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Wood as a Construction Material 43-23

43.10 Wood and the Environment

Trees are nature’s only renewable resource for building materials. Trees use energy from the sun and

carbon dioxide to create cellulose while cleansing the atmosphere and giving off oxygen. Wood is a

signiﬁcant “storehouse” for carbon, and it does all this with little or no input from people. Converting

trees into useful products requires much less energy than is needed for other construction materials.

Considering any other structural material in terms of production costs to the environment and use

through recycling and ultimate disposal, wood is certainly the most environmentally benign material in

use today. It is renewable, available, easily converted into products, recyclable, and biodegradable with

no toxic residues.

What is the current status of the forest resource? What are the factors affecting the resource and its

use? The answers seem to depend upon whom you ask, because there are no explicit, easy answers.

Environmental concerns and the “green movement” have resulted in national policy shifts regarding

forest use. Large acreages have been set aside as wilderness areas to remain totally unavailable for timber

management and harvest. Harvesting on national timberlands has been drastically curtailed. This will

necessarily shift harvesting pressure more to industrial and private commercial forestland. While com-

mercial forestland, which is held by numerous wood products companies, is quite productive and provides

more product per acre than other sources, privately held timberlands tend to be the least managed of

the nation’s forested acres. Thus, even though a valid argument can be made for sequestering national

timberlands or reducing their production of timber products, shifting the nation’s demand for wood to

the private, noncommercial sector may not be wise in the long run. On a brighter note, in many traditional

timber states, regulations require that cut areas be properly and promptly restocked and waste greatly

reduced. It has been estimated that in several western states six trees are planted for each one that is

harvested (but under normal forest growth patterns only one or two of the six survive to reach maturity).

In most regions of the U.S. increases in tree growth signiﬁcantly exceed harvest and mortality due to

ﬁre, old age, and disease. In other words, there is more timber being produced annually than is being

harvested by a signiﬁcant margin. But that is not the whole story. Average tree diameter has been steadily

declining as we have harvested the biggest, best, and most economically available trees. Likewise, indi-

vidual tree quality has been decreasing, and many forested areas are inaccessible or uneconomical for

harvesting operations. Balanced against these factors is the impact of advancing technology. Such products

as structural composite lumber, LVL, OSB, glu-lam, and other products, as well as improved grading and

strength assessment techniques, have stretched the resource remarkably. We use nearly 100% of the tree

for useful products; waste has been signiﬁcantly reduced. Lesser-known species are being utilized; for

example, aspen, a “junk tree” species not long ago, is now the mainstay for many OSB plants in the upper

Midwest because of its low density, availability in large quantities, and desirable panel properties. Thin-

kerf saws, improved kiln-drying technology, environmentally friendly preservatives, waste conversion to

fuel energy (modern integrated paper mills may be over 90% energy self-sufﬁcient, paper industry average

energy self-sufﬁciency value is 56%), and more reliable product grading and QC programs are just a few

successful resource-stretching innovations.

New technologies have made tremendous strides and old technologies are being updated. In some

areas it is predicted that by the turn of the century wood will be a signiﬁcant fuel source. Fast-grown

tree plantations of hardwoods represent an economical fuel source that does not require mining and is

there when the sun is not shining or the wind is not blowing. Wood chips added to coal signiﬁcantly

reduce sulfur and carbon emissions. Over 1000 wood-burning plants are in operation, and their combined

output is reputed to be the equivalent of three large nuclear reactors (Anonymous, 1993). Wood waste

from manufacturing operations and demolition refuse may become a valuable, environmentally accept-

able fuel source.

Although controversy regarding just how we are to allocate the nation’s timber resource to provide

for endangered species, increased demand for “wild” areas, and increasing numbers of products made

from wood will certainly continue, it is possible to retain many of the “natural” aspects of forests and

still obtain products from this remarkable resource on a sustainable basis if attention is paid to proper

43-24 The Civil Engineering Handbook, Second Edition

management and skillful utilization. Current (1990s) controversy over environmental policies will lead

to acceptable compromise in time. However, the nature of trees and all the wood products derived from

them will assure wood a prominent place as a highly preferred, environmentally desirable, and econom-

ically competitive building material.

