source 2 aisi specification. introduction housed in the construction group of the american iron and...

SOURCE 2

AISI SPECIFICATION

INTRODUCTION

• Housed in the construction group of the American Iron and Steel Institute (www.steel.org)

• ANSI approved specification for the design of cold-formed steel structural members

• Serves 4 primary industries:• Metal buildings (www.mbma.com)• Steel studs (www.ssma.com)• Racks (www.rmi.com)• Metal decks (www.sdi.org)

http://www.steel.org/

http://www.mbma.com/



http://www.ssma.com/



http://www.rmi.com/

http://www.sdi.org/

http://www.sdi.org/

http://www.sdi.org/

AISI SPECIFICATION EDITIONS

• 1996 Edition• In primary use today• Basis for current LSF manual

• 1999 Supplement• New web crippling and shear capacity

calculations for C-sections with holes• Changes to Base Test

• 2001 North American Edition• Combination of Canada, Mexico, and U.S.

• Broad philosophical changes• U.S., Canada &Mexico (ASD, LRFD, LSD)• Load combinations removed from the Specs.• Rational analysis clause when outside scope

• Detailed changes of interest• Effective width changes

• webs revised based on h/b ratio• flanges with multiple intermediate stiffeners revised

(decks)• flanges with one edge stiffener cleaned up a bit

• Web crippling completely revised• Fastener edge distances = 1.5d (vs. 3d before)• Fatigue provisions provided

full list at www.umr.edu/~ccfss

2001 NORTH AMERICAN SPECIFICATION

http://www.umr.edu/~ccfss

2001 NORTH AMERICAN SPECIFICATION

(from Section A1.1)

• Typical sections are not doubly-symmetric (Torsional-flexural buckling possible)

• Local buckling & post-buckling strength• Effective width

• effective width = f(stress,geometry)

• stress = f(effective properties: e.g., Aeff, Ieff)

• iteration results• Web crippling calculations

AISI SPECIFICATION COMPLICATION REASONS:

AISI SPECIFICATION PRESENTATION

Basic overviewof behavior

(focusing onC Sections)

DESIGN OF COLD-FORMED STEEL STRUCTURES

USING THE 2001 AISI SPECIFICATION

A. GENERAL PROVISIONSB. ELEMENTSC. MEMBERSD. STRUCTURAL ASSEMBLIESE. CONNECTIONS AND JOINTSF. TESTS FOR SPECIAL CASESG. CYCLIC LOADING (FATIGUE)

• MATERIAL • TYPICAL APPROVED STEELS • OTHER STEEL AND DUCTILITY

REQUIREMENTS

• DESIGN BASIS• ASD • LRFD

• LOAD FACTORS AND LOAD COMBINATIONS

• STRENGTH INCREASE DUE TO COLD FORMING

A. GENERAL PROVISIONS

• REQUIRED DUCTILITY(Section A2.3.1)

• Fu/Fy 1.08

• Elongation 10% (two-inch gage) 7% (eight-inch

gage)



REQUIREMENTS





• ASD STRENGTH REQUIREMENTS (Section A4.1.1)

R Rn/

• LRFD STRENGTH REQUIREMENTS (Section A5.1.1)

Ru Rn


• MATERIAL • TYPICAL APPROVED STEELS • OTHER STEEL AND DUCTILITY REQUIREMENTS


• LOAD FACTORS AND LOAD COMBINATIONS(More on this later)




REQUIREMENTS





Increase in yield and ultimate strength due to cold-work


LOCAL BUCKLING

PLATE BUCKLING

BUCKLING OF COMPONENT PLATE ELEMENTS

POST LOCAL BUCKLING STRENGTH

P= 0.07 k 3.2 k 3.8 k 4.9 k 7.2 k 7.6 k Pult= 7.9 k

Photo shows post buckling behavior and interaction of local and overall buckling

The effective width, b, shall be determined from the following equations:

where

w = Flat width

is a slenderness factor determined as follows:

/y crF F

EFFECTIVE WIDTH CONCEPT

0.673b w for 0.673b w for

(1 0.22 / ) /

Actual Stresses Effective Section

EFFECTIVE SECTION FOR COLUMNS

EFFECTIVE SECTION FOR BEAMS

Actual Stresses Effective Section

Flexural Buckling

Torsional-flexural buckling

MODES OF BUCKLING

AXIALLY LOADED COLUMNS

Column just bends during buckling

Column twists and bends during buckling

LOCAL DISTORTIONAL

LATERAL

BEAMS

INTERACTION OF LOCAL AND OVERALL BUCKLING

• Find long column elastic buckling stress Fe based on full section, Fe = min (flexural and flexural-torsional)

