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A large number of engineered buildings were severely damaged and collapsed during the Richter magnitude 6.8 earth quake that struck Erzincan, Turkey, on March 13, 1992. The building inventory of the city contained structures similar to those built worldwide at the time of their construction. The study of the effects of this earthquake on structures is of particular significance to Canada, since an earthquake of approximately the same magnitude is likely to occur in many parts of the country, where similar structural systems are used. This paper reports on the findings of a 4-day reconnaissance visit conducted by the authors. A brief description of seismological aspects of the earthquake is presented, including characteristics of the ground motion. The emphasis is placed on structural performance during the earth quake, and possible causes of observed structural failures and collapses. Key words: building code, concrete, earthquake, Erzincan, masonry, precast concrete, reinforced concrete, seismic design, structures, structural design, steel, unreinforced masonry.
Eccentricities between the centres of rigidity and centres of mass in a building cause torsional motion during an earthquake. Seismic torsion leads to increased displacement at the extremes of the building and may cause distress in the lateral load-resisting elements located at the edges, particularly in buildings that are torsionally flexible. For an equivalent static load method of design against torsion, the 1995 National Building Code of Canada specifies values of the eccentricity of points through which the inertia forces of an earthquake should be applied. In general, the code requirements are quite conservative. They do not place any restriction on the torsional flexibility, however. New proposals for 2005 edition of the code which simplify the design eccentricity expressions and remove some of the unnecessary conservatism are described. The new proposals will require that a dynamic analysis method of design be used when the torsional flexibility of the building is large. Results of analytical studies, which show that the new proposals would lead to satisfactory design, are presented.Key words: torsional response to earthquake, natural torsion, accidental torsion, design for torsion, National Building Code of Canada, interdependence of strength and stiffness.
Indeterminate truss models are proposed for the analysis of low-rise walls retrofitted using a special system of vertical and diagonal steel strips. An equilibrium diagram is also presented to calculate the ultimate strength of walls based on a lower bound theorem of plasticity. Step-by-step analyses were conducted to establish force-displacement relationships of walls that compared well with those obtained experimentally. A simple retrofit design procedure is formulated and illustrated using a numerical example.
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This research describes an experimental investigation carried out to verify the use of fiber reinforced polymer (FRP) grids as transverse reinforcement in concrete structures. Emphasis was placed on concrete confinement and seismic performance. Large-scale column specimens were tested under simulated seismic loading. Column reinforcement cages consisted of ordinary steel reinforcement as longitudinal bars and FRP grids as transverse reinforcement. Test parameters included grid spacing and pattern, the volumetric ratio of grid reinforcement, and the level of axial load. Results indicate improved deformability of columns, when confined by properly designed grids. Other findings are also reported and discussed in detail.
This research describes an experimental investigation carried out to verify the use of fiber reinforced polymer (FRP) grids as transverse reinforcement in concrete structures. Emphasis was placed on concrete confinement and seismic performance. Large-scale column specimens were tested under simulated seismic loading. Column reinforcement cages consisted of ordinary steel reinforcement as longitudinal bars and FRP grids as transverse reinforcement. Test parameters included grid spacing and pattern, the volumetric ratio of grid reinforcement, and the level of axial load. Results indicate improved deformability of columns, when confined by properly designed grids. Other findings are also reported and discussed in detail.
This study implements a two-dimensional nonlinear finite-element approach for the assessment of the shear behavior of a post-tensioned bridge girder whose tendons and reinforcement suffered severe corrosion during its service life of almost 60 years. The girder is lightly reinforced with vertical stirrups and its tendons follow a parabolic profile, with many of them anchored at the top of the girder; as such, the contribution of post-tensioning is significant to its shear capacity. The objective of the study is to understand the various aspects of the progressive damage of the girder due to increased corrosion of its prestressing steel. Through the finite-element modeling tool, different approaches have been considered to model the corrosion of post-tensioning tendons and reinforcement to assess the shear response of the girder in the degraded state.
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The use of high-strength concrete (HSC) in seismically actively regions poses a major concern because of the brittle nature of material. The confinement requirements for HSC columns may be prohibitively stringent since they require proportionately higher confinement than columns of normal-strength concrete. An alternative to conventional confinement reinforcement is the use of fiber reinforced polymer (FRP) casings, in the form of a stay-in-place formwork, which can fulfill multiple functions of; i) formwork, ii) confinement reinforcement, and iii) protective shell against corrosion, weathering and chemical attacks. This paper investigates the use of stay-in-place FRP formwork as concrete confinement reinforcement for HSC columns with square and circular cross-sections. Large-scale specimens with 270 mm cross-sectional dimension and 90 MPa strength concrete, were tested under constant axial compression and incrementally increasing lateral deformation reversals. FRP casings were manufactured from carbon fiber sheets and epoxy resin. One of the square columns was equipped with internal FRP crossties, a new technique introduced by the authors, to provide well-distributed lateral restraints along the column face, thereby improving the mechanism of confinement. The results indicate that the deformation capacity of HSC columns can be improved significantly by using FRP casings. The results further indicate that the confinement effectiveness of FRP-confined square sections can be substantially increased with the use of FRP crossties.
The characteristics of confined-concrete columns under strain gradient were examined. Twelve columns were tested under two different levels of end eccentricity. The test parameters included the arrangement, spacing, and volumetric ratio of confinement reinforcement. Columns with well-distributed longitudinal reinforcement, laterally supported by closely spaced transverse reinforcement, showed extremely ductile behavior, developing inelastic displacements exceeding 4% of the drift ratio without a significant loss of strength. Those with a low volumetric ratio and wide spacing of transverse steel developed strength decay immediately after the peak load. A confined concrete model, developed on the basis of column tests under concentric loading, was used to compute analytical moment-curvature relationships of the critical column section. The analytical relationships are compared with those recorded experimentally, for columns with different eccentricity of loading and parameters of confinement. The comparisons indicate that the flexural behavior of confined-concrete columns can be computed reasonably accurately by a confined-concrete model developed for concentric loading, provided that the relevant parameters of confinement are incorporated in the model, and that strain hardening in reinforcement is considered.
Strength and ductility of high-strength concrete columns were investigated by examining available test data. The results are summarized in terms of load and ductility capacities of columns. Experimental observations indicate that strength and ductility of high-strength concrete columns can be improved to levels that are usually expected of normal-strength concrete columns, provided that the core concrete is adequately confined by transverse reinforcement.
