BRIDGE STEELS BASIC AND TUTORIALS

BRIDGE STEELS BASIC INFORMATION
What Are The Type Of Steels Used In Constructing Bridges?


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.

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 Below)

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 21⁄2 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.)

SPACE TRUSSES BASICS AND CIVIL ENGINEERING TUTORIALS

SPACE TRUSSES BASIC INFORMATION
What Are Space Trusses?

A space truss is the three-dimensional counterpart of the plane truss described in the three previous articles. The idealized space truss consists of rigid links connected at their ends by ball-and-socket joints.

Whereas a triangle of pin-connected bars forms the basic noncollapsible unit for the plane truss, a space truss, on the other hand, requires six bars joined at their ends to form the edges of a tetrahedron as the basic noncollapsible unit.

In Fig. 4/13a the two bars AD and BD joined at D require a third support CD to keep the triangle ADB from rotating about AB. In Fig. 4/13b the supporting base is replaced by three more bars AB, BC, and AC to form a tetrahedron not dependent on the foundation for its own rigidity.

We may form a new rigid unit to extend the structure with three additional concurrent bars whose ends are attached to three fixed joints on the existing structure. Thus, in Fig. 4/13c the bars AF, BF, and CF are attached to the foundation and therefore fix point F in space.

Likewise point H is fixed in space by the bars AH, DH, and CH. The three additional bars CG, FG, and HG are attached to the three fixed points C, F, and H and therefore fix G in space. The fixed point E is similarly created.

We see now that the structure is entirely rigid. The two applied loads shown will result in forces in all of the members. A space truss formed in this way is called a simple space truss.

Ideally there must be point support, such as that given by a balland- socket joint, at the connections of a space truss to prevent bending in the members.

As in riveted and welded connections for plane trusses, if the center lines of joined members intersect at a point, we\ can justify the assumption of two-force members under simple tension and compression.