CHAPTER 5 119-132
6.2. T EST M ODEL AND I SOLATION S YSTEM
A two-storey un-reinforced masonry building model of one-fifth scale is considered for the experimental study. Brick masonry is commonly used building material in urban and rural India and hence bricks are used for construction of the test model. Size of the test model has been finalized considering capacity and size of the available shake table.
Scaled model of FREI is designed to carry estimated gravity load and horizontal displacement simultaneously. Details of testing of the model FREI under cyclic load have been presented in Chapter 5. Details of the test model and model FREIs are presented in the following sub-section.
6.2.1. Scaling and Similitude
A scale factor of 1:5 is considered for the present study considering the size of shake table (2.5m X 2.5m) and its payload capacity. The laws of similitude [Harris and Sabnis, 1999] are followed to arrive at appropriate input motion, dimensions of test model as well as for finalization of additional mass requirement for gravity load simulation. The similitude requirements are derived from dimensional analysis which depends on physical characteristics like length, force, time, temperature etc. Table 6.1 shows the derived scaling relationship of the parameters relevant to the present study.
Table 6.1 Similitude scaling relationship Parameters Scale Prototype
1/5-scale model
Length S 5
Mass S2 25
Displacement S 5
Time √ 2.236
Acceleration S 5
The mass of the prototype building is 121.4 x103 Kg, whereas the total mass of the test model is 970 kg. Hence the additional mass of 3,885 kg required for fulfilling similitude requirement is uniformly distributed over different floors of the model.
6.2.2. Test Model and its Construction Details
A storey masonry building is constructed for shake table testing subjected to different input earthquake accelerations. The geometrical details of the building are given in Table 6.2. In the model building, beams are provided at the base level, which rest on four U-FREIs placed at four corners of the building. The plan and elevation of the building mounted on U-FREIs are shown in Fig. 6.1.
Table 6.2 Building Parameter
Parameter Model Building
Length (m) 1.5
Width (m) 1.1
Height of storey (m) 0.6
Thickness of slab (m) 0.08
Dimension of base beam (m x m) 0.125x0.150 Total weight of building (kg) 971
Material used for construction of test model are M25 grade of concrete, Fe415 steel reinforcement and fly ash brick. The dimensions of building elements of test model are obtained by scaling of corresponding element of the prototype as per similitude requirement. This elements are design as per load prescribed in various Indian Standard and Seismic Zone-V as per seismic zoning map of India. Lifting hook are provided at the base ring beams for handling and placing of test model during different stages of experiments. Base beam and lift hooks are designed to take care the loads of entire test structure and these are shown in Fig. 6.2a.
Fig. 6.1 Plan and elevation of model building
Brick masonry wall are constructed directly above the base level slab. On the top of brick wall first floor and roof level slabs are placed directly without any joint. The storey height of the model is 0.60m. The total height of the test model is 1.51m including the base ring beam and two level slabs. The base level as well as first floor and roof level slab has plan area of 1.50m x 1.10m. There are 8 nos. of hooks of 20mm diameter provided as at corner of base beam as shown in Fig. 6.2a for handling of the model. Details of reinforcement provided in base level ring beam and all three slabs are shown in Fig. 6.2b.
6.2.3. Properties of U-FREI
U-FREIs are designed for low horizontal stiffness so as to achieve the desired horizontal frequency and high vertical stiffness so that no undesirable vertical or rocking mode is excited. Stability of bearings under combined gravity load and imposed lateral displacement is ensured. A BI building is generally designed for a natural frequency of 0.5 Hz for deflecting energy associated with higher modes of vibration, while keeping the base displacement within acceptable limit. FREIs for the isolated scaled test model of un-reinforced masonry building has been designed and extensively tested for the evaluation of mechanical properties. U-FREIs have shown excellent potential in reduction of dynamic response of structures mounted on such isolators.
240
1100 1500
200 200
240
150 Shake Table Isolator
Additional Mass
Additional Mass
80
600
80
600
80
1500
(a) Base ring beam and lifting hook (b) Base ring beam reinforcement
(c) Slab reinforcement details
Fig. 6.2 Base ring beam plan, reinforcement detail of beam and slab
Geometrical and material properties of isolators designed to achieve effective isolation are listed in Chapter 3. Each layer of elastomer is interleaved with bi-directional (00/900) carbon fibre reinforcement layer. Details of FREI bearing used for seismic isolation of the test model is shown in Chapter 3.
Experimental investigation on behaviour of U-FREI under horizontal cyclic displacement (along X-axis and 450 to X-axis) are already carried out and presented in Chapter 5. Lateral force vs displacement hysteresis behaviour of U-FREIs are also shown in Chapter 5. It may be observed that the stiffness of U-FREI reduces with increase in the amplitude of horizontal displacement. The reduction in stiffness is about 50% of initial stiffness at maximum horizontal displacement.