Now we shall move on with a solved example. This will help you understand the methods in a better way. I suggest that you do them yourselves too. Practice will help you make your concepts more concrete and clear.
An reinforced concrete 300mm x 600mm effective dimensions is provided with tensile and compressive reinforcement of 1256mm2 each. The compressive steel is placed 30mm from the top edge of the beam. If σcbc = 7N/mm2, σst = 190N/mm2 and m = 13.33, find the moment of resistance of beam by following two methods:
Sections that have tensile as well as compressive reinforcement are called doubly reinforced sections.
Necessity of design of doubly reinforced sections
When the dimensions of the beam are restricted for architectural or structural considerations, the section has insufficient area of concrete which results in inability of the beam to take sufficient compressive stresses. If not paid attention to, it could result in structural failure.
To solve this problem, steel is placed in the compressive area of the section to help the concrete section in resisting compressive stresses. (Steel is good at taking up both compression and tension.)
In this way, the moment of resistance of the section is increased without altering its dimensions.
Three important conditions where doubly reinforced sections are to be used:
1) When the dimensions of the beam are restricted for architectural or structural purposes.
2) Sections that are subjected to the reversal of bending moment (piles, braces in water towers etc.
3) The portion of the beam over middle support in continuous T beams has to be designed as doubly reinforced section.
We are now going to begin with a series of articles on “Design of Doubly reinforced sections”. In our previous series of articles for “Singly reinforced sections“, we have covered every step in detail for the design and analysis of Singly reinforced sections.
We would be covering the following for “Doubly reinforced Sections”: