Spannbetonsilos mit hohen Vorspanngraden unter instationären Temperatureinwirkungen

Post-tensioned silo walls with high post-tensioning degree and transient temperature effects

When post-tensioning a cylindrical silo wall in ring direction usually a compromise of two limit states has to be considered: For an empty silo high compression stresses will occur in combination with high restraint stresses due to the temperature difference between inside and outside when filling the silo with hot bulk material. This means a maximum for the compression stresses in ring direction at the inner face of the wall. When the silo is filled to capacity circumferential tension and restraint bending moments due to the hot bulk material will cause maximum tensile stresses at the outer face of the wall. Excessive compression stresses can damage the wall concrete, while excessive tensile stresses cause wide cracks, which endanger the durability.
Until now no indications were found for damages at the inner face of silo walls due to excessive compression stresses in ring direction, but damages (delaminations) at the outer face of some silo walls with very high post-tensioning suggest a risk, which has been underestimated until now eventually. Transient temperature stresses due to the sun radiation will increase the compression stresses due to post-tensioning, which causes very high transverse tensile stresses in the range between outer reinforcement and tendons. This was proved by a non-linear analysis with the FE-program MASA 3. If further influence from silo operation causes tensile stresses above the ultimate limit state, this might be an explanation for the delaminations, which have been detected at the outer face of silo walls with high post-tensioning degree and intensive sun radiation.