Resistance Factor/Losses tab
You can modify the resistance factors and prestress losses by clicking the Resistance Factor/Losses tab to display the appropriate screen. The default resistance values shown are as specified in LRFD Art. 5.5.4.2 and LFD Art 9.14. You can modify these default values by typing over them. In the LRFD mode, please see the Theory topic for further explanation.
Phi factors are strength reduction factors, defined by the code as equal to the ratio of the design strength to the nominal strength. In AASHTO LRFD, phi for tension controlled and compression controlled sections are to be specified. The strain limits (compression and tension epsilon values) can be modified by the user; Precast/Prestressed Girder uses these limits to determine if the section is tension-controlled, compression-controlled, or is in transition and then determines appropriate phi values for its behavior.
Resistance Factor Terms
| Term | Description |
|---|---|
| φ (phi) | Resistance factors |
| εcl | compression-controlled strain limit in the extreme tension steel (in./in.) |
| εtl | tension-controlled strain limit in the extreme tension steel (in./in.) |
There are primarily two methods you can use to calculate the prestress losses: AASHTO or Manual. In the LFD mode, Standard (LFD) Specifications Article 9.16 is used to compute prestress loses at release and final stage. In the LRFD mode, the AASHTO method is as specified in LRFD Article 5.9.5, and a choice can be made to use either the Approximate Estimate or the Refined Estimate of Time Dependent Losses. The Manual method allows you to specify the loss percentages at release and final. Clicking on the Relative Humidity link displays Annual Ambient Average relative Humidity charts from AASHTO so that an accurate value can be input in the text boxes.
In the LRFD mode, there is an option to use an alternate closed form solution for computing Elastic Shortening (Es) according to Eq. C5.9.5.2.3a-1.In the LRFD mode, you can also select to use either the older method of computing loses as per the 2004 LRFD code, or select the approximate method to compute losses according to the provisions of LRFD Art. 5.9.5.3
- Approximate Estimate of Time Dependent Losses:
Please see the theory topic and hand calculations provided for Tutorial 5 for more details on the LRFD 2005 Interims option. To allow for transition of projects, you can also select to use the older method of computing losses as per the 2004 LRFD code.
- Refined Estimate of Time Dependent Losses:
If this option is selected, the computations are done according to Art. 5.9.5.4 of the LRFD specifications. Additional time parameters are required as input. Under this method, when computing the relaxation of prestressing strands, the users also have an additional option to use a more accurate equation as per Dr. Tadros in equation C5.9.4.2c-1.
Loss Screen Terms
| Term | Description |
|---|---|
| Release Time | Time between pouring of concrete and release of strands, in days. |
| td | Age at deck placement (days) (5.9.5.4.2b) |
| tf | Age of concrete at Final Age (days) (5.9.5.4.2a) |
| KL | Factor account for type of steel taken as 30 for low relaxation strands, and 7 for other prestressing steel unless more accurate manufacturer’s data are available (5.9.5.4.2c) |
| K’L | Factor account for type of steel, 45 for low relaxation and 10 for stress relieved steel. (C5.9.5.4.2c) |