Theoretical background - decompression

The check of decompression is done according to chapter 7.3.1(5) EN 1992-1-1 in Scia Engineer. Together with check of decompression the limited values of crack width for prestressed were implemented. Check of decompression is done only for prestressed concrete 1D member. The check of decompression means the prestressing strand or tendon lie in the predefined zone (z_dec)of the concrete which is in compression. The decompression distance (z_dec,max) where the concrete is verified is set in the EN 1992-1-1 and default value is 25mm. This value is measured from the edge of the strand (pre-tensioning) or edge of the duct (post-tensioning).

Decompression check calculation

The idea is to verify the distance around the strand to be in compression in proper distance. This check is required according to chapter 7.3.1(5) from EN1992-1-1. The check is performed as the following

• Calculation z_dec is the distance from the edge of the strand or edge of the duct to the neutral axis.
• Value z_dec,max is set in NA
• Calculation check value the z_dec,max / z_dec
  • IF z_dec,max / z_dec ≤ 1,0 → Check is OK
  • IF z_dec,max / z_dec > 1,0 → Check is NOT OK

The check is performed for one or two plane bending. In case of two planes bending the distances from the strands are measured perpendicularly to the neutral axis.

Performing of the check YES /NO

If the check of decompression is performed depends on several items:

• Type of structure – only prestressed members
• Type of combination – frequent or quasi-permanent
• Type of exposure class – from XC2 to XS3

It means the program verifies the type of the structure, the selected combination in the service and defined exposure class on whole structure or on the selected member. The decision when the check of decompression is done is based on the special table in the national annexes. Table is placed in Concrete > SLS > General as value w_max – for prestressed concrete.

Using three dots button you get access to the table of limit crack width (see chapter 3) and also table with information about check of decompression (the second table).

You can see from the table above the decompression check is performed only for prestressed member and exposure class XC2-XC4 together with quasi-permanent combination and for the exposure class XD1-XS3 together with frequent combination.