M. Haskett

Mechanics-Based Hinge Analysis for Reinforced Concrete Columns

AbstractThe lateral deformation behavior of a RC column is particularly important because it not only magnifies the moment but also affects the ability of the column—and, subsequently, the frame—to sway and absorb energy at all stages of loading. The lateral deformation is affected by disturbed regions, such as tensile cracks or compression wedges, which are often simulated with the help of hinges whose properties are derived empirically. Being empirical, these hinges can only be used within the bounds of the tests from which they were derived, and in this respect are of limited use. In this paper, a mechanics-based hinge is developed that can be used at all stages of loading (that is, at serviceability through to ultimate) and also during failure. The mechanics-based model is based on the principle of plane sections remaining plane, shear-friction theory that quantifies the behavior of RC across sliding planes, and partial-interaction theory that allows for slip between the reinforcement and the encasing...

FRP-Reinforced Concrete Beams: Unified Approach Based on IC Theory

In general, steel-reinforced concrete involves a ductile steel material and a very strong and ductile bond between the steel reinforcement and concrete, so that debonding rarely governs the design. In contrast, fiber-reinforced polymer (FRP) reinforcement is a brittle material with a weak and brittle bond, making debonding a major issue. Consequently, there has been an extensive amount of research on FRP debonding and in particular intermediate crack (IC) debonding. This paper shows that the very good research by the FRP research community on the mechanics of IC debonding can be applied to a wide range of apparently disparate reinforced concrete behaviors to produce a unified approach. Hence, a single mechanism, or unified approach, based on IC debonding is proposed in this paper for dealing with moment rotation, tension stiffening and deflections, member ductility and moment redistribution, shear capacity, confinement, and fiber concrete for FRP RC beams.

Characteristics of Confined Blast Loading in Unvented Structures

Confined blast loading occurs in many scenarios and the effects of confined blast loading may result in more serious damage to buildings due to multiple shock reflections (Shi et al. 2009). However, spherical charges are assumed for all confined explosive-effects computations in modern standards for blast-resistant design such as UFC-3-340-02 (2008) and the soon-to-be published ASCE Standard for the Blast Protection of Buildings (ASCE forthcoming) without consideration of effects of charge shape on the distribution of reflected overpressure and impulse. As confinement is an aggravation factor of explosion effects, analysis and design of infrastructure under critical scenarios of confined blast loading should take the aggravation factor into consideration. This paper is to develop a numerical model for prediction of blast loads inside unvented structures as a result of variation of the charge shape, charge orientation, geometries and volumes of confined chambers. A finite element program, AUTODYN (Century Dynamics, 2003), is utilized extensively to generate a model which is capable of being calibrated with the experimental results conducted by Wu et al. (2010) in external conditions and by Zyskowski et al. (2004) in a confined small box. The calibrated AUTODYN model is then used to conduct parametric studies to analyze the effects of the variation of charge shape, charge orientation, chamber geometry and chamber volume on the peak reflected overpressure and impulse on the walls of the chamber. The quasi-static overpressure for fully confined blast loading is characterized and the simulated results are used to derive the relationships between the quasi-static overpressure and scaled distance for the fully confined blast loading. Discussion is made on characteristics of fully confined blast loading inside chambers.

Derivation of Normalized Pressure Impulse Curves for Flexural Ultra High Performance Concrete Slabs

In previous studies, a finite-difference procedure was developed to analyze the dynamic response of simply supported normal reinforced concrete (NRC) slabs under blast loads. Ultra high performance concrete (UHPC) is a relatively new material with high strength and high deformation capacity in comparison with conventional normal strength concrete. Therefore, the finite-difference procedure for analysis of conventional reinforced concrete members against blast loads needs to be significantly adapted and extended to accommodate UHPC. In this paper, an advanced moment-rotation analysis model, employed to simulate the behavior of the plastic hinge of an UHPC member, is incorporated into the finite-difference procedure for the dynamic response analysis of reinforced UHPC slabs under blast loads. The accuracy of the finite-difference analysis model that utilized the moment-rotation analysis technique was validated using results from blast tests conducted on UHPC slabs. The validated finite-difference model was then used to generate pressure impulse (PI) curves. Parametric studies were then conducted to investigate the effects of various sectional and member properties on PI curves. Based on the simulated results, two equations were derived that can be used to normalize a PI curve. Further numerical testing of the normalization equations for UHPC members was then undertaken. The generated normalized PI curve, accompanied by the derived normalization equations, can be used for the purposes of general UHPC blast design.

