stress-strain curves were calculated using 18 micro columns in each buckling layer (0 =

20 degrees).

 8 -
 -0- Exper. Peak Stress
 -9- Calculated Peak Stress



 6 <


 5


 4
 0 20 40 60 80
 No of Micro Columns


Figure 8-9. Sensitivity of model to the number of micro columns

 The stress-strain curve of the concrete cylinder was constructed by plotting the

critical stress against the strain. The parameters of the model such as the crack spacing

and the lengths of the micro columns are unknown and no experimental data were found

in the literature to indicate their value. Therefore, an assumption about the value of the

parameters was made. The parameters of the model were tuned to the specific

experimental data set of the grid confined cylinders from chapter 3. The parameters were

chosen to yield a curve that matched the experimental data from chapter 3. The set of

parameters that best matched the experimental data are presented in table 8-1. It was

assumed that the modulus of elasticity of concrete was equal to 4700 ksi (experimental

concrete strength 6.8 ksi), longitudinal cracks were oriented at an angle of 20 degrees

(approximate spacing 1.2 in. on the exterior face) and radial cracks at 0.15 in. The critical

buckling length of the exterior micro columns was assumed to be approximately 3.6 in.

Using these values the stress-strain curve of the concrete cylinder was constructed (see

Fig. 8-10).