Steel design

Design for strength

[edit]ASD

In this method, the engineer uses the ASD load combinations (below) to determine the required strength of a member and arranges for the allowable strength to satisfy this equation:

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.

[edit]LRFD

In this method, the engineer uses the Load and Resistance Factor Design (LRFD) load combinations (below) to determine the required strength of a member and arranges for the allowable strength to satisfy this equation:

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.

[edit]ASD versus LRFD

As per the AISC SCM, 13 ed., either design method is allowed by the AISC SCM 13th edition. A common misconception about the two methods is that ASD gives a more conservative value. In reality, ASD is more conservative in designs with a live to dead load ratio of 3 or lower. With a higher ratio, LRFD is more conservative.

The two design methods are related through 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:

The value of these factors vary according to the country codes.

[edit]Load combination equations

[edit]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 + (0.75W 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.

[edit]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.

For the wind consideration, the ASCE allows a "position correction factor" which turns the coefficient of wind action to 1,36:

1,2D + 1,36W + .... the same above or 0,9D - 1,36W

[edit]AISC Steel Construction Manual

American Institute of Steel Construction (AISC), Inc. publishes the AISC Manual of Steel Construction (Steel construction manual, or SCM), which is currently in its fourteenth edition. Structural engineers use this manual in analyzing, and designing various steel structures. Some of the chapters of the book are as follows.

• 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

[edit]References

1. ^ Steel Construction Manual (13th ed.). American Institute of Steel Construction. 2006. ISBN 156424055X.