Question

Sketch and explain the two components of primary and secondary moments. Explain how we use design and the material properties of rebar and concrete to counter the two moment types.

Answer 1

these terms are related to pre-stress concrete

1.primary moment: this moment is produced due to TENDON ECCENTRICITY.generally moment diagram due to tendon follows the same shape as tendon.for example if tendon follows parabolic path then bending diagram for prestress is also parabolic.if tendon is below centroidal axis then it produces hogging moment and vice versa

2. secondary moment: this moment is due to internal reaction of member subjected to external load.it only depends on external; loads(live and dead loads) .

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while designing, the bending stresses in both cases should not exceed permissible values.the concrete may fail due to prestress(PRIMARY MOMENT) and applied loads(SECONDARY LOAD)..

STEEL tendon should exceed yielding strain before concrete ultimate strain to make sure of ductile failure

generally minimum of M40 grade concrete and fe500 steel tendon are used in design