Deﬁning Terms

Bark — The outside covering of a tree which protects the tree from invasion by insects, disease, and decay.

Cellulose — A long-chain molecule that constitutes the major building block of plant material. Cellulose

is the element responsible for wood’s strength along the grain.

Check — Often referred to as a “seasoning check”; a separation of the wood cells as a result of shrinkage

during drying. Checks tend to reduce shear strength.

Decay — Results from fungal attack of wood. Fungi may attack either the wood cellulose and hemicel-

lulose or the lignin; in either case even the early stages of decay seriously reduce wood strength

and make it unsuitable for any structural purpose.

Density — We ight per unit volume of wood. The weight is always in the oven-dry condition (i.e., 0%

MC); the volume, however, may be measured at any stated MC (most often green, 12%, or

oven-dry). Since wood shrinks as it dries, density values vary, and the MC base must be stated

as, for example, “volume measured at 12% MC.” Similarly, speciﬁc gravity values will vary

depending on the MC at which volumetric measurements are made.

Duration of load (DOL) — The ability of wood members to sustain larger loads for shorter periods of

time without failure than they can sustain without failure for longer periods of time. Structures

that will have design loads imposed upon them for shorter than “normal” DOL are permitted to

have an increase in allowable design stresses in bending, compression parallel to the grain, tension

parallel to the grain, and shear. Normal DOL is deﬁned as a 10-year cumulative loading duration.

Earlywood — The portion of a growth ring that is formed early in the growing season. It normally

contains larger cells with thinner walls. Earlywood is relatively low in density and is followed

by latewood as the growing season progresses. See Latewood.

Fiber saturation point (fsp) — A point (about 25% MC, depending on species) in the drying of wood

at which the hollow centers of the wood cells have lost their moisture, leaving the cell walls

fully saturated. Wood does not begin to shrink until its MC has dropped below fsp.

Flat-sawn — Lumber that has its wide face in a tangential plane (i.e., cut approximately tangent to the

annual growth rings); also called plain-sawn. See Quarter-sawn.

Glu-lam — A structural product made by gluing structural boards together; the grain of all pieces is

oriented along the long axis of the member. Glu-lam may be manufactured into a variety of

curved beams, arches, or irregular shapes. See also Laminated-veneer-lumber.

Growth ring — The portion of wood of a tree produced during one growing season. In the temperate

zones this is also called an annual ring.

Hardwood — Trees that are deciduous; that is, trees whose leaves are broad and are generally shed each

year in the temperate zones. Typical hardwoods include oaks, maples, and poplar.

Heartwood — The innermost growth rings of a tree; may be darker in color than the outermost growth

rings (called sapwood ). Contains phenolic compounds that in some species impart decay

resistance to the heartwood.

Hemicellulose — A long-chain molecule similar to cellulose, but more easily soluble in dilute acid or

basic solutions.

Incising — Perforation of the surface of a pole or wood member by a series of chisel-like knives prior

to preservative treatment to permit deeper and more uniform infusion of the preservative into

the wood.

Wood as a Construction Material 43-25

Knots — The result of a branch having been cut in the manufacture of a board. If the branch is cut at

a right angle to its length, the knot appears as a branch cross-section; if the branch is cut along

its length, appearing to go across the face of a board, it is termed a spike knot.

Laminated-veneer-lumber (LVL) — A board product made by gluing pieces of thin lumber or veneer

together to make a larger member, usually formed into long ﬂat panels suitable for remanufac-

ture into common lumber sizes. The grain of all pieces is oriented along the long axis of the

panel. See also Glu-lam and Plywood.

Latewood — The portion of a growth ring that is formed later in the growing season. Cells tend to be

smaller in size and have thicker, denser cell walls than in earlywood. See Earlywood.

Lignin — A highly complex molecule that acts as an adhesive to bond wood cellulose units together.

Although plastic in nature, especially at elevated temperatures, lignin gives wood its rigidity.

Microﬁbrils — Groups of cellulose or hemicellulose molecules that form long, threadlike macro-mol-

ecules. Layers of microﬁbrils form the wood cell walls and are ultimately responsible for wood’s

anisotropic properties.

