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Balanced section, tension controlled section and compression controlled section of reinforced concrete beams

RC beams where reinforcing steel yields at the same time in which concrete strain at extreme compression fiber is 0.003(concrete reach it is flexural capacity at this value of strain) called balanced section.
Figure 1

in compression controlled section the strain at extreme fiber for concrete reached 0.003 before steel yield. in this case, the section is over reinforced. for compression controlled section slight and unnoticeable deflections will occur to RC beams. this will result in a sudden failure without any warning in case of overloading. these members called as brittle members and it should be avoided.

Figure 2

in tension controlled members. reinforcing steel yields before the concrete strain at extreme compression fiber reaches 0.003. in this type of member, significant deflections will occur before failure. giving sufficient time and warning for the looming failure. these members can be called as a ductile section also. member with εt≥0.005 is fully ductile.  ACI code selects this value for all type of reinforcing steel used for concrete member including prestressing steel. for members where εyεt≤0.005 considered in transition between tension controlled and compression controlled sections. for reinforcing steel grade 60 (420) εy considered as 0.002.



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