fig 21 stress-strain curve for concrete
fig 22 stress block parameter
how fig 22 is comes from fig 21 ?
For design purposes. the compressive strengthof concretein the structure shall be assumed tobe 0.67times the characteristic strength. The partial safety factor = 1.5
Area of stress block =Area of stress block for (rectangular portion+parabolic portion)=(0.1928+0.6428)fckXu=0.364fckXu
sure...from fig 21, you will observe that stress-strain diagram is parabolic upto strain of 0.002. from strain of 0.002 to 0.0035, the diagram is straight line. the value of stress is 0.67fck/gamma m. i.e 0.67fck/1.5=0.446fck. this is the value of stress in the fig 22. then (0.002/0.0035)xu part is parabolic and remaining ((0.0035-0.002)/0.0035)xu part straight line. area of the stress block
straight portion
(3/7)xu x 0.446fck= 0.191 fck xu
parabolic portion
2/3 x (4/7)xu x 0.446fck= 0.1699 fck xu
total area of the stress block = 0.191 fck xu + 0.1699 fck xu = 0.360 fck xu
so from fig 21
you know the stress value = 0.67 fck/gamma m
nature of the stress strain curve..i.e parabolic portion and straight portion
these data is required to calculate the area of the stress block of fig. 22
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