Concrete Behavior

This elastic modulus of concrete is defined by the user in the materials window. You can choose to use the code equation of clause 6.2.3.1 or a specified value.

When you directly specify values, there must be two elastic modulus values:

E_ci = value for initial service (transfer) cross section analyses
E_c = value for all other conditions

When the IS 456 code equation is selected the following values are used:

E_{c i} = 5, 500 \sqrt{f_{c u i}}

E_{c} = 5, 500 \sqrt{f_{c u}}

where

f_cui	=	cube strength at stressing
f_cu	=	28 day cube strength

For calculations based on the "concrete section", concrete is assumed to be a perfectly linear-elastic material with no stress or strain limits.

For detailed cross section analyses, three different stress strain curves are used. All three stress-strain curves are parabolic-linear curves as detailed in IS456 Fig 21. The transition strain from the parabolic to the linear curve is at 0.002.

For initial stress conditions, the peak stress in the stress strain curve is
0.67f_cui
For service stress conditions, the peak stress in the stress-strain curve is
0.67f_cu
For strength conditions, the peak stress in the stress-strain curve is
0.67f_cu / 1.5

The strength stress-strain curves are truncated at a strain of 0.0035. The other stress-strain curves have no limit strain.

Note: Calculations on the gross cross-section always use the Ec values calculated above, while the cracked cross-section strain analyses use the stress strain curve of IS 456 Fig 21. The elastic modulus for these two conditions will therefore be different for most concrete strengths. This may have an effect on initial concrete strains and ECR calculations.

For service design crack width calculations and for service design cracked stress analyses, a tension stiffened concrete stress strain curve is used:

Use of this curve is similar, but not technically equivalent, to the provisions of IS 456 Annex F, Fig. 28. A comparison of the stress diagrams for the Code provision and the Concept implementation are shown below:

Figure 3.1 provision

RAM Concept implementation

Since RAM Concept ’s crack width design does a cracked stress analysis (with a non tension stiffened concrete stress strain curve) for the 0.8f_y provision, the concrete and rebar stress results for members on which a crack width design is done will represent the range of results between the tension stiffened and the non tension stiffened concrete stress strain curve. Concrete and rebar stress results for all other members will represent use the tension stiffened concrete stress strain curve only.