Stavroula J. Pantazopoulou

Direct Tension Pullout Bond Test: Experimental Results

Results from 50 bond tests conducted using the so-called direct tension pullout specimen type are presented. Anchorages of steel bars with machined deformations were tested so as to enable a targeted study of the effect of rib height and related rib area on bond behavior; tests were conducted with or without the combined presence of external confinement over the embedded length. The novel specimen form presented in the paper was designed to simulate the state of stress arising in usual bar anchorages in the tension zones of flexural members where both cover concrete and bar are stressed in tension. This development was motivated by the need to eliminate spurious influences of the test setup on specimen behavior, which are known to interfere with bond mechanics in conventional bond tests leading to unconservative estimates of bond strength and misleading interpretations as to the true parametric dependencies of the bond problem. Additional parameters studied in the experimental program were the development length, the cover thickness, the effect of confinement, and the tensile strength of concrete. Data reduction local bond strength and slip estimates was possible by fitting the exact solution of the differential equations describing the state of stress along the anchorage to the test measurements, while accounting for important phenomena such as yield penetration or debonding, and bond plastification. Next, the obtained values for local bond strength were used for calibration of the frictional analog for bond strength a Mohr-Coulomb failure criterion. Milestone values for bond and slip were estimated with reference to the code limit-state model for bond.

Load-History Effects on Deformation Capacity of Flexural Members Limited by Bar Buckling

Buckling of reinforcement is one of the possible phenomena that limit the deformation capacity of reinforced concrete members under reversed cyclic loading. Previous experimental research suggests that occurrence of buckling is linked to displacement history, a parameter that is not explicitly accounted for in the available expressions for ultimate drift or curvature ductility capacities. This problem is explored in the present paper, by following through analytical expressions that relate the critical buckling strain as defined by the hysteretic stress-strain model of the reinforcement and the imposed cyclic history in terms of displacement. The analytical expressions thus derived are evaluated parametrically in order to establish behavioral trends. It is shown that when controlled by bar buckling, deformation capacity cannot be defined uniquely as it varies with the path of applied load. A primary conclusion of the research is that any quantifiable indices of deformation capacity referred to in the framework of displacement-based design using deterministic approaches need be adjusted to represent conservative lower bounds rather than approximations to the actual values of nominal failure.

Seismic Rehabilitation of Substandard R.C. Buildings with Masonry Infills

ABSTRACT Seismic deformation demands are localized in areas of stiffness discontinuity, such as in the soft storys of frame structures, where disproportionate damage is often reported in post-earthquake reconnaissance. In many parts of the world, this damage pattern is mitigated using strengthening schemes that include addition of stiffness in the structure so as to limit the magnitude of drift demands. A low-cost retrofitting method is the addition of masonry infills to increase the stiffness of soft storys in low- to mid-rise reinforced concrete (R.C.) structures. This is an easily replaceable remedy in the event of damage that may prove advantageous over R.C. structural systems, owing to the lower forces imparted to the foundation in this retrofit option as compared to more thorough interventions, thereby avoiding extensively invasive retrofit operations in the foundation. Behavioral mechanisms mobilized by masonry infills in successful retrofits are shown to emulate confined masonry behavior. It is also shown that despite their brittleness, well-connected infills can successfully mitigate the occurrence of catastrophic damage by diverting damage localization from the vulnerable regions of the building. The main objective of the current paper is to present a rapid retrofit design methodology, where masonry infills are utilized for strengthening existing substandard constructions in order for their R.C. load-bearing elements to behave elastically in the event of the design earthquake. To facilitate the retrofit design, practical design charts have been derived, to link drift demand to the ratios of infills’ area in plan to the total plan area in the critical floor of the structure. Performance criteria, such as target distributions of interstory drift demand, a target estimate of the fundamental period, as required by the designer, and a limit on acceptable displacement ductility in terms of demand for the retrofitted structure, are necessary design decisions that guide the proposed retrofit strategy. Application of the retrofit design through infills is demonstrated through example case studies.

