creep and shrinkage at crack section 2

[PowerRanger]

Hi Fellows

I am designing the bearing pad support the bridge beam. And I have a question about the creep and shrinkage displacement at the bottom of the beam.

My case is, the prestress beam I designed is allow the cracking under the full loading, and my question is after the beam cracked at the bottome of the beam, will there be any more shrinkage and creep at the bottom of the beam to cause movement at the support (which betweem the beam and the pad), or the shrinkage and creep only affect the top of the beam to the neutral axis in a crack section?

In my situation, the creep by prestress at the bottom of the beam is at the same way as the shrinkage before the section crack under the dead load itself.(member hog at this time, bottom stress act compressive, so the strain is toward each other at the support). After traffic loading, the section crack, this is what I am not understand the creep and shrinkage strain movement at the support after the concrete is flexure cracked.

For designing the bearing pad size, I need to find out the shear strain due to tangential movements and forces.

Thank you

 

[Dinosaur]

Your English is very poor or you are in too much of a hurry.

Yes, you have to design the bearings to accommodate the creep and shrinkage of the concrete. Flexure cracking does not change this.

 

[tjhubo]

Actually creep and shrinkage are two different phenomena. Creep is the time-dependent deformation which has the same sign with the original deformation. Say if you have a tensile strain in concrete, without external constraints, the creep effect will result in a larger tensile strain as time goes by.

Shrinkage, as the word suggests, will always result in a decrease of the dimensional sizes of a concrete member.

In the design of bearing pads, if you have a simply supported beam, which means you do not have any axial force at the neutral axis, you will not have any creep deformation along the axial direction of the beam. But you will definitely have the shrinkage deformation.

Although you may argue that the rebar and crack will affect the actual creep and shrinkage effects, within the AASHTO framework, you don't need to do that.

BTW: Thermal expansion seems to be crucial in designing the bearing pads.

 

[Dinosaur]

If you are talking about prestressed concrete, as I assumed you are, you will have compression stress causing creep and shrinkage which must be accommodated in the bearing design.

 

[tjhubo]

Yeah. Dinosaur made a good point on the prestressed concrete design.

 

[PowerRanger]

Hi Fellows

Thank you for the reply. Yes, my beam is partially prestressed, and it will have the flexural crack under full traffic loading. What I really want to know is that, before the beam crack, we know the end displacement is = strain * length of the member, but after the beam crack, at the bottom part of the member, we can assume they are a series of small sections separate by the crack, and their length is the crack spacing. Then in this time, will the shrinkage & creep(by prestress) still affect the whole beam member, or only reduce the small section's lengths by increase the crack width?

Thank you

 

[Dinosaur]

Creep and shrinkage is normally computed using gross section properties and the effects of cracking are not considered. I guess, since we don't expect the crack to propegate to the NA of the girder, the shrinkage and creep are effectively acting on the section. Good Luck