On Minimum Reinforcement in Concrete Structures

The usual criteria to fix minimum reinforcement in concrete structures require first, that the cross section of reinforcement be dimensioned so that the tensile stresses acting on the ``embedment section'' of concrete, when concentrated on the steel after cracking, do not exceed its elastic limit, and second, the crack width be kept within established limits. But these rules neglect some important aspects as soon as cracking provokes significant redistribution of stresses. The incidence of redundancy is decisive in the presence of imposed deformations. An interesting example is the case of a well in which thermohygrometric effects generates a diffused state of tension. From a finite elements analysis it appears that, in the mid-vertical section, a limited opening of the crack enduces a marked relaxation, the sum of the tensile stresses along the section being much smaller than before crack formation. This result suggests the idea to replace evenly distributed bars with series of well-reinforced chords (or prestressed prismatic elements) spaced so that they may keep within acceptable limits the opening of cracks in the intermediate zones and allowing to exploit the release of stresses to cut down load effects in restraining elements.