B3Design basis

Design shall be such that no applicable strength or serviceability limit state shall be exceeded when the structure is subjected to all applicable load combinations.

Design for strength shall be performed according to the provisions for load and resistance factor design (LRFD) or to the provisions for allowable strength design (ASD).

User Note: The term “design,” as used in this Specification, is defined in the Glossary.

B3.1 Design for Strength Using Load and Resistance Factor Design (LRFD)

Design according to the provisions for load and resistance factor design (LRFD) satisfies the requirements of this Specification 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 Specification, except for those in Section B3.2, shall apply.

Design shall be performed in accordance with Equation B3-1:

(B3-1)

where

$R_{u}=$ required strength using LRFD load combinations

$R_{n}=$ nominal strength

$\phi \quad=$ resistance factor

$\phi R_{n}=\text { design strength }$

The nominal strength, $R_{n}$, and the resistance factor, $\phi$, for the applicable limit states are specified in Chapters D through K.

B3.2 Design for Strength Using Allowable Strength Design (ASD)

Design according to the provisions for allowable strength design (ASD) satisfies the requirements of this Specification 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 Specification, except those of Section B3.1, shall apply.

Design shall be performed in accordance with Equation B3-2:

(B3-2)

where

$\begin{aligned} R_{a} & =\text { required strength using ASD load combinations } \\ R_{n} & =\text { nominal strength } \\ \Omega & =\text { safety factor } \\ R_{n} / \Omega & =\text { allowable strength }\end{aligned}$

The nominal strength, $R_{n}$, and the safety factor, $\Omega$, for the applicable limit states are specified in Chapters D through K.

B3.3 Required Strength

The required strength of structural members and connections shall be determined by structural analysis for the applicable load combinations, as stipulated in Section B2.

Design by elastic or inelastic analysis is permitted. Requirements for analysis are stipulated in Chapter C and Appendix 1.

B3.4 Design of Connections and Supports

Connection elements shall be designed in accordance with the provisions of Chapters J and K. The forces and deformations used in design of the connections shall be consistent with the intended performance of the connection and the assumptions used in the design of the structure. Self-limiting inelastic deformations of the connections are permitted.

At points of support, beams, girders, and trusses shall be restrained against rotation about their longitudinal axis unless it can be shown by analysis that the restraint is not required.

User Note: Code of Standard Practice Section 3.1.2 addresses communication of necessary information for the design of connections.

B3.4a Simple Connections

A simple connection transmits a negligible moment. 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.

B3.4b Moment Connections

Two types of moment connections, fully restrained and partially restrained, are permitted, as specified below.

• (a) Fully restrained (FR) moment connections A fully restrained (FR) moment connection transfers 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 initial angle between the connected members at the strength limit states.
• (b) Partially restrained (PR) moment connections Partially restrained (PR) moment connections transfer moments, but the relative rotation between connected members is not negligible. In the analysis of the structure, the moment-rotation response characteristics of any PR connection shall be included. The response characteristics of the PR connection shall be based on 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 such that the moment-rotation response can be realized up to and including the required strength of the connection.

B3.5 Design of Diaphragms and Collectors

Diaphragms and collectors shall be designed for forces that result from loads, as stipulated in Section B2. They shall be designed in conformance with the provisions of Chapters C through K, as applicable.

B3.6 Design of Anchorages to Concrete

Anchorag between steel and concrete acting compositely shall be designed in accordance with Chapter I. The design of column bases and anchor rods shall be in accordance with Chapter J.

B3.7 Design for Stability

The structure and its elements shall be designed for stability in accordance with Chapter C.

B3.8 Design for Serviceability

The overall structure and the individual members and connections shall be evaluated for serviceability limit states in accordance with Chapter L.

B3.9 Design for Structural Integrity

When design for structural integrity is required by the applicable building code, the requirements in this section shall be met.

• (a) Column splices shall have a nominal tensile strength equal to or greater than $D+L$ for the area tributary to the column between the splice and the splice or base immediately below,

where

$D=$ nominal dead load, kips (N)

$L=$ nominal live load, kips (N)

• (b) Beam and girder end connections shall have a minimum nominal axial tensile strength equal to (i) two-thirds of the required vertical shear strength for design according to Section B3.1 (LRFD) or (ii) the required vertical shear strength for design according to Section B3.2 (ASD), but not less than 10 kips in either case.
• (c) End connections of members bracing columns shall have a nominal tensile strength equal to or greater than (i) 1% of two-thirds of the required column axial strength at that level for design according to Section B3.1 (LRFD) or (ii) 1% of the required column axial strength at that level for design according to Section B3.2 (ASD).

The strength requirements for structural integrity in this section shall be evaluated independently of other strength requirements. For the purpose of satisfying these requirements, bearing bolts in connections with short-slotted holes parallel to the direction of the tension force and inelastic deformation of the connection are permitted.

B3.10 Design for Ponding

The roof system shall be investigated through structural analysis to ensure stability and strength under ponding conditions unless the roof surface is configured to prevent the accumulation of water.

Ponding stability and strength analysis shall consider the effect of the deflections of the roof's structural framing under all applicable loads present at the onset of ponding and the subsequent accumulation of rainwater and snowmelt.

The nominal strength and resistance or safety factors for the applicable limit states are specified in Chapters D through K.

B3.11 Design for Fatigue

Design of members and their connections shall consider fatigue in accordance with Appendix 3. Fatigue need not be considered for seismic effects or for the effects of wind loading on typical building lateral force-resisting systems and building enclosure components.

B3.12 Design for Fire Conditions

Design for fire conditions shall satisfy the requirements stipulated in Appendix 4.

Two methods of design for fire conditions are provided in Appendix 4: (a) by analysis and (b) by qualification testing. Compliance with the fire-protection requirements in the applicable building code shall be deemed to satisfy the requirements of Appendix 4.

User Note: Design by qualification testing is the prescriptive method specified in most building codes. Traditionally, on most projects where the architect is the prime professional, the architect has been the responsible party to specify and coordinate fire protection requirements. Design by analysis is a newer engineering approach to fire protection. Designation of the person(s) responsible for designing for fire conditions is a contractual matter to be addressed on each project. This section is not 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.

B3.13 Design for Corrosion Effects

Where corrosion could impair the strength or serviceability of a structure, structural components shall be designed to tolerate corrosion or shall be protected against corrosion.