Page 392 - Forensic Structural Engineering Handbook

STEEL STRUCTURES 11.23

apparent, a nonlinear analysis should be carried out. Many programs have the capability to
account for nonlinear characteristics of the material and the structural system, including
plasticity, large strain-rotation, and contact.

Design Codes: Loads and Resistance Factor Design
Increasingly, design codes are moving toward load and resistance factor design (LRFD). 29,30
The basic limit states are strength and serviceability. For structural steel buildings, the AISC
LRFD Specifications provides a sound means of assessing the strength of structural members
and connections. Similarly, the AASHTO Specifications provides a sound basis for assessing
the strength and performance of highway bridges. The serviceability limit state of fatigue and
fracture often is the cause of cracking under the random variable live loads that a structure is
subjected to. The HS15 fatigue truck represents the root mean cube load that is characteristic
of the variable load spectrum that highway bridges will be subjected to. It is based on the lin-
ear damage hypothesis attributed to Miner. The load cycles from vehicle passage produce
stress range cycles that can be related to life in the exponential model given in Eq. (11.13).
The specification uses an impact factor for fatigue equal to 0.15 which is based on experi-
mental observations of in-service bridges. Steel railroad bridges are designed by American
Railway Engineering and Maintenance-of-Way Association (AREMA) 31 Specifications.
These remain an allowable stress design for strength.

Other Major Codes

Several other codes are used to design and assess steel structures. Some are relied upon by
several of the design codes for specific issues. For example, the American Society for
Testing and Materials provides standardization of materials and methods of testing which
are relied upon by the AISC, AASHTO, and AREMA Specifications for designing build-
ing and bridge structures. Likewise, the American Welding Society (AWS) Specifications
are utilized to identify welding electrodes and fluxes, to qualify welders and weld
processes, and to provide quality assurance. Steel railroad bridges are designed and rated
by Committee 15 of the AREMA Specifications. These remain in the allowable stress
design format. Serviceability limits for fatigue and fracture use a design reference load for
new construction. Actual traffic is used to rate and assess existing bridges. Other codes such
as the AISI Load and Resistance Factor Design Specification for Cold-Formed Steel
Members deals with light structural building components such as purlins, columns, and
joists; metal deck provides a means of assessing lightgauge components and systems. The
ASME Pressure Vessel Code and the API Code are useful for special structures and have
recently published comprehensive fitness-for-service standards (Ref. 3), which can be used
to evaluate many of the failure-damage mechanisms described above.

CRITICAL CHARACTERISTICS OF STEEL
STRUCTURES

Design Parameters
Members such as beams, columns, and their connections are prepositioned to resist design
loads. The applicable design limit states must be satisfied to ensure that the component is
capable of resisting the applied loads. Beams, for example, must satisfy the applicable limit