CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 1

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

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 2

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

Design of steel/reinforced concrete/timber bridges

www.aashto.org

Other 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

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 3

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

GRADES OF STEEL

Numerous grades of steel are available in the marketplace. The choice is dependent on

Application

Yield strength

Composition

See the summaries on the following sheets from the textbook of Gaylord et al.

ASTM A36, A53, A242, A572, A709

Tensile properties

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 4

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).

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 5

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

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 6

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 PfA

=

Engineering stress

Strain is denoted ε and is calculated as LL

ε ∆=

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 7

Engineering strain

The figure above shows 4 ranges of response

Elastic

Plastic (yield plateau)

Strain hardening

Necking and failure (strain softening)

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

0

0

( )fL Le

L

−=

where

fL =

0L =

An idealized stress-strain relationship is shown in the figure on the following page from Segui. The important descriptors are

yF : the yield point

uF : the ultimate tensile strength

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 8

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

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 9

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

Chemical composition

The chemical composition of a steel determines its mechanical properties of

Strength

Ductility

Hardness (resistance to plastic deformation)

Closely related to ultimate strength

Toughness

The principal components of steel, an alloy, are iron (large %) and carbon (smaller %). Carbon contributes to strength but not ductility. Other components include Manganese (Mn), Silicon (Si), Chromium (Cr), Molybdenum (Mo), Vanadium (V), Nickel (Ni), and Copper (Cu).

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)CE=C+ + +6 5 15

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 10

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

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 11

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

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 12

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

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 13

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 14

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

100F 800F 1200F 1800F

yF

yF

E

Temperatures below room temperature do not have an adverse effect on yF but can have a significant effect on ductility.

Behavior will transform from ductile to brittle at a threshold temperature range known as the Ductile-to-Brittle Transition Temperature (DBTT) range.

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 ( 1 2h h− ) in the figure.

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

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 15

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

NDT?

Rate effects?

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 16

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

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 17

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 18

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.

Plastic design

Plastic design is based on a consideration of failure conditions rather than load conditions.

Rarely used in practice

Similar to LRFD that is described next

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 19

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 nQ Rγ φ≤∑

where

iγ = Load factor

iQ = Load effect (force or moment)

nR = Nominal resistance of component

φ = Resistance factor

nRφ = Design strength

Load factors are not constant for a given load effect (D, L, S, R, W, E, for example)

Arbitrary point-in-time values

Lifetime maximum

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 20

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 variation

CIE 428 Module A Instructor: Andrew Whittaker

8/21/2002 8:39 AM 21

Probability density function (pdf)

Randomness and uncertainty in loads and resistances

( ), ( ), ( )Rf Q f R f InQ

Cumulative distribution function (cdf)

Reliability index

Members

Connections

Structure?

Values calibrated to the then current practice.