Ioannis A. Tegos

Cyclic Load Behavior of Low-Slenderness Reinforced Concrete Walls: Design Basis and Test Results

This study addresses the problem of cyclic shear in squat reinforced concrete walls and attempts to assess the validity of current design provisions, both in Europe--Eurocode 8 (EC8)--and in the United States--American Concrete Institute 318 (ACI318). This paper describes a comprehensive experimental program involving 11 wall specimens, six with shear span ratios of 1.5 and five with 1.0, detailed to the provisions of EC8. Problems in applying these provisions are pointed out and comparisons with the corresponding ACI318 provisions are also made. The wall specimens are reinforced against shear, either conventionally (orthogonal grids of web reinforcement) or with cross-inclined bars. The effects of web and edge reinforcement ratio, of axial load level, and of the quality of construction joints are also investigated. Results show that properly designed and reinforced walls can reach their flexural capacities, even when their aspect ratio is as low as 1.0, that sliding shear in this category of walls is not a major problem, and that cross-inclined (bidiagonal) web reinforcement can effectively and economically control sliding and the subsequent pinching of the hysteresis loops, particularly when these bars intersect close to the critical section.

Seismic Resistance of Type 2 Exterior Beam-Column Joints Reinforced With Inclined Bars

An experimental investigation of the behavior of external beam-column joints with inclined reinforcing bars under seismic conditions is presented. A simple technique to prevent these elements from falling in premature, explosive cleavage shear fracture was implemented for the first time. Twenty full-scale reinforced concrete exterior beam-column subassemblies were tested. The primary variables were the amount of inclined bars, the ratio of the column-to-beam flexural capacity, and the joint shear stress. Test results showed that use of crossed inclined bars in the joint region is one of the most effective ways to improve the seismic resistance of exterior reinforced concrete beam-column joints.

Strengthening of a Multistory R/C Building under Lateral Loading by Utilizing Ties

The scope of the study is to examine of the possibility of applying steel ties, which can consist of common steel wires, as means to restrain the seismic displacements of buildings structural systems. The method seems to be competitive to the conventional ones, since fewer disturbances are created during the retrofitting works, i.e. the installation of ties as compared to the conventional strengthening methods. The method also proved to be more economic than the conventional method of constructing and infilling concrete walls in the vulnerable structural system. Initially, the efficiency of the ties was studied in multistory building models, such as frame, combined frame-wall and combined frame-wall without beams. Based on the results of the analytical study, the method was applied on a 5 storey existing and seismically deficient building. More specifically, the long side of the building was chosen for the method’s application. The last choice was deemed more preferable due to the fact that it has no openings which allowed the easy application of straight ties. The one end of the ties is anchored in the concrete slab of the upper stories, while the other end on the basement’s upper concrete slab. Although the seismic demand was higher for the implemented solution of the steel wires due to multiple loading, caused by the low value of the seismic performance coefficient equal to 1.5, an adequate level of earthquake resistance was achieved. The results of the case study confirmed the applicability of the proposed method. It should be underlined that the ties method looks promising in the field of structures retrofitting and could be studied analytically in campaigns, either independently or in combination with the conventional in concrete shear wall method.

FIBER REINFORCED CONCRETE BEAMS WITH CIRCULAR SECTION IN TORSION

An experimental investigation carried out to assess the effect of incorporating steel fibers on the torsional behavior of beams with circular cross sections is reported. The principal variables of the testing program were: (a) the amount (percent by volume) of steel fibers, (b) the aspect ratio of steel fibers, and (c) the composition of concrete mix. Eighty (forty pairs) 250 mm diameter specimens were tested. The testing program also served to help develop and verify an ultimate strength model to predict the torsional strength of concrete reinforced with short discontinuous straight steel fibers randomly oriented and uniformly dispersed in a concrete matrix.

Shaking table study of the seismic interaction of an isolated bridge deck with the abutment utilizing small-scale models and numerical simulations

It has been recognized that an isolated deck develops horizontal displacements of considerable amplitude during a strong earthquake. In this case the possibility of mobilizing the abutments in moderating such large amplitude horizontal response is beneficial for the safety of the structure. Thus, apart from lowering the seismic forces by the low-stiffness isolator units, the interaction between the deck and the abutments in the form of pounding for large horizontal deck response amplitudes aims at limiting through this mechanism excessive horizontal deck displacements. Such a problem was examined at the laboratory of Strength of Materials and Structures of Aristotle University using a small-scale physical representation that retains in a qualitative way the following important features: 1. A relatively stiff steel platform, representing the bridge deck, which is supported on a shaking table by two flexible supports, representing the isolator units; it is subjected to simulated horizontal earthquake motions developing large amplitude horizontal displacement response. 2. The possibility of bridge deck pounding on the abutment was introduced through a connector device that became active after the deck response exceeded a certain amplitude, introducing an initial gap within this connector. Despite the fact that these two basic response mechanisms, flexibility of isolator units and connector force-displacement characteristics, are crude small-scale representations of the actual mechanisms that are mobilized in a prototype bridge deck, the qualitative characteristics of this problems are retained. A number of simulated earthquake tests provided the necessary measured acceleration and displacement response of the model steel platform of the small-scale model and the force-displacement response of the connector and the flexible supports of the steel platform with the shaking table. This was next utilized to validate numerical simulations of this small-scale experimental representation of the bridge-deck pounding problem. By comparing the numerical predictions with the measured response of this small-scale experimental representation of the bridge-deck pounding problem it can be concluded that such numerical simulations can yield quite accurate predictions provided that the force-displacement characteristics of the isolator units as well as the force-displacement characteristics of the mechanism representing the bridge deck-abutment pounding are defined with reasonable accuracy for the prototype bridge.

