design forces / actions from shrinkage

[blihpandgeorge]

hi

I have a suspended slab that is restrained at opposite ends by shear cores & I want to determine some design forces from shrinkage to check the core design. I am comfortable in checking crack widths & reinforcing the slab, it is determining the forces on the core that I need. As the shrinkage forces are generated from an applied strain, and the code gives me a design strain. I intend on applying the design strain to an analysis model by converting the design strain to a temperature and applying a temperature analysis.

I have other options such as pour strips / movement joints etc to control shrinkage, however it is a slab without pour strips tied into cores that i would like to investigate

A few specific questions i have:
1) is there a documented procedure / design guide for this sort of thing?
2) i assume (perhaps incorrectly) that distributed cracking in the slab will act to reducing the design strain, is this correct? how can i quantify this?
3) my design code is the Austalian code AS 3600, is there better guidance in another code? (ACI/Eurocode?)

thanks in advance

 

[SethGuthrie]

I think a thermal analysis is OK for basic analysis, but if you are looking for something more refined you might try Ram Concept which offers a long term load history analysis of the structure with full consideration of shrinkage effects. There is some supplementary information online our the methods used in Concept here:

http://communities.bentley.com/othe...ber_folders/m/jason_colemans_files/87861.aspx

 

[ishvaaag]

Attached go the design pages of old (never prescriptive) code NTE ECR 1973 that stated shrinkage moments and shears at the first level of RC buildings; since rheological forces they used to be combined with all the other forces at service level as well.

It is of course a particular assumption of the shrinkage effects of interest. The NTE codes attempted to be safe rules for building design in the seventies' here.

You best will understand how to proceed by examining the third page in the document that has the example. It looks to me it seems to give conservative values... but that was the intent in the NTE codes.

Will try to get back with more info.

 

[asixth]

Are they masonry cores or concrete cores. I think the shrinkage would affect the slabs more but I have seen masonry cores (corefilled and reinforced) crack moderately from slab shrinkage pulling away from the core.

I think you are on the right path. Converting the shrinkage strains to temperature and then performing a global analysis.

I think the AS3600 values might be a bit high. I always request a shrinkage test from the concrete supplier and typically I get the 56 day result around 450-500 microstrain.

 

[ishvaaag]

Shrinkage according to the EP-93 prescriptive code for prestressed structures. Even if superseded, holds more info than some current codes prescriptive by now ... quite likely close to CEB-FIP publications.

Se article 35.8

0.30 mm/m in general is suggested for normal concrete weakly reinforced in environments that are neither overly dry or humid. 0 mm/m is suggested for submerged structures.

Spain's bridges' code of the seventies' stated between 0.15 and 0.40 mm/m engineering shrinkage.

 

[ishvaaag]

pdf printout of Mathcad 2000 Professional for the evaluation of free shrinkage (not enginering shrinkage) strain as per CEB-FIP 1990

 

[ishvaaag]

pdf printout of Mathcad 2000 Professional worksheet establishing the relationship between enginering shrinkage strain and free shrinkage strain for prestressed structures; source unquoted but quite likely some Spain's code.

 

[ishvaaag]

pdf printout of Mathcad 2000 Professional worksheet for the evaluation of free shrinkage strain according to EHE 1998 (superseded, ruled both reinforced and prestressed concrete).

I have some evaluations more for free shrinkage but stop since enough to get an appraisal for design. One could also ask for a prediction of (at least free) shrinkage strain from the concrete provider.