Moisture content (MC) — The amount (weight) of water in a piece of wood, expressed as a percent

of the weight of an oven-dry (0% MC) piece.

Nominal size — A convenient size nomenclature that approximates the size of a member in a log prior

to being sawn into lumber; a nominal 2-by-4, when sawn, dried, and surfaced, has an actual,

usable size of 1½ ¥ 3½ in., for example.

Normal duration of load — Te n years, cumulative, of the load for which a member or structure has

been designed. Allowable design stresses may be modiﬁed for shorter or longer load periods.

Oriented-strand board (OSB) — A ﬂat board product made from wood ﬂakes that are long and

narrow and are oriented along the length of the panel, and a waterproof adhesive, so that the

properties and characteristics resemble those of solid lumber.

Oven-dry weight — The weight of a piece of wood at 0% MC; normally determined by drying a small

block of wood at 103˚C until repeated weighings indicate that all moisture has been removed

from the block.

Particleboard — A ﬂat board product made from wood particles or ﬂakes mixed with an adhesive and

formed under pressure and elevated temperature. Not sold for structural use. See also Oriented-

strand board.

Preservative — A chemical, usually in solution form, that is forced into wood (usually under pressure)

to preserve the wood from attack by insects and decay organisms.

Plywood — A ﬂat glued panel made from thin sheets of veneer with alternate plies having grain direc-

tions oriented 90 degrees to adjacent plies.

Quarter-sawn — Lumber that has its wide face in a radial plane (i.e., cut approximately parallel to the

wood rays). Lumber is quarter-sawn to accentuate the wood ﬁgure due to the large wood rays

in some species. See Flat-sawn.

Repetitive member use — A structural situation wherein three or more bending members in sizes of

2 to 4 in. in nominal thickness (as for ﬂoor joists) are fastened together and stabilized, as by

ﬂooring, to form an interactive unit. Repetitive members are permitted a 15% increase in

allowable bending stress.

Sapwood — The outermost growth rings of a tree; always light brown to cream-colored in all species;

never decay- or insect-resistant. Sapwood and heartwood together make up the “wood” of

commercial use.

Shake — A lengthwise separation of the wood that may occur between or through annual growth rings.

Slope of grain — A deviation of cell orientation from the longitudinal axis of a member; measured as

rise/run (as in 1 in 10). Slope of grain may be a growth phenomenon (as when grain deviates

around a knot) or a manufacturing defect caused by failure to cut a member “along the grain.”

43-26 The Civil Engineering Handbook, Second Edition

Softwood — Typically “evergreen” trees with needle-like leaves. Includes Douglas ﬁr, pines, spruces, cedars,

and hemlock.

Speciﬁc gravity (SG) — The ratio of the density of a material to that of an equal volume of water. See

Density.

Split — A separation of wood cells along the grain; splits are deeper and more serious than cracks or

checks in that splits penetrate completely through the thickness of a member. Splits most often

occur at the ends of a member and reduce shear strength.

Strength ratio (SR) — A factor applied in lumber stress grading to account for wood defects. It repre-

sents a ratio of the strength of a piece with all its defects to the strength the piece would have

if no defects were present.

Structural composite lumber (SCL) — A panel product made of thin layers of veneer or wood strands

mixed with a waterproof adhesive. See LVL .

Wane — A lack of wood that occurs in lumber manufacture when the edge of a member intersects the

periphery (bark) of a tree so that the edges are not “square.”

Warp — Any deviation from a true or plane surface, including bow, crook, cup, and twist. Bow is a

deviation ﬂatwise from a straight line drawn from end to end of a piece. Crook is a deviation

edgewise from a straight line drawn from end to end of a piece. Cup is a deviation in the face

of a piece from a line drawn from edge to edge of a piece. Tw ist is a deviation ﬂatwise, or a

combination ﬂatwise and edgewise, of a piece, resulting in the raising of one corner of a piece

while the other three corners remain in contact with a ﬂat surface.

Wood cells — Long, thin, hollow units that make up wood. Most cells are oriented with their long axis

roughly parallel to the axis of the tree.

