Hancock G.J., Murray Th.M., Ellifritt D.S. Cold-Formed Steel Structures to the AISI Specification

Подождите немного. Документ загружается.

Probably 90% of the 1996 speci®cation deals with ®nding

failure loads or nominal capacities. The process is the same

whether one is using ASD or LRFD. The decision as to

which method to use is only made at the end, when the

nominal capacity is either divided by a safety factor O or

multiplied by a resistance factor f. Of course, in ASD the

capacity is compared to the service loads or forces and in

LRFD it is compared to factored forces.

1.8.1 Allowable Stress Design

The basis for ASD is the use of fractions of the material's

yield stress as a limiting stress. Put another way, the yield

stress is divided by a safety factor to determine the maxi-

mum stress allowed for a particular structural action. For

example, the allowable bending stress may be 0:6  yield

stress. The inverse of this coef®cient, 1.67, is O. Each type

of stress, tension, compression, bending, shear, etc., has its

own corresponding allowable stress which cannot be

exceeded by the actual stresses. Ever since engineers

have been calculating the forces and stresses in structures,

this has been the method used. The only changes have been

in the way the allowable stress is determined, and this is

the function of a design standard such as AISC or AISI

speci®cations.

1.8.2 Load and Resistance Factor Design

The philosophy of LRFD is that all the variabilities that the

old safety factor was supposed to take into account should

be divided into two factors: one representing the load

variability and the other accounting for all those things

that go into making variability in resistance, such as

dimensional tolerances, material variations, errors in fabri-

cation, errors in erection, analytical assumptions, and

others. These are called, respectively, the load factor and

resistance factor. Typical frequency distribution curves for

Chapter 1

load effects and corresponding resistances are shown in

Figure 1.16.

In addition, the load factor is not constant for all types

of loading. The load with least variability, such as dead

load, can have a smaller factor. This is an advantage for

structures with heavy dead load, one that is not available in

ASD. In most cold-formed steel structures, the dead load

tends to be a small percentage of the total gravity load, thus

partially negating the advantages of LRFD. In order to

produce designs that are consistent with the more tradi-

tional ASD, the LRFD speci®cation was ``calibrated'' at a

live=dead load ratio of 5=1.

1.9 LOAD COMBINATIONS

The load combination in Section A5.1.2 for allowable stress

design and in Section A6.1.2 for load and resistance factor

design follow the standard ASCE-7-98 (Ref. 1.10), but with

a few important exceptions. The traditional 1=3 stress

increase is no longer permitted in ASCE-7 for ASD, but a

0.75 reduction factor may be used when wind or earth-

quake loads are used in combination with other live loads.

FIGURE 1.16 Frequency distribution curves.

Introduction

One application that is not found in hot-rolled steel is

that of a steel deck which supports a concrete ¯oor. A

special load combination for erection purposes, including

the weight of the wet concrete as a live load, appears in the

commentary to A6.1.2 in the AISI speci®cation. Another

difference involves a reduced wind load factor for secondary

members, that is, sheeting, purlins, girts and other

members. These members can be designed with a lower

reliability index than primary structural members of long-

standing practice.

REFERENCES

1.1 American Institute of Steel Construction, Load and

Resistance Factor Design Speci®cation for Structural

Steel Buildings, 1993.

1.2 American Iron and Steel Institute, Cold-Formed Steel

Design Manual, 1996.

1.3 Yu, W-W, Wolford, D. S., and Johnson, A. L., Golden

Anniversary of the AISI Speci®cation, Center for

Cold-Formed Steel Structures, University of

Missouri-Rolla, 1996.

1.4 Standards Australia=Standards New Zealand, Cold-

Formed Steel Structures, AS=NZS 4600:1996.

1.5 British Standards Institution, Code of Practice for the

Design of Cold-Formed Sections, BS 5950, Part 5,

1986.

1.6 Canadian Standards Association, Cold-Formed Steel

Structural Members, CAN=CSA S136-94, Rexdale,

Ontario, 1994.

1.7 Comite

Europe

en de Normalisation, Eurocode 3:

Design of Steel Structures, Part 1.3: General Rules,

European Prestandard ENV 1993-1-3, 1996.

1.8 Gaylord, E. H., Jr, and Gaylord, C. N., Design of Steel

Bins for Storage of Bulk Solids, New York, Prentice-

Hall, 1984.

Chapter 1

1.9 Bethlehem Steel Corporation, Technical Bulletin,

Coating Weight and Thickness Designations for

Coated Sheet Steels, March 1995.

1.10 American Society of Civil Engineers, Minimum

Design Loads for Buildings and Other Structures,

1998.

Introduction

Materials and Cold Work of Forming

2.1 STEEL STANDARDS

The AISI Speci®cation allows the use of steel to the follow-

ing speci®cations:

ASTM A36=A36M, Carbon structural steel

ASTM A242=A242M, High-strength low-alloy (HSLA)

structural steel

ASTM A283=A283M, Low- and intermediate-tensile-

strength carbon steel plates

ASTM A500, Cold-formed welded and seamless

carbon steel structural tubing in rounds and shapes

ASTM A529=A529M, High-strength carbon-manga-

nese steel of structural quality

ASTM A570=A570M, Steel, sheet and strip, carbon,

hot-rolled, structural quality

ASTM A572=A572M, High-strength low-alloy colum-

bium-vanadium structural steel

ASTM A588=A588M, High-strength low-alloy struc-

tural steel with 50 ksi (345 MPa) minimum yield

point to 4 in. (100 mm) thick

ASTM A606, Steel, sheet and strip, high-strength,

low-alloy, hot-rolled and cold-rolled, with improved

atmospheric corrosion resistance

ASTM A607, Steel, sheet and strip, high-strength,

low-alloy, columbium or vanadium, or both, hot-

rolled and cold-rolled

ASTM A611 (Grades A, B, C, and D), Structural steel

(SS), sheet, carbon, cold-rolled

ASTM A653=A653M (SS Grades 33, 37, 40, and 50

Class 1 and Class 3; HSLAS types A and B, Grades

50, 60, 70 and 80), steel sheet, zinc-coated (galva-

nized) or zinc-iron alloy-coated (galvannealed) by

the hot-dip process

ASTM A715 (Grades 50, 60, 70, and 80) Steel sheet

and strip, high-strength, low-alloy, hot-rolled, and

steel sheet, cold rolled, high-strength, low-alloy with

improved formability

ASTM A792=A792M (Grades 33, 37, 40, and 50A),

Steel sheet, 55% aluminum-zinc alloy-coated by

the hot-dip process

ASTM A847 Cold-formed welded and seamless high-

strength, low-alloy structural tubing with improved

atmospheric corrosion resistance

ASTM A875=875M (SS Grades 33, 37, 40, and 50

Class 1 and Class 3; HSLAS types A and B,

Grades 50, 60, 70, and 80) steel sheet, zinc-5%

aluminum alloy-coated by the hot-dip process

Tubular sections manufactured according to ASTM A500

have four grades of round tubing (A, B, C, D) and four

grades (A, B, C, D) of shaped tubing. Details of stress

grades are shown in Table 2.1.

Steels manufactured according to ASTM A570 cover

hot-rolled carbon steel sheet and strip of structural quality

Chapter 2

ABLE

2.1 Minimum Values of Yield Point, Tensile Strength, and

Elongation

Permanent

ASTM

(ksi) (ksi) elongation in

designation Product

Grade (min) (min

=max) 2 in. (min) (min)

A500-98

Round

1.36

tubing

1.38

1.35

1.61

Shaped

1.15

tubing

1.26

1.24

1.61

A570=

Sheet

1.63

A570M-96

and strip

1.58

1.47

1.38

1.33

1.30

A607-96

Sheet and strip

Class 1 Class 1 Class 1

60 HR 23, CR 22 1.33

65 HR 20, CR 20 1.30

70 HR 18, CR 18 1.27

75 HR 16, CR 16 1.25

80 HR 14, CR 15 1.23

85 HR 12, CR 14 1.21

Materials and Cold Work of Forming

ABLE

2.1 Continued

Permanent

ASTM

(ksi) (ksi) elongation in

designation Product

Grade (min) (min

=max) 2 in. (min) (min)

Class 2 Class 2 Class 2

55 HR 23, CR 22 1.22

60 HR 20, CR 20 1.20

65 HR18, CR 18 1.18

70 HR 16, CR 16 1.17

75 HR 14, CR 15 1.15

80 HR 12, CR 14 1.14

A611-97

Sheet

1.68

1.50

C types

1 and 2 33

1.45

D types

1 and 2 40

1.30

A653

=A653M-97 Sheet

(galvanized

1.36

1.41

galvannealed) 40

1.38

50 (Class 1) 50

1.30

50 (Class 3) 50

1.40

A792=A792M-97 Sheet

1.36

aluminium-

1.41

zinc alloy-

1.38

coated

50A

1.30

Chapter 2

in cut lengths or coils. It is commonly used for purlins in the

metal building industry. Details of stress grades are shown

in Table 2.1.

Steels manufactured to ASTM A607 cover high-

strength low-alloy columbium, or vanadium hot-rolled

sheet and strip, or cold-rolled sheet, and is intended for

applications where greater strength and savings in weight

are important. Details of stress grades are shown in Table

2.1. Class 2 offers improved weldability and more form-

ability than Class 1.

Steels manufactured to ASTM A611 cover cold-rolled

carbon steel sheet in cut lengths or coils. It includes ®ve

strength levels designated A, B, C, D, and E. Grades A, B,

C, D have moderate ductility, whereas Grade E is a full

hard product with minimum yield point 80 ksi (550 MPa)

and no speci®ed minimum elongation. Details of stress

grades are shown in Table 2.1.

Steels manufactured to ASTM A653=A653M-95 cover

steel sheet, zinc-coated (galvanized) or zinc-iron alloy

coated (galvannealed) by the hot-dip process. Structural

quality and HSLA grades are available. Details of struc-

tural quality stress grades are shown in Table 2.1. SQ

Grade 80 is similar to ASTM A611 Grade E.

Steels manufactured to ASTM A792 cover aluminum-

zinc alloy-coated steel sheet in coils and cut lengths coated

by the hot-dip process. The aluminum-zinc alloy composi-

tion by weight is normally 55% aluminum, 1.6% silicon, and

the balance zinc. Details of stress grades are shown in

Table 2.1.

Steels manufactured to ASTM A875 cover zinc-alumi-

num alloy coated steels coated by the hot-dip process with 5%

aluminum. SS Grade 80 is similar to ASTM A611 Grade E.

2.2 TYPICAL STRESS-STRAIN CURVES

The stress-strain curve for a sample of 0.06-in.-thick sheet

steel is shown in Figure 2.1. The stress-strain curve is

Materials and Cold Work of Forming