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For sections where λp≤ 0.68 and Lb≤ 0.75Lp, the design flex-

ural strength, ϕbMn,csm, and the allowable flexural strength, Mn,csm

/ 

The nominal flexural strength at the limit state of yielding,

Mn,csm, is given by:

(a) Major axis bending

M M E E S Z S Z n,csm,x p,x sh x x csm y x x csm y = + ⎛ − ⎝ ⎜⎜ ⎞_⎠⎟⎟ − −⎛_⎝⎜ ⎞_⎠⎟ 1 ε 1 1 ε ε ε ⎛⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ 2 (A-7) (b) Minor axis bending

M M E E S Z S Z n,csm,y p,y sh y y csm y y y csm y = + ⎛ − ⎝ ⎜⎜ ⎞_⎠⎟⎟ − −⎛_⎝⎜ ⎞ ⎠ ⎟ 1 ε 1 1 ε ε ε ⎛⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ α (A-8) where

Mp = plastic bending moment, kip-in. (N-mm)

Z = plastic section modulus about axis of bending, in.3

(mm3)

S = elastic section modulus about axis of bending, in.3

(mm3)

α = 2.0 for rectangular HSS or 1.2 for I-shaped sections

Lb and Lp are defined in Section 6.2 and ϕb and Ω b are

given in Section 6.1.

AISC DESIGN GUIDE 27/ STRUCTURAL STAINLESS STEEL / 71

Appendix B

Commentary to the Design Provisions

B.1 INTRODUCTION

B.1.1 Purpose of the Commentary

This Appendix describes the work undertaken to derive the design provisions in this Design Guide. This Appendix will also facilitate the development of revisions to the design rules as, and when, new data become available.

B.1.2 How Does the Structural Performance of Stainless Steel Differ from Carbon Steel? The structural performance of stainless steel differs from that of carbon steel because stainless steel has no definite yield point, shows an early departure from linear elastic behav- ior, and exhibits pronounced strain hardening. This impacts design rules in the following ways:

• The design strength is based on the 0.2% offset yield strength.

• There is a different buckling response for members sub- ject to compression, unrestrained bending and shear buckling (also different levels of residual stresses for welded members).

• Greater deflections will occur in beams at high strains (the secant modulus is generally used for estimating these deflections).

• Different rules for the bearing strength of connections are necessary in order to limit deformation.

B.1.3 Design Specifications for Structural Stainless Steel

Specifications for the design of cold-formed structural stain- less steel are available in the U.S.  (ASCE, 2002), Austra- lia/New  Zealand  (AS-NZS, 2001), South Africa  (SABS, 1997), and Japan  (SSBA, 2005). However, there are only European (CEN, 2006a) and Japanese (SSBA, 1995) speci- fications which cover the design of structural sections made from thicker walled material (welded, hot rolled, structural hollow sections). The Japanese specification is not available in English. A comparison of the various structural design standards for stainless steel is made in Baddoo (2003).

Eurocode  3: Design of Steel Structures, Supplementary

Rules for Stainless Steels, Part  1-4 (EN  1993-1-4) (CEN, 2006a) gives rules which can be applied to welded, hot-rolled and cold-formed stainless steel members. It is a supplement rather than a standalone document, referring extensively to the following parts of Eurocode 3:

EN 1993-1-1 Design of Steel Structures: General Rules and Rules for Buildings

EN 1993-1-2 Design of Steel Structures: Structural Fire Design

EN 1993-1-3 Design of Steel Structures: General Rules: Supplementary Rules for Cold-Formed Members and Sheeting

EN 1993-1-5 Design of Steel Structures: Plated Structural Elements

EN 1993-1-8 Design of Steel Structures: Design of Joints EN 1993-1-9 Design of Steel Structures: Fatigue

EN 1993-1-10 Design of Steel Structures: Material Tough-

ness and Through-Thickness Properties

The design rules in the 1996 public draft of EN  1993- 1-4 were initially based on the first edition of the European

Design Manual for Structural Stainless Steel (Euro Inox and SCI, 1994), following a European joint industry proj- ect. The rules were derived on the basis of an extensive test program and took into account all known work carried out in Europe, U.S., South Africa and Australia. The Design Man-

ual included a commentary which explains the basis of the development of the design rules and presents the results of the relevant test programs. Since 1994, the European Design

Manual has been revised and extended two times, taking into account the results of further European research projects and new work from other parts of the world. EN 1993-1-4 (CEN, 2006a) aligns with the recommendations in the current third edition of the European Design Manual (Euro Inox and SCI, 2006a), except in the area of fire resistance where the rules in the Design Manual are less conservative.

B.1.4 Scope of the Design Guide

The intention at the start of writing this Design Guide was to modify the structural stainless steel rules in EN 1993-1-4 and present them in a format aligned with the AISC Speci-

fication for Structural Steel Buildings. However, due to the fundamental differences between the design rules in the AISC Specification and EN 1993-1-4, this approach was not possible and the following procedure was implemented: 1. Compare the rules for carbon steel and stainless steel in

Eurocode 3.

2. Compare the rules for carbon steel in the AISC

72 / STRUCTURAL STAINLESS STEEL / AISC DESIGN GUIDE 27

Specification against all available stainless steel test data on members and connections.

3. Modify the AISC Specification carbon steel rules to suit the stainless steel data where necessary.

4. Calculate the stainless steel resistance factors to use with the recommended stainless steel design rules.

An Evolution Group has been established for each part of Eurocode 3 to oversee maintenance and future development activities and it is expected that revisions to all parts of Euro- code 3 will be issued in the future. The Evolution Group for EN 1993-1-4 is considering a number of developments to the standard, most of which will lead to less conservative design rules due to the far greater body of test data which is now available for structural stainless steel. These proposed devel- opments have been taken into account during the preparation of this Design Guide.

The Design Guide gives guidance that a designer familiar with designing to the AISC Specification should be able to use easily. Where stainless steel behaves in a similar way to carbon steel, the Design Guide simply refers to the relevant section in the AISC Specification. Where the guidance in the AISC Specification would be unconservative or unduly con- servative when applied to stainless steel, specific rules for stainless steel have been presented in a format as close as possible to the equivalent expressions in the AISC Specifica-

tion for carbon steel.

The assumptions made and data used in order to calculate the resistance factors by means of a reliability analysis are described in Section B.2. Sections B.3 to B.11 deal with dif- ferent aspects of structural design. In each section the design provisions in Eurocode 3 for both carbon steel and stainless steel are presented and compared to the provisions for car- bon steel in the AISC Specification. Stainless steel data are then compared to the AISC provisions and new provisions for stainless steel presented where necessary. The results of the reliability analysis are then given.

It should be noted that the Design Guide is applicable to hot-rolled materials. Structural design of cold-formed stain- less steels (including cold-worked austenitic stainless steels) are covered by ASCE/SEI 8-02, Specification for the Design

of Cold-Formed Stainless Steel Structural Members (ASCE, 2002). See also Section B.1.3.