Section 8.4.B.3.a of the ‘Aluminum Windos and Doors, and Glazing’ specifications states the following:
The AAMA standard dealing with storefront design is ‘AAMA SFM-1-87: Aluminum Storefront and Entrance Manual’.
Engineering Section 6.0 (shown below) directs us to the current Aluminum Association’s Aluminum Design Manual (2010):
The Aluminum Design Manual, section F.2 (shown below), indicates that when framing members are subject to lateral buckling, the nominal flexural strength must consider the un-braced length of the framing in question:
We want you to have this information to give you a better understanding of why we are designing open shaped verticals mullions to prevent lateral-torsional buckling. This increases the stability and life of the storefront wall installation providing a higher level of client satisfaction to the end user.