Simple and explicit formulae are presented for the analyses of the accumulated dead load deflection óda, and the instantaneous live load defection ój; they are applicable to both reinforced and partially prestressed concrete beams under repeated loading. The total deflection at a given number of load repetitions is obtained by summing óda and ój. In the analysis, óda, is computed as the product of the initial dead load deflection, ój, and the intensive creep factor k. For reinforced beams, a semi-empirical formula for k has been presented previously. With the use of the additional deflection data from five partially prestressed box bearns tested herein, a unified equation for k is derived statistically. An interpolation formula developed earlier for estimating the equivalent moment of inertia (hence ój) of reinforced beams is confirmed to be equally applicable to purtiully prestressed beams under repeated loading. In the light of the authors’ own experimental data, and those published by other researchers, comparisons of results are made which indicate that the overall accuracy of the proposed procedure is good. It is also more versatile and more reliable than two other recently published methods of analysis. Y.C. Loo and Y.W. Wong
The fatigue strength of composite in situ and precast concrete slabs has been studied following the failure of a suspended floorslab at a West Midlands factory. Nine short (600 mm long) lengths cut from standard precast units (400 mm wide) have been placed side by side and used to cast 17 ‘transverse’ specimens 600 mm wide tested on a simple span of 3.18 m (i.e. normal to the main span direction of the standard floors). Professor B.P. Hughes and C. Dundar
When the voussoirs of a masonry arch are idealised as a set of rigid blocks abutting against each other, classical energy and virtual work principles commonly used in structural analysis are not valid for determining either equilibrium or stability. This paper establishes that stable equilibrium is achieved through a V-shaped, rather than a U-shaped, curve of potential energy against displacement. The possibility that such a form of equilibrium may exist is not mentioned in textbooks or papers on structural analysis, as far as the author is aware. Yet recognition of this form of equilibrium is essential to the understanding of masonry arch stability, since the use of the second differential of potential energy to investigate stability gives the incorrect answer that all arches except very thick ones are unstable even when subject to just their own self-weight. Professor A. Jennings
This paper gives details of a joint research project into the economic design of M-beam bridge decks. The results are presented of serviceability tests on the standard 160 mm reinforced concrete slab of a M-beam deck with the beams at up to twice the normal spacing. Within the slab the level of reinforcement ranged from 0.25% to 1.7% with the bars at a constant spacing of 150 mm. J. Kirkpatrick, G.I.B. Rankin and Professor A.E. Long