Wood r ays — Groups of cells whose long axes are oriented horizontally and that radiate outward from

the center of a tree. Wood rays are responsible for horizontal translocation of ﬂuids in a tree

and are partially responsible for the difference in radial vs. tangential shrinkage and swellage

in wood.

References

American Forest and Paper Association. 1991. National Design Speciﬁcation for Wood Construction and

Supplement. American Forest and Paper Association, Washington, D.C.

American Plywood Association. 1992. APA Product Guide, Grades and Speciﬁcations. American Plywood

Association, Tacoma, WA.

American Plywood Association. 1992. Guide to Wood Design Information. American Plywood Association,

Ta coma, WA.

Anonymous. 1993. Electric Utilities Study an Old, New Source of Fuel: Firewood. Wall Street J., Dec. 2,

1993, p. A1.

ASTM. 1992. Standard Practice for Establishing Structural Grades and Related Allowable Properties for

Visually Graded Lumber, ASTM Designation D245-92. American Society for Testing and Materials,

Philadelphia, PA.

ASTM. 1988. Standard Test Methods for Establishing Clear Wood Strength Values, ASTM Designation

D2555-88. American Society for Testing and Materials, Philadelphia, PA.

Forest Products Laboratory. 1987. Wood Handbook: Wood as an Engineering Material. Agricultural Hand-

book 72, USDA, Washington, D.C.

Tr iche, M. H. and Suddarth, S. K. 1993. Purdue Plane Structures Analyzer 4. Purdue Research Foundation,

Purdue University, West Lafayette, IN.

Wood as a Construction Material 43-27

Further Information

Dietz, A. G. H., Schaffer, E. L., and Gromala, D. J., eds. 1982. Wood as a Structural Material. Vol. II of

the Clark C. Heritage Memorial Series on Wood. Education Modules for Materials Science and

Engineering Project. The Pennsylvania State University, University Park, PA.

Freas, A. D., Moody, R. C., and Soltis, L. A. 1986. Wood: Engineering Design Concepts. Vol. IV of the Clark

C. Heritage Memorial Series on Wood. Materials Education Council, The Pennsylvania State Uni-

versity, University Park, PA.

Hoadley, R. B. 1980. Understanding Wood: A Craftsman’s Guide to Wood Technology. The Taunton Press,

Newtown, CT.

Hoyle, R. J., and Woeste, F. E. 1989. Wood Technology in the Design of Structures, 5th ed. Iowa State

University Press, Ames, IA.

Wangaard, F. F. 1981. Wood: Its Structure and Properties. Vol. I of the Clark C. Heritage Memorial Series

on Wood. Educational Modules for Materials Science and Engineering Project, The Pennsylvania

State University, University Park, PA.

American Forest and Paper Association

1250 Connecticut Avenue NW

Washington, D.C. 20036

The American Institute of Timber Construction

11818 SE Mill Plain Blvd.

Suite 415

Vancouver, WA 98684-5092

American Plywood Association

PO Box 11700

Ta coma, WA 98411

American Wood Preserver Association

PO Box 849

Stevensville, MD 21666

Forest Products Laboratory

U.S. Department of Agriculture

One Gifford Pinchot Drive

Madison, WI 53705-2398

National Particleboard Association

18928 Premiere Court

Gaithersburg, MD 20879

Wood Truss Council of America

111 E. Wacker Drive

Chicago, IL 60601

Structural Steel

44.1 Properties and Processes

Types of Steel • Structural Properties • Heat Treatment •

Welding

44.2 Service Performance.

Brittle Fracture • Fatigue • Performance in Fire • Creep and

Relaxation • Corrosion and Corrosion Protection • Non-

destructive Testing

44.1 Properties and Processes

Types of Steel

The term

structural steel

is generally taken to include a wide variety of elements or components used in

the construction of buildings and many other structures. These include beams, girders, columns, trusses,

ﬂoor plates, purlins and girts. Many of these elements are made from standard hot rolled structural

shapes, cold formed shapes or made up from plates using welding. They are joined at connections made

using plate, structural shapes, welding, and fasteners. Fasteners include bolts, nuts, washers and stud

shear connectors. For a more complete listing of

structural steel

elements see the American Institute of

Steel Construction (AISC) Code of Standard Practice for Steel Buildings and Bridges, Section 2.1.

