A rectangular plate will not experience any shear lag effect when connected to a gusset plate.
Shear lag is the non-uniform straining of member due to tension and it occurs when action and reaction do not pass through the centre of gravity of cross section.
The shear lag effect can be reduced if there is symmetry in connection i.e. eccentricity due unsymmetrical connection is minimum as much as possible.
When double angle member placed symmetrically to gusset plate i.e. placed back to back on opposite side of gusset plate, no shear lag effect is developed.
An I Section with bottom flange Connected to gusset plate, An angle member with one leg connected to gusset plate and two angle members placed back to back on same side of gusset plate , these are unsymmetrical sections, where action and reaction do not passes through the CG as shown in figure, so shear lag effect may occur.
(A) → CG of section/Reaction
(B) → Action of load
e → eccentricity
‘e’ causes shear lag in all above members