sji 1.15 factor for moi

huangyhg

sji 1.15 factor for moi
can someone give me a reference why the sji says to reduce the moment of inertia by 1.15 when calcing deflection of steel trusses?
thanks

the factor is used to account for the affect of the axial deformations in the webs and for joint eccentricity.  
a discussion and an alternate means of this calculation can be found here:
i believe it is because a joist isn't really a "beam", so to speak, and the traditional deflection formula (5wl4/(384ei)) won't capture the difference in behavior.