Robert L. Vollum

Punching of Reinforced Concrete Flat Slabs with Double- Headed Shear Reinforcement

Twelve slabs, 11 of which contained double-headed studs as shear reinforcement, were tested supported by central column and loaded concentrically. Their behavior is described in terms of deflections, rotations, strains of the concrete close to the column, strains of the flexural reinforcement across the slab width, and strains of the studs. All failures were by punching, in most cases within the shear reinforced region. The treatments of punching resistance in ACI 318, Eurocode 2 (EC2), and the critical shear crack theory (CSCT) are described, and their predictions are compared with the results of the present tests and 39 others from the literature. The accuracy of predictions improves from ACI 318 to EC2 to CSCT—that is, with increasing complexity. However, the CSCT assumptions about behavior are not well supported by the experimental observations.

Bearing Capacity of Partially Loaded Concrete Elements

There are numerous situations where compressive loads are introduced into members over small areas. The stress distribution in these zones, which are depicted D for disturbed, is non-uniform. The flow of compressive stresses into the surrounding member induces transverse tensile stresses which can cause premature splitting failures. D regions are typically designed using the strut-and-tie method (STM). The STM design recommendations provided by ACI 318 (2014), fib Model Code (2010) (2013) and Eurocode 2 (2004), limit bearing stresses to admissible values which depend on a strut efficiency factor (β s ). This paper uses a database with 162 tests on partially loaded concrete prisms without and with crack-control reinforcement to evaluate these provisions and to discuss the influence of parameters like the concrete strength (f c ), the crack-control reinforcement ratio (ρ), concentration ratio (b/a) and aspect ratio (h/b). It also presents results of four tests on diamond-shaped concrete prisms carried out to evaluate the effect on resistance of increasing ρ. Experimental results show that crack-control reinforcement increases both ductility and resistance. Existing design provisions are shown to be conservative.

On the Distribution of Shear Forces in Non-axisymmetric Slab-Column Connections

The majority of punching tests, based on which the provisions for slab-column connections in design codes are developed, were performed on symmetric test specimens where the support reaction was centric with respect to the supported area. However, most slab-column connections in practice are subjected to asymmetric loading. This may lead to non-uniform distribution of bending moments, deformations and shear forces on a control perimeter around the column or transfer of bending moments between the slab and the column. A non-linear finite element approach was developed in this research to investigate the distribution of internal forces around the columns in non-axisymmetric connections. The focus of this paper is on analyzing the interaction between redistributions of bending moments and shear forces while maintaining the equilibrium of internal forces in the slab. Selected test specimens from the literature were modelled with different types of asymmetric loading. The modelling results show that shear redistribution along the perimeter of the connection, which would lead to a more uniform shear distribution, is possible only in certain cases depending on the bending moments and flexural capacity of the slab. It is also shown that uneven distribution of shear forces can also occur in connections with no eccentricity, being related to the non-linear flexural behavior of the slab. The results obtained from the analyses are compared to the approaches suggested in the design codes or by other authors.

Investigating the Need for Long Laps in Reinforced Concrete Elements

The current Eurocode 2 (EN 1992-1-1) detailing rules can lead to considerably greater lap and anchorage lengths than previous design recommendations such as the superseded British Standard BS 8110-1. Moreover, fib Model Code 2010 (MC 2010) requires even longer laps than EN 1992-1-1. This research is motivated by complaints from industry that designing to the current EN 1992-1-1 detailing rules leads to numerous construction issues such as reinforcement congestion, as well as cost and sustainability implications, with no apparent justification. This paper presents the experimental programme which was conducted by the authors with a view to justifying reduced lap lengths more commensurate with previously proven UK experience. To this end, a series of three point bending (3PB) and four point bending (4PB) tests were designed and tested in the Structures Laboratory at Imperial College London to investigate bond stress distributions along laps of different lengths consisting of lapped reinforcing bars of the same or different bar diameters. In particular, experiments were aimed at quantifying the effectiveness of very long laps in transferring forces between two lapped bars. The 3PB tests were aimed at investigating whether anchorage capacity of laps is enhanced at high shear locations.

Nonlinear Finite Element Analysis of Reinforced Concrete Flat Slabs Subjected to Reversed-Cyclic Loading

Flat slabs are only permitted to be used as gravity-load carrying systems in regions of high seismicity because of poor resistance to lateral deformation and punching shear under reversed cyclic loading. This paper considers the influence of reverse cyclic loading on the punching resistance of internal slab column connections without shear reinforcement. Currently, ACI 318-14 determines the deformation capacity of slab-column connections using a best-fit line based on test data from relatively thin slabs, with average thickness of 110 mm, and flexural reinforcement ratios of around 1%. Consequently, the ACI 318-14 (2014) design recommendations require further validation for slab thicknesses and reinforcement ratios outside this range. A possible tool for doing this is the mechanically-based critical shear crack theory (CSCT) of Muttoni (2008). The model is based on considerations of equilibrium and kinematics for an isolated axis-symmetrical slab. The model gives good predictions of punching resistance under concentric loading but its applicability to the design of flat slabs subject to reversed-cyclic loading requires further consideration. The paper presents the results of a parametric study which was carried out with the finite element program ATENA (Cervenka et al. 2007) in order to obtain an improved understanding of the influence of cyclic degradation on punching resistance. Maximum slab rotations are shown to increase under cyclic loading with a consequent degradation in unbalanced moment resistance and ultimate slab rotation. This finding is consistent with the predictions of the CSCT.

Headed Bar Connections Between Precast Concrete Panels Loaded in Bending

As part of a wider study into headed bar connections, this research concerns tests on precast concrete specimens connected by means of a joint developed and patented by Laing O’Rourke, known as the “E6 joint”. The joint consists of lapped headed bars that, along with additional vertical shear studs and transverse bars used as confining reinforcement, allow a narrow joint width of only 200 mm to be used. The first part of this research included testing of small scale joint specimens subjected to direct tension, while the specimens presented here are tested in flexure. Variables tested include; the concrete strength in the joint, vertical misalignment of the precast panels, and absence of shear studs. Results show that tests in bending achieve a higher ultimate capacity than calculated from strengths of corresponding direct tension tests, whilst showing similar failure mechanisms. The experimental results are used to validate a three dimensional non-linear finite element model (NLFEM) of the joint. A strut-and-tie model (STM) used to determine the E6 joint design strength has been found previously to give safe predictions for specimens in tension whilst not fully capturing the observed joint behaviour. The STM gives even more conservative results for specimens in bending. The results presented in this paper will be used in future research to further optimise the joint design and reduce the degree of conservatism associated with the design model.