Tests On Prototypes Of Masonry Panels

2.1 Experimental investigation

This section reports some of the results of the wide experimental campaign developed at the Laboratory of Materials and Structural Testing of the University of Naples "Federico II" on masonry panels, which are symmetrical, with a central hole covered by a steel architrave, and having upper part characterized by a concrete fascia lightly reinforced by steel (Baratta & I. Corbi 2003, 2004, 2006).

The geometry of the panels is shown in Figure 1.a. A laboratory prototype of masonry panel is referred to, made of tuff bricks (type "yellow tuff of Naples", Italy)

jointed to each other by a pozzolana mortar in order to confer a light additional resistance to the masonry; the masonry itself is characterized by unit weight Y = 10300 Nm and Young modulus E = 5.5 GPa. As regards to the loading condition, a varying force is applied in the middle of the left side of the panel, in such a way to mitigate the proneness of the panel to sliding of bricks with respect to each other, and some loading/unloading cycles are developed up to the collapse condition.

Once reached the crisis, the panel is reinforced by directly laminating on the masonry some FRP strips according to the provision scheme shown in Figure 1.b, at the same time with the impregnation of the fibers by means of a special bi-component epoxy resin, and a further experimental investigation is developed on the reinforced structure by re-executing some loading/unloading cycles.

The adopted reinforcement, produced by FTS, is a BETONTEX system GV330 U-HT, made of 12 K carbon fiber, jointed by an ultra light net of thermo-welded glass. The mechanical characteristics of the employed carbon fibers are: tensile limit stress aflp = 4.89 GPa, elastic modulus in traction Efrp = 244 GPa, limit elongation afrp = 2%. The FRP strip is characterized by thickness of 0.177 mm and depth of 200 mm. The induced displacements at some selected points [the transducers 1, 2, 3 and 4 in Figure 1.a] of the panel both for the not reinforced and for the lightly reinforced panel are recorded by a monitoring equipment consisting of: 4 transducers, placed at different locations of the panel in order to record the absolute displacements, and 15 strain-gauges, arranged in 3 blocks of5 strain-gauges, in such a way that each block is devoted to record the related strain situation. In details two transducers are located horizontally at two different heights on the panel right side (transducers 1 and 2), and two are placed in correspondence of the opening, one in horizontal position at the top of the left side of the hole (transducer 3) and the other one under the architrave, which is devoted exclusively to control the panel deflection (transducer 4). The displacements s(mm) versus the varying force F(N) monitored by the transducers during the experiment in the not-reinforced and in the reinforced case with some horizontally applied C-FRP strips are shown in Figures 2.a-c and 2.d-f respectively, as regards to the first loading cycle. By the diagrams in Figure 2, which report the displacements s(mm) vs the varying force F(N) read by the transducers 1-3, some considerations can been made.

With reference to the panel's reinforcement by means of the application of some C-FRP strips, the major effect of the C-FRP intervention is the reduction of the stress in the masonry. In general lower displacements at the locations monitored by the transducers can be recorded in the consolidated case with comparison to the unconsolidated case.

To this regard, the pretty light type of reinforcement allows to read the influence of even a small provision on the panel response, which, on the counterpart, cannot be expected to be macroscopic.

One should emphasize that the first objective of this application is, then, to show the sensitivity of the NRT model even to small changes in the structural response, very differently from the elastic model, which, on the contrary, for the specific case, is unable to detect any difference in the behaviour of the wall. A number of more effective reinforcements have also been tested by the authors obviously resulting in more appreciable results and a much higher performance (Baratta & I. Corbi 2006).

In the specific case, one can notice that, with reference to the same load intensity [e.g. in correspondence of the load value 3000 N in Figures 2.a-c], lower displacements can be recorded in case of FRP insertions. Moreover, the increase of the overall stiffness of the panel results in a higher loading capacity with respect to the not-reinforced wall. In particular the trend of each curve, shows that it is closer to the x-axis (representing the load variable), thus indicating an increase in the stiffness which is also related to an higher collapse value of the load.

2.2 Experimental/theoretical comparison

Actually the application of the general theory of NRT structures to the considered case of the masonry panel, also in the presence of FRP reinforcements, can produce numerical results which are in good agreement with the results obtained by the above reported experimental campaign (Baratta & I. Corbi 2006). The specialization of the general problem to the case of masonry walls requires the definition of a discrete model coupled to the real structural model, the set up of the energetic problem (in the case of masonry panels the potential energy approach is to be preferred) for the discrete problem, which, for masonry material, results in a Non Linear programming problem to be solved by means of Operational Research tools, and, finally, the search of the numerical solution of the set up OR problem by means of a suitably implemented calculus code (Baratta & I. Corbi 2004, 2006).

Once followed the above described steps, the numerical results can be compared to the ones coming out from the experimental investigation, for the final validation of the theoretical set up.

For the specific case one may compare the results relevant to the first loading cycle with those related to experiments. As shown in Figures 2, the theoretical data (continuous lines) are in good agreement wit the experimental ones (dotted lines) both as regards to the

Figure 2. Comparison between the numerical (continuous line) and the experimental (dotted line) at the monitored positions 1, 2, 3 for the not reinforced panel (a, b, c) and for the reinforced panel (d, e, f).

Experimental data - Theoretical results

Experimental data - Theoretical results

0 100 200 300 400 500 F (Kg)

• Experimental data -Theoretical results

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