Masurement of imperfections in cylindrical silos for buckling strength assessment

The cylindrical sections of a steel silo are subject to axial compression induced by the downward frictional drag from the stored bulk solid and unsymmetrical bulk solid pressures. Buckling of the cylindrical shell wall under axial compression is the most common mode of failure for steel silos and is therefore a very important design consideration. Since small geometric imperfections in the silo wall can drastically reduce the buckling strength, knowledge of the shape and amplitude of possible imperfections is vital to any strength assessment of the structure. This paper describes the background to a research project in which the imperfections in full scale steel silo walls are measured. A full description of the imperfection measurement project is given in a companion paper. Here, a brief review is presented of existing work on imperfection sensitivity in shell buckling, as it applies to steel silos. Past research on imperfection measurement has mainly been concerned with laboratory models, aeronautical and offshore shells, where the imperfections bear little relation to those in steel silos because the fabrication procedures are so different. Measurements of the imperfections in steel silos are fundamental to assessing adequate buckling strengths. Known measurement techniques for steel silos in service are briefly examined, highlighting the substantial difficulties in precise measurement of a three dimensional curved surface. Improvements to overcome these difficulties are discussed. Finally the challenging task of exploiting the measured three-dimensional imperfections both in structural buckling analyses and in developing rational design rules is examined.