ACI 318-14 Material Behaviors

This section explains how RAM Concept models the concrete, non-prestressed reinforcement and prestressed reinforcement when using ACI 318-14.

Concrete Behavior

You define the concrete elastic modulus in the materials window. You can choose to use code equation 19.2.2.1a with the inclusion of w_c 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 code equation is selected the following values are used:

E_{c i} = w_{c}^{1.5} 33 \sqrt{f_{c i}}

E_{c} = w_{c}^{1.5} 33 \sqrt{f_{c}}

where

f_ci	=	cylinder strength at stressing
f_c	=	28 day cylinder strength

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

See Concrete Stress-Strain Curves for a description of the stress-strain curves used in a detailed cross section analysis.

The strength stress-strain curves are truncated at a strain of 0.003. The other stress-strain curves have no limit strain.

Where applicable, the lightweight concrete modification factor is calculated using the values in Table 19.2.4.2 as follows:

Lightweight concrete (assumed density of 105 pcf): 0.75
Sand lightweight concrete( assumed density of 115 pcf): 0.85
Normalweight concrete (assumed density of 145 pcf): 1.0

Linear interpolation is used between these values, which is also explicitly permitted by the footnotes in the table. The maximum density for the cross section is used in the calculation.

(Non-prestressed) Reinforcement Behavior

This material is described in Non-prestressed Reinforcement Stress-Strain Curves in "Section Design Notes".

Bonded Prestressed Reinforcement Behavior

This material is described in Post-tensioning Material Stress-Strain Curves and Relationship of Bonded Post-tensioning Strains to Cross-Section Strains in "Section Design Notes".

Unbonded Prestressed Reinforcement Behavior

For service conditions, RAM Concept assumes that unbonded tendon stresses are not affected by cross section strains.

For ultimate resistance moment calculations, RAM Concept ’s general approach to unbonded tendon stress-strain curves is detailed in Unbonded Post-tensioning Stress-Strain Curves – Program Implementation in "Section Design Notes".

For ACI 318-14, the maximum unbonded tendon stress, f_limit, is defined by equations described in Table 20.3.2.4.1. In the calculation of ρ_p, RAM Concept assumes that the tendons are placed on the more beneficial side of the cross section centroid (the same limiting stress value is used for both positive and negative moment capacity calculations at each cross section).