Temperture gradient effect

Question:

Hello,


I am currently working on a grillage model with concrete slab on prestress girders of a integral abutment bridge. when I try to read the results of some temperature gradient load cases (ie. Creep DL in attached model), I noticed that the results of those temperature effect will vary a lot when if there are tendon prestress  forces assigned on the girders (in a totally separated load case). Please see the snap shot below as an example, a temp gradient force of -32.2 is assigned to the girder in load case "creep DL", the secondary restraint moment I get is 7320kN.m. However, in the same model if I simply delete the prestress load case without touching anything else, the result I then get from "creep DL" will be reduced to ~6877kN.m.

Could you please help me diagnose the reason why this is happening?


Answer:




Hello,

Good job noticing that!
Actually that happens due to the way in which the tendon properties are considered in the section.  This could be controlled from construction stage analysis control.

What this option does could be referred in our online help manual.  The relevant part is as below.

When the prestressing tendons are not defined, there is no duct deduction or transformation of section and hence no change in section properties.  So, without prestressing tendon, the section property remain same as what is defined.  This change causes change in forces.
If this change is not required, the beam section property option could be set to constant.
Kindly let us know if further assistance would be required.

Regards,
Nandeep
Technical Manager, MIDAS


Hi Nandeep,


Thanks for your answer, in my case I am not planning to use the construction stage function. Is there a way to get around the transformed section property in the basic stage? I tried this setting but it doesn't help in the basic stage. which means I still have to manually delete the strands to get the non-transformed section property throughout all the loadcases?




Hello,

In that case, I'd recommend creating a dummy construction stage as done in the attached model.  Just activate the structure completely with all required boundaries in the same construction stage with 0 duration.  No load to be activated in stage.
Then, simply the beam section property change option could be set to constant and the required results could be obtained.  The modified model file is attached.  Kindly let us know if further assistance would be required.

Regards,
Nandeep
Technical Manager, MIDAS
Creation date: 10/17/2018 10:52 PM (dklee@midasit.com)      Updated: 10/18/2018 8:22 AM (dklee@midasit.com)
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