Design Requirements
Home > GERTC Online Reference > Structural Engineering > Steel Design > Design Requirements
SECTION 502 DESIGN REUIREMENTS
502.1 General Provisions
The design of members and connections shall be consistent with the intended behavior of the framing system and the assumptions made in the structural analysis. Unless restricted by this chapter, lateral load resistance and stability may be provided by any combination of members and connections.
502.2 Loads and Load Combinations
The loads and load combinations shall be as stipulated by this chapter. In the absence of a building code, the loads and load combinations shall be those stipulated in Chapter 2. For design purposes, the nominal loads shall be taken as the loads stipulated by this chapter.
502.3 Design Basis
Designs shall be made according to the provisions for Load and Resistance Factor Design (LRFD) or to the provisions for Allowable Strength Design (ASD).
502.3.1 Required Strength
The required strength of structural members and connections shall be determined by structural analysis for the appropriate load combinations as stipulated in Section 502.2.
Design by elastic, inelastic or plastic analysis is permitted. Provisions for inelastic and plastic analysis are as stipulated in Appendix A-1, Inelastic Analysis and Design. The provisions for moment redistribution in continuous beams in Appendix A-1, Section A-1.3 are permitted for elastic analysis only.
502.3.2 Limit States
Design shall be based on the principle that no applicable strength or serviceability limit state shall be exceeded when the structure is subjected to all appropriate load combinations.
502.3.3 Design for Strength Using Load and Resistance Factor Design (LRFD)
Design according to the provisions for LRFD satisfies the requirements of this chapter when the design strength of each structural component equals or exceeds the required strength determined on the basis of the LRFD load combinations. All provisions of this chapter, except for those in Section 502.3.4, shall apply.
Design shall be in accordance with Eq. 502.3- 1:
Design according to the provisions for ASD satisfies the requirements of this chapter when the allowable strength of each structural component equals or exceeds the required strength determined on the basis of the ASD load combinations. All provisions of this chapter, except those of Section 502.3.3, shall apply.
Design shall be in accordance with Eq. 502. 3-2:
502.3.5 Design for Stability
Stability of the structure and its elements shall be determined in accordance with Section 503.
502.3.6 Design of Connections
Connection elements shall be designed in accordance with the provisions of Sections 5 IO and 511. The forces and deformations used in design shall be consistent with the intended performance of the connection and the assumptions used in the structural analysis.
502.3.6.1 Simple Connections
A simple connection transmits a negligible moment across the connection. In the analysis of the structure, simple connections may be assumed to allow unrestrained relative rotation between the framing elements being connected. A simple connection shall have sufficient rotation capacity to accommodate the required rotation determined by the analysis of the structure. Inelastic rotation of the connection is permitted.
502.3.6.2 Moment Connections
A moment connection transmits moment across the connection. Two types of moment connections, FR and PR, are pem1itted, as specified below.
1. Fully-Restrained (FR) Moment Connections
Fully-restrained (FR) moment connections transfer moment with a negligible rotation between the connected members. In the analysis of the structure, the connection may be assumed to allow no relative rotation. An FR connection shall have sufficient strength and stiffness to maintain the angle between the connected members at the strength limit states.
2. Partially-Restrained (PR) Moment Connections
Partially-restrained (PR) moment connections transfer moments, but the rotation between connected members is not negligible. In the analysis of the structure, the force-deformation response characteristics of the connection shall be included. The response characteristics of a PR connection shall be documented in the technical literature or established by analytical or experimental means. The component elements of a PR connection shall have sufficient strength, stiffness, and deformation capacity at the strength limit states.
502.3.9 Design for Serviceability
The overall structure and the individual members, connections, and connectors shall be checked for serviceability. Performance requirements for serviceability design are given in Section 512.
502.3.10 Design for Ponding
The roof system shall be investigated through structural analysis to assure adequate strength and stability under ponding conditions, unless the roof surface is provided with a slope of 20 mm per meter or greater toward points of free drainage or an adequate system of drainage is provided to prevent the accumulation of water.
