Design criteria for structural design of silage silo walls

The national guidelines for design of silage bunker silo walls are not available today. The present calculation and design is based on previously issued guidelines. These were elaborated to be used in designing silo wall heights of maximum 3 m. There are reasons to believe that these guidelines overestimate the forces and pressures, mainly from silage juice, that are occurring especially for silo walls > 3 m. This could result in over-sizing, material waste and increased investment costs. The aim of the project is to determine silage physical properties of importance for the horizontal wall pressure and evaluate maximum silage juice level of silos with a wall height of 3 m or more. The data will form a basis for new national design guidelines and a revised Swedish Standard. The ultimate goal is to lower the investment costs for silage bunker silos. A pressure profile was acquired by means of transducers mounted on a “ladder rack” placed vertically along the inside silo wall. The pressure on the transducers was recorded by a data acquisition system displaying static and total load (pressures imposed by silage matter and compacting tractor). The silage juice levels were measured by slotted 16 mm steel pipes placed vertically along the silo walls. The level inside the pipe was determined by a measuring stick. Additionally the juice level could be registered in one of the legs of the “ladder rack”. Silage juice level measurement were carried out during two seasons, all together in 24 silos, while pressure profiles were measured during 10 grass and maize harvests in one season, with approximately 400 pressure profiles at each harvest. The static loads were recorded with no tractor present and total loads as a tractor was passing in front of the transducer racks. The difference between the static and the total load was defined as the dynamic load. Preliminary results show that the silage juice level can vary considerably between harvests within the farm due to weather conditions and geographic location in Sweden. Mean of peak levels was approximately 40% of the silage height. A direct relation was found between grass silage juice level and silage total solid content. The silage juice level tends to remain stable during the storage but can be radically redistributed between layers within the silo. Tentative pressure profile results show that the dynamic pressure created by the compacting tractor is largely concentrated to the first half metre below the vehicle when driving 0.1-0.5 m from the silo wall. Pressure measurements after filling the silo show an increase in wall pressure. The pressure profile results are presently under evaluation. Key words: Silo, wall, silage, pressure, force.