Vật liệu thép theo ASTM

Vật liệu thép theo ASTM

MODULE A: INTRODUCTION

This module of CIE 428 covers the following subjects

Specifications for design of steel structures

Structural steel

Grades of steel

Steel shapes

Properties of structural steel

Concepts in structural steel design

Basis of load and resistance factors

READING: Chapters 1 and 2 of Segui

Chapters 1 and 2 of Bruneau et al

AISC LRFD Manual of Steel Construction, 3rd

Ed

SPECIFICATIONS

There are two key specifications for the design of steel structures

1 American Institute of Steel Construction (AISC)

Design of steel buildings and connections

www.aisc.org

2 American Association of State Highway and Transportation Officials (AASHTO)

Design of steel/reinforced concrete/timber bridges

www.aashto.orgOther specifications are available from

American Iron and Steel Institute (AISI)

Cold-formed steel structures

www.steel.org

American Railway Engineering Association

Steel railway bridges

STRUCTURAL STEEL

History of engineered construction using metals

Iron

Chief component of steel

Wrought iron first used for tools around 4000 BC

Produced by heating ore in a charcoal fire

Cast and wrought iron used in the late 18C and early 19C

in bridges

Steel

An alloy of primarily iron and carbon

Fewer impurities and less carbon than cast iron

Began to replace iron in construction in the mid 1800s First steel railroad bridge in 1874

First steel framed building in 1884

STEEL SHAPES

Hot-rolled shapes are produced from molten steel in a furnace that

is poured into a continuous casting where the steel solidifies but does not cool completely The partially cooled steel is then passed through rollers to achieve the desired shape

Common structural steel shapes are shown below (from Segui)

A sample designation of a steel shape is

W18x50

Bar, plate and HSS shapes are shown below

Hollow steel sections (HSS) are fabricated by either bending plate material into the desired shape and seam welding or hot-working to produce a seamless shape

PROPERTIES OF STRUCTURAL STEEL

Stress-strain relationship

The stress-strain relationship is the best-known characterization of steel See the figure below from Segui

Stress is denoted as f or σ , and is calculated as f P

Necking and failure (strain softening)

Many steels are ductile Ductility is a measure of post-yield elongation, where elongation is calculated as

0 0

E: Modulus of elasticity or Young’s modulus (29,000 ksi)

For high-strength steels, the stress-strain relationships are often similar to that shown below (from Segui)

Note from the above figure that

Elastic range

No well-defined yield point

Ultimate tensile strength

Because steel design makes use of yield strength and a tensile strength, a definition of yield strength is needed for these steels

0.2% offset (residual strain) method used

Hardness (resistance to plastic deformation)

Closely related to ultimate strength

The concept of carbon equivalent (CE) was introduced to convert into equivalent carbon content the effect of other elements known

to increase the hardness of steel The AWS definition of CE is:

(Mn+Si) (Cr+Mo+V) (Ni+Cu)

where C is the % carbon, etc

If strength increases, hardness increases, ductility decreases, and weldability decreases

If CE is high, say 0.4 to 0.5, then the potential for cracking

in the HAZs of welded connections is increased

Limits on CE not found in ASTM standards but other limits are used to control maximum % of elements, etc

Structural steels are often grouped by broad composition, namely,

Plain carbon steels Mostly iron and carbon, less than 1% C Low-alloy steels

Iron, carbon and other components (<5% by volume) Increase in strength, reduction in ductility

High-alloy or specialty steels

For example, ASTM A36 steel is a plain carbon steel with the following components other than iron:

Carbon: 0.26% maximum Phosphorous: 0.04% maximum Sulphur: 0.05% maximum

Consider the figure from Bruneau on the following page that shows generic stress-strain relationships for different steels

What are the key observations from this figure?

Effect of temperature on the properties of structural steel

Elevated temperatures generally degrade the properties of structural steel Threshold temperatures vary as a function of mechanical property under consideration See the figure below from Bruneau et al for sample information on the effect of

temperature on yield stress, tensile strength, and Young’s modulus

Consider A36 steel What is the % reduction in yield stress, tensile strength, and Young’s modulus at 100F, 800F, 1200F, and 1800F?

Charpy V-notch test introduced to determine the transition temperature

Specimen and hammer shown on the following page (from Bruneau et al.)

Result of the test is a value for notch toughness (CVN) given by ** ft-lb at ** F This is a characterization of the energy absorbed by the notched specimen as given

by (h1−h2) in the figure

NDT is the nil-ductility temperature Below the NDT the steel is considered to be brittle under conditions of impact loading

Sample CVN data are shown on the following page from Barson and Rolfe

NDT?

Rate effects?

Effect of strain rate on mechanical properties of steel

Strain rate will affect the shape of the stress-strain relationship

Yield stress and tensile strength will generally increase with strain rate

Strain rate effects only significant for blast engineering

% increase in yield stress and tensile strength is dependent on temperature

See the figure from Bruneau et al on the following page for sample information

CONCEPTS IN STRUCTURAL STEEL DESIGN

Steel structures can be designed using one of three approaches

Allowable stress deign (ASD) Plastic design

Load and resistance factor design (LRFD)

Each of these is summarized in turn below CIE 428 will focus exclusively on LRFD

Allowable stress design

Allowable stress design (ASD) is also called working stress design

Working stresses are calculated from the working loads using best estimates of the applied loads

Allowable stresses (under working loads) are calculated

by dividing the yield stress or tensile strength by a factor of safety

No information on how safe is the design

Load and resistance factor design (LRFD)

Load and resistance factor design is similar to plastic design in that strength (the failure condition) is considered, where

Factored load Factored strength≤

where the factored load is the sum of the load effects multiplied

by the load factors and the factored strength is equal to the

resistance multiplied by the resistance factor In this approach, the factored loads bring the member to its limit

The above relationship can be written as follows:

i i Q R n

γ ≤φ

∑where

Load combinations summarized from text below

ASCE-7 now provides load combinations for use with the LRFD specification

BASIS OF LOAD AND RESISTANCE FACTORS

The load and resistance factors are based on extensive analytical studies and assessment of in-service conditions

Load factors account for randomness in load effects

Resistance factors account for randomness in material properties and uncertainties in analysis and design theory, and fabrication and construction practices The presentation on the probabilistic basis

of the AISC load and resistance factors is taken from Segui Consideration is given to

Mean, variance, standard deviation, and coefficient of

Probability density function (pdf)

Randomness and uncertainty in loads and resistances ( ), ( ), ( R)