Streamlining Information Transfer between Construction and Structural Engineering Edited by Shiau, J., Vimonsatit, V., Yazdani, S., and Singh, A. Copyright © 2018 ISEC Press ISBN: 978-0-9960437-7-9 STR-03-1 ON THE REDUCTION OF PRESTRESSING FORCE NEAR SUPPORTS IN PARTIALLY PRESTRESSED CONCRETE FLEXURAL MEMBERS NAZAR OUKAILI Dept of Civil Engineering, University of Baghdad, Baghdad, Iraq Straight tendons in pretensioned members can cause high-tensile stresses in the concrete extreme fibers at end sections because of the absence of the bending stresses due to self-weight and superimposed loads and the dominance of the moment due to prestressing force alone. Accordingly, the concrete tensile stresses at the ends of a member prestressed with straight tendons may limit the service load capacity of the member. It is therefore important to establish limiting zone in the concrete section within which the prestressing force can be applied without causing tension in the extreme concrete fibers. Two practical methods are available to reduce the stresses at the end sections due to the prestressing force. The first method based on changing the eccentricity of some tendons by raising them towards the end zone. The second method is based on bond prevention by encasing some of the tendons in plastic sheathing, effectively moving the point of application of prestressing force inward toward midspan for part of tendons. The present study focuses on a proposed third method to reduce the effect of the prestressing force near end supports by using straight strands with limited initial prestressing value in compression zone. New equations were suggested for the cracking moment and the prestressing force which consider the prestressed tendons in compression zone. Keywords: Pretension, Strands, Stress, Top fibers, Compression zone, Cracking. 1 INTRODUCTION It should be recognized that almost prestressed concrete members implicit significant amount of nonprestressed bonded reinforcement in addition to the prestressed steel. The nonprestressed reinforcement may be included in the flexural member to facilitate its fabrication, or due to strength and serviceability consideration. Such concrete members, which reinforced with combination of prestressed and nonprestressed reinforcement, are called partially prestressed concrete members. They occupy the whole spectrum of reinforcing range between fully reinforced and fully prestressed concrete members (Libby 1990), (Lin and Burns 1982), (Tadros et al. 1985). Prestressing can be used to best advantage by varying the position of the prestressing force. Tendons whether bars, wires, strands, or made up cables may be used either straight or curved. Straight steel tendons are still the most commonly used tendons in pretensioned concrete members. Continuously curved tendons are used primarily in posttensioning applications. Cast- in ducts are positioned in the concrete unit to a continuous curve chosen to suit the varying bending moment distribution along the members.