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*The following are approximate values for all the steels: Modulus of elasticity—29 X 103 ksi. Shear modulus—11 X 103 ksi. Poisson's ratio—0.30.

Yield stress in shear—0.57 times yield stress in tension. Ultimate strength in shear—% to 3/4 times tensile strength.

Coefficient of thermal expansion—6.5 X 10—6 in per in per deg F for temperature range —50 to +150°F. Density—490 lb/ft3. f See ASTM A6 for structural shape group classification.

Where two values are shown for yield stress or tensile strength, the first is minimum and the second is maximum. §The minimum elongation values are modified for some thicknesses in accordance with the specification for the steel. Where two values are shown for the elongation in 2 in, the first is for plates and the second for shapes. 1 Not applicable.

*The following are approximate values for all the steels: Modulus of elasticity—29 X 103 ksi. Shear modulus—11 X 103 ksi. Poisson's ratio—0.30.

Yield stress in shear—0.57 times yield stress in tension. Ultimate strength in shear—% to 3/4 times tensile strength.

Coefficient of thermal expansion—6.5 X 10—6 in per in per deg F for temperature range —50 to +150°F. Density—490 lb/ft3. f See ASTM A6 for structural shape group classification.

Where two values are shown for yield stress or tensile strength, the first is minimum and the second is maximum. §The minimum elongation values are modified for some thicknesses in accordance with the specification for the steel. Where two values are shown for the elongation in 2 in, the first is for plates and the second for shapes. 1 Not applicable.

These relative corrosion ratings are determined from the slopes of corrosion-time curves and are based on carbon steels not containing copper. (The resistance of carbon steel to atmospheric corrosion can be doubled by specifying a minimum copper content of 0.20%.) Typical corrosion curves for several steels exposed to industrial atmosphere are shown in Fig. 1.2.

For methods of estimating the atmospheric corrosion resistance of low-alloy steels based on their chemical composition, see ASTM Guide G101. The A588 specification requires that the resistance index calculated according to Guide 101 shall be 6.0 or higher.

A588 and A242 steels are called weathering steels because, when subjected to alternate wetting and drying in most bold atmospheric exposures, they develop a tight oxide layer that substantially inhibits further corrosion. They are often used bare (unpainted) where the oxide finish that develops is desired for aesthetic reasons or for economy in maintenance. Bridges and exposed building framing are typical examples of such applications. Designers should investigate potential applications thoroughly, however, to determine whether a weathering steel will be suitable. Information on bare-steel applications is available from steel producers.

A572 specifies columbium-vanadium HSLA steels in four grades with minimum yield points of 42, 50, 60, and 65 ksi. Grade 42 in thicknesses up to 6 in and grade 50 in thicknesses up to 4 in are used for welded bridges. All grades may be used for riveted or bolted construction and for welded construction in most applications other than bridges.

A992 steel was introduced in 1998 as a new specification for rolled wide flange shapes for building framing. It provides a minimum yield point of 50 ksi, a maximum yield point of 65 ksi, and a maximum yield to tensile ratio of 0.85. These maximum limits are considered desirable attributes for seismic design. To enhance weldability, a maximum carbon equivalent is also included, equal to 0.47% for shape groups 4 and 5 and 0.45% for other groups. A supplemental requirement can be specified for an average Charpy V-notch toughness of 40 ft • lb at 70°F.

FIGURE 1.2 Corrosion curves for structural steels in an industrial atmosphere. (From R. L. Brockenbrough and B. G. Johnston, USS Steel Design Manual, R. L. Brockenbrough & Associates, Inc., Pittsburgh, Pa., with permission.)

1.1.3 Heat-Treated Carbon and HSLA Steels

Both carbon and HSLA steels can be heat treated to provide yield points in the range of 50 to 75 ksi. This provides an intermediate strength level between the as-rolled HSLA steels and the heat-treated constructional alloy steels.

