Timothy Ibell

Shear design of circular concrete sections using the eurocode 2 truss model

The introduction of the Eurocodes for concrete design will alter the way that shear is approached for concrete structures. BS EN 1992-1-11 has adopted the variable angle truss model for shear, a more theoretically consistent approach than that used in BS 8110-12. The model is confidently applied to rectangular sections, but its applicability to irregular sections is less clear. In particular, the behaviour of circular concrete sections is not well defined. This paper is intended to satisfy a requirement for design guidance on this topic that has been recognised by key BSI Committees. Using both experimental and theoretical data, the Eurocode variable angle truss model for shear design is assessed and extended to circular columns.

Novel shear reinforcement for Fiber-Reinforced Polymer-Reinforced and prestressed concrete

This article presents the findings of research conducted into the shear behavior of fiber-reinforced polymer (FRP)-reinforced and FRP-prestressed concrete beams containing continuous FRP helical transverse reinforcement. The authors contend that for FRP materials to proliferate as internal reinforcement and prestressing tendons for concrete structures, they should be used rationally, lest the indisputable advantages that these materials possess be lost due to poor design. The article reports on 12 tests that were conducted on ordinary reinforced beams and 15 that were conducted on FRP-prestressed concrete beams. The results showed that full-depth unbonded rectangular helixes were more effective than unbonded circular helixes for equal quantities of material. When used to resist shear, fully unbonded circular and rectangular helixes had to be spaced at a closer pitch in comparison with fully bonded or intermittently bonded rectangular helixes, to provide a similar increase in failure capacity. In addition, prestress aids FRP-reinforced concrete beams to a greater extent in increasing shear capacity than it does in the case of the equivalent steel-reinforced situation. The authors conclude by presenting comparisons between experimental results and existing design guideline predictions, demonstrating the conservative nature of the ACI-440.1R-03 shear clause.

Innovative Reinforcement for Fabric Formed Concrete Structures

Using fabric formwork, it is possible to cast architecturally interesting, optimised structures that use up to 40% less concrete than an equivalent strength prismatic section, thereby offering significant embodied energy savings. This paper reports on the latest techniques for the design, optimisation and shape prediction of fabric formed concrete beams before new test results of an innovative anchorage method for both steel and fibre reinforced polymer longitudinal reinforcing bars are presented. Two 2m span beams were tested and the ‘helically confined splayed bar’ was shown to provide full anchorage in both cases. The two beams both exceeded their design capacity and showed remarkably similar behaviour at the serviceability limit state, with the steel reinforced section going on to display considerable ductility. Potential areas of future development are then highlighted, with the use of woven advanced composite fabrics as participating formwork for both beam and shell elements being of particular interest.

Innovative concrete structures using fabric formwork

© 2010 Taylor & Francis Group, London. This paper examines the state of the art in fabric formed beam design, providing a summary of previous work, experimental data and optimisation processes. The future of fabric formwork is considered and current work at the University of Bath is presented.

Behavior of Fiber-Reinforced Polymer-Reinforced Anchorage Zones for Post-Tensioned Concrete Structures

Due to their corrosion resistance and strength-to-weight ratio, fiber-reinforced polymers (FRP) have been suggested as an alternate to steel for use in structural concrete. The relatively low stiffness of FRP, compared with steel, implies that concrete structures containing FRP ought to be prestressed. In this way, the serviceability of the structure is improved, and higher ultimate strength is attained. Much research into FRP-prestressed concrete has now been conducted, but the design of FRP-reinforced posttensioned anchorage zones has received very little attention. This research concentrates on investigating the feasibility of using aramid FRP (AFRP) helical reinforcement in local anchorage zones for wholly nonmetallic, efficient posttensioned concrete structures. 47 patch-loaded concrete specimens (of circular and rectangular cross section) have been tested and relevant results are provided herein. Lab-made AFRP has been used as reinforcement in a circular helical form. Test results are compared with predictions based on an existing design approach that has been modified for the case of FRP reinforcement. It is shown that the relative diametrical dimension between the reinforcing helix and the bearing plate is a critical factor, influencing the post-elastic behavior. Further, by combining helical and mat reinforcing systems, both higher ultimate capacity and real ductility of the anchorage zones are achieved.

