Steel design

Steel design, or more specifically, structural steel design, is an area of knowledge of structural engineering used to design steel structures. The structures can range from schools to homes to bridges.

There are currently two schools of thought in steel design. The oldest is the Allowable Strength Design (ASD) method. The second, and most recent, is the Load and Resistance Factor Design (LRFD) method. See "AISC Steel Construction Manual" (SCM), 13 ed., 2006, pp. 2-8.

Design for strength

ASD

Design according to the provisions for ASD satisfies the requirements of the AISC Specification when the allowable strength of each structural component equals or exceeds the required strength determined on the basis of the ASD load combinations. This explanation in symbols is:

:R_a ≤ R_n/Ω

where
R_a = required strength,
R_n = nominal strength, specified in Chapters B through K of the AISC SCM,
Ω = safety factor, specified in Chapters B through K of the AISC SCM,
R_n/Ω = allowable strength.

LRFD

Design according to the provisions for LRFD satisfies the requirements of the AISC Specification when the allowable strength of each structural component equals or exceeds the required strength determined on the basis of the LRFD load combinations. This explanation in symbols is:

:R_u ≤ φ·R_n

where
R_u = required strength,
R_n = nominal strength, specified in Chapters B through K of the AISC SCM,
φ = resistance factor, specified in Chapters B through K of the AISC SCM,
φ·R_n = allowable strength.

ASD versus LRFD

It should be noted that, as per the AISC SCM, 13 ed., either design process is allowed in the design of steel structures. A common misconception about the two design procedures is that ASD gives a more conservative value as opposed to LRFD. In reality, ASD is more conservative when designing structures with a live to dead load ratio of 3 or lower. With a higher live to dead load ratio, LRFD will give the more conservative design.

Furthermore, the two design processes can be related between the Ω factor of ASD and the φ factor of LRFD. While these factors have different uses, they are always related by the following expression:

:Ω = 1.5/φ

Load combination equations

Allowable Strength Design

For ASD, the required strength, R_a, is determined from the following load combinations (according to the AISC SCM, 13 ed.):

D
D + L
D + (Lr or S or R)
D + 0.75L + 0.75(Lr or S or R)
D ± (W or 0.7E)
D ± 0.75(W or 0.7E) + 0.75L + 0.75(Lr or S or R)
0.6D ± (W or 0.7E)

where
D = dead load,
L = live load due to occupancy,
Lr = roof live load,
S = snow load,
R = nominal load due to initial rainwater or ice, exclusive of the ponding contribution,
W = wind load,
E = earthquake load.

Load and Resistance Factor Design

For LRFD, the required strength, R_u, is determined from the following factored load combinations:

1.4D
1.2D + 1.6L + 0.5(Lr or S or R)
1.2D + 1.6(Lr or S or R) + (0.5L or 0.8W)
1.2D + 1.6W + 0.5L + 0.5(Lr or S or R)
1.2D ± 1.0E + 0.5L + 0.2S
0.9D ± (1.6W or 1.0E)

where the letters for the loads are the same as for ASD.

AISC Steel Construction Manual

American Institute of Steel Construction (AISC), Inc., is currently on their thirteenth edition of the "AISC Steel Construction Manual" (SCM). This manual is used by structural engineers in analyzing, designing, and renovating various steel structures. The manual contains several chapters of which includes:

Dimensions and properties of various types of steel sections available on the market (W, S, C, WT, HSS, etc.)
General Design Considerations
Design of Flexural Members
Design of Compression Members
Design of Tension Members
Design of Members Subject to Combined Loading
Design Consideration for Bolts
Design Considerations for Welds
Design of Connecting Elements
Design of Simple Shear Connections
Design of Flexure Moment Connections
Design of Fully Restrained (FR) Moment Connections
Design of Bracing Connections and Truss Connections
Design of Beam Bearing Plates, Column Base Plates, Anchor Rods, and Column Splices
Design of Hanger Connections, Bracket Plates, and Crane-Rail Connections
Specifications and Codes
Miscellaneous Data and Mathematical Information
General Nomenclature