Elastic Shortening/Shrinkage on Elevated PT Slabs 1

[slickdeals]

Folks,
How many of you explicitly model the effects of elastic shortening and concrete shrinkage in your lateral analysis models (ETABS/RAM/STAAD etc.) to account for additional column moments? The effects will be considerable at least for the first floor which is typically the stiffest. I was always told to ignore these, but I don't certainly buy into it.

Thoughts?


We are Virginia Tech
Go HOKIES

 

[ash060]

Check Chapter 3 of the PCI Handbook 6th edition. There is some good information regarding shinkage. There are also some examples and design tables.

 

[rowingengineer]

can of worms,

Volume change due to thermal, shrinkage and creep cause forces and strains to build up in restrained concrete members these actions should not be “ignored” in analysis. These strains can cause tensile stress in beams and slabs and shear/ moments in columns. Since the volume changes take place over a period of time, the effect of shortening on shear and moments is reduced due to creep and micro-cracking effects, this causes the estimation of restraint forces is problematic at best, with assumptions for connections, footing ect playing a major role. Such as if you assume fixed foundation supports the forces will be overestimated conversely if you assume pin foundations the forces will be under estimated. This slab restraint cracking is the most common cause of deflection problems. The question is how you determine the amount of restraint. Some programs account for shrinkage restraint caused by the reinforcing, however very few account for restraint forces created by columns and shear walls.

The PCI method involves the use of an equivalent shortening principle. This will allow you to compute a tensile force in the slab. This can then be used to adjust the expected tensile strength of the concrete. However it is very rough at best.

Most commercial concrete FEA programs are not going provide any information in regards to these forces. Due to the use of plates instead of shell, plate elements do not allow axial forces to be generated.

My approach is to place expansion (or more correctly contraction) joints as required and try and place columns and shears wall locations to be reduce shrinkage forces.

An expert is a man who has made all the mistakes which can be made in a very narrow field