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Calculating deformations due to Creep

Calculating deformations due to Creep

Calculating deformations due to Creep

(OP)
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.

RE: Calculating deformations due to Creep

asixth,

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

RE: Calculating deformations due to Creep

(OP)
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.  

RE: Calculating deformations due to Creep

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.

RE: Calculating deformations due to Creep

(OP)
Rapt,

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

RE: Calculating deformations due to Creep

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

Mike McCann
MMC Engineering

RE: Calculating deformations due to Creep

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.

RE: Calculating deformations due to Creep

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

RE: Calculating deformations due to Creep

(OP)
chicopee,

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

RE: Calculating deformations due to Creep

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?

RE: Calculating deformations due to Creep

(OP)
Already have

RE: Calculating deformations due to Creep

(OP)
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.

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