At ultimate stage, why NA moves upward in concrete beam?

Why N.A. moves upward at ultimate stage in concrete beam?

Today, i started the series of questions ans answers related to design of concrete structures. I hope, you will enjoy these videos.
Following questions are included in this video
1- What is the min nos of steel bars provided in rectangular, circular and octagonal columns and what is the min diameter of steel bars should be used for columns?
2- How piles transfers the load to the soil and what are the types of it?
3- What assumption are taken for retained earth during the design of retaining wall?
4- What is drop panel and why it is provided?
5- What is the purpose of shear reinforcement in concrete beam and why it is provided?
6- For design purpose, how the depth and width of concrete beam is assumed?
7- What are the difference between under-reinforced beam, balanced beam and over reinforced beam?
8- At what section of concrete beam, chances of shear failure is dominant?
9- What is creep of concrete and what are the factors effecting the concrete creep?
10- Why thermal stresses produced in concrete and how they can be prevented?
11- What is the difference between singly reinforced beam and doubly reinforced beam?
12- Why neutral axis moves upward in concrete beam at ultimate stage?

Watch the full video and if you like it, please comment.

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