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).