DYWIDAG Bar Post-Tensioning System - Steel Stress Level

Bar Post-Tensioning System

Steel Stress Level

strongModulus of Elasticity/strongbr /The modulus of elasticity E is an intrinsic property of steel whose magnitude remains basically constant and is little affected by normal variations in mill processes. For THREADBARS sup®/sup, this value has been determined to be 29,700 ksi (205 MPa).br /br /strongRelaxation/strongbr /Relaxation is defined as the loss of prestress load in a post-tensioning steel subjected to a specified initial stress while maintaining the length and the temperature constant. Relaxation tests are usually conducted at an initial load equal to 70% of the strand's actual ultimate strength (see chart below). The tension loss after 1,000 hrs for a THREADBAR sup®/sup initially stressed to 70% of guaranteed ultimate strength can be assumed between 1.5 and 2%.Tests indicate that the relaxation losses in cold drawn, cold threaded bars are significantly higher. img style=BORDER-BOTTOM: medium none; BORDER-LEFT: medium none; PADDING-BOTTOM: 0px; PADDING-LEFT: 0px; WIDTH: 300px; PADDING-RIGHT: 0px; HEIGHT: 187px; BORDER-TOP: medium none; BORDER-RIGHT: medium none; PADDING-TOP: 0px src=uploads/RTEmagicC_dsi_threadbar_relaxation.jpg.jpg alt= / img style=WIDTH: 300px; HEIGHT: 269px src=uploads/RTEmagicC_dsi_threadbar_stress_characteristics.jpg.jpg alt= /
strongStress-Strain Characteristics/strongbr /A typical stress-strain curve for a stretched and stress relieved bar is substantially different from a typical curve produced for a cold drawn, cold threaded bar. Samples of each are illustrated below.The most notable feature is the lack of a definite yield point characteristic of cold drawn bars.br /br /strongFatigue Resistance/strongbr /Under normal circumstances, fatigue is not a primary design consideration for prestressing steels. However, all DYWIDAG bars and accessories have been tested and proven to exceed the fatigue requirements specified by the Post-Tensioning Institute.br /br /strongTemperature characteristics/strongbr /Tests have demonstrated that no significant loss of strength occurs when bars are subjected to elevated temperatures up to 1,100 degrees F (593° C). Only the yield strength is reduced when temperatures exceed approximately 750 degrees F (399° C). Bar ductility is not significantly affected by temperatures down to -60 degrees F (-51° C).br /br /strongSusceptibility to stress corrosion cracking and hydrogen embrittlement./strongbr /All prestressing steel is susceptible to stress corrosion cracking and hydrogen embrittlement in aggressive environments and therefore must be properly protected. However, accelerated tests have demonstrated that while A 416 strand failed after 5 to 7 hours, bars still held their load when testing was discontinued at 200 hours.br /br /strongBond/strongbr /The deformations on the DYWIDAG THREADBAR sup®/sup exceed the deformation requirements of A 615. Consequently, bond strength is at least equivalent to A 615 reinforcing bars.br /br /strongShear/strongbr /High strength bars are not usually used to resist transverse shear loads. However, their untensioned shear strength is similar to that of any other steel. nbsp;img style=WIDTH: 60px; FLOAT: left; HEIGHT: 35px src=uploads/RTEmagicC_formular_shear_loads.jpg.jpg alt= /