See Appendix A-2, Design for Ponding, for methods of checking ponding.
502.3.11 Design for Fatigue
Fatigue shall be considered in accordance with Appendix A-3, Design for Fatigue, for members and their connections subject to repeated loading. Fatigue need not be considered for seismic effects or for the effects of wind loading on normal building lateral load resisting systems and building enclosure components.
502.3.12 Design for Fire Conditions
Two methods of design for fire conditions are provided in Appendix A-4, Structural Design for Fire Conditions: Qualification Testing and Engineering Analysis. Compliance with the fire protection requirements in this Chapter shall be deemed to satisfy the requirements of this section and Appendix A-4.
Nothing in this section is intended to create or imply a contractual requirement for the engineer-of-record responsible for the structural design or any other member of the design team.
502.3.13 Design for Corrosion Effects
Where corrosion may impair the strength or serviceability of a structure, structural components shall be designed to tolerate corrosion or shall be protected against corrosion.
502.4 Member Properties
502.4.1 Classification of Sections for Local Buckling
Sections are classified as compact, non-compact, or slender-element sections. For a section to qualify as compact its flanges must be continuously connected to the web or webs and the width-thickness ratios of its compression elements must not exceed the limiting width-thickness ratios λp from Tables 502.4.1 and 502.4.2. If the width-thickness ratio of one or more compression elements exceeds λp, but does not exceed λr from Tables 502.4. l and 502.4.2, the section is noncompact. If the width-thickness ratio of any element exceeds λr, the section is referred to as a slender-element section.
502.4.2 Unstiffened Elements
For unstiffened elements supported along only one edge parallel to the direction of the compression force, the width shall be taken as follows:
For flanges of I-shaped members and tees, the width b is one-half the full-flange width, bf.
For legs of angles and flanges of channels and zees, the width b is the full nominal dimension.
For plates, the width b is the distance from the free edge to the first row of fasteners or line of welds.
For stems of tees, d is taken as the full nominal depth of the section.
User Note: Refer to Table 502.4.1 for the graphic representation of unstiffened element dimensions.
502.4.3 Stiffened Elements
For stiffened elements supported along two edges parallel to the direction of the compression force, the width shall be taken as follows:
For webs of rolled or formed sections, h is the clear distance between flanges less the fillet or comer radius at each flange; he is twice the distance from the centroid to the inside face of the compression flange less the fillet or comer radius.
For webs of built-up sections, h is the distance between adjacent lines of fasteners or the clear distance between flanges when welds are used, and he is twice the distance from the centroid to the nearest line of fasteners at the compression flange or the inside face of the compression flange when welds are used; hp is twice the distance from the plastic neutral axis to the nearest line of fasteners at the compression flange or the inside face of the compression flange when welds are used.
For flange or diaphragm plates in built-up sections, the width b is the distance between adjacent lines of fasteners or lines of welds.
For flanges of rectangular hollow structural sections (HSS), the width b is the clear distance between web less the inside comer radius on each side. For webs of rectangular HSS, h is the clear distance between the flanges less the inside comer radius on each side. If the comer radius is not known, b and h shall be taken as the corresponding outside dimension minus three times the thickness. The thickness, t, shall be taken as the design wall thickness, per Section 502.3.12.
User Note: Refer to Table 502.4.2 for the graphic representation of stiffened element dimensions.
For tapered flanges of rolled sections, the thickness is the nominal value halfway between the free edge and the corresponding face of the web.
503 .1 Design Wall Thickness for HSS
The design wall thickness, t, shall be used in calculations involving the wall thickness of hollow structural sections (HSS). The design wall thickness, t, shall be taken equal to 0.93 times the nominal wall thickness for electric resistance welded (ERW) HSS and equal to the nominal thickness for submerged-arc welded (SAW) HSS.