Shrinkage analysis 1

[slickdeals]

Folks,
I need to study the shrinkage of a box girder and associated restraint forces. I have never done one before and I am unable to figure out a good starting point?

Can anyone give me good pointers? Design examples etc.?

 

[dgkhan]

Basically you need one formula to account for shrinkage in concrete and creating loss in prestressing force. WInyter Nilson has agood book on it, prestress concrete by W&N

 

[asixth]

I am doing a temperature, creep and shrinkage analysis of a precast prestressed girder right now. What I have done is calculated a temperature variation that can be modelled simply as a temperature load, I then calculate the expected free movement from shrinkage and creep and covert this to an equivalent temperature load. I have a 18000mm girder (20') and the free shrinkage from creep and shrinkage is approximately 10mm (4") from 4 months to 30 years (erection of the precast members occurs after 4 months).

The restraint for my situation is a elastomeric bearing which I have modelled as a spring restraint. If you don't allow adequate movement, the built up stressed from these parameters will be very large.

 

[slickdeals]

@asixth,
I am trying to do a similar process to yours. I am trying to compute a shrinkage strain and use that as a coefficient of thermal expansion and use a 1 degree temperature gradient to compute the restraint forces.

However, I am having difficulty computing the shrinkage strain. I have not been able to find good examples to do the same. Do you have any references that you don't mind sharing? I think the key for me is to get to a shrinkage strain value at different ages of concrete.

 

[rapt]

ACI 209

 

[asixth]

If you look at the McGregor textbook that I have used, you need to calculate a basic shrinkage strain, for my example the prestressed girder was being installed in a particulary humid environment so the total long-term shrinkage strain was approximatley 400 micro-strain. On page 62 of the text, it gives a term that represents the developed on strain with time. As an approximate, I calculate approximatley 15% of shrinkage occurs after 1 month of curing, 25% after 4 months, 40% after 1 year and 55% after 2 years.

My girder was being installed after 4 months, so only 75% of the total shrinkage strain would occur (300 micro strain). So the free movement of the girder was 5mm (2"), of which there was very little restraint provided against movement so the girder was free to shrink that amount.

If you have a prestressed member you best check creep shortening as well because this can be of same magnitude to shrinkage.

 

[hokie66]

asixth,

Your conversions from mm to feet and to inches are all wrong.

 

[asixth]

hokie,

your right, 18000 is more like 60'.

 

[PST09]

Hokie66 i think he means cm... 2" =appx. 5cm, 4" =appx. 10cm

 

[asixth]

Thanks guys,

I'm don't work with the imperial units often but try to use both units on this forum.

The precast beam I mentioned above was only moving 5mm at the bearing which is 1/5".

 

[druminman]

One thing to keep in mind with prestressed members is that creep strains will not be uniform on the cross section, and will also result in a loss of prestress which will increase deflections beyond those calculated. Shrinkage is approximately uniform if the drying is uniform, but not so if the cross section of the section is much thicker in some parts than others.

 

[ishvaaag]

Respect shrinkage I remember free shrinkage is NOT a good measure of the shrinkage amount to be considered when analyzing structures. In normal structures shrinkage is restrained, resulting in cracking that then reduces the amount of shortening imparted by the shrinkage. In free shrinkage the piece glides through the interface without cracking. From somewhere in my mind I recall 220 microstrain average shrinkage for restrained structures in my practice (buildings, spain).

Bridges may be understood to have less restriction to free shrinkage if upon free supports; without looking farther some more recent codes, the "Instrucción relativa a las acciones a considerar en el proyecto de puentes de carreteras", a code that stood about 5 decades ruling the construction of road bridges in Spain in the second half of the XX century, shrinkage should be taken between 150 and 400 microstrain, the first value for humid zones that have less shrinkage, and 400 for dry and desertic areas, that have more.