Why the tendon stress doesn't decrease in CSM?

According to CSM manual

Due to creep, shrinkage and tendon steel relaxation the tendon stress will decrease by ΔσP.

This is part of my CSM

PROG CSM
HEAD
CTRL DL AUTO
CTRL RELZ AUTO
CS NO TYPE T RH NCRE TITL
11 G_1 0 70 1
12 P 0 70 1
13 C 10 70 1
21 C 60 70 1
31 G_2 0 70 1
32 C 30 70 1
41 C 3650 70 1
GRP NO ICS1 ATIL T0 TS
1 11 - 14 3

“RELZ AUTO” means tendon relaxation, C in CS means creep analysis
But after it is done, the axial force due to prestress are the same during the whole period.

Another question is: where to input the parameter for low-relaxation prestress reinforcement?

The easy one:
Relaxation of prestressing steel is in the material definition (AQUA)

I’m assuming you are checking a beam:

  • In that case you have to use the 7000 series.
    Read the CSM manual chapter 2.5.
  • Before you ask: the 6000 series is “hidden”.
    You see it in the AQB report of used forces and you can also find it in the cdb (record 104/LC)

@Sofistik: Both wingraf and the result viewer are a bit lacking on this topic. It should be easier to access the 6000 results. (Correct me if I’m missing something)

Correct me if I’m wrong:
As for relaxation, looks like I need to specify these two values in Aqua to get relaxation.
REL1: Coefficient of relaxation (0.70 fpk)
REL2: Coefficient of relaxation (0.55 fpk)

Even if I didn’t include relaxation in my original Aqua, the creep and shringkage should also cause tendon stress to decrease. The creep did cause concrete beam to deform over time, but axial force in the beam due to prestress didn’t change accordingly.

Normally the standard relaxation will be used (e.g. eurocode: class 2 low relaxation), check the aqua report.

If you wish to define it explicitly:

  • Either numeric values for Rel1 and Rel2
  • Or a literal for Rel2, e.g. ENC2 for eurocode low relaxing steel

Are you using a beam system?

Yes I use beam.

I forgot one thing:
I’m modeling pretension bars, not post-tension bars, so i simply make BETA =0 MUE=0

SYSP NOPS MAT ZV AZ LITZ MINR BETA MUE ECC SP DO
1 11 180.7 139 1 10 0 0 0 0 15.2

Is it correct for pretension bars?

I found decreasing axial force in 7000 load cases.
But one thing is weird to me: the pretension force is 180.7kN (ZV), while in the report, it shows 183.3
Nominal/extremal force Po : 181 kN
Max. perm. force Po,max : 183 kN