Analysis model of Creep and Shrinkage of a reinforced concrete beam
Analysis model of Creep and Shrinkage of a reinforced concrete beam
(OP)
I try to design of a long continuous reinforced concrete beam (about 100m long) which is supported by the bored piers (8m high) at 5m span. I am not sure the way to check shrinkage of the beam.
I worked out the axial displacement of 25mm due to shrinkage and input it into the computer model, I end up very bending moment at the top of pier (say 800KNm). Axial force is more than 24000KN.
Is it the right way to do by input pre-displacement due to shrinkage into the beam model ?
I worked out the axial displacement of 25mm due to shrinkage and input it into the computer model, I end up very bending moment at the top of pier (say 800KNm). Axial force is more than 24000KN.
Is it the right way to do by input pre-displacement due to shrinkage into the beam model ?






RE: Analysis model of Creep and Shrinkage of a reinforced concrete beam
80% of shrinkage occurs within the first year.
RE: Analysis model of Creep and Shrinkage of a reinforced concrete beam
Don't know, but did you try with temperature load?
RE: Analysis model of Creep and Shrinkage of a reinforced concrete beam
I think I gonna do the equivalent temperature due to shrinkage. Say Shrinkage strain =500 micro strain is equivalent to 45 degree. Then I add temperature and shrinkage effects (20+45=65 degree) into the computer model to analyse the beam.
RE: Analysis model of Creep and Shrinkage of a reinforced concrete beam
RE: Analysis model of Creep and Shrinkage of a reinforced concrete beam
So, I would recommend the following:
Pour breaks / joints – to allow some movement
Pinned connection to piers – to allow them to rotate (and not throw moment back into the beam)
Small pier size as possible – this reduces the restraint (cracks/rotates pier) and limits the maximum moment that can be induced in the beam (if a rigid connection) to the section capacity of the pier.
Don't forget the 25mm shrinkage you have determined is total (ie 12.5mm each side).
Cheers
RE: Analysis model of Creep and Shrinkage of a reinforced concrete beam
1. Equivalent thermal load or axial distortion/pre-strain (if your software has this option) will be most appropriate.
2. Reduce modulus of elasticity to Ect = Ec/(1+ct), where ct is the creep factor. Safely Ect = Ec/2.
3. Do not overestimate ultimate shrinkage strain.
4. Specify a couple of construction joints, so initial shrinkage will be less restrained.
5. Nominally post-tension if crack control is very important.
RE: Analysis model of Creep and Shrinkage of a reinforced concrete beam
RE: Analysis model of Creep and Shrinkage of a reinforced concrete beam
I will disagree on the 5 and last item. Prestress is not good at providing crack control for restraint effects. You are much better to provide closely spaced reinforcement.
PT is great for crack control until the concrete cracks and then it is awful and the cracks are basically unrestrained.
As the amount of PT to stop the concrete cracking is normally very large, and in the case of rigid end restraints (cores) is basically infinite, normal reinforcement at about 200mm centres is normally much more effective.
RE: Analysis model of Creep and Shrinkage of a reinforced concrete beam
- put more expansion joint and the beam is now 22m long with. Piers are 5m spacing and 1m cantilever each end.
- use equivalent temperature to check shrinkage (thermal coefficient=11x10^-6), then combined Live load+Shrinkage+Temperature (Shrinkage just like permanent load). The results give reasonable axial forces (about 100KN not like 24000KN due to pre_displacement).
- The piers will take bending moment at the top and compression then check the pier capacity. If pier is not strong enough, we may increase the pier diameter or reinforcement etc. or maybe put more expansion joints.
We end up 450-600mm diameter of the pier with 8-11m long.
I agree with Rapt that PT maybe not a good option becasue it's expensive.
RE: Analysis model of Creep and Shrinkage of a reinforced concrete beam
RE: Analysis model of Creep and Shrinkage of a reinforced concrete beam
AS3600 gives the thermal co-efficient of concrete as 10x10^-6 (+/-20%), while AS5100 gives 11x10^-6 (+/-20%)
RE: Analysis model of Creep and Shrinkage of a reinforced concrete beam