Post Tension Beam Design

[saberali.akbar]

Hi, 

I have several questions regarding designing PT beam in SBD;

1 - My parabolic shaped tendon A seems like not symmetry and incorrect where when i check the losses results, the mid point and end point are different. ( Stress results also doest look symmetry)  

2 - The top and bottom stress differ quite significantly compared to manual calculation at mid point during stage 1 and 2.

Stage 1 - Top 2.36Mpa (manual) 2.74Mpa (SBD)

                 Bot 7.24Mpa (manual) 5.73Mpa (SBD)

Stage 2 - Top -1.01Mpa (manual) 0.146Mpa (SBD)

                 Bot 22.13Mpa (manual) 17.403Mpa (SBD)

I attached the model as well as calculation for ease of reference. Appreciate help or suggestion from the team.

Regards 

[karoly_tarcsi]

Hi,

1)

Anchor slip, friction and wobble losses are set for Tendon A on the "Loss of prestress" tab, therefore the total force after all losses vary along the beam. Set these losses to zero if you want uniform force or stress the tendon from both ends to get a symmetrical force plot.

2)

Here is how ASBD calculates stresses using the stage one top fiber as an example:

ft = F/A + M/Zt + Msw/Zt

where:

F: prestressing force: 4208.6*0.875=3682.52kN in the manual, 3248.58kN in ASBD = (Jacking force) - (Immediate losses) - (time dependent losses) as set on the losses tab.

M: bending moment from prestressing: 3918.207kNm in the manual vs. 3474.2kNm in the software due to the mismatching force,

A: area of cross section: 8.11*10^5mm2 in the manual vs. 8.01*10^5mm2 in ASBD, because the area of the tendon ducts are deducted from the area of the girder,

Zt: 5.073*10^8mm3 vs 5.023*10^8mm3 in ASBD due to the deducted ducts,

Msw: self weight moment: 2811kNm vs 2813kNm.

As a result of these differences:

ft = 3248.58/8.012/100-3474.2/5.023/100+2813/5.023/100 = 4.05-6.92+5.60 = 2.74N/mm2.

Note:

- Your slab thickness is 230mm instead of 180mm specified in the manual,

- Immediate and time-dependant losses cannot be set individually at construction sequence. In your example when the tendons are stressed to 50%, half of the total losses are applied.

Regards,