Our obsession with curvature in RC beam modelling

Much of the early research in reinforced concrete dealt with steel reinforcement that was both ductile and had a very strong bond with the concrete. Hence partial-interaction, that is slip between the reinforcement and concrete and subsequently debonding, has not been a major issue. This has allowed researchers to develop the two-dimensional full-interaction moment-curvature approach to model the three-dimensional behaviour of reinforced concrete. It is shown in this paper that this two-dimensional full-interaction moment-curvature approach relies on a large amount of empirical calibration to ensure a safe design. Furthermore, it is shown that a three-dimensional partial-interaction moment-rotation approach can lead to more advanced structural mechanics models of reinforced concrete behaviours and subsequently better accuracy and more versatile models.

Design for Moment Redistribution in RC Beams Retrofitted with Steel Plates

It is now common practice to retrofit reinforced concrete members by adhesively bonding steel or fibre reinforced polymer plates to their surfaces. However, tests have shown that these plated RC structures tend to have less member ductility, or rotational capacity, than the unplated structure because of premature plate debonding. In this paper, structural mechanics approaches are described for both: quantifying the moment rotation capacity, or member ductility, of steel plated RC flexural members; and quantifying the moment redistribution capacity from the moment rotation capacity. It is shown how the moment redistribution structural mechanics model can be used to design for member ductility directly and, furthermore, it is applied to both externally bonded and near surface mounted steel plates. As would be expected, it is shown that steel plating produces more ductile members than fibre reinforced polymer plating.

Analysis of Moment Redistribution in Fiber-Reinforced Polymer Plated RC Beams

Ductility of RC structures has always been a classical area of concrete research. Given the complexity of the problem, the great mass of research investigating ductility, and specifically, moment redistribution and rotational capacities, has used empirical approaches to quantify moment redistribution and invariably assumed that concrete crushing is the singular mode of failure. With the advent of new reinforcement materials such as fiber reinforced polymers, these empirical approaches are not necessarily appropriate as failure modes other than concrete crushing can occur. In this paper, the empirical approaches to moment redistribution are replaced by a structural mechanics approach that incorporates moment rotation directly into moment redistribution. A structural mechanics method for determining moment and rotation at failure for any RC section with any material properties is first presented and this is followed by a structural mechanics model for moment redistribution; these enable the moment redistribution capacities of any RC section to be quantified. Moment redistribution capacities of various sections are analyzed and it is shown that plated sections can have significant moment redistribution capacities much of which can be used in design.

Moment Redistribution in RC Beams Retrofitted by Longitudinal Plating

The ability to redistribute moment within a reinforced concrete frame or structure is an intrinsic requirement in design. This is generally dealt with using the established neutral axis depth factor, the ku approach, with fixed hinge lengths, which require the flexural member to fail by concrete crushing and which, in turn, requires large strains in the tension reinforcement. As longitudinally plated flexural members tend to fail by plate debonding or plate fracture before the concrete crushes, an alternative approach is presented which is based on variable hinge lengths, which can cope with beam failure at any tension reinforcement strain, and which can be applied to both longitudinally plated and unplated structures.

Discrete Rotation in RC Beams

AbstractThe ability of a RC beam to rotate is extremely important at both the serviceability and ultimate limit states as it affects deflection, moment redistribution, and the absorption of energy. The rotation in a RC beam can be considered to have two components: the continuous rotation that occurs in the homogenous portions of the beam and that can be determined by straightforward integration of the curvature and the discrete rotation due to the rigid body displacement between crack faces. In this paper, generic closed-form solutions are derived for the discrete rotation across crack faces, which are limited by debonding or fracture of the reinforcement in tension, or softening of the concrete in compression. It is shown how closed-form solutions can be derived for the moment/discrete rotation for the simplest case of a RC beam with a single layer of reinforcement, which could be used as the starting position for much more complex scenarios. The technique developed is generic as it can cope with any ty...

Influence of Bond on the Hinge Rotation of FRP Plated Beams

Fibre reinforced polymer (FRP) plate reinforcement is a brittle material which has a brittle interfacial bond with concrete. This can lead to the misconception that all FRP retrofitting techniques provide brittle members and, hence, limited rotational capacity which has severe limitations for structural applications. This paper shows that the FRP reinforcement behaviour is but one of three components that govern the rotational capacity of plated reinforced concrete beam hinges. It is shown that FRP retrofitted beams and slabs can achieve ductile behaviour and provide rotational capacity and, furthermore, that the rotational capacity of FRP plated members depends very importantly on the interface bond characteristics.