bridges. These ideas, in turn, are disseminated to the engineers of the Nation through the publications of revised editions of "Standard Plans for Highway Bridge Superstructures."

Most State highway departments have prepared and use standard bridge plans similar to those published by the Bureau of Public Roads. To have a good set of standards makes for great savings in engineering time and in the cost of preparing project plans.

The steel industry has been active in the area of standardized bridges since as early as 1900. They have made designs and plans, and have updated them, to include rolled steel beam section bridges, deck girder bridges, and various truss-type bridges. The highway industry has made use of these standardized steel bridges in many areas, particularly in distress areas caused by storms and floods, for the quick erection of quality structures. They have also been used by bridge builders for preparing estimates and submitting proposals. Standard bridge components. Some progress has also been made in the marketing of standard bridges. Composite timber-concrete girder bridges are now available for delivery in complete "package" form. Each unit can be ordered by span lengths from 30 to 100 feet for any desired loading or roadway width. Each unit consists of girders, girder bracing, wheel guards, handrails and posts, all necessary hardware, and framing materials cut to size and ready for use. There is much more that can be accomplished toward using economical mass production methods for standard bridges.

Steel in precast, prestressed bridges

Good progress has been noted in the development and use of standardized bridge components. Much of this progress results from the extensive use of precast concrete in bridge construction.

Precast concrete is not a special kind of concrete but a method of fabrication. The term means any concrete member, either plain or reinforced, that is cast in forms somewhere other than its final position. Usually the casting is done in a shop or plant that makes a specialty of such work. The use of precast concrete members has been increasing steadily during the past few years. This subject is discussed further under portland cement concrete.

Corrugated steel culvert pipe

Galvanized steel culvert pipe is widely used for disposing of surface runoff and for carrying small water courses. It is estimated that approximately a quarter of a million tons will be used in 1962 for highway construction and maintenance purposes. Corrugated metal culverts are fabricated in both round and eliptical shapes and are available with bituminous coated inverts or an asbestos bonded bituminous coating with or without paved inverts to minimize deterioration from chemical agents and abrasions.

In 1955, with the aid of a special grade asphalt, a smooth pipe section was developed to increase flow capacity. These sections have been used for storm sewers and longitudinal drainage on a number of

expressways.

Until recently, the maximum diameter of the helical corrugated steel pipe was 21 inches. Due to improved manufacturing techniques, the maximum diameter of these pipes has been increased to 36 inches. These new designs are being tried experimentally on Federal forest.

and park projects under the supervision of the Federal Highway Projects Division of the Bureau of Public Roads.

Improved jacking and boring methods made it possible to install culverts and conduits through existing highways without disrupting traffic. Recently developed horizontal boring equipment make it possible to drill holes up to 36 inches in diameter and 150 foot lengths in some types of soil.

Steelplate arches and pipes. In addition to the smaller riveted sections fabricated from sheet steel, the industry manufactures large plate arches and pipe arches which are fabricated in sections from galvanized steel structural plate and are erected with steel bolts.

The plate arches may be used in place of small bridges up to spans of 25 feet for H-20 loading conditions and up to 30-foot spans with no live load conditions.

The pipe arches may be used for single lane underpasses and cattle passes, as well as for watercourses, and are available in spans up to 16 feet and maximum rise or vertical clearance of 10 feet. conforms to current AASHO specifications.

This

A new pipe arch section was developed a few years ago in which the maximum span has been increased to 20 feet and the rise increased to over 13 feet. These sections have been used on selected projects after careful analysis of the design stresses by the compression ring method has assured adequate longitudinal seam strength. This method of design analysis is being used to a greater extent, and in some cases it results in a more economical installation. Perhaps the most noteworthy installation to date was made on the Lakeman Expressway in Cleveland, Ohio, last year. On this project, three adjacent pipe arches, each about 1,000 feet long and each with a 20-foot width, were installed under a 20-foot fill. A considerable saving over an alternate design method is reported.

Federal specification development. At the request of the General Services Administration, the Bridge Division of the Bureau of Public Roads is preparing the Federal specifications for corrugated metal culvert pipe. The Bridge Division is also cooperating with AASHO in developing a bridge and culvert construction manual. This manual will include installation requirements for corrugated metal pipe culverts.

