Nikola Grgić

Effect of the joint type on the bearing capacity of a multi–drum column under static load

ABSTRACT The results of an experimental study on the effect of the joints between the blocks on the ultimate bearing capacity of a multi-drum column loaded to centric vertical force and horizontal force in the middle of its height are shown. The column is approximately 2.5 m high, with one hinge at the top and another hinge at the bottom. Four types of joints between the blocks were considered: a dry joint (column C-DJ) and joints with stone powder (column C-SPJ), lead (column C-PBJ), and epoxy (column C-EPJ). The applied vertical and horizontal forces, horizontal displacement, vertical strains, and horizontal circumferential strains in the middle of the column height, as well as shortening of the column, were measured. Under axial compression, ratios between the ultimate load bearing capacities of tested columns were C-EPJ: C-DJ: C-SPJ: C-PBJ = 1: 0.68: 0.59: 0.51. The bearing capacity of the tested columns with regard to the horizontal force depended on the level of the applied centric compression force. Columns with soft joints (C-PBJ, C-SPJ) had the largest shortening and the largest horizontal displacements for the equal forces.

Numerical Model for Static and Dynamic Analysis of Masonry Structures

Firstly, the main problems of numerical analysis of masonry structures are briefly discussed. After that, a numerical model for static and dynamic analyses of different types of masonry structures (unreinforced, reinforced and confined) is described. The main nonlinear effects of their behaviour are modelled, including various aspects of material nonlinearity, the problems of contact and geometric nonlinearity. It is possible to simulate the soil-structure interaction in a dynamic analysis. The macro and micro models of masonry are considered. The equilibrium equation, discretizations, material models and solution algorithm are presented. Three solved examples illustrate some possibilities of the presented model and the developed software for static and dynamic analyses of different types of masonry structures.

Behavior of fiber reinforced mortar composites under impact load

This paper presents results of the experimental study on the behavior of plain and fiber reinforced cement mortars with different fibers under static and impact compressive load. Glass, polypropylene and carbon fibers were used in equal dosage by mass. The impact test was conducted using an impact tower with drop hammer, which represented the modification of the split-Hopkinson pressure bar system, with strain rates ranging from approximately 35 to 60 s-1. The results of the static test and impact test with two different drop heights were compared and discussed. Among other, it has been concluded that the tested fiber reinforced mortars had no greater static and impact strength compared to the plain mortar. Only their ductility was increased at both static and impact failure. Strengths and ductility of all composite specimens were similar, i.e. without the effect of fiber type. With the increase of strain rate, compressive strength is increased and ductility is decreased for all tested specimens.

Comparison of Developed Numerical Macro and Micro Masonry Models for Static and Dynamic Analysis of Masonry-infilled Steel Frames

First, the basic characteristics of macro and micro masonry models in numerical analyses of masonry structures are discussed. Afterwards, developed numerical macro and micro masonry models, intended for the nonlinear static and dynamic analysis of unreinforced and confined masonry walls, as well as of masonry-infilled frames, are briefly presented. The models are tested on previously performed experimental tests of masonry-infilled steel frame under horizontal static force and masonry-infilled steel frame on a shake-table. The precision of both models is compared. It is concluded that both numerical models can provide reliable results. However, the macro model has more advantages for wide practical application.

Effect of the Shear Force on the Failure of Spatial Concrete Framework Structures

Firstly, the previously developed numerical model for static analysis of spatial concrete frame structures is briefly described. In this model, cross-sections of structural elements can be of arbitrary shape and formed by various materials, with arbitrary normal stress normal strain relations. This model only includes the effect of normal stresses on the structure failure. Here, it was improved by including the effect of shear forces on the failure of reinforced concrete beam elements. Shear bearing capacity of reinforced concrete section includes the concrete capacity, as well as the shear bearing capacity of longitudinal, transversal and inclined reinforcement bars. The developed numerical model and appropriate software were verified on experimental shear test of a concrete beams. Good agreement was obtained between the experimental and the numerical results. However, further verifications of the presented numerical model are needed.

