Jm Rotter

SYSTEMATIC AND RANDOM FEATURES OF MEASURED PRESSURES ON FULL-SCALE SILO WALLS

Abstract It is well known that the pressures exerted by bulk solids on silo walls vary between filling and discharge. It is less frequently observed that these pressures vary all over the silo wall both in space and in time during discharge. The pressure at any point may be understood as having two components: one systematic and the other random. The systematic component relates to the stored solid, the silo geometry, geometric imperfections in the silo wall, the eccentricity of filling and discharge, and the flow pattern. It can be attributed to causal phenomena. The random component leads to pressure variations with time at any point on the wall which are not related to each other and which are normally ignored in design. In this paper, careful measurements of pressures on the walls of a prototype silo subject to both concentric and eccentric flow are studied to extract the systematic and random components of pressure. The form of the systematic pressure distribution and the magnitudes of the random variations are related to Janssen pressures. The results lead to pressure distributions which can be used as a more rigorous basis for future silo design codes.

The sensitivity of silo flow and wall stresses to filling method

The pattern of solids flow during discharge of a silo and the pressures against the silo wall are commonly assumed to depend only on the measurable properties of the solid, including its interaction with the silo wall. A small number of previous studies have noted that the method of filling the silo may affect the flow pattern, generally altering it from mass to mixed flow. It has been suggested that this change may be caused by altered mechanical properties of the solid. However, the significance of the filling method for the practical design of silos has rarely been considered. In this experimental study, barley and plastic pellets were tested in a pilot scale aluminium silo with a flat bottom. The paper outlines the results of the experiments, in which flow pattern observations were made from the top surface, and wall stresses were measured using strain gauges. The filling process is seen to have a marked effect on the flow pattern and wall stresses found for plastic pellets, but very little effect on those for barley. The implications for silo design are discussed briefly.

Experiments on a square planform steel silo

Rectangular planform silos have not been studied as often as their circular counterparts, and relatively little detailed information exists in the literature on pressure patterns which develop on their walls. This study used a pilot-scale square planform funnel flow unstiffened steel silo with relatively flexible walls to investigate the patterns of pressures on filling, during storage and during discharge. Two different free-flowing solids, sand and pea gravel, were used. The silo was instrumented with wall pressure cells, free field cells, strain gauges and displacement devices. From the extensive collected data, some examples of measured pressures during filling and discharge are presented, together with an introduction to the effects of eccentric discharge.

Measurement of solids flow patterns in a gypsum silo

Abstract The pattern of solids flow out of a silo has a strong influence on the wall pressures, and consequently on the silos structural integrity. However, measuring the pattern of flow is very difficult because the material is opaque and movement cannot be easily detected from outside the structure. Most flow pattern studies have used laboratory models but exploration of these observations to full scale industrial sios is most uncertain. The flow patterns described occurred in a recently constructed 250 tonne steel silo of circular planform storing ground gypsum mineral powder. Radio frequency tags were placed in a systematic pattern in the solid using a specially designed deployable seeding device. Many tags were accurately placed in the solid at different levels, giving up to 300 measures of the movement of the solids during discharge. Three methods (contours of residence time, contours of mean velocities and computer visualisation) were used to infer the flow pattern from the radio tag marker data. The tests show that commercial discharge aid can have a major pattern from which is different from any documented in the literature.

Patterns of wall pressure on filling a square planform steel silo

Abstract This paper reports the filling pressures measured in a rectangular planform silo with flexible walls. Many experiments were conducted on a pilot-scale square unstiffened steel silo with a pyramidal hopper. The stress state in the solid stored in the silo was thoroughly investigated during filling using both wall pressure cells and free field pressure cells. The results show that the pressure distribution at any given depth at the end of filling may be far from uniform, though most existing silo pressure theories and design codes assume uniformity in any horizontal plane. High pressures were found to develop in the corners of the silo, with corresponding lower pressures at the centre of the walls. An explanation is offered for the observed pressure pattern and a design proposal is made.