Scope of use

ALUMINUM

An example of technological advances in adapting existing materials to new uses in the highway field is provided by aluminum. Some of the more common uses are lighting standards, warning, regulatory, and directional signs, signposts, guard and guide rails, gratings, auxiliary structural hardware, and, to a limited extent, for bridge superstructures and deck replacements. Some States have made experimental installations of aluminum pipe as drainage culverts. Bridges

With a few notable exceptions, the use of aluminum for major components of bridges has been quite limited. Structural aluminum would appear to warrant consideration as an alternate to steel, whenever it is vital to save deadweight-as in long-span bridges, in movable bridges, and in the reconstruction of the floor system or portions of existing bridges.

The suspended floor, including floor beams, stringers, roadway, and sidewalks, on two 360-foot lenticular truss spans of the 50-yearold Smithfield Street Bridge over the Monongahela River in Pittsburgh was replaced with aluminum, reducing the deadweight of the structure by 700 tons. This marked the earliest large-scale use of aluminum for highway bridges.

In the field of movable bridges, an aluminum double-leaf trunnion bascule bridge, aggregating 121 feet between bearings, was completed in England in 1948. This was followed by construction in 1949 of an aluminum bridge over the Saguenay River in Canada to connect the towns of Arvida and Shipshaw. Weighing 400,000 pounds, this latter structure consists of a main 290-foot arch span (with a 47.5-foot rise) and five approach spans on the north and south ends.

The world's first welded aluminum girder highway bridge was dedicated in Des Moines, Iowa, on September 24, 1958. Built as a development project under joint sponsorship of the State of Iowa and three leading aluminum manufacturers, this four-span continuous girder structure totals 222 feet in length and 36 fest in width, and contains 75,000 pounds of structural aluminum.

Semimonocoque designs.-A new type of aluminum bridge, employing proven aircraft design principles, has been developed. Known as semimonocoque (or stressed skin) construction, it consists primarily of a thin shell which is reinforced by a network of stiffening members. Test installations of this type of structure have demonstrated potentials, particularly for intermediate span lengths. Actual bridges of this type have been completed in Maryland and Virginia. Under conditions of mass production, the cost-benefit ratios related to dead load weight factors could provide significant savings in the future.

FIGURE 11.-Placing an aluminum V-beam of semimonocoque construction. Top of the beam

provides the bottom deck forming for the concrete deck.

Culverts

Experimental installations of corrugated aluminum pipe culverts have been made to test them for deformation under stress and for possible evidence of either erosion or corrosion. Pipes used in these installations were corrugated and fabricated by independent suppliers from flat aluminum sheets rolled by the manufacturer.

From these tests the feasibility of aluminum culvert is being evaluated at least for the smaller diameters, and for a range of fill heights comparable to corresponding criteria for the more conventional materials. As a further extension of this experimentation, additional test installations were arranged by the producer at both State and local highway department levels. Twenty States have now installed, and another eighteen States have made provisions to install, aluminum culverts on a trial basis.

Preliminary reports and appraisals of initial aluminum culvert test installations in California, Georgia, and elsewhere confirm the structural strength of aluminum culvert and indicate no measurable corrosion at test installations in natural soils having pH values from 4 to 9. Another preliminary report from California indicates both untreated aluminum and galvanized steel culverts corroded in a highly acid soil having a pH factor as low as 2.5.

Closely allied with the relatively new use of aluminum for culvert is the adaptation of this material for guardrail. At least seven State highway departments provide for aluminum guardrail installation through the medium of standard specifications. In addition, a number of States authorize this type of design and construction by special provisions.

Aluminum-magnesium alloys

Magnesium appears to offer possibilities for future lightweight materials for construction. Currently used aluminum-magnesium alloys such as 5083 plate or sheet and 5183 filler wire alloys, seem to be virtually indispensable for use in welded aluminum structures. The quality provided by the magnesium in these alloys is that of retention. of strength after welding. As a companion of aluminum in the light metal field, and as the lightest structural metal, magnesium is regarded as on the threshold of a new and expanding career. Usage for beams in the textile industry, coupled with varied applications ranging from handtrucks and ladders to lawnmowers and handtools, may well lead to more extensive use and translation to the highway field.