A variety of steel types are used to produce these structural shapes, plates and other components

depending on the intended use and other factors such as the importance of cost, the weight of the

structure and corrosion resistance. The properties of steel products result from a combination of the

chemical composition, the manufacturing processes and the heat treatment. The properties most com-

monly used as a basis for speciﬁcation and design are the speciﬁed minimum yield stress (yield point or

yield strength) and the speciﬁed minimum tensile strength (or ultimate strength), both obtained from

tensile tests on small representative specimens of the steel (see below). Other properties also required

are ductility, weldability, and fracture toughness (or notch ductility) although these requirements are

often not explicitly stated. The steels most commonly used are:

• carbon steels

• high-strength low-alloy steels

•corrosion resistant, high-strength low-alloy steels

•quenched and tempered alloy steels

The elastic modulus is virtually the same for each of these types of steel, but their yield stress and

ultimate strength vary widely. The range of strengths for each of these types of steel is shown along with

the typical products and uses in Table 44.1.

The production of structural steel structures and buildings is a complex process involving making the

steel, processing it into useful products, fabricating these products into useful assemblies or structures,

Ian Thomas

Victoria University

-2

The Civil Engineering Handbook, Second Edition

and erecting and assembling these components, assemblies, and structures into buildings, bridges, cranes,

and the myriad of other structures and pieces of machinery and equipment constructed using structural

steel. It requires a team effort on the part of specialists who understand in detail the processes and pitfalls

that are associated with each step in the overall process. Such is the complexity of each of these areas

that specialists are required in each area and each in turn must contribute appropriately for the ﬁnal

product to be successful.

The ﬁrst step in the production process, steel making and rolling to form ﬁnished products suitable

for use in buildings and structures, are themselves very complex processes requiring extensive knowledge

of the metallurgy and mechanical forming of steel. Designers should be aware of the importance of

certain parameters and processes because they can have a major effect on the performance of a steel

structure, but they normally do not specify or need details of precisely how the steel is produced, rolled

or formed. Indeed structural steels are usually speciﬁed in terms of a very limited number of parameters

that describe their essential attributes.

Structural steels are usually produced by rolling steel cast from the steelmaking process after reheating

it to the austenizing range (above 850°C). Rolling consists of passing the steel through a series of rolls

that form the cast steel into the shape and/or thickness required. A very wide range of shapes and sizes

are currently rolled or available in the U.S. (Table 44.2). The shapes are designated as follows:

•W shapes are parallel ﬂanged I or

sections used as beams and columns

•S shapes are I sections with inside ﬂange surfaces that slope used as beams

• HP shapes are parallel ﬂanged

sections used as beams with ﬂanges and webs of similar thickness

•M shapes are I sections not qualifying as W, S or HP shapes

TA B L E

44.1

Types and Usage of Steel Used In Construction

Steel Type

ASTM

Speciﬁcation Grade Products Usage

Carbon A36 36 Shapes, plates and bars Riveted, bolted or welded bridges,

buildings and general structural

purposes

A529 50 Shapes, plates and bars Riveted, bolted or welded buildings and

general structural purposes55 Shapes, plates and bars

High-strength, low-alloy A572 42 Shapes, plates, sheet

piling and bars

Riveted, bolted or welded structures

Riveted or bolted bridges, or riveted,

bolted or welded construction in other

applications

A992 50 W shapes Building framing

Corrosion-resistant,

high strength low-alloy

A242 50 Shapes, plates and bars Riveted, bolted or welded construction

for weight saving or added durability

A588 50 Shapes, plates and bars Riveted, bolted or welded construction,

primarily for welded bridges or

buildings for weight saving, added

durability or good notch toughness

Quenched and tempered

alloy steels

A852 70 Plates Riveted, bolted or welded construction,

primarily for welded bridges or

buildings for weight saving or added

durability

A514 90 Plates to 6 in thickness Welded bridges and other structures

100

A913 50 Shapes Riveted, bolted or welded bridges,

buildings and other structures60

Structural Steel

-3

•C shapes are channels with inside ﬂange surfaces that slope similarly to S shapes

