LITERATURE REVIEW
Despite a vast amount of experiments being carried out on silos over the past century the area of pressures within the silo are still not very well understood. Huge quantities of data are recorded during these experiments and it is up to the person carrying out these tests to decide what data should be looked at in greater detail and analysed. For the most part it is the highest/peak pressure which is recorded. Unsymmetrical pressure regimes dominate real failure modes, however most silo experiments assume that the highest observed pressure is the worst event. The presence of significant asymmetry in silo pressures was first recognised by Askengaard, Nielsen and Munch-Andersen (1, 2 & 3) following their work on full scale damaged concrete silos in Sweden. In ‘Critical Pressure Conditions in Silos’ (4) Rotter et al. propose that it is the unsymmetrical pressures and low pressures that represent the worst loading condition. From data collected during the testing carried out for this paper, the author noted that the structure has the ability to spread the effect of symmetrical local high pressures. This theory is backed up in much of their work carried out and papers produced such as ‘Guide for the Economic Design of Circular Metal Silos’ (5).
Some initial tests were carried out in silos containing only one pressure cell at each level. This gave a very inaccurate idea of the pressures acting within the silo. Over time the number of cells within a silo have increased, however this adds expense to the experiments, so the cells are widely spread out. The pressure values between measuring points can only be calculated by interpolation, but this involves large amounts of uncertainty. Over the past number of years new techniques have been developed to determine local unsymmetrical load patterns in a much more cost effective way. One method is described by J.F. Chen, J.M. Rotter & J.Y. Ooi (6). It uses the strains measured on the isotropic shell walls. This method is reasonably robust against random errors in strain readings and is believed to be one of the most comprehensive and detailed pictures of pressure distributions on silo walls.
One researcher who developed theories which are still used today is Janssen (1895). His theory is almost universally used as the single most reliable reference point even today. This theory is the main descriptor of filling pressures within the silo. One disadvantage to this theory is that it does not take into account the surface profile in defining wall pressures near the surface. This is important in squat silo geometries (EN 1991-4, 2007), (9). It was assumed that after filling the solid was in a Rankine active state, giving a low lateral pressure ratio, K, and leading to lower pressures, however, by the 1960’s it was widely recognised that this was an underestimate of K. Some experimentalists assumed the solid changed from an active state to a passive state during discharge. This change was termed as the “switch”. This theory is questioned more recently by Rotter, 1999 (7), who believes this change is much slower and the peak pressures are much less than expected using the switch theory.
One thing common to almost all papers is that the pressures during discharge of a silo are still poorly understood and can not yet be predicted with great certainty. Throughout our thesis we are hoping to get a better understanding of these pressures.
Note: The references for this review have been represented by a number in a bracket and these correspond to the references listed on the web site.
