Tests On Prototypes Of Masonry Arches

3.1 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 arches, consolidated or not by means of FRP strips (Baratta & O. Corbi 2003b, 2005b).

The geometry of the portal arch (Figs 3 and 4) is symmetrical and is characterized by span

» Dial Gauge Gi t^J Inclinometer I[ J Transducers T[,T2 Q Extesemeter Ei Q Deformometric cells dk, k=1...30

Figure 3. The portal arch model with the monitoring equipment: sketch of the monitoring equipment.

L = 1900 mm, rise f = 660 mm, arch thickness d = 240 mm, piles thickness b = 385 mm, piles height h = 1700 mm; the arch shape is a semi-ellipse. The arch depth is 400 mm, whilst the two abutments are 480 mm deep. The masonry is characterized by unit weight y = 12300 N • m-3 and Young modulus E = 5.5 GPa.

As mentioned, in the above, in the second stage of the experimental campaign one also considers some FRP continuous reinforcement applied on the arch length. In this case, the FRP reinforcements consist of continuous mono-directional FRP strips applied on the extrados of the arcade.

The adopted reinforcement, produced by FTS, is a BETONTEX system GV330 U-HT, made of 12 K carbon fibre, jointed by an ultra light net of thermo-welded glass.

The mechanical characteristics of the employed fibres are: tensile limit stress Ofrp = 4.89 GPa, elastic modulus in traction Efrp = 244 GPa, limit elongation efrp = 2%. The FRP strip is characterized by thickness of 0.177 mm and depth of 100 mm.

After roughly preparing the masonry support in order to render the application surface smoother, the FRP is directly laminated on the masonry, at the same time with the impregnation of the fibres by means of a special bi-component epoxy resin.

As regards the execution the tests, the structure is subject to its constant own weight and to a lumped horizontal force F, applied on the top right side of the right abutment in the rightward direction in the increasing phase (Figs 3 and 4), which is transmitted by means of a loading equipment consisting of a load cell placed on the right side of the portal arch.

This force is able to potentially produce collapse of the structure according to a mechanism that is typical of earthquake failures of arch-portals (Fig. 6), and it

Figure 4. The portal arch model with the monitoring equipment: picture from laboratory tests.

is intended to represent a pseudo-seismic action, able to yield a measure of the structure attitude to sustain earthquake shaking.

The monitoring stuff (Figs 3 and 4) consists of:

- 1 dial gauge Gj, placed on the left side of the left abutment, finalized to the monitoring of the absolute displacement of the pile;

- 2 transducers T1 and T2, vertically placed on the front side of the left abutment, finalized to the monitoring of the length variation of both edges of the pile;

- 1 inclinometer I1 , placed on the top of the left abutment, finalized to the monitoring of the pile average rotation;

- 1 extensometer Ej, placed between the two abutments, finalized to the monitoring of the relative piles' displacement;

- 30 deformometer cells, placed on the front of the arch, finalized to the monitoring of the arcade deformation.

For the un-reinforced structure (Baratta and Corbi, 2003a,b) the critical condition is related to the activation of a collapse mechanism composed by four hinges distributed as follows:

- 1 at the keystone on the extrados,

- 2 at the reins on the intrados,

- 1 at the bottom of the right pile on the extrados.

Figure 5. Unreinforced portal arch: pile displacement u versus load F-numerical/experimental comparison.

The collapse condition is reached at F~80 N; the low failure value of the force shows that, due to the chosen elliptical shape of the arch, the funicular line compatible with the applied loads and admissible (i.e. interior to the arch profile) is already very close to the upper and lower bounds of the arch profile at the rest condition.

The experimental force-displacement diagram is reported in Figure 5.

After reaching the collapse condition, the portal arch is then unloaded in order to be prepared for the subsequent experimental tests on FRP reinforcements. After completing the unloading process, the portal arch is prepared for laboratory tests on FRP reinforcements, which are finalized to the evaluation of the benefits induced on the model response by the application of carbon fibre strips.

The reinforcement consists of a continuous FRP strip bonded on the extrados of the arch. Since the collapse mechanism of the not reinforced simple portal arch is characterized, as described in the above, by the formation of two intrados hinges at the reins of the arch, corresponding to the fractures d4-d5and d12-d13 at the extrados, the major effect of this intervention is supposed to be the prevention of these fractures, and, therefore, a wide increase in the model loading capacity.

The funicular line is now free to exceed the lower contour of the portal arch cross section.

In this case the critical condition is related to the activation of a collapse mechanism composed by four hinges, distributed as follows:

- 1 at the top of the left pile on the intrados,

- 1 at the keystone on the extrados,

- 1 under the load cell on the intrados of the right pile

(where shear occurs),

- 1 at the bottom of the right pile on the extrados.

The collapse is reached at F~800 N with an increase in the loading capacity of the portal arch of approximately 10 times with respect to the unconsolidated case. The experimental force-displacement diagram is reported in Figure 7.

3.2 Experimental/theoretical comparison

Actually the application of the general theory of NRT structures to the considered case of the masonry portal arch, 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 & O. Corbi 2005a, 2005b, 2007).

The specialization of the general problem to the case of masonry arches 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 arches the complementary energy approach is to be preferred) for the discrete problem, which, for masonry material, results in a Non Linear programming problem (which in the specific case can be reduced to a 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 & O. Corbi 2003a, 2003b, 2005a).

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.

Numerical investigation on the portal arch model experimentally tested results in the possibility of appreciating the skill of the NRT model to capture the major features of the structure behaviour. Moreover also the correct modelling of the reinforcement and of its coupling with the main structure can be evaluated. Figure 5 reports the numerical/experimental comparison relevant to the right pile top displacement u (mm) versus the varying load F (N) for the considered un-reinforced arch.

A very good agreement between the numerical and experimental data can be observed. The calculus code is demonstrated to be able to capture the behaviour of the portal arch following the whole loading path up to collapse; Figure 6 depicts the collapse mechanism of the structure as it appears directly from the calculus code, clearly due to the formation of four hinges: one at the keystone on the extrados, two at the reins on the intrados, one at the bottom of the right pile on the extrados.

Moreover one reports in Figure 7 the numerical/experimental comparison relevant to the right pile

Figure 6. Unreinforced portal arch: picture of the collapse Figure 8. Portal arch with extrados reinforcement: picture mechanism captured from the calculus code. of the c°llapse mechanism captaed from the cdculus code.

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