• MC shapes are channels that do not qualify as C shapes

•L shapes are equal and unequal legged angles

The groups in Table 41.2 are convenient groups based on the thickness of the shape at the point where

standard tension test (see below) specimens are taken and are used in the ASTM materials speciﬁcations

and in design speciﬁcations such as the American Institute of Steel Construction Load and Resistance

Factor Design Speciﬁcation for Structural Steel Buildings. They are useful because in some material

speciﬁcations the speciﬁed minimum tensile properties differ for different thicknesses of steel and care

must be exercised in design to use appropriate properties and fabrication and welding procedures for

the shapes used.

The properties of steel largely result from the inﬂuence of microstructure and grain size though other

factors such as non-metallic inclusions are also important. The grain size is strongly inﬂuenced by the

cooling rate, to a lesser extent by other aspects of heat treatment and by the presence of small quantities

of elements such as niobium, vanadium and aluminium. The chemical composition of steel is largely

determined when the steel is liquid but for a given chemical content the structure is largely determined

by the rate at which it is cooled and may be altered by subsequent reheating and cooling under controlled

conditions. The microstructure of steel is classiﬁed in terms of several crystal structures called ferrite,

austenite, pearlite, and cementite. Phase diagrams are used to map the steel structure in relation to

temperature and carbon content and transformation diagrams are used to map the structure in terms

of temperature and time. If steel is allowed to cool slowly, the crystal lattice structure and microstructure

reaches equilibrium dependent on the steel temperature and the carbon concentration (represented by

the phase diagram). If it is cooled faster, the structure is dependent on the cooling rate and the chemical

content of the steel (represented by transformation diagrams).

TA B L E

44.2

Range of Structural Shapes Available in the U.S.

Shape Size Groupings for Tensile Property Classiﬁcation\

(as given in ASTM A6/A6M – 1998)

Group 1 Group 2 Group 3 Group 4 Group 5

W40

149 to 268 W40

277 to 328 W40

362 to 655

W36

135 to 210 W36

230 to 300 W36

328 to 798 W36

920

W33

118 to 152 W33

201 to 291 W33

318 to 619

W30

90 to 211 W30

235 to 261 W30

292 to 581

W27

84 to 178 W27

191 to 258 W27

281 to 539

W24

55 & 62 W24

68 to 162 W24

176 to 229 W24

250 to 492

W21

44 to 57 W21

62 to 147 W21

166 to 223 W21

248 to 402

W18

35 to 71 W18

76 to 143 W18

158 to 192 W18

211 to 311

W16

26 to 57 W16

67 to 100

W14

22 to 53 W14

61 to 132 W14

145 to 211 W14

233 to 550 W14

605 to 873

W12

14 to 58 W12

65 to 106 W12

120 to 190 W12

210 to 336

W10

12 to 45 W10

49 to 112

10 to 48 W8

58 & 67

9 to 25

16 & 19

M shapes to 18.9 lb/ft

S shapes to 35 lb/ft

HP shapes to 102 lb/ft HP shapes over 102 lb/ft

C shapes to 20.7 lb/ft C shapes over 20.7 lb/ft

MC shapes to 28.5 lb/ft MC shapes over 28.5 lb/ft

L shapes to

in L shapes over

in to

in L shapes over

Te es cut from W, M and S shapes remain within the group of the shape from which they are cut.

-4

The Civil Engineering Handbook, Second Edition

There are several terms associated with cooling and heat treatment that will now be brieﬂy introduced.

Normalized steel is allowed to cool in air from temperatures above 850

∞

C, developing a reasonably ﬁne

grain structure, moderate strength and good toughness. Controlled rolling involves rolling the steel to

the desired form within a comparatively narrow temperature range and then allowing it to cool naturally

in air. This results in higher strength than achieved through conventional rolling. Quenching is rapid

cooling by immersing the steel in water or oil to produce a higher strength but less ductile and tough

product. Tempering is a subsequent process in which the temperature is again raised, but not to the

austenitizing range, and cooled more slowly resulting in a reduction in strength but increased ductility

and toughness. All of these processes are used in producing the steel types listed in Table 44.1. In addition

some structural steel shapes are produced by a patented process of quenching and self-tempering (QST)

which produces shapes with mechanical properties equivalent to those that would normally be obtained

using reheating after rolling.