A633 is a normalized HSLA plate steel for applications where improved notch toughness is desired. Available in four grades with different chemical compositions, the minimum yield point ranges from 42 to 60 ksi depending on grade and thickness.

A678 includes quenched-and-tempered plate steels (both carbon and HSLA compositions) with excellent notch toughness. It is also available in four grades with different chemical compositions; the minimum yield point ranges from 50 to 75 ksi depending on grade and thickness.

A852 is a quenched-and-tempered HSLA plate steel of the weathering type. It is intended for welded bridges and buildings and similar applications where weight savings, durability, and good notch toughness are important. It provides a minimum yield point of 70 ksi in thickness up to 4 in. The resistance to atmospheric corrosion is typically four times that of carbon steel.

A913 is a high-strength low-allow steel for structural shapes, produced by the quenching and self-tempering (QST) process. It is intended for the construction of buildings, bridges, and other structures. Four grades provide a minimum yield point of 50 to 70 ksi. Maximum carbon equivalents to enhance weldability are included as follows: Grade 50, 0.38%; Grade 60, 0.40%; Grade 65, 0.43%; and Grade 70, 0.45%. Also, the steel must provide an average Charpy V-notch toughness of 40 ft • lb at 70°F.

1.1.4 Heat-Treated Constructional Alloy Steels

Steels that contain alloying elements in excess of the limits for carbon steel and are heat treated to obtain a combination of high strength and toughness are termed constructional alloy steels. Having a yield strength of 100 ksi, these are the strongest steels in general structural use.

A514 includes several grades of quenched and tempered steels, to permit use of various compositions developed by producers to obtain the specified strengths. Maximum thickness ranges from 1V4 to 6 in depending on the grade. Minimum yield strength for plate thicknesses over 2V2 in is 90 ksi. Steels furnished to this specification can provide a resistance to atmospheric corrosion up to four times that of structural carbon steel depending on the grade.

Constructional alloy steels are also frequently selected because of their ability to resist abrasion. For many types of abrasion, this resistance is related to hardness or tensile strength. Therefore, constructional alloy steels may have nearly twice the resistance to abrasion provided by carbon steel. Also available are numerous grades that have been heat treated to increase the hardness even more.

1.1.5 Bridge Steels

Steels for application in bridges are covered by A709, which includes steel in several of the categories mentioned above. Under this specification, grades 36, 50, 70, and 100 are steels with yield strengths of 36, 50, 70, and 100 ksi, respectively. (See also Table 11.28.)

The grade designation is followed by the letter W, indicating whether ordinary or high atmospheric corrosion resistance is required. An additional letter, T or F, indicates that Charpy V-notch impact tests must be conducted on the steel. The T designation indicates that the material is to be used in a non-fracture-critical application as defined by AASHTO; the F indicates use in a fracture-critical application.

A trailing numeral, 1, 2, or 3, indicates the testing zone, which relates to the lowest ambient temperature expected at the bridge site. (See Table 1.2.) As indicated by the first footnote in the table, the service temperature for each zone is considerably less than the Charpy V-notch impact-test temperature. This accounts for the fact that the dynamic loading rate in the impact test is more severe than that to which the structure is subjected. The toughness requirements depend on fracture criticality, grade, thickness, and method of connection.

A709-HPS70W, designated as a High Performance Steel (HPS), is also now available for highway bridge construction. This is a weathering plate steel, designated HPS because it possesses superior weldability and toughness as compared to conventional steels of similar strength. For example, for welded construction with plates over 2V2 in thick, A709-70W must have a minimum average Charpy V-notch toughness of 35 ft • lb at -10°F in Zone III, the most severe climate. Toughness values reported for some heats of A709-HPS70W have been much higher, in the range of 120 to 240 ft • lb at - 10°F. Such extra toughness provides a very high resistance to brittle fracture.

(R. L. Brockenbrough, Sec. 9 in Standard Handbook for Civil Engineers, 4th ed., F. S. Merritt, ed., McGraw-Hill, Inc., New York.)

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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