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.

In structural engineering, a structure is a body or combination of pieces of rigid bodies in space to form a fitness system for supporting loads. Structures such as buildings, bridges, aircraft and ships are all examples under steel structure. The effects of loads on structures are determined through structural analysis. Steel structure is steel construction material, a profile, formed with a specific shape or cross section and certain standards of chemical composition and mechanical properties.

There are currently two common methods of steel design: The first (and older) method is the Allowable Strength Design (ASD) method. The second (newer) is the Load and Resistance Factor Design (LRFD) method.^[1]

Design for strength

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:

$R_a \le {R_n \over \Omega}$

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

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:

$R_u \le \phi * R_n$

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 = design strength.

ASD versus LRFD

As per the AISC SCM, 14th ed., either design method is allowed by the AISC SCM 14th 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:

$\Omega = \frac{1.5}{\phi}$

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

Load combination equations

Allowable Stress Design

For ASD, the required strength, R_a, is determined from the following load combinations (according to the AISC SCM, 13 ed.) and:^[2]

D + F
D + H + F + L + T
D + H + F + (L_r or S or R)
D + H + F + 0.75(L + T) + 0.75(L_r or S or R)
D + H + F ± (W or 0.7E)
D + H + F + (0.75W or 0.7E) + 0.75L + 0.75(L_r or S or R)
0.6D + W + H 0.6D ± (W or 0.7E)

where:

• D = dead load,
• D_i = weight of Ice,
• E = earthquake load,
• F = load due to fluids with well-defined pressures and maximum heights,
• F_a = flood load,
• H = load due to lateral earth pressure, ground water pressure, or pressure of bulk materials,
• L = live load due to occupancy,
• L_r = roof live load,
• S = snow load,
• R = nominal load due to initial rainwater or ice, exclusive of the ponding contribution,
• T = self straining load,
• W = wind load,
• W_i = wind on ice..

Special Provisions exist for accounting flood loads and atmospheric loads i.e. D_i and W_i

Load and Resistance Factor Design

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

1.4(D + F)
1.2(D + F + T) + 1.6(L + H) + 0.5(L_r or S or R)
1.2D + 1.6(L_r or S or R) + (L or 0.8W)
1.2D + 1.0W + L + 0.5(L_r or S or R)
1.2D ± 1.0E + L + 0.2S + 0.9D + 1.6W + 1.6H
0.9D + 1.6 H ± (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

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 14th 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
• General Nomenclature
• Specifications and Codes
• Commentary on Specifications and Codes
• Miscellaneous Data and Mathematical Information

CISC Handbook of Steel Construction

Canadian Institute of Steel Construction publishes the "CISC Handbook of steel Construction". CISC is a national industry organization representing the structural steel, open-web steel joist and steel plate fabrication industries in Canada. It serves the same purpose as the AISC manual, but conforms with Canadian standards.

References

1. Steel Construction Manual (13th ed.). American Institute of Steel Construction. 2006. ISBN 1-56424-055-X.
2. http://peer.berkeley.edu/~yang/courses/ce248/CE248_CN_Loading_and_Gravity_loads.pdf