MOMENT DISTRIBUTION WHEN PARTIALLY RELEASING MOMENTS IN BEAM

Question

How does Moment distribution and release for beams work in STAAD Pro ? 
 While designing Secondary Beams, I&rsquo;m releasing the moments MX, MY, MZ to a 100% at their ends. 
 if I keep the ends fixed, I get a hogging moment say &lsquo; X &rsquo; kN-m at the ends. 
 However, if I partially release (say 40% of) the moments at ends keeping MXY and MXZ to 0.4, why am I not getting hogging moment 60% of &lsquo; X &rsquo; moment. My value in this case is lesser but closer to X, but not 0.6 times X. 
 Somebody please elucidate. 
 Thank you. 
 
 Robin Thomas

Modestas · Accepted Answer

The partial moment factor that you specify is used as a multiplication factor on the corresponding term in the stiffness matrix. The superfluous stiffness amount is then removed from the stiffness matrix and load vector using the process of static condensation. 
 There is no guarantee that a 50% reduction in the stiffness term will bring down the moment by 50%, primarily because that moment is redistributed as a shear and moment to other points on the structure, and the moment at that end of the member will be based on the reduced stiffness. You may need to use a higher factor like 0.75 to achieve a 50% reduction in stiffness. 
 You could refer to books such as &ldquo;Matrix Structural Analysis&rdquo; by Ronald Sack in which he describes the static condensation technique in the context of member releases. While the author only talks about &ldquo;full releases&rdquo;, the technique used in STAAD is the same, except that the stiffness can be reduced by any fraction using the partial moment factor.

MOMENT DISTRIBUTION WHEN PARTIALLY RELEASING MOMENTS IN BEAM

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