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Short-term losses can be calculated in advance, before the solver is run:
Anchorage set loss
Losses due to deformation of stressing bed
Losses due to steel relaxation
Losses caused by the temperature differences in between prestressing steel and the stressing bed.
Besides of those losses, calculation of some other losses is included in the method for the structural analysis itself. Three codes are supported for the calculation of losses in SCIA Engineer:
EC2,
CSN 73 6207,
CSN 73 1201.
The losses are calculated according to assumptions given in these codes.
CSN 736207 art. 9.2.5 stipulates that a curved part of a duct with the radius larger than ratio mju/k is considered to be a straight part the length of which is equal to the length of the curved part.
This condition is not taken into account in the algorithm in the program. Such configuration occurs only occasionally and therefore is not handled separately. The problem can be simply sorted out by the users themselves – they may define such a value of the friction coefficient in a curve so that mju*alpha=k*l.
The introduction of EN code is accompanied with the preparation of a general algorithm in which k*l is considered an unintended change of tendon angle with l equal to the sum of lengths of straight and curved parts. This means that the described problem is in the calculation according to EN automatically sorted out.
There are two simplifications used in calculation of anchorage set loss:
EC2: we introduce effective cumulative angle g=a+kx over a distance x, where x is horizontal co-ordinate, a is intended angle over a distance x, kx is unintended angle over a distance x.
CSN 73 1201 and CSN 73 6207: the exponential functions for friction calculations are approximated by first two members of power function.
The losses of prestressing caused by steel relaxation are introduced at three levels.
At first level - the correction of relaxation is calculated, namely the relaxation which appears during keeping the stress constant before anchoring. In fact, this is not a loss of prestressing. On the contrary the total relaxation considered in the structural analysis is decreased by this value. The relaxation at this level is applied for types 1, 2 and 3 of stressing sequence only (see Properties of pre-tensioned tendons > Type of stressing).
At second level – the short-term relaxation loss is calculated. The calculation is performed according to procedure of stressing and anchoring for types 1, 2 and 5 (see Properties of pre-tensioned tendons > Type of stressing).
At third level – the long-term relaxation loss is calculated for all five types of stressing sequence. This relaxation loss will occur after anchoring. Therefore it has an impact on the long-term behaviour of building structure and it should be applied as one of the loads in time-dependent analysis. In SCIA Engineer implementation the strain equivalent to relaxation to be passed in long-term is applied in several time nodes following ttr (see Properties of pre-tensioned tendons > Type of stressing).
The calculation of all of the relaxation losses mentioned above is based on the following principles. The differences between the procedures applied for different national codes are only in different definition of material characteristics. Firstly the final relaxation loss is calculated, which depends on the level of stress at given section related to the characteristic tensile strength. In the second step - the relative decrease of stress related to the final relaxation loss is calculated according to the duration of time interval when the stress is applied. The only simplification is that there is no effect of the changes of steel stress during the interval on the magnitude of relaxation in this time interval. For example - at third level – the losses of prestressing due to creep and shrinkage of concrete do not influence the amount of steel relaxation (insignificant).
The final relaxation loss is not defined in EC2 (only the relaxation up to 1000 hours). Therefore the estimate of the relaxation up to 30 years is used according to CEB FIP 1990 Model Code [2].
Having set all the input data described above, the losses can be calculated.
Select the beam strand pattern for which the losses should be calculated.
The properties of the strand pattern are shown in the Property window.
Click button [Edit strand patterns].
The editing dialogue for the selected strand pattern is opened on the screen.
Select one strand which you are interested in.
Its properties are displayed in the bottom right corner of the dialogue.
A preview window is displayed. The preview window is split into two parts. In first part some details of tendon parameters are displayed together with the table of results. Using the toolbar at the top of the window, all the information can be exported to a file (HTML, TXT, PDF, RTF) or directly to printer. In the second part a diagram, the distribution of various losses along the length of the tendon is shown. It is possible to change the scale of the diagram or the text. And the pop-up menu (pressing right mouse button), offers some basic functions for the picture: zoom, print, copy to clipboard or save to an external file.
