Prestressing Steel Properties
Description
Definition of the material properties for a section or beam is a three step process:
- Add the material type (concrete / reinforcing steel / etc.)
- Define a set of properties for that material type (compressive strength etc.)
- Allocate the material properties defined in 1 and 2 to the relevant parts of the section or beam section component.
This form performs the second of these 3 stages for prestressing strand. As data is entered the program checks that values are within the expected range for steel. This check will highlight any values in error by a factor of 10 more. The displays for Strength Limit State and Service Limit State are to the same scale, and are updated as the values in the fields are changed.
For the Ultimate Limit State, stress-strain distribution is assumed to be linear elastic up to a certain strain defining the onset of yield, and fully plastic beyond a higher full yield strain. A linear relationship is assumed between the two.
Alternatively, to define a non-linear curve, select the ULS Stress / Strain Curve Parameters option 'Define Non-linear' and enter or paste in the negative stress-strain values.
The data for initial prestress force, relaxation and age at stressing is used only in the beam analysis parts of the program.
Form Graphic
Related Topic
Prestressing Strand Stress/Strain Curve
Field Help
Tensile strength fp
Enter the tensile strength of the prestress tendons. Suitable values are given in AS 5100.5-2017 Table 3.3.1 (2004 Table 6.3.1). A warning will be issued if the value entered is unrealistic, but the value will not be rejected.
The default value is appropriate for Grade 270 strand.
ULS stress/strain curve parameters
To select a stress / strain curve for the prestress strand, click on this field to display a drop-down list containing the 3 options as follows:
Default
This is based upon the ULS stress strain curve defined in the UK bridge design code BS 5400.
Define Linear
To define a stress / strain relationship of the same form as the default above, but with different parameters.
Define Non-linear
To define a non-linear curve, enter or paste in the negative stress-strain values.
Modulus of Elasticity
Enter the modulus of elasticity of the strand. The default value is 200,000 as given in AS 5100.5-2017 Clause 3.3.2 (195,000 MPa in AS 5100.5-2004 Clause 6.3.2) for stress relieved steel strand to AS 1311.
Initial Prestress force
Enter the prestress force immediately prior to transfer as a percentage of the tensile strength. The default value is 75%.
Max Relaxation after 1000 hours
Enter the value of calculated relaxation after 1000 hours as a percentage of the initial prestress force.
FOR CONCRETE CURED AT AMBIENT TEMPERATURES:
The calculated relaxation is equal to the basic relaxation multiplied by the coefficients k4, k5 and k6, where:
k4 is always 1.0 here (1000 hours),
k5 is 1.0 for tendons stressed initially to 70% of the tensile strength, and
k6 is 1.0 when the average annual temperature is 20 degrees C.
The basic relaxation may be taken as follows for Australian manufactured materials:
low relaxation wire 1%
low relaxation strand 2%
alloy steel bars 3%
For further details of basic relaxation and the parameters, refer to AS 5100.5-2017 Clause 3.3.4 (2004 Clause 6.3.4).
The default value is 10%, which is likely to be conservative in service, but not for transfer. Alternatively a value of 2% may be appropriate, which is the value for low relaxation strand initially loaded to 70% of the characteristic strength, where the average annual temperature is not greater than 20 degrees.
FOR CONCRETE CURED AT ELEVATED TEMPERATURES:
The value should be determined from suitable test data, or taken as 10% for low relaxation strand stressed to 0.8fp in accordance with AS 5100.5-2017 Clause 3.3.4.4 (2004 Clause 6.3.4.4).
Relaxation at transfer
Enter the value of relaxation at transfer as a percentage of the total (1000 hour) calculated relaxation.
Density
Enter the density of the Prestress Strand.
Property name
The program will supply a reference name which provides an identifier to the material property set which is usually unique. This value may be edited to give a more readily recognisable name if required.