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The concrete stave farm silo is a pre-stressed concrete structure in which the hoops are tensioned to pre-compress the stave wall circumferentially. However, current construction procedures, the building code (Canadian Farm Building Code 1977), and other standards (Ontario Silo Association 1974, National Silo Association) governing the design of these structures do not adequately treat the stave silo as a pre-stressed concrete structure. In similar pre-stressed concrete structures such as standpipes, much greater emphasis is placed on ensuring that proper tension exists in the steel pre-stressing tendons during the expected service life of the structure, and on minimizing the inevitable tension losses. It is believed that several wind-induced failures of concrete stave farm silos can be attributed to improper regard of initial hoop tension requirements and the ensuing hoop tension losses. The subject of hoop tension and hoop tension losses has been examined perfunctorily by only a few investigators since the first concrete stave silo was built circa 1910. The American Concrete Institute (ACI) Committee 714 (1946) suggested that silo hoops should be tensioned in three steps: (I) the hoops were to be uniformly tightened to 50% of the design stress, (2) after joint grouting the hoops were to be tensioned further to full stress and (3) the hoops of the lower two thirds of the silo were to be retightened prior to filling the silo. Johnson et. al. (1971) instrumented a number of hoops on a newly erected 6.1-m diameter silo. They determined that after tightening, the tensions in six 14.3-mm diameter hoops ranged between 9.4 and 21.4 kN, and averaged 12.9 kN. This constitutes a tensile stress of only 80 MPa. Sadler (1972) states that only I0-40%of the capacity of the hoop is developed when kinking at the lug occurs. Kinking refers to the formation of a plastic hinge in a hoop at the lug during tensioning. Many contractors apply only sufficient tension for this kinking to commence.
stave silo hoop design, hoop tension and hoop tension losses
Kleywegt, H.S. and J.C. Jofriet 1979. STAVE SILO HOOP DESIGN, HOOP TENSION AND HOOP TENSION LOSSES. Canadian Agricultural Engineering 21(2):91-96.
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