Long Term Deflection due to creep.

Structures are built to last. Most modern structures have a life expectancy much more than a regular person. So it's only natural to assume Engineers designed for it. We do design checks in this regard too. 

It was not always a thing though, at least that's what I believe. And I will tell you why I think so. Civil Engineering is the oldest branch and has gone through some vey serious updates.


My trade initially started with the Working Stress Method. It is also called the traditional method. In this, the design is based only within the elastic limit. Which means, the materials used for construction behaves in a linear elastic manner. You know how an elastic rubber works, it won't break unless stretched through its capacity. The philosophy is similar. As long as it is within the limit, it won't fail. But to make the structure not reach its limit, the materials required was much more. This, at times, leads in very un-conservative designs. This Method is good for critical structures like Dams, Nuclear plants etc.

Then there was the Ultimate Load Method and now the Limit State Method. LSM is the one we work on. The codes around the world is based on this. Although one can argue there are specific codes, this is the one we use on a daily basis.

Now let's get on to the topic - Long Term Deflection. As time passes on, a structure loses some of its strength. It can be caused mainly due to differential shrinkage, due to temperature variation and creep caused by sustained loading. Creep plays a major role in this. This blog will be on Creep.

What is Creep? It can be defined as the deformation of structure under sustained loading. This sustained loading has a very high tenure. 

IS Code: The long term deflection is calculated by the change in Young's modulus of the structure. After a certain level of creep, the strain changes are more significant for a slight change in stress. This is depicted in the image below. Hence, the new Young's modulus:


The procedure for calculation can be found in the IS code Annex C. Here you can find deflection due to Shrinkage too.

ACI Code: If not done a comprehensive analysis, the long term deflection is calculated by multiplying the short term deflection by a factor.
where rho denotes the percentage of steel.
The effect of shrinkage and creep is better studied by the ACI code and that will be for a different blog.

Now, how does one implement a LTD check on ETABS. You can find it on the internet. Just remember to set the modifiers to 1.
https://wiki.csiamerica.com/display/safe/Cracked-section+analysis

Always ready for discussion.





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