Antonio Borri

Strengthening techniques tested on masonry structures struck by the Umbria-Marche earthquake of 1997-1998

The results of experiments carried out on structures damaged in the Umbria–Marche earthquake of 1997–1998 are presented. These tests were carried out in situ on masonry panels of various dimensions, which had been strengthened with either traditional or innovative materials and techniques. Concerning traditional methods, panels injected with new limed-based mixes were tested. Other tests were realized by gluing to the wallettes sheets of monodirectional carbon fiber (CFRP) or fiber glass (GFRP) with epoxy resins. In both cases the purpose of the tests was to analyze the effectiveness of the intervention, above all as a technique of seismic-upgrading against in-plane mechanisms of collapse. The results show a significant increase in strength. The experiments carried out allowed to obtain interesting indications for their practical utilization of the studied technique. The injection technique is substantially more efficient when used as a method of repair damaged panels, confirming that a preliminary evaluation of dimension and distribution of voids is necessary before adopting this technique. The experimental work showed that the use of composite materials on double-leaf roughly cut stone masonry is more effective when conducted with other stabilization schemes. The failure of the double leaf roughly cut stone panels strengthened with composite materials resulted from the separation of the two masonry leaves. In both cases the strengthening showed remarkable benefits in terms of increase in strength, providing the masonry with greater shear strength. The increase in stiffness following the intervention, as well as its effect, was also analyzed.

Experimental study on the determination of strength of masonry walls

The results of a research project carried out on masonry panels obtained from structures struck by the Umbria-Marchigiano earthquake of 1997–1998 are presented. The project consists of two parts: tests were performed in the laboratory, and in situ in order to determine the correct parameters describing masonry behavior. With regard to the laboratory tests, several compression tests were performed on cylindrical stone samples. Stone samples were obtained from the panels on which in situ tests had been previously carried out. Depending on the three types of in situ tests carried out (compression test, diagonal compression test, shear–compression test), different dimensions of panels were used using an appropriate cutting technique in order to leave the panels undisturbed. The shear strength and the Young and shear elastic modulus were measured. These results were compared with the values suggested by different standards. The experimental research allowed to characterize the mechanical properties of some typical masonry walls in old buildings of Umbria. These results are reported, together with an analysis of the masonry textures and sections.

Strengthening of Brick Masonry Arches with Externally Bonded Steel Reinforced Composites

The objective of this study is to investigate the efficiency of an innovative technique for strengthening masonry arches, based on the use of high strength steel cords embedded in either an epoxy (steel reinforced polymer) or mortar matrix (steel reinforced grout). Ten prototypes of brickwork arches strengthened by composite laminates were tested under a monotonic vertical load applied at the quarter-span. Load tests were performed to compare the behavior up to collapse of strengthened masonry arches; the influence of the types of reinforcement (steel and carbon fibers) and matrices (epoxy and cementitious), as well as location of the strengthening layer (intrados, extrados, and both) and the presence of anchorage systems has been investigated. The experimental results highlight the enhanced strength of the arches reinforced with steel cords, as well as the role of the mechanical anchoring with regard to the resulting final strength.

Structural reliability analysis using a standard deterministic finite element code

A method for performing a reliability analysis of structural systems within a standard finite element code is presented. This numerical procedure can be implemented in any finite element (f.e.) code having an internal optimization routine. The design points of structural problems are determined by calculating the minimum distance from the origin to the failure surface in a set of normalized variables, by using the minimization routine of the f.e. code. In order to test the procedure, simple structural systems are solved and the results are compared with those obtained by using different approaches. Some examples of application of the procedure for the reliability analysis of real structures are presented.

A method for the analysis and classification of historic masonry

It is known that the mechanical behavior of masonry material depends on many factors, such as compressive or shear strength of components (mortar and blocks), blocks shape, volumetric ratio between components and wall texture, that is the result of applying a series of construction devices which form the “rule of the art”. Taking into account the complexity of the problem due to the great number of variables, a fair assessment of the load carrying capacity of masonries can be made only with in situ test. Being in situ test a semi-destructive methods, not always viable, a numerical estimate of the mechanical parameters of the walls can be made on the basis of a qualitative criteria evaluation. The method here proposed, called Masonry Quality Index method, consists in evaluating the presence, the partial presence, or the absence of certain parameters that define the “rule of the art”, namely a set of construction devices that, if executed during the construction of a wall, provides a good behavior and ensure the compactness and the monolithicity.

