horizontal deflection calculation of building frame

huangyhg

horizontal deflection calculation of building frame
for a typical one bay one story building frame. there is a equation to calculate the lateral deflection (like story drift) of the frame. vh^3/(12eic)+vh^2*l/(12eig). in this equation, i can understand the first item. but could somebody explain the second item to me. i konw the second must come from the rotation of the joint and has something to do with the beam stiffness. is this formula from the stiffness marix (6x6) of a beam member?
thx.  
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yes, that term is being derived from the moment at the joints and the stiffness of the girder. the terms listed are derived from the flexural parts of the stiffness matrix of the structure assuming axial and shear deformations are negligible.