TYPES OF PRESTRESSED BEAM BASED ON MODE OF

TYPES OF PRESTRESSED BEAM BASED ON MODE OF FLEXURAL FAILURE : UNDER REINFORCED PT BEAMS: For the Under reinforced beams, Steel reaches its permissible tensile stress (yield stress) before concrete reaches its crushing strength (permissible compressive stress), so failure is initiated by yielding of tensile steel (tendon). The associated large tensile strains permit widening of cracks and upward migration of Neutral axis towards compressive face along with considerable deflection of member before failure. Such failure are not sudden and gives ample warning before failure. This is an important safety consideration. Under reinforced beams are always DUCTILE. The increased concrete stresses acting on reduced compressive area result in “secondary” compression failure of concrete, even though failure is initiated by yielding of steel. n 1

OVER REINFORCED PT BEAMS: For the Over reinforced beams, concrete reaches its crushing strength (permissible compressive stress) before steel reaches its permissible tensile stress (yield stress), so failure is initiated by crushing of concrete. The associated large compressive strain in concrete results in downward migration of Neutral axis towards tensile face. Such failure are sudden and gives no warning before failure. Under reinforced beams are always BRITTLE. n 2