Creep of concrete is a complex phenomenon that has proven difficult to model. Nevertheless, for many reinforced and prestressed concrete applications, a reasonably accurate prediction of the magnitude and rate of creep strain is an important requirement of the design process. Although laboratory tests may be undertaken to determine the deformation properties of materials, these are time consuming, often expensive and generally not a practical option. In addition, this is not often an option at the design stage of a project when decisions about the actual concrete to be used have not yet been taken. National design codes therefore rely on empirical prediction models to estimate the magnitude and development of the creep strain. This paper considers the suitability of nine 'design code type' creep prediction models when compared with the actual strains measured on a range of concretes under laboratory control conditions. The concretes tested incorporate three aggregate types and two strength grades for each aggregate type. The results are compared with the predictions of creep using models contained in BS 8110 (1985), SABS 0100 (1992), SABS 0100 (1992) modified, ACI 209 (1992), AS 3600 (1988), CEB-FIP (1970, 1978 & 1990), the RILEM Model B3 (1995) methods. The results indicate that the CEB-FIP (1970) and BS 8110 (1985) methods provide suitably accurate predictions over all the concretes tested. These methods yielded overall coefficients of variation of approximately 18 % and 24 %, respectively. The least accurate method was the CEB-FIP (1978) which yielded a coefficient of variation of approximately 96 %. The results of this investigation led to recommending the BS 8110 (1985) model for South African conditions.
Ambiguities associated with shear in reinforced concrete have led to a misunderstanding of what shear is and how it affects structural members. This paper therefore explores the notion of shear, using mechanics and finite elements as a tool to describe this force. A series of postulates are also presented in order to define the nature and characteristics of shear. The theory is then applied to structural elements such as beams and slabs. Various shear concepts, such as shear enhancement, shear in slabs and punching shear are also examined.
In timber roofs, multiple-ply trusses are placed next to each other and the web members are held together with nails. This paper presents various methods, among which is a simple spring model, whereby the effective stiffness of these composite web members may be calculated, based on the stiffness of the connectors and the individual web members. The spring model can be used for any number of connectors and members as long as the members are loaded within the elastic range of both the members and the connectors. Results from a limited number of finite element simulations show that this method has merit. It makes it possible to determine the bending and axial stiffness to an acceptable degree of accuracy. The paper also addresses the calculation of the stresses in the individual members.