Statistical inference of unsymmetrical silo pressures from comprehensive wall strain measurements

Abstract A thin cylindrical shell structure which is subjected to local or unsymmetrical loading often displays a very complex pattern of response, involving multiple alternative potential failure conditions in different parts of the structure. The loading may therefore need to be defined with great precision. In the field of silo structures, it is widely recognised that such local loads often exist, but experimental observations of the patterns of load are very difficult to obtain because of the expense of instrumentation and the need to use full-scale testing to avoid granular solid scale errors. This paper presents a newly developed technique which permits these local unsymmetrical load patterns to be determined in a much more cost-effective way. In addition, because the loading is deduced from the structural response, the method has an inherent robustness in that when the deduced loadings are generalised and used to predict a structural response, it is more likely to be close to the real response. The same cannot be said for loading patterns deduced from single discrete observations of loading with imaginative interpolations between them, which form the basis of most current design rules. The paper describes a rigorous procedure for inferring the complete pressure distribution from a large body of strain observations on the silo wall. The method is outlined and a simple practical example, involving unsymmetrical loads, is used to explore the effect of observation errors on the inferred pressures. A sample set of pressures in a specially built full-scale test silo under eccentric solids discharge is also derived.

Elastic predictions of pressures in conical silo hoppers

Abstract Theoretical techniques for predicting the pressures on the walls of conical silo hoppers have generally depended on the assumption that the mass of material within the hopper is in a plastic state of stress. The fact that this imposes considerable restrictions on the stress history and strain state has almost always been ignored. Moreover, few investigators have undertaken checks to verify the assumption. In this study, the alternative simple assumption is made that the material stored within the hopper is in an elastic state. The study is heuristic in character, as it is not claimed that an elastic state does exist within the hopper, though the assumption is not unreasonable for the initial filling condition. Nevertheless, a number of interesting discoveries are made about the stress distribution within the stored mass, and concerning the influence of various parameters on the hopper wall pressures. The analysis is conducted using a finite element analysis which includes the effects of hopper wall friction and hopper wall flexibility, and these influences are investigated.

A rigorous statistical technique for inferring circular silo wall pressures from wall strain measurements

The pressures exerted by particulate solids on circular silo walls are often measured using pressure cells mounted in the silo wall. Recent research has shown that the patterns of pressure are often very complex, and that the pressures can vary rapidly with position. Thus, a comprehensive picture can only be obtained at great expense, principally because many cells are needed, and also due to the high cost of stiff pressure cells and the installation procedures. Some attempts have been made to infer wall pressures from wall strain measurements, but gross errors of interpretation have often been made by misunderstanding the complex structural behaviour of the circular silo as a thin shell structure. This paper sets out a procedure to infer the wall pressures with reasonable accuracy from wall strain measurements on a cylindrical silo whose wall can be described as an isotropic thin shell.

Prediction of static wall pressures in coal silos

Abstract Coal silos have been among the most problematic with regard to flow and structural failures. This paper describes part of a project to improve understanding of the mechanical behaviour of coal and its effect on wall pressures and solids arching. The parameters for several constitutive models were derived from material tests on ground Shannon coal and used in finite element calculations of static wall pressures. The finite element predictions were compared with both existing classical theories and codified design methods.

Long-Term Performance and Assessment of Circular Reinforced Concrete Silos

Abstract This paper presents an investigation into specific aspects related to the performance of reinforced concrete circular silos. Firstly, two case studies of recent failures in circular silos in the UK are described and significant observations are highlighted. Methods used for estimating internal horizontal pressures acting on silo walls are discussed, and recommended values of design parameters are compared. A brief account of the results of an analytical investigation, on typical silo configurations selected from the case studies, is also given. The study reflects the level of uncertainty in calculating internal pressures considering possible variability in the properties of stored material and silo walls. Additionally, an assessment of stresses induced in steel reinforcement and associated cracking in concrete within critical wall sections is summarized. Finally, the expected causes of observed damage in this type of structure are outlined. In particular, it is concluded that inadequate consideration of crack control and durability requirements increases the vulnerability of the structure and may lead to premature failures especially in the long term.