Calculating deformations due to Creep

[asixth]

Hi guys,

I posted a similar thread in the precast/prestressed forum but it doesn't seem to be getting any replies.

Basically I have a prestressed concrete bridge girder that hogs 30mm (1.25") at transfer. The concrete strength at transfer is 40MPa (5800 psi).

What I want to calculate is the hog after 28 days, assuming the girder sits in the stressing yard for this time. When calculating deformations due to creep and shrinkage, should I reduce the Young's modulus to account for these two phenomena. Or is there another model that is generally accepted throughout the industry, if so, can someone please provide me with a reference paper/text. I think I may need to account for some concrete strength gain and pretension losses when calculating the deformations at 28 days.

 

[rapt]

asixth,

I think the data in RAPT can be adjusted to do this for you.

 

[asixth]

I can't uderstand the results I have, the only two load combinations I have modelled are prestress and selfweight, the system is always under negative moment even after I have considered final losses, however the longterm deflection is less than at transfer (less hogging that is)>

The 28 day hog should be around 1.6 x transfer hog.

 

[Dinosaur]

Have you contacted any DOTs? They check this condition all the time. I would recommend FLDOT because they use more P/S girders than anyone else I know of. They have decided Mathcad is the way to go so they have many publicly available routines that run in Mathcad. If FLDOT doesn't have what you want, then I would check PENNDOT. They are known to be conservative, but they are also known to be very thorough.

 

[asixth]

Rapt,

Can you please be more specific on how to alter the data so I can calculate the hog at 28 days.

 

[msquared48]

1.6X does not seem unreasonable. I have always figured 1.5 to 2X.

Mike McCann
MMC Engineering

 

[civilperson]

Concrete creep and shrinkage is the mechanism for losing prestress curvature. The shrink formula takes into account size, humidity and time. The creep is accounted for with a reduced initial stiffness, Ec eff.

 

[chicopee]

Colloquialism sometimes is often used in the construction industry and each trade in construction have them. Please explain to me the term "hog".

 

[asixth]

chicopee,

I have been using the term "hog" referring to the upward deflection due to prestress.

 

[rapt]

asixth,

Sorry,

I have not been monitoring Engtips for the last 6 weeks as I have not had good internet access available while travelling.

Why don't you email your questions to me directly trather than through the forum?

 

[asixth]

Already have

 

[asixth]

Rapt,

Thanks for the help by the way. We sent the design out late last year. I still would like some reference material on the topic for my own eductation.