Structural Steel

Design Methods

The AISI Specification is structured such that nominal strength equations are given for various types of structural members such as beams and columns. For allowable stress design (ASD), the nominal strength is divided by a safety factor and compared to the required 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. These procedures and pertinent load combinations to consider are set forth in the specification as follows.

ASD Requirements

ASD Strength Requirements. A design satisfies the requirements of the AISI Specification
when the allowable design strength of each structural component equals or exceeds the
required strength, determined on the basis of the nominal loads, for all applicable load
combinations. This is expressed as

ASD Load Combinations. In the absence of an applicable code or specification or if the applicable code or specification does not include ASD load combinations, the structure and its components should be designed so that allowable design strengths equal or exceed the effects of the nominal loads for each of the following load combinations:

1. D
2. D + L + (Lr or S or Rr)
3. D  (W or E)
4. D + L + (Lr or S or Rr) + (W or E)

Wind or Earthquake Loads for ASD. When the seismic load model specified by the applicable code or specification is limit state based, the resulting earthquake load (E) is permitted to be multiplied by 0.67. Additionally, when the specified load combinations include wind or earthquake loads, the resulting forces are permitted to be multiplied by 0.75. However, no decrease in forces is permitted when designing diaphragms.

Composite Construction under ASD. For the composite construction of floors and roofs using cold-formed deck, the combined effects of the weight of the deck, the weight of the wet concrete, and construction loads (such as equipment, workmen, formwork) must be considered.

LRFD Requirements

LRFD Strength Requirements. A design satisfies the requirements of the AISI Specification when the design strength of each structural component equals or exceeds the required strength determined on the basis of the nominal loads, multiplied by the appropriate load factors, for all applicable load combinations. This is expressed as

 

LRFD Load Factors and Load Combinations. In the absence of an applicable code or specification, or if the applicable code or specification does not include LRFD load combinations and load factors, the structure and its components should be designed so that design strengths equal or exceed the effects of the factored nominal loads for each of the following combinations:

 

Several exceptions apply:
1. The load factor for E in combinations (5) and (6) should equal 1.0 when the seismic load model specified by the applicable code or specification is limit state based.
2. The load factor for L in combinations (3), (4), and (5) should equal 1.0 for garages, areas occupied as places of public assembly, and all areas where the live load is greater than 100 psf.
3. For wind load on individual purlins, girts, wall panels and roof decks, multiply the load factor for W by 0.9.
4. The load factor for Lr in combination (3) should equal 1.4 in lieu of 1.6 when the roof live load is due to the presence of workmen and materials during repair operations.
Composite Construction under LRFD. For the composite construction of floors and roofs using cold-formed deck, the following additional load combination applies:

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