Effect of Anisotropy of Masonry on the Behaviour of Unreinforced and Confined Masonry Walls Under Ground Motion

This paper presents the numerical test results of the effect of anisotropy on the behaviour of unreinforced and confined masonry walls under horizontal harmonic ground acceleration. Two-storey masonry walls with different lengths (3, 6, 12 m) are analysed. The wall foundations are supported on a rigid base, with possibility of lifting. A non-linear macro model of masonry is used. Cases of strong masonry (high strength and stiffness) and weak masonry (low strength and stiffness) are analysed. Different coefficients of anisotropy of masonry ca (0.2, 0.4, 0.6, 1.0) are considered. The excitation period is taken to correspond to the fundamental period of the individual elastic wall. Horizontal displacements at the top of the walls, vertical stresses in masonry and concrete at the bottom of the walls and stresses in the longitudinal bars at the bottom of the vertical ring beams are shown for each analysed case. It is concluded that the ultimate bearing capacity and deformability of walls, as well as stresses in masonry, concrete and reinforcement under dynamic excitation significantly depend on the anisotropy of masonry. It is also concluded that, with the same coefficient of anisotropy, the behaviour of masonry walls depends on its type (unreinforced, confined), height to length ratio (0.5, 1.0, 2.0), and the quality of masonry (strong, weak).

Effect of the Drum Height on the Seismic Behaviour of a Free-Standing Multidrum Column

The results of a shake-table study on the effect of the drum height on the seismic behaviour and bearing capacity of small-scale free-standing multidrum columns are presented. Columns of equal height with one, three, and six drums through their height were considered for the case of their self-weight only and for the case with an additional weight on the top of the column. The columns were exposed to a horizontal base acceleration of three accelerograms by successively increasing the maximum acceleration to their failure. The characteristic displacements and accelerations of the column were measured. It was concluded that an increase in the number of blocks in the column can significantly increase or decrease its ultimate bearing capacity, depending on the type of the applied accelerogram. It is expected that the experimental database can be useful in the validation of nonlinear numerical models for the dynamic analysis of multidrum columns.

The Use of Limestone Sand for the Seismic Base Isolation of Structures

The possibility of the use of a layer of natural material under foundations for seismic base isolation was investigated. The dissipation of seismic energy of a low-cost natural material with adequate thickness, bearing capacity, and lateral and vertical stiffness, which can serve as an optimal solution for seismic base isolation under the foundations of many structures, was tested. This paper presents the results of a brief experimental study to determine the effectiveness of ordinary limestone sand under the foundation of a cantilever concrete column to increase its seismic resistance. The behavior of small-scale columns with three substrates below the foundation (rigid base, the thin layer of limestone sand, and the thick layer of limestone sand) was investigated by the shake table. The column was exposed to a set of horizontal base accelerations until structure collapse. It was concluded that a layer of limestone sand of appropriate thickness and compressibility can serve as the means a seismic base isolation. The nonlinear numerical model for the dynamic analysis of planar concrete structures coupled with soil is briefly presented and verified by the performed experimental tests.

Retrofitting of Three Historic Stone Arch Bridges over the Cetina River in Croatia

Abstract The basic characteristics of the three historic stone arch bridges over the Cetina River in Croatia are presented first. The bridges are under the protection of the Ministry of Culture of the Republic of Croatia as valuable architectural heritage. The current state of the bridges is then presented, and the main solution of the renovation and strengthening of the bridges is briefly explained. A conclusion is reached that the renovation of the bridges was performed successfully, with excellent cooperation between the engineers and the conservators.

Experimental testing of concrete beams with different levels of prestressing

The behaviour of actual prestressed concrete beams under static load up to failure has been experimentally investigated. The experimental testing includes five types of precast concrete beams with different ratios of prestressed and classical reinforcements (only prestressed reinforcement, dominant prestressed reinforcement and classical reinforcement, equal prestressed and classical reinforcements, dominant classical reinforcement and prestressed reinforcement, and only classical reinforcement) i.e. with different levels of prestressing force. Twenty-three beams were tested. The total amount of reinforcement in each beam was selected to provide approximately equal ultimate bending bearing capacity. The deflections, concrete stresses, stresses in classical and prestressed reinforcements and concrete cracks were monitored until the beams collapsed. It was concluded that by increasing the level of prestressing, the serviceability limit state of the beams was improved. The optimal level of prestressing should be selected based on the relevant criteria for each individual structure.