EndBlock dialog
The program provides the following options to design end blocks in post-tensioned concrete box girders. This option is available in the Design tab of the program.
You are allowed to manually input the stirrup information.
The number of Stirrups is also shown graphically in the End Block dialog box.
Then finally, Bearing stress check is done as per Clause 18.6.2.1 IRC: IS1343.
| Setting | Description |
|---|---|
| Plate Anchorage | Size of the plate anchorage is user-defined. |
| Bearing Area | A br is the same as the area of End block. This is user-defined. |
| Punching Area | Apun is the same as area of contact of the bearing plate. This is user-defined. |
| Bearing Stress | Bearing Stress is calculated as per IS1343 - 18.6.2.1. The bearing stress needs to be smaller of or 0.8×fci. |
| Actual Bearing Stress | Total force of the cable divided by the bearing area. The actual bearing stress should be less than the calculated bearing stress for the design to be valid. |
The default P-Jack forces are calculated based on the analysis results of the PT girder, and the program allows you to input your own P-Jack forces to override the default ones. Changing values of other design parameters are also allowed. After all design parameters are input, a you need to click the Auto-Design button to see the design results.
End Block Design
End Block Design needs to be done for Bursting Tensile Force of cables and shall be designed to distribute the concentrated prestressing force at the anchorage as per Vide Clause 17.2 IRC: 18-2000.
End Block Design is generally done for Post tensioned structures at the point where a prestressing cable is anchored the full prestressing force in that cable is transmitted to the structure. The reinforcement is provided perpendicular to the axis if the prestressing cable and immediately behind the anchorage region to withstand the splitting force arising from the wedge like action of the anchorage.
Then additional stirrups are distributed to the whole length of blister to take care of different prevailing forces in different zones of blister.
End Block Design Procedure
This is part of the box girder design.
The bursting forces in the end blocks should be assessed on the basis of the ultimate tensile strength. The bursting tensile force , Fbst existing in an individual square end block loaded by a symmetrically placed square anchorage or bearing plate , This can be derived from Table 8 of IRC:18-2000. Length of end block in no case shall be neither being less than 600 mm nor less than its Flange width.
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This force Fbst will be distributed in a region extending from 0.2Yo to 2 Yo from the loaded face of the end block as showing in Fig 4 of IRC: 18-2000.
To calculate Fbst, firstly Ypo/Yo is calculated and then based on Table 8, Fbst / Pk is calculated by linear interpolation.
Once Fbst/Pk value is obtained, finally Fbst needs to be calculated and the units are measured in Tons.
Fbst = (Pjack x (Ypo/Yo)) Tons
Reinforcement provided in this region to sustain the bursting tensile force may be calculated based on a Permissible tensile strength of 0.87×fy.
Area of steel required = Fbst / (0.87fy) in2.
Based on Ast required information, the you can Auto-Design the Number of Stirrups required, along with the number of legs, Rebar Size and Spacing of the stirrups.
Finally Ast required (mm^2) needs to be compared against Ast provided (mm^2).
If the Ast provided is less than Ast required then a red flag will be raised.