
23、 one-half scale. For the sake of simplicity, ramps required in a parking structure have not been considered in the selected prototype structure. Their use, requiring large openings in the floor system, would have required a very complex model of the floor system for both linear and nonlinear analysi
24、s of the structure.A detailed description of the dimensions, materials, design procedures, and construction of the test structure can be found elsewhere.6 A summary of this information is given below. The dimensions and some characteristics of the test structure are shown in Fig. 3. The longitudinal
25、 and transverse are shown in Fig3a. Also, the exterior (longitudinal) frame containing the wall (Column Lines 1 and 3) are termed the lateral frame (see Fig, 3b), and the internal (longitudinal) frame with the single tee (Column Line 2) are termed the central frame. Doable tees spanning in the longi
26、tudinal direction are supported by L-shaped precast beams in the transverse direction as shown in Fig3a. The structure uses precast frames and precast structural walls, the latter elements functioning as the main lateral load resisting system. Fig. 4 shows an early phase of the construction of the t
27、est structure. As can be seen, the windows in the columns and walls are left in these elements for a later assemblage with the precast beams.The unfastened design base shear required by the Mexico City Building Code (MCBC, 1993)2 is 0.2WT, where WT is the total weight of the prototype structure, ass
28、uming a dead load of 5,15 KPa (108 psi) and a live load of 0.98 KPa (20.5 psi). The prototype structure was designed using procedures of elastic analyses and proportioning requirements of the MCBC, In these analyses, the gross moment of inertia of the members in the structure was considered and rigi
29、d offsets (distances from the joints to the face of the supports) were assumed for all beams in the structure except for beams in the central frame, which had substandard detailing as will be described latch. Results from these analyses indicated that the structural walls in the test structure would
30、 take about 65 percent of the design lateral loads. A review of the nominal lateral resistance of the structure using the MCBC procedures showed that this resisting force was about 1.3 times the required code lateral resistance (0,2Wr), This is one of several factors, later discussed, that contribut
31、ed to the over-strength of the structure.The longitudinal reinforcement in all the structural elements of the test structore was deformed bars from Grade 420 steel. Table 1 lists the concrete compressive cylinder strengths for different members of the prototype structure. Fig. 5 shows typical reinfo
32、rcing details for precast beams spanning in the direction of the applied lateral load (see Fig. 3). Figs. 6 and 7 show reinforcing details for the columns, and for the structural wails and their foundation, respectively. It should be mentioned that the test structure was designed with the requiremen
33、ts for moderately ductile structures specified by the MCBC. According to these provisions, the test structure did not require special structural walls with boundary elements such as those specified in Chapter 21 of AC1 318 02.The precast two-story columns were connected to the precast foundation by
34、unthreading them in a grouted socket type connection. The reinforcing details of the foundation, as well as its design procedure and behavior in the test structure are discussed in the companion paper? Tae beam-to-cadmium joints in file test structure were cast-in-place to enable positioning the lon
35、gitudinal reinforcement of the framing beams. The beam top reinforcement was placed in sum on top of the precast beams. Fig. 8 shows typical reinforcing details for the joints in the double tees of the central frame. Since these tees and their supporting L-shaped beams in Axes A or C (see Fig. 3) ha
36、d the same depth, the hooked bottom longitudinal bars in the double tees could not pass through the full depth of the column because of interference with the bottom bars from file transverse beam (see Fig. g).As a result, these hooked bars possessed only about 55 percent of the development length re
37、quired by Chapter 21 of ACI 318-02. In an attempt to anchor these hooked bars, some designers in Mexico provide closed hoops around the hooks, as shown in Fig. 8. The effectiveness of this approach was also studied in the companion paper.3 Beam to-column joints in the lateral frames of the test stru
38、cture had transverse beams that were deeper than the longitudinal beams. This made it possible for the top and bottom bars of the longitudinal beams to pass through the full joint, and, therefore, these bars achieved their required development length.Cast-in-place topping slabs in the test structure
39、 were 30 mm (1.18 in.) thick and formed the diaphragms in January-February 2005 Fig. 3. Plan and elevation of test structure: (a) Plan; (b) Lateral frame; (c) Transverse frame. Dimensions in mm. Note: 1 mm - 0.0394 in. the structural system. Welded wire reinforcement (WWR) was used as reinforcement
40、for the topping slabs. The amount of WWR ill the topping slabs was controlled by the temperature and shrinkage provisions of the MCBC. which are similar to those of AC 318-02.It is of interest to mention that the requirements for shear strength in the diaphragms given by these provisions, which are
41、similar to those of ACI 318-89, did not control the design. A wire size of 6 x 6 in. 10/10 led to a reinforcing ratio of 0.002 in the topping slab. The strength of the WWR at yield and fracture obtained from tests were 400 and 720 MPa(58 and 104 ksi),respectively.TEST PROGRAM AND INSTRUMENTATIONTest
42、 ProgramThe test structure was subjected to simulate seismic loading in the longitudinal direction (see Fig. 3a). Quasitatic cyclic lateral loads FI and F2 were applied at the first and second levels of the structure, respectively (see Fig. 3b). The ratio of F2 to FI was held constant throughout the
43、 test., with a value equal to 2.0. This ratio represents an inverted triangular distribution of loads, which is consistent with the assumptions of most seismic codes including the MCBC.The test setup is shown in Fig. 9.The structure had Hinges A, B, and Cat each slab level as shown in Fig. 9b.The pu
44、rpose of the hinges was to avoid unrealistic restrictions in the structure by allowing the ends of the slabs to rotate freely during lateral load testing. As can be seen in Fig 9, the lateral loads were applied by hydraulic actuators that work in either tension or compression.When the actuators work
45、ed in compression, they applied the loads directly at one side of the structure. However, when the actuators applied tensile loads at one side of the test structure, they were convened to compression loads at the other side by means of four high strength reinforcing bars for each actuator see D32 (1
46、 in) reinforcing bars in Fig. 9. Both ends of these bars were attached to 50 mm (2 in.) thick steel plates At each of the floor levels, two of these plates were part of Hinge A, and the other two plates at the actuator side were part of Hinge B (see Fig. 9b).As can be seen in Fig 9b before the appli
47、cation of tensile loads in the actuators, the latter end of the plates left a clear space with the end of the slab. This space at zero lateral load was about 50 mm (2 in.), and it allowed for beam elongation of the structure which occurs during the formation of plastic hinges in the beams? For the c
48、ase of compressive loads on the actuators acting on the transverse beam at the side of Hinge B (see Fig. 9b), the system also allowed 50 mm (2 in.) of beam elongation. These particular features of the test setup allowed the application of compressive loads at points in the slabs with no special rein
49、forcement at these points. If tensile forces had been applied to loading points in the slabs, it is very likely that these loading points would have required unrealistic, special reinforcing details that are not represent five of those in an actual structure. The gravity load was represented by 53 steel ingots, acting at each level of the structure, with the layout shown in Fig. 9a, The weight per unit area of the ingots per level was 2.79 KPa (58.3 psf), which added to the self-weight of tile sla