Page 391 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch09.qxd 9/29/05 5:25 PM Page 9.3

COLD-FORMED STEEL DESIGN

COLD-FORMED STEEL DESIGN 9.3

working and strain aging effects as discussed in Art. 1.10. The strength increase, which may be
small for sections with few bends, can be conservatively neglected. Alternatively, subject to cer-
tain limitations, the AISI NAS includes provisions for using a section-average design yield stress
that includes the strength increase from cold-forming. Either full-section tension tests, full-section
stub column tests, or an analytical method can be employed. Important parameters include the tensile-
strength-to-yield-stress ratio of the virgin steel and the radius-to-thickness ratio of the bends. The
forming operation may also induce residual stresses in the member, but these effects are account-
ed for in the equations for member design.

9.3 NOMINAL LOADS

The nominal loads for design should be according to the applicable code or specification under
which the structure is designed or as dictated by the conditions involved. In the absence of a code or
specification, the nominal loads in the United States and Mexico should be those stipulated in the
American Society of Civil Engineers Standard Minimum Design Loads for Buildings and Other
Structures, ASCE 7. In Canada, the loads are specified in the National Building Code of Canada. The
following loads are typically considered.
D = dead load, which consists of the weight of the member itself, the weight of all materials of
construction incorporated into the building which are supported by the member, including built-in
partitions, and the weight of permanent equipment
E = earthquake load
L = live loads due to intended use and occupancy, including loads due to movable objects and
movable partitions and loads temporarily supported by the structure during maintenance (L includes
any permissible load reductions; if resistance to impact loads is taken into account in the design,
such effects should be included with the live load)
L r = roof live load
S = snow load
R r = rain load, except for ponding
W = wind load
The effects of other loads, such as those due to ponding, should be considered when significant. Also,
unless a roof surface is provided with sufficient slope toward points of free drainage or adequate
individual drains to prevent the accumulation of rainwater, the roof system should be investigated to
assure stability under ponding conditions.

9.4 DESIGN METHODS

The AISI NAS is structured such that nominal strength equations are given for various types of struc-
tural members such as beams and columns. For allowable strength design (ASD), the nominal
strength is divided by a safety factor and compared to the allowable strength based on nominal loads.
For load and resistance factor design (LRFD), the nominal strength is multiplied by a resistance
factor and compared to the required strength based on factored loads. In Canada, this latter method
is known as limit states design (LSD). Both ASD and LRFD are used in the United States and
Mexico. LSD is the mandatory method in Canada. Procedures and pertinent load combinations to
consider are set forth in the specification as follows.

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