Carbon steels are largely composed of iron with up to 1.7% carbon, but the addition of relatively small

quantities of other elements greatly inﬂuences its behavior and properties. For structural purposes it is

desirable that steel be ductile and weldable, and consequently most structural carbon steels are “mild

steel” with carbon in the range 0.15 to 0.29% and may include small quantities of manganese, silicon

and copper.

High-strength low alloy steels include alloying elements such as chromium, copper, nickel, silicon and

vanadium. In addition to their higher strength these steels may also possess greater corrosion resistance

than carbon structural steels.

Corrosion resistant, high-strength low-alloy steels, due to the addition of greater quantities of copper,

have corrosion resistance properties substantially greater than carbon and low-alloy structural steels.

Under suitable environmental conditions, including several cycles of wetting and drying, a stable layer

of corrosion products is developed on the surface of these steels and further corrosion is inhibited.

Quenched and tempered alloy steels contain greater concentrations of alloying elements than carbon

and low-alloy structural steels and speciﬁc heat treatments are incorporated in the manufacturing process

primarily to increase their strength. Generally this increased strength is obtained at the cost of reduced

ductility and great care must be exercised in their use.

Table 44.1 summarizes the range of structural steels available in the U.S. according to their American

Society for Testing and Materials (ASTM) speciﬁcation, the grades available and the products and uses

for which they are intended. It should be understood that many shapes, plates and bars are not readily

available in all of these grades and that their availability should be checked with suppliers or fabricators

before they are speciﬁed in designs. The more readily available types and grades are:

•W shapes A992

•M, S, HP, C and L shapes A36 (and increasingly A572 Grade 50)

• HSS A500 Grade B for round and A501 Grade B for square and

rectangular (and increasingly Grade C for both)

• Plate A36 and A572 Grade 50

Structural Properties

As mentioned above structural steels are primarily designated in terms of their strength as measured in

tensile tests. In these tests normally conducted in accordance with ASTM 370 (Test Methods and Deﬁ-

nitions) specimens of the steel cut from the shape or plate are tested in tension. Figure 44.1 shows a typical

stress-strain diagram obtained from such a test for a carbon steel. It shows a well deﬁned yield point and

yield plateau, with subsequent strain hardening and substantial elongation before rupture. Figure 44.2

shows the low initial (low strain) portion of the stress-strain diagram shown in Fig. 44.1 in more detail.

The behavior shown in Figs. 44.1 and 44.2 with an extended yield plateau and substantial strain

hardening results in structures that have a great capacity for deformation and redistribution of stresses

before failure.

Structural Steel

-5

Higher strength structural steels (low alloy and quenched and tempered) do not show the well-deﬁned

yield plateau seen in Figures 44.1 and 44.2. The form of stress-strain curve obtained for such steels is

shown in Fig. 44.3. The yield stress for these steels is determined from the yield strength, which is the

stress at which a nominated strain is reached. The yield strength is determined by either the 0.2% offset

method or the 0.5% extension-under-load method as shown in Fig. 44.4. Steels with this form of stress-

strain curve do not exhibit the same elongation capability and stress redistribution capability as mild steels.

FIGURE 44.1

Typical stress-strain curve for carbon steel.

FIGURE 44.2

Detail of low strain region of stress-strain curve shown in Fig. 44.1.

FIGURE 44.3

Form of stress-strain curve for high strength steels.

600

500

400

300

200

100

0 0.1 0.15 0.2

0.25

0.05

Strain ()

Linear elastic

region

Yield plateau

Strain hardening

region

Upper yield

peak

Stress (MPa)

Tensile

strength

600

500

400

300

200

100

0 0.01 0.015 0.02 0.0250.005

Strain (−)

Yield plateau

Stress (MPa)

Strain hardening region

E = elastic modulus

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Strain (–)

Stress (Ksi)

100