AECT250-Lecture 31

AECT250-Lecture 31 - Lecture 31 Serviceability...

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Lecture 31 - Page 1 of 7 Lecture 31 – Serviceability Serviceability refers to the structural “performance” of the finished building under service loads. Beam deflection Lateral drift Vibration We will be focusing our discussion on beam deflection. The ACI 318-02 Code dictates that the deflections be checked on the basis of effective moment of inertia, I e , under service loads . Before we can determine the value of the effective moment of inertia, we must first have an understanding of the gross moment of inertia, I g , and the cracked moment of inertia, I cr . Gross Moment of Inertia I g : The gross moment of inertia is not appropriate for reinforced concrete beams because the concrete under the neutral axis is in tension and is ineffective. Since tension is carried by the steel rebar, the beam becomes composite and therefore must be analyzed as such (See AECT 210 – Lecture 6). The calculated value of gross moment of inertia is higher than what is actually present. h b I g = Gross moment of Inertia = 12 3 bh
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Lecture 31 - Page 2 of 7 Cracked Moment of Inertia I cr : The cracked moment of inertia takes into consideration the composite action between the concrete and steel rebar. This assumes that the concrete in the tension zone is totally ineffective, which is overly conservative.
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AECT250-Lecture 31 - Lecture 31 Serviceability...

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