CEMP-E
TI 809-07
NOVEMBER 1998
the serviceability limit state would include deflections and flange curling. Many times section will
look distressed, buckled or distorted and still have a great deal of strength remaining. Therefore,
serviceability limit states are set to reduce distortion effects.
g. Design Thickness. Designers need to calculate the sectional properties using the
design or nominal thickness of the material. The AISI design equations do account for the
minimum or delivered thickness difference. When using the AISI specification the equation for
calculating the nominal strength of the section are the same for Allowable Strength Design (ASD)
or Load Factor Resistant Design (LRFD) methods. The method of calculation of section
properties is unique to the AISI specification as effective sections properties are used in many of
their procedures. Thin materials and high strength steels combine to make sections that are
subject to not only local buckling, lateral buckling, but also lateral-torsional buckling, and
therefore, sections are not always fully effective. Non-fully effective sections need to have their
effective section properties calculated. This calculation is a iterative process for C and Z
sections, when the neutral axis is located nearer to the tension flange of the section, and the
section properties are calculated based on the compression flange yielding first. The effective
section is based on the effective flat widths of the compression-flange, the flange-lip, and the
section's web. All sections are to be designed to develop their full strength using an all steel
design. This means that all cold-formed steel sections are to be braced with steel to prevent
lateral or lateral-torsional buckling created by lateral or twisting loads. Sheathing and gypsum
wall board are not allowed to brace the stud section. When the section is fully effective the
nominal moment capacity is equal to the fully effective section modulus times the yield strength of
the steel. Typically, cold-formed sections are too thin to develop plastic sections and cannot
redistribute plastic moments. Also, web crippling is usually required at concentrated loads and at
beam supports, and web reinforcement may be required around pipe openings. Around these
areas of the beam designers should check the requirements for beam web stiffening with
reinforcement plates at pipe opening or web crippling at supports and concentrated loads as
necessary.
4. DESIGN OF STRUCTURAL ELEMENTS
a. AISI Specification. The AISI Specification for the Design of Cold-Formed Structural
Members-1996 applies to the design of all cold-formed steel members.
b. Preliminary Member Selection. A good report to use when designing cold-formed steel
sections is the AISI Report CF 93-1, "Preliminary Design Guide for Cold-Formed C and Z
Members", June 1993. This report uses Gross section properties and reduced stresses to size
cold-formed sections. It has in it design procedures for strength in bending, strength in shear,
strength in combined bending and shear, strength in web crippling, strength of concentrically
loaded compression members, and strength in combined axial and bending. Designers using this
procedure will normally be conservative by 5 to 15%.
c. Element Behavior. Designers will quickly learn about the characteristics of thin
compression elements, elastic buckling, and post buckling strength. Within the AISI criteria
stresses are referred to as flat plate elements and plate stresses, these are in reference to the flat
rolled elements of the cross section being analyzed. Designers should note that AISI assumes
that flat plate stresses are assumed to be uniform across the plate element's width as the plate
reaches the critical plate buckling stress, which has a nonuniform distribution. Sine wave ripples
along the length of the member characterize buckling of a thin compression element. Thin plate
elements go into an elastic buckling mode when the longitudinal plate stresses exceed fcr, the
critical buckling stress of the element as calculated by Equation 2-1. As flat elements buckle the
plate stresses are redistributed to the stiffened edges of the plate, and edge stresses approach
the yield stress of the steel. Therefore, the effective width is a function of the design stress. This
redistribution effect was developed by Mr. George Winter at Cornell University, and has been
used by the airline industry since the 1940's. This is the basis of the effective width concept used
to design cold-formed steel section. The constant (lambda) as shown in Equation 2-2 is used to
calculate the effective width of the flat element. Values of less than or equal to 0.673 indicate a
2-5
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