ANALYSIS AND DESIGN OF STAIRCASES AGAINST SEISMIC LOADINGS

In this work all types of staircases are studied with respect to: First, earthquake design requirements and second, their complex interactions with the multistory space frame in which they belong. In the first case the essential influence of the vertical component of the earthquake’s acceleration on the structure’s performance is examined. In the second case the staircases’ behavior is studied, as well as the important role that they play as structural seismic connections, in the response and behavior of space structures. In particular, external staircases, which connect structurally independent multistory systems are studied. In addition, in this work, a multistory external staircase that connects buildings in an area of high seismicity is analyzed. Staircases that present special design challenges due to gravity and earthquake loadings are also investigated. These structures include staircases with a free landing as well as helical cases. This work, beyond its theoretical interest, may prove important to practicing engineers as well, since it contains (almost) all types of staircases.

A new scheme for the seismic retrofit of multi-span simply supported bridges

There are two alternative strategies that a designer may adopt and combine when faced with the retrofitting of a bridge: (a) the increase in the capacity or (b) the reduction in the actions of the structure. In this article, a new scheme, based on the second strategy, is proposed for the retrofit of existing multi-span simply supported (MSSS) bridges. The reduction in the actions of the bridge was mainly achieved by utilising an external restraining system consisting of I-shaped steel piles driven in the backfill soil and a slab that is the pile-cap of the piles. The restraining system was preliminarily designed and assessed in an existing MSSS bridge system, whose deck slab was made continuous. The existing and the retrofitted bridge were analysed by means of non-linear dynamic time history analysis and their response was compared in terms of serviceability and earthquake resistance performance. The study showed that the retrofitting scheme enhanced effectively the earthquake resistance of the existing bridge.

Connection of Bridges with Neighborhooding Tunnels

A large number of bridges are constructed between tunnels. This co-existence can be developed in order to reduce the seismic actions of bridges, as their end parts can be restrained by the tunnels. This restrain requires the accommodation of the resulting serviceability problems, which are possible to be arranged by means of appropriate approach elements and expansion joints. In the present study, an appropriately configured approach element is proposed with which a semi-connection of the bridge with both tunnels is achieved. This approach slab is designed in a manner to accommodate both serviceability and earthquake resistance of the bridge. The proposed semi-connection of the bridge with the neighborhooding tunnels was proven to be efficient as the parametric investigation showed that the interaction of the bridge with the stiff tunnels can lead to reductions in the seismic actions of the bridge.

ANALYTICAL AND EXPERIMENTAL RESEARCH ON IMPROVEMENT OF THE DUCTILITY OF BUILDINGS’ R/C SEISMIC WALLS THROUGH SLIP PREVENTION WITH THE USE OF STOPPERS

Abstract. Several researchers propose placing diagonal reinforcing bars at the base of the wall to treat the shear slip, while others have suggested various ways to address this problem associated with the halting of the effects incurred by the through-crack in the base of the wall during recycling of loading. An indicative proposal of the bibliography is using large diameter reinforcement bars in the web of the wall as vertical reinforcements, so that through the dowel action of these bars to be able to better control the shear action. The two aforementioned proposals, while adequately address the phenomenon of shear slip, present significant disadvantages: The use of diagonal reinforcement is very difficult to be constructed, because of the density of existing reinforcement in the base of the walls, which involves compromising good concrete condensation. Also, the use of large diameter vertical reinforcement along the length of the whole wall section, including its web, leads to a strongly uneconomical solution. This works examines a solution without the aforementioned side effects. Innovation of the present work is the fact that it positions stoppers in combination with the use of conventional reinforcing bars at positions of the critical zones of the walls, in order to further prevent the expected slip along the through-crack in the base of the rigidly supported wall. The work is mainly experimental and includes investigation of the seismic mechanical properties of a wall specimen with conventional reinforcement according to EC8. This study presents the investigation on the effect of the shear span as resistance parameter, on the design of concrete interfaces. In the first part of the study the shear transfer between concrete interfaces, in which the value of the shear span is equal to zero, is investigated. The experimental investigation is extended to include values of the shear span greater than zero. These values are usually observed at bridge seismic stoppers. The experimental results presented in this study are used to derive an analytical expression of the resistance of bridge seismic stoppers.

A Proposal for the Improvement of the Earthquake Resistance of Multi-span Precast I-Beam Bridges

This study proposes an innovative restraining system for the improvement of the earthquake resistance of precast concrete I-beam bridges mainly in the longitudinal direction. The proposed method is based on the connection of the continuous deck slab of the bridge through the sidewalks with both abutments and the elimination of the end expansion joints. In this way, the bridge obtains four restraints (one per wing-wall), which behave as tension ties during the deck contraction and have the ability to reduce the seismic movements of the deck. The resultant in-service requirements due to creep, shrinkage and thermal effects are properly accommodated. Each sidewalk is divided to zones that are anchored to the deck and zones that have the ability to slip on the deck. The efficiency of the proposed method is investigated by utilizing a precast I-beam bridge of Egnatia Odos Motorway in Greece. This bridge has been converted to a ductile bridge system by using active seismic stoppers at the head of the piers.