- 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}/Ωwhere

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

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