CONTRIBUTION OF THE INTEGRITY AND ENERGY DISSIPATION TO THE SEISMIC RESPONSE OF MASONRY

Ramiro A. Sofronie of Bucharest and Emilia Juhasova of Bratislava

1.        Concretization of masonry

Under dynamic lateral actions, both bearing structural members and infills are easily cracked or even crashed and dislocated. The brittleness of masonry is higher when cored bricks are used. These thin walled ceramic units of reduced weight are mainly appreciated for thermal insulation properties and some advantages during the construction process. Cement mortars do not help to decrease the brittleness of modern masonry. Masonry is the only construction material built up manually with the aid of gravity and which remains dependant on gravity throughout is service life.

The buildings damaged by recent earthquakes in Turkey, Greece, Taiwan, El Salvador and India or by war in Iraq had shown again how brittle is the masonry. Masonry is not at all a composite material but only an association of two materials with similar proprieties: brittle ceramic bricks and brittle cement mortars. It is an antagonistic association because bricks are produced by fire while mortars by water. This artificial stone with light aggregates is in fact concrete with the color of masonry. In other words with the aid of advanced technologies the masonry was definitely concretized. This is far away from the conceptual philosophy of the original masonry.

2.        The sandwich effect

The original masonry is essentially different from the modern types. The two associated materials, solid bricks and plastic mortars always have had complementary proprieties. This means that around structural faults, like vertical joints between bricks, concentrations of stresses occur resulting in plastic strains in mortars. Spontaneously and according to the Principle of minimum compulsion the stresses located around geometrical imperfections are gradually redistributed to neighbouring less heavily loaded areas. By this phenomenon of adaptation, known also as “sandwich effect”, the original masonry protects itself against overloads and is long lasting.  In the particular case of dynamic actions like seismic ones there is usually no time for plastic strains to develop. Then up to some extent the induced dynamic energy is stored in the bricks as potential energy that later is gradually released and dissipated through mechanical work. So far, for masonry there were produced many computing methods called either linear or nonlinear but none, without any exception, was able to include the miraculous phenomenon that occurring between the solid bricks and plastic mortars is leading to the “sandwich effect”. It is a global effect that combines the contribution of stresses and strains and it is only energetically best controlled.

3.        Masonry: Quo Vadis?

It is interesting to note that since 1964 the architects, gathered to the International Charter of Conservation and Restoration endorsed in Venice, accepted to restrain themselves from association the masonry with reinforced concrete in the buildings of Cultural Heritage.

Since History never returns and the existing factories producing cement or ceramic bricks cannot be closed, the alternative is to restore the conceptual philosophy of the original masonry.  At the existing levels of both knowledge and technology, one of the practical solutions immediately available consists in reinforcing masonry with polymer grids. One assumes that under compressive and shear forces, when the ductile mortar reaches its ultimate limit state, bricks suddenly expel it. The polymer grids, with slender ribs and solid integrated joints, inserted in the bed layers are uniformly distributing the tensile stresses and by the “sandwich effect”, any stress concentrations are prevented. Thus reinforced the mortar is able to cooperate with all types of cored or solid bricks in any structural members of masonry. If further the masonry, either plain or reinforced one, is wrapped around with the same polymer grids and plastered then it becomes a composite material with higher bearing capacity and better behaviour under seismic actions. Laboratory tests and numerical analyses validated this innovative method. The results of static, pseudo-dynamic and seismic tests are now available together with two study cases recently completed in Romania. The method is easily applied and financially attractive while the existing database supports any conceptual design. The most important outcome of the idea is that masonry becomes again a challenging construction material. New directions of research by real and virtual simulations are also opening.

4.        Dynamic behaviour in view of seismic response

Masonry ultimate strength is controlled by strengths of two associated materials bricks and mortar. Under the dynamic loading the cracks appear either in mortar or in bricks or in both depending of the used material properties. Non-returning deformations are related to the degree of dynamic excitation. Bricks drop out from the wall in or out of plane.

The properly chosen polymeric grids create the third material component in the masonry material association. The benefit is from grid tension capacity and the ability to substantially increase the integrity of the masonry. With increasing dynamic loading the cracks can appear but they are of different nature than that in plain masonry. Localised horizontal cracks are limited through shear deformation of polymeric grids. The dissipation of energy contributes to the main dynamic response features.

In optimisation approach the property of any component can be varied. In addition to the basic crucial parameters as stiffness, strength and damping, the integrity is another parameter that should be taken into account.

Thus, the collapse limits reach quite another dimension. The structure is rather able to survive the strong seismic shocks without the collapse, however, with the prescribed option of repair.

It is actually promising that in most cases the seismic carrying capacity shall increase several times.