modeling time effects

[ido3778]

Hello,

My Software I use for bridge modeling do not take into considerations Time effects like creep and shrinkage.
For my knowledge these kind of effects can be modeled as a temperature loads.
My problem is to find and prove the magnitude of the temperature load to model the creep and shrinkage effects.
I am talking here about simple girders bridges, bridges with precast aashto beams (or similar) and cast in place deck.
I saw an example that these effects has been take care using -40 degrees Celsius load over the whole superstructure, but no calculations for it.

I do not mind being conservative here, but I need a procedure to show that I am conservative with that temperature load.

Please advice.

 

[wiktor]

-40 degree Celsius looks way to high. German literature recommends -25 Celsius, and physical shrinkage is listed at 3 to 4 x 10^-4, so you may calculate your equivalent temperature drop. I will recommend using lower number, as your composite deck will be cured.

 

[rapt]

I doubt that you could model creep as a temperature load!

 

[asixth]

Depends on what aspect of design you are looking at:

Creep and Shrinkage effects on deflections of the superstructure.

To design for this you will need to perform a cross-sectional analysis at frequent intervals along the bridge. This can be done by hand calculations or you could use a program which you have independently reviewed and deemed fit-for-purpose. Remember that the software developer will not take responsibility for incorrect calculations. There are plenty of good concrete design programs which can be used for this design and I'm sure that your company will have a license for one.

Creep and Shrinkage effects or Bridge Restraints (such as Bearings and Abutments).

I think this must be what you are referring to when you say model creep and shrinkage as an equivalent temperature load. I have myself done this in the past.

What you need before you start is an acceptable model for creep and shrinkage. I.e. ACI209, CEB-FIP or even AASHTO itself.

1. Firstly, you must establish the construction methodology of the bridge:

CREEP & SHRINKAGE

1. Precast girders are cast at the precast plant. They usually sit there for 3-4 months with no restraint to the units so they are allowed to shrink freely.

2. The girders are then brought to site, placed on bearings and the concrete deck is placed. You now have a composite section where the deck is unstressed and the prestressed girders are aged 3-4months.

You will need to look at the creep and shrinkage model which you are using for design. Establish how much more shortening from creep and shrinkage the girder is going to experience during it's design life. The bridge deck should be modeled with spring restraints at each support to allow for the flexibility of bearings and abutments. The calculated shortening from creep and shrinkage can be converted to an 'equivalent' temperature load to establish the actions on the supports from creep and shrinkage. From memory, the thermal expansion for reinforced concrete is something like 11x10^-6mm/degC.

TEMPERATURE

1. Obviously you have already calculated your temperature range (40degC) which is the difference between the minimum and maximum uniform temperature the bridge will be subject to for thermal expansion/contraction. Myself, I use this temperature difference for design purposes. Normally, a 40degC expansion will not offset the shortening from creep and shrinkage, so I consider the thermal contraction resulting from a -40degC temperature variation.

I attended a workshop on bridges recently where the presenter (who worked for another company) mentioned that their company does not use the full temperature variation for design but will make an assessment of what a reasonable temperature variation would be. This depends on what time of year construction occurs, say 30degC. I do not recommend this approach and would say to you to use the full temperature variation.

LOAD COMBINATIONS FOR LRFD

Please be aware of the load combinations for creep, shrinkage and temperature effects as proposed in AASHTO and they are not equal to unity for strength design.

Let us know if you need us to clarify any of the notes above.

All the best.

 

[ido3778]

Thank you Very much for the help.
Probably I will have a few more questions soon.

Thanks again