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:

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:

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

where:

Special Provisions exist for accounting flood loads and atmospheric loads i.e. Di and Wi

Load and Resistance Factor Design

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

1.4(D + F)
1.2(D + F + T) + 1.6(L + H) + 0.5(Lr or S or R)
1.2D + 1.6(Lr or S or R) + (L or 0.8W)
1.2D + 1.0W + L + 0.5(Lr 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.

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
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