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What are Ec (S),
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Ec (D)
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, and
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Ec (V)
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in composite beam design?
Answer: Values for Ec (S), Ec (D), and Ec (V) are given as follows:
- Ec (S) is the modulus of elasticity for strength design.
- For LRFD, Ec (S) is taken directly as the modulus of elasticity of the material assigned to the composite section.
- For ASD, Ec (S) is calculated as 33 * gammac1.5 * √f 'c (ksi), where gammac is the weight per unit volume (kips/in3) of the material assigned to the composite section.
- For LRFD, Ec (S) is taken directly as the modulus of elasticity of the material assigned to the composite section.
- Ec (D) is the deflection modulus of elasticity, given as Ec (S) / (creep factor), where the creep factor is specified in the Preference menu.
- Ec (V) is the vibration modulus of elasticity, given as 1.35 * Ec (S). This amplification coefficient is suggested in AISC Guide 11, page 12.
Since these modular ratio are different, transformed inertia varies between strength, deflection, and vibration analysis.
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Related Incident: Creep factor 195684 - For both LRFD and ASD, Ec should be computed directly from ACI 8.5, though ETABS does not currently follow such process while implementing this formulation. This article reflects the current ETABS application process. Regardless, results are not affected since concrete density is manually input according to that specified for deck material within ACI 8.5. |