• Find nominal column buckling stress Fn using Fe

• Find effective column area Ae at stress Fn

• Column strength considering local buckling is AeFn

STRUCTURAL ASSEMBLIES

Bolted connections

Welded connections

Screw connections(more on these topics during the numeric examples)

CONNECTIONS AND JOINTS

TESTS FOR SPECIAL CASES

- Tests for Determining Structural Performance

LRFD (Calculation of resistance factors)

ASD (Calculation of factors of safety)

- Tests for Confirming Structural Performance

- Tests for Determining Mechanical Properties

Resistance to be evaluated for:• Cold-formed corners and sheared edges of

sections• Longitudinal and transverse fillet welds• Spot welds• Bolt and screw connections

Evaluation of fatigue resistance is not required for wind and seismic loads

FATIGUE DESIGN

An excellent reference for hand calculations.

(Available from the AISI)

Similar document is in preparation for Europe using Eurocode

COLD-FORMED STEEL PROVIDES OPTIMUM SOLUTIONS

HOT-ROLLED

(heavy)

COLD-ROLLED

(efficient and elegant solutions)

(comparison for European sections)

AISI SPECIFICATION EXAMPLE

“Simple” axially loaded column

800S163-54, 50ksi

• h = 8 in.

• b = 1.625 in.

• d = 0.500 in.

• t = 0.0566 in.

• r = 0.0625 in.

t

h

b

r

d

Problem Geometry:

• Lx = 96 in. (8 ft.)

• Ly = 48 in.

• Lt = 48 in.

• Kx = Ky = Kt = 1

A A

AA

Column & support conditions:

AISI Procedure

• Find gross properties• Find long column elastic buckling stress (Fe)

• Fe = min (flexural and flexural-torsional)• Find nominal column buckling stress (Fn)

• launder Fe through AISC column curve →Fn

• Find effective column area Ae at stress Fn

• effective width of web, heff

• effective width of flange, beff

• effective width of lip, deff

• Ae=t(heff+2beff+2deff)• Column strength is AeFn

• A centerline approximation of the geometry, ignoring corners, is allowed(centerline approximations tend to overestimate flexural and flexural-torsional buckling but are conservative on local buckling (Ae))

• 800S163-54, 50ksi

• hCL= h - t = 8 - 0.0566 in.

• bCL = b - t = 1.625 - 0.0566 in.

• dCL = d - t/2 = 0.500 - 0.0566/2 in.

• t = 0.0566 in.

Centerline approximation:

example completed in Mathcad®

Gross Properties

Long Column Buckling

Nominal Buckling Stress

AISC column curve

• Effective width of each element of the cross-section must be determined. The steps are

• find appropriate plate buckling coefficient, k

• determine local plate buckling slenderness, • calculate effectiveness ratio • effective width = x full width

• To find k, we must know what kind of element we have (and what kind of loading – in this case pure compression)

• web = stiffened element

• flange = edge stiffened element

• lip = unstiffened element

Effective Area at Fn:

Elements

stiffened element, supported on both edges, k = 4 used, assumes element is simply supported on all 4 sides for local buckling consideration

edge stiffened element, supported on one edge fully, other edge by a stiffener, 0.43 < k < 4, depending on stiffener size and slenderness of flange itself

unstiffened element, supported on only one edge, k =0.43, assumes element is simply supported on 3 sides for local buckling consideration

Effective WidthWeb:

Edge Stiffened Elements (fun):

as low as it goes (unstiffened)

adequate stiffener size

sensitivity to stiffener ratio

stiffener adequacy ratio

lip/flange interaction reduction

final reduction to get k

k aisi 3.772

Flange:

• Determined at Fn

• heff = 3.089 in.• beff = 1.596 in.• deff = 0.472 in.

• Ae=t(heff+2beff+2deff)

Ae=0.409 in2

• Ag=0.684 in2

Fn

Effective Area:

Capacity

• Pn = AeFn = (0.409)(29.35) = 12 kips

• ASD

Pallowable = Pn/ = (12)/(1.8) = 6.7 kips

compare vs. unfactored load combinations

• LRFD

Pnominal = Pn = (0.85)(12) = 10.2 kips

compare vs. factored load combinations

How is a beam different

• Mn=SeffFn

• Fn = nominal lateral-torsional buckling stress• Seff = effective section modulus

• Seff determination (iteration)• Seff = Ieff / ycg-eff

• web heff = function of stress gradient• stress gradient = function of ycg-eff

• Even symmetric sections become unsymmetric when effective width of compression flange is less than full width… iteration…

• Calculations become quite tiresome

source 2 aisi specification. introduction housed in the construction group of the american iron and...

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