Masonry Columns Confined by Steel Fiber Composite Wraps

The application of steel fiber reinforced polymer (SRP) as a means of increasing the capacity of masonry columns is investigated in this study. The behavior of 23 solid-brick specimens that are externally wrapped by SRP sheets in low volumetric ratios is presented. The specimens are subjected to axial monotonic load until failure occurs. Two widely used types of masonry columns of differing square cross-sections were tested in compression (square and octagonal cross-sections). It is concluded that SRP-confined masonry behaves very much like fiber reinforced polymers (FRP)-confined masonry. Confinement increases both the load-carrying capacity and the deformability of masonry almost linearly with average confining stress. A comparative analysis between experimental and theoretical values computed in compliance with the Italian Council of Research (CNR) was also developed.

Determination of Shear Strength of Masonry Panels Through Different Tests

This study addresses the problem of evaluation of strength of masonry walls. In-plane behavior of masonry panels has been studied under monotonic diagonal-compression and shear-compression loading in quasi-static test facility. The results of 35 laboratory and in situ tests are analyzed to show that in the case of the diagonal compression test results are lower than the strength of masonry walls evaluated trough the shear-compression test, highlighting the problem of choosing the test which best simulates to the real behavior of the masonry when stressed by lateral loads. A presentation is also given of the results of a F.E. investigation for shear strength evaluation of masonry walls. F.E. modeling non-linear procedure was used for the representation of masonry panels. The numerical simulations are compared with experimental results and the reliability of the different finite element models is discussed, thus confirming the different shear strength values measured in the experimental campaign.

Durability analysis for FRP and SRG composites in civil applications

In this study, an effort was made to develop an experimental protocol to study the effects of accelerated ageing on composite materials based on bamboo and steel fibers. The physic-mechanical properties of different types of steel and bamboo fibers were investigated. Specimens were subjected to environmental agents. Mechanical and physical tests were used to measure the retained properties and to observe the causes of damage and strength reduction. The experimental data showed that resin properties may strongly influence the durability of FRP reinforcement, environmental combined cycles did not take to significant damage of conditioned specimens; steel fibers are sensitive to alkaline attack when resin does not provide adequate protection to fibers.

Bond Strength of Composite CFRP Reinforcing Bars in Timber

The use of near-surface mounted (NSM) fibre-reinforced polymer (FRP) bars is an interesting method for increasing the shear and flexural strength of existing timber members. This article examines the behaviour of carbon FRP (CFRP) bars in timber under direct pull-out conditions. The objective of this experimental program is to investigate the bond strength between composite bars and timber: bars were epoxied into small notches made into chestnut and fir wood members using a commercially-available epoxy system. Bonded lengths varied from 150 to 300 mm. Failure modes, stress and strain distributions and the bond strength of CFRP bars have been evaluated and discussed. The pull-out capacity in NSM CFRP bars at the onset of debonding increased with bonded length up to a length of 250 mm. While CFRP bar’s pull-out was achieved only for specimens with bonded lengths of 150 and 200 mm, bar tensile failure was mainly recorded for bonded lengths of 250 and 300 mm.

Masonry Confinement Using Steel Cords

The objective of this study is to investigate the effectiveness of an alternative technique for strengthening masonry columns, based on the use of steel cords. Although fiber-reinforced polymer (FRP) strips or jackets are successfully used as strengthening elements for this kind of application, such a strengthening method represents a new opportunity to restore ambit, with considerable development in unreinforced masonry wrapping. Thus, to assess the potential of steel cord wrapping, this paper presents the results of an experimental program consisting of 48 solid-brick specimens subjected to uniaxial compression load. Different cross-section geometry (octagonal, square, and rectangular) and different amounts and different schemes for confining reinforcement were investigated. The primary experimental outcomes are presented and discussed in the paper considering the mechanical behavior of specimens and the axial stress–axial strain relationships. Test results have shown that the investigated confining systems are able to provide significant gains, both in the compressive strength and in the deformation capacity of masonry columns.