G ASTOS EXIGIBLES AL C ONTRATISTA QUE NO SON OBJETO DE ABONO INDEPENDIENTE

The geometrical scale of the model was selected on the basis of the characteristics of the seismic shaking ta- ble and the precisely defined objectives of testing, i.e., based on the following criteria:

K1 – proportions of the shaking table (4.5 m x 4.5 m)

K2 – allowed total height of the model (10m) K3 – allowed total weight of the model (400 kN) K4 – realistic reproduction of nonlinear behaviour K5 – realistic reproduction of the failure mecha- nisms

Satisfying these criteria, the following three main scales were adopted:

– geometrical scale Ir = 1:2,

– scale for the bulk density of the material rr = 1,

– scale for the stresses Er = 1.

Computed were the scales of all the remaining physical quantities in model analysis of problems on dynamics of structures. The considered structures have relatively low levels of axial stresses at the base which justifies the adoption of a model with neglected gravity forces (gr ¹ ar, gravity acceleration cannot be simulated).

In such a case, the scales of all the quantities of interest are expressed only in relation to the geometrical scale lr

adopting a material identical to that of the prototype.

Figure 5.1. 3D view of the model to a scale 1:2

The comparison between the character- istics of the model and the hypothetical pro- totype, i.e., the comparison of the designed and the obtained scales is given in Table 5.1. By satisfying the scale for the bulk density in constructing the model to a scale of 1 : 2, used were original construction materials with physical-mechanical characteristics of ma- terial almost equal to those of the prototype that were prepared according to the designed proportions. Scanning the main characteris- tics, it is clear that almost ideal similarity be- tween the model and the prototype has been achieved by modeling. Thus, conditions were created to interpret the results obtained from testing of the dynamic response of the model relating them directly to the prototype, and further to the large number of such buildings

in the considered region, which is the main pur- pose of the performed investigations.

On the basis of the designed model of hypo- thetical prototype and using the parameters de- scribed in the previous items, a model to a scale of 1 : 2 was constructed. Bricks with designed proportions of 12.5 x 6 x 3.25 cm have been used for the construction of the walls. It was decided to procure the required number of bricks with stand- ard dimensions, 25 x 12 x 6.5 cm, and prepared the bricks for model construction in the required dimensions by cutting. Lime-cement mortar which is the main bonding material for the structures that represents the hypothetical structure has been pre- pared with lime: cement: sand ratio equal to 1: 1: 3. The river sand with fraction 0-2 cm was used as filler.

In accordance with the designed proportions, the model was constructed in the Dynamic Testing Laboratory of IZIIS using a traditional technology of construction of masonry structures particularly simulating the walling pattern in constructing the walls. A reinforced concrete platform with propor- tions of 3.26 m x 4.76 m and a thickness of 0.30m was constructed as a foundation for the model for the purpose of its construction and transport, (Fig. 5.2). The required number of holes was left for an- choring the model with the shaking table.

Dimension of the model are as follows: length 4.24m, width 3.06m and height 3.30m. Structural system of the model consists of five bearing walls, four facades and one middle wall. The walls were constructed in a running bond, (Fig. 5.3). The thickness of the walls is 12.5 cm, while the thickness of the vertical and horizontal mortar layers is 0.5 cm. The floor and roof struc- ture are constructed as reinforced concrete one, with the thickness of 15 cm. Additional 12 cm of RC plate was constructed because of satisfying the criteria for normal stresses, (sr=1). The additional

12cm thick plate is constructed in such a way that it is not connected to the walls and has a role of additional load. Thus the level of normal stress in the walls is very similar as that in the prototype structure, (Table 5.1).

The construction of model was finished in three weeks. After being completed and dried for a period of 30 days on the place where it was con- structed, the model was transported and connected to the shaking table by 90-tons auto crane, (Fig. 5.3). A total of 26 anchors were used for anchor- ing the model with the shaking table.

The model response was monitored by high speed data acquisition system consisting of 12 ac- celerometers (ACC), 20 displacement transducers (LVDT) and 6 linear potentiometers (LP), provid- ing information about accelerations at different levels and points, relative displacements, defor- mations at selected points, (Fig. 5.4).

Table 5.1. Main characteristics of the model and the prototype

Characteristics Unit of Proto-

type Model Design

measure Xp Xm Xp/Xm scale Proportions at plan, L / W m 8.49/6.11 4.24 /3.06 2 2 Total height m 6.60 3.30 2 2 Total volume – walls – plates m 3 _39.50 14.50 4.93 3.15(+3.3) 8 4.5 (2.3) 23=8 Bulk density -masonry -concrete kN/m 3 _18.50 25.00 19.50 25.00 0.94 1 1 Total weight kN 1093.2 256.5 4.2 22=4

Area of the walls at plan m2 _{8.45 2.11 4}

22=4

Average o for the walls kN/m2 _{129.4 121.6 1.06 1}

Total mass -masonry -concrete kNs2/m 74.536.9 9.8 8 (+8.4) 7.6 4.6 (2.25) 23=8 Compressive strength of:

– mortar MPa - 15.50 - 1

– brick - 20.10 - 1

Bending strength of:

– brick MPa - 11.80 - 1

– mortar - 11.40 - 1

Frequency in E-W direc- tion Frequency in N-S direction Hz - 11.0 16.5 - 2-1 =0.5

Figure 5.2. Reinforced concrete foundation of scaled model during construction

Figure 5.3. Construction and transportation of the brick masonry model, BM

Figure 5.4. Instrumentation of the BM model

The shaking table tests of 1:2 scaled model BM re- quired special testing program consisting of several test phases, considering the expected information about the dynamic behaviour of the prototype and the effective- ness and justification of applied strengthening method and technology. The same testing procedure was applied for original, (BM) and for retrofitted model (BM-SR), consisting of two main phases:

– Tests for definition of dynamic characteristics of the model, before and after performing seismic tests at each phase, in order to check stiffness degradation of the model produced by micro or macro cracks developed during the tests;

– Seismic testing by selected earthquake record until heavy damage. The tests are performed in several steps, increasing the input intensity of the earthquake in order to obtain the response in linear range, as well as to define the initial crack state, development of failure mechanism and possible collapse of the model.

Table 5.3. Specification of selected experimental test on the non-retrofitted model BM

Earthquake span % aссmaxinput (g) accmax top (g) LPtop (mm)

LPtop_-LPfoun. (mm) damage El Centro 50 0,14 0,22 10,5 0,494 - Petrovac 36 0,16 0,26 6,4 0,34 Northridge 16 0,18 0,34 13,5 0,26 - El Centro 65 0,18 0,26 13,7 0,89 Petrovac 40 0,18 0,29 6,9 0,58 El Centro 75 0,21 0,35 16,4 0,91 initial cracks Petrovac 45 0,20 0,32 7,8 0,77 Northridge 20 0,21 0,42 17 0,98 El Centro 80 0,27 0,52 11,6 1,2 development Northridge 25 0,23 0,47 21 1,04 Petrovac 50 0,22 0,41 8.9 1,14 of Petrovac 70 0,32 0,61 12,2 1,29 cracks Petrovac 75 0,35 0,71 13,9 1,54