STRENGTH AND BEHAVIOR IN SHEAR OF CONCRETE BEAM-AND-SLAB BRIDGES

Shear tests on four concrete beam-and-slab bridge specimens are reported. Quantities of longitudinal steel and shear stirrups were varied in an attempt to determine the strength and behavior in shear. These tests show that significant enhancement in shear strength of the beams is provided by the surrounding concrete slab. In addition, critical shear collapse mechanisms are described. These failure patterns have been used as the basis for an upper-bound plasticity approach to the problem of shear assessment of concrete beam-and-slab bridges.

Prediction of Capacity for Moment Redistribution in FRP-Strengthened Continuous RC T-Beams

AbstractBecause of the premature debonding of fiber-reinforced polymer (FRP) materials that results in a reduction in ductility, the problem of how to exploit moment redistribution (MR) in FRP-strengthened continuous reinforced concrete (RC) structures is unresolved. To date, limited research has been conducted into MR in such structures; a reliable and rigorous solution for quantifying MR throughout the loading cycle remains elusive. This paper aims to quantify MR and predict the capacity at reasonable accuracy, to encourage the use of FRP for the strengthening of existing continuous RC structures. Experiments conducted on 12 continuous T-beams are reported, and the findings are discussed. Strengthening configuration and anchorage scheme are the main variables. A new analytical strategy is described for quantifying MR, and the analytical results are then validated against the experimental results. Both experimental and analytical results confirm that there is no reason to restrict MR into strengthened zo...

Fabric formed concrete: physical modelling for assessment of digital form finding methods

© 2016, A.A. Balkema Publishers. All rights reserved. Fabric formwork is a novel concrete construction method which replaces conventional prismatic moulds with lightweight, high strength sheets of fabric. The geometry of fabric formed structures is therefore dictated by the behaviour of fabric under hydrostatic loading. While there are numerous examples of digital and physical modelling of this problem, there have only been limited efforts to link the two through measurement. In this investigation, a number of small scale fabric formed beams were manufactured using both ‘free hanging’ and ‘keel mould’ methods, and the resulting forms were accurately measured with a digital 3D scanner. Computational form finding tools were also developed, enabling a comparison to be made between the predicted and build geometries. This allowed assessment of both the accuracy of the construction methods and the limitations of the form finding techniques used. The data collected provides a useful assessment of existing form finding techniques and will be used as a reference data set as these are developed further.

Shear strength theories for beams of variable depth

Flexibly formed reinforced concrete beams usually have varying cross sections along their longitudinal axis, capitalising on the fluidity of concrete to create optimised geometries. According to Orr et al. [1], these new shapes have led to challenges for shear design, especially when the depth of the beams is relatively small. It is crucial to be able to accurately determine the shear strength of such beams to maintain structural safety whilst achieving material optimisation. The effective shear force method is adopted for tapering beams in many design codes. Recent work by Paglietti et al. [2] has highlighted concerns over the use of such an approach. In this paper, the theoretical basis for stress distributions in tapered beams built by Timoshenko [3] and Oden [4] in their elastic range is reviewed and then extended to included cracked behaviour. It is found that the effective shear force method used in design codes does not accurately account for the stress distribution in a section both in elastic and cracked stage of concrete, underestimating the peak shear stress for beams with inclined soffits. This is important for flexibly formed beams, and has implications for designers As a result of this work, a new calculation and design method for shear reinforcement is proposed. Keywords: variable depth beam, shear strength, shear stress distribution, flexible formwork.

Fabric formwork for ultra high performance fibre reinforced concrete structures

Using fabric formwork, it is possible to cast architecturally interesting, optimised structures that use up to 40% less concrete than an equivalent strength prismatic section, offering potentially significant embodied energy savings in new concrete structures. Fabric formwork allows elegant designs to be realised but its use also presents some unique challenges, including the practical provision of transverse reinforcement in slender, non-prismatic beam elements. This paper details how these challenges have been overcome with the use of ultra-high performance fibre reinforced concrete. Methods for the design, optimisation and construction of such elements cast using fabric formwork are illustrated and structural tests data are presented.