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Answer:
Explanation:
Prestressing of Concrete:
Prestressing of concrete is an idea of subjecting the concrete section to compressive force before exposing to service load so that after the service load is applied, the whole section does not subject to any tensile stress even below the neutral axis.
There are three different stress in a prestressed section: (I) Direct compressive stress due to prestressing, (II) stress due to external loading and (III) stress due to eccentricity of the prestressing force.
The resultant stress in the section is the sum of these three stresses.
Calculation:
The different stresses of the prestressing section are shown in the following diagram:
F1 N.M 12.8.20 Pallavi D 5
Here
P is the total prestressing force, M is the bending moment due to external loading,
A is the cross-sectional area, I is moment of inertia of the section,
e is the eccentricity of the pre-stressing force from neutral axis and
y is the distance from neutral axis.
So, resultant stress at the bottom face of the beam is,
Now, Total prestressing force, P = effective prestress × area of prestressing wires
Given,
From diagram,
Given, stress at the bottom face must to be zero.
Hence,
Hence, the maximum sagging moment that can be applied without causing tensile stress is
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