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Re: 2D and 3D design

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This message is being sent in the hopes of finding possible research sources regarding 2 dimensional versus 3 dimensional design problems/approaches.  I am currently studying to receive my B.S in civil engineering, and I am primarily interested in structures.  My senior project for California Polytechnic State University at San Luis Obispo is an analysis of simple frames in 2D and the identical model with identical loading in 3D.  I am comparing the results obtained from SAP 2000 and Risa 2D and I am coming up with optimum design scenarios regarding top story deflection and steel weight.  The designs are performed as per LRFD.  I am also comparing the effects of bracing a system versus not.  I am wondering how practicing engineers primarily perform 2, 2D analyses when from my studies I have found that in 3D for an identical model from 2D, the stress combinations are generally larger.  My steel member sizes increase from a 6x25 to  say a 8x35 in some cases due to the out of plane or weak axis bending in 3D that is not taken into account in 2D.  Also, the out of plane stiffness of other members contribute to the in plane stiffness, thus attracting larger forces for the in plane members, which in some cases increases the member sizes.  If some of you out there have any comments, suggestions, or sources of research on the subject it would be very appreciated as I am eager to learn more about these types of problems and how to successfully solve them.
Andrew Eric Arnold