Testing Procedure for Determining the Bond-Slip Law of Steel Bars in Strain Hardening Cementitious Composites

In order to advance the use of Strain Hardening Cementitious Composites (SHCC) in structural applications it is essential to understand the mechanics of bond of reinforcement anchored in this type of material. The strength of the cover against splitting as well as the strain development capacity of bars anchored in SHCC are studied in the present experimental study through tests conducted on specially designed tension pullout specimens. Parameter of study was the steel bar diameter and the anchorage length as well as the bar cover. The experimental results obtained from a series of 24 specimens were used to calibrate a mechanistic frictional model for bond along reinforcement anchorages embedded in SHCC. It was shown that the sustained tensile resistance of the SHCC up to large levels of tensile strain was engaged confining the bar-cover interface, thereby preventing through-splitting of the cover as would normally occur in plain concrete matrices. This enabled the contribution of a frictional component to the anchorage strain development capacity that is directly traceable to the deformation capacity of the SHCC matrix. This modelling approach was motivated by the experimental evidence where splitting failures may be mitigated even with short anchorage lengths and relatively small covers (cover thickness = bar diameter) up to large levels of pullout slip of reinforcement.

A MECHANISTIC APPROACH IN DEFINING INELASTIC ROTATION CAPACITY OF RC COLUMNS

When a reinforced concrete (RC) column is subjected to lateral sway as a result of earthquake action, the large strain demand in the end sections is supported by development of strains in the anchorage. This causes the bars to displace (or slip) relative to the anchoring concrete at the column fixed end(s). The lumped slip causes rigid-body rotation of the column, thereby alleviating partially the column deformation. This reinforcement slip is assumed to occur in the tension bars only and cause the rotation about the neutral axis. Development of flexural yielding and large rotation ductilities in the plastic hinge zones of frame members is synonymous with the spread of bar reinforcement yielding. Yield penetration in the anchored reinforcing bar inside the shear span of the column where it occurs, destroys interfacial bond between bar and concrete and reduces the strain development capacity of the reinforcement. This affects the plastic rotation of the member by increasing the contribution of bar slippage. In order to establish the plastic rotation in a manner consistent with the above definition, this paper uses the explicit solution of the field equations of bond over the shear span of a column. Through this approach, the bar strain distributions and the extent of yield penetration from the yielding cross section towards the shear span are resolved and calculated analytically. By obtaining this solution the aim is to illustrate the true parametric sensitivities of plastic hinge length as a design variable for practical use in seismic assessment of existing structures. Results obtained from the analytical procedures are compared with experimental evidence from tests conducted on reinforced concrete columns under seismic loading reported in the literature. 2739 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 2739-2752

VULNERABILITY OF TORSIONALLY SENSITIVE HISTORICAL BUILDINGS UNDER SEISMIC LOADS

Abstract. The paper explores the effects of asymmetry in plan owing to the occurrence of slender wings in massive monumental and historical URM buildings that belong to the postrenaissance neoclassical school of architecture. Although it is common in buildings with flexible diaphragms to disregard the implications of torsion, in this particular case these effects are significant causing important differential displacements and therefore a high risk of damage in the wings. To study the problem, three-dimensional finite element models of a neoclassical building with asymmetric plan shape located in Thessaloniki, Greece, are analyzed to a strong ground motion that has been recorded in the region and their maximum seismic response is explored. This response is then compared with results obtained from application of the rapid seismic assessment procedure of unreinforced masonry buildings, which has been recently introduced by the authors and conclusions on the procedure’s accuracy are derived. Emphasis is placed on the sensitivity of the torsional aspects of the response to the stiffness of horizontal diaphragms that is provided by the architectural forms of the era under investigation. Application of this rapid methodology enables easy inspection over the building’s geometry to identify those characteristics that may predispose the tendency for a strong torsional component of response, but it may also help identify the efficacy of retrofitting through the addition of diaphragms as a strengthening approach that may mitigate the occurrence of damage.