Technical Paper Morteza Aboutalebi* Amir M. Alani Joseph Rizzuto Derrick Beckett
DOI: 10.1002/suco.201300043
Structural behaviour and deformation patterns in loaded plain concrete ground-supported slabs The work presented in this paper is considered to be an attempt to contribute towards a better understanding of the structural behaviour of plain concrete slabs under step loading conditions. The Concrete Society Technical Report TR34 “Concrete Industrial Ground Floors” is in its 3rd edition (2003) and is currently under review. TR34 covers the design of concrete ground-supported slabs containing fibres, both steel and synthetic, as an alternative to mesh reinforcement. This work reports on tests carried out at different critical loading locations, including the centre, edges and corners of a 6.0 × 6.0 × 0.15 m deep plain concrete slab. The test results are compared with theoretical values derived using available design codes and other information sources. The results show a notable difference between the test results and the theoretical values. Keywords: ground-supported slab, displacement, crack propagation, bending, punching
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Introduction
More than 30 tests on ground-supported slabs were undertaken at the University of Greenwich between 1989 and 1999 using a test rig capable of applying a concentrated load of up to 600 kN (60 t) at any location within an area having maximum dimensions of 11 m long and 3 m wide. The results of these tests were summarized in the proceedings of the 4th and 5th International Colloquiums on Industrial Floors [1], [2]. Further tests using an upgraded ground slab test rig with similar loading capabilities but significantly larger maximum dimensions, 12 m long and 6 m wide, were published in the proceedings of the 2007 Industrial Floors Colloquium [3]. The majority of the test slabs had dimensions of 3 × 3 × 0.15 m deep and were subject to internal, edge and corner loading. In all cases, the loading plates were 100 × 100 mm, which was intended to simulate single- or double-racking leg loads. The loading tests included plain concrete, fabric (mesh), steel and synthetic fibre reinforcement. The majority of tests were on slabs containing steel fibres of variable geometry, including plain, undulating and hook-ended, with fibre contents ranging between 20 and 40 kg/m3.
These tests and others [4] formed the basis for an appendix in the 2nd edition of TR34 (1994) [5], which introduced a plastic approach to the thickness design of concrete ground slabs based on the work of Meyerhof [6]. The 1st edition of TR34 (1988) [7] used an elastic analysis approach for slab thickness design based on the pioneering work of H. M. Westergaard (1926) [8]. The 3rd edition of TR34 was published in March 2003 and the sections on thickness design and the worked examples were undertaken by Beckett and Clarke [9]. These were in an ultimate limit state format and in line with the draft version of Eurocode 2 [10]. The 3rd edition of TR34 pays greater attention to crack control, deflections and load transfer across joints. In addition, significant emphasis is paid to the use of fibres, steel and synthetic, as an alternative to fabric (mesh) reinforcement. The final version of Eurocode 2 [11] was published in 2004 and now incorporates the Jan 2008 corrigendum. The 2003 edition of TR34 is currently being revised and there are several issues that need resolving. These include the fact that the equation for the characteristic flexural strength of plain concrete in the 2004 edition of EC2 gives significantly lower values than those given in TR34 (2003), which uses the draft EC2 formula. TR34 (2003) recommends that the characteristic flexural strength of plain concrete should be taken as fctk.fl = [1 + (200/h)0.5] fctk(0.05) ≤ 2 fctk(0.05) Eq. 9.1 TR34 (2003) where: h total slab thickness in mm (h > 100 mm) fctk.fl characteristic flexural strength of plain concrete fctk (0.05) characteristic axial tensile strength of plain concrete (5 % fractile) EC2 (2004) recommends that the following relationship may be used for calculating the mean axial tensile strength of reinforced concrete: fctm(fl) = max [(1.6 – h/1000) fctm]
Eq. 3.23 EC2 (2004)
* Corresponding author: aboutalebi@greenwich.ac.uk
where: h fctm
Submitted for review: 12 June 2013 Revised: 12 June 2013 Accepted for publication: 6 July 2013
The relation given in Eq, (3.23) also applies for the characteristic tensile strength values. Other issues include the
total member depth in mm mean axial tensile strength (TR34, Table 3.1)
© 2014 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin · Structural Concrete 15 (2014), No. 1
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