How does the fabricant of prestressed pre-tensioned beams calculate the equivalent pressure in the hydraulic jack (in bars) to the design prestress force (in kN)? Are there factors to multiply like a jack efficiency factor or are there some losses we must add before setting the right pressure to the jack?
Just to give a very rough narrative.
Let's say we have a 20 ft long beam with strands of prestressed eccentricity of 1 ft. and our goal is to prestress this beam for 100 kips.ft moment at midspan.
So we have to provide 100 kips tension in the enter provided by the strands.
Ideally in a lossless situation we would need to apply 100kips tension at the jack.
But there are many factors that cause the loss of prestress which need to be counted for and added to the jack's force.
Concrete beam will strain under the compression stress caused by the strands and cause relaxation. this depends on the quality of concrete and section properties.
Shrinkage of the concrete again depending on the concrete and the humidity of environment and some more factors.
Creep of the concrete. depends on concrete and environment, humidity, the beam will perform.
Mechanical losses at the jack bulkhead and material properties and details of the jack.
The above relaxing factors can be complicated and sometimes are just assumed arbitrarily from past performances. Or sometimes there are codes by Caltrans, for example, specifying the loads and factors and equations that apply in each case.
Below is the flow chart that indicates the required considerations in determining the final "effective force level". You can reverse the flow to get the necessary jacking force. Ref.