Blister dialog

The program provides the following options to design blisters in a 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 Blister dialog box.

Additional information which explains the headings shown in the Blister Design dialog box.

Setting	Description
Blister ID	This shows the ID of the Blister (1, 2, …) along with the Tendon Type of that particular Blister.
Start Station	This shows the Start station of the blister, basically where the tendon is cut in-between the span.
End Station	This shows the End station of the blister, the default length is set as 1m and can be modified.
Height	This shows the height of the blister at the start station, Default height is set at 1m.
No of Cables	This is user-defined.
P-Jack	Final Pjack force of all the cables in the blister.

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, you must click the Auto-Design button to see the design results.

Blister Design

Blister design is based on shear friction theory. A crack is assumed to occur at intersection of blister and slab. Shear is transferred by reinforcement crossing that section. Total reinforcement required for transfer of shear from blister to slab is calculated in the Blister Design dialog box.

Then additional stirrups are distributed to the whole length of blister to take care of different prevailing forces in different zones of blister.

The Design Procedure is shown in the below. This is part of the box girder design.

Calculation of Reinforcement

Total reinforcement required (AsT) for transfer of shear from Blister to slab

AsT = F×cosθ / fst

where

F	=	anchorage force
θ	=	exit angle
fst	=	allowable stress in steel = 0.87×fy

Based on Ast required information, the users can Auto-Design the Number of Stirrups required, along with the number of legs, Rebar Size and Spacing of the stirrups.

Finally Ast required (mm²) needs to be compared against Ast provided (mm²).

If the Ast provided is less than Ast required then a red flag will be raised.