Page 34 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 1 OVERVIEW OF BRIDGE ENGINEERING 9

• Weathering steel: Using ASTM A242, A588 or A709 “W” grade steel does not require
painting, except at the ends.
• Special steel: Using special chemical composition steel or with special heat treatment,
such as quenched and tempered plate (usually A514 or A517 grade).
• Hybrid steel section: This is used to describe a section, which is composed of more than

one type of steel (e.g., web is composed of grade 50W steel and the flanges are composed
of 70W).
Type 4–Aluminum, wrought iron, or cast iron. Wrought iron and cast iron were commonly
used before 1900.
Type 5–Stone or brick masonry.
2. Selection of modern materials: A choice can be made for arriving at optimum solution for
the use of available construction materials.
Steel versus concrete:
Structural steel has the advantage of being a permanent material with repetitive uses extend-
ing over decades. When recovered from the demolition of a disused bridge, it is salvaged and
sent to steel mills and foundries for re-rolling. With the expansion in transportation systems,
there are supply shortages, resulting in higher price as the demand for steel increases.
Recent trends are for greater use of precast prestressed concrete girders in small and medium
span ranges, although steel girders are applicable to any span length. HPS 70W steel welded
girders are increasingly being used for long spans. Both steel and reinforced concrete are
essential materials for bridge construction as indicated by the use of steel in the form of
steel reinforcing bars, prestressing strands, rolled steel joists, or welded plate girders.
• Epoxy coated reinforcing steel bars—Substructure rebars are generally plain uncoated
bars. Galvanized steel rebars are used in marine environments. Both epoxy coated and
galvanized steel bars are more expensive than plain bars. While local bond properties
between concrete and uncoated bars is known, tests need to be performed to determine
local bond of different types of chemical used in coatings.

• Modern concrete materials—Composites, fiber reinforced polymer concrete, and exodermic
decks are being successfully used.
• Recycling of construction materials—Good quality aggregate has the potential to be
cleaned and reused for all types of concrete construction. Recycled concrete aggregate
from debris can be washed and reused. So far the practice has been to use the debris in

landfills and to some extent in pavement construction.
• Normal weight concrete
• Lightweight concrete
• High performance concrete
• High performance steel
• Aluminum
• Glue laminated timber: Glulam hardwood bridges are economical for small spans.
The following precautions are also applicable to timber bridges:
• Timber friction piles can be damaged while driving in fi rm ground.
• Timber pile piers in rivers should not be subjected to debris accumulation.
• Timber bridges are posted for low speed.

1.3.3 Span Classifi cation
In practice, most bridges are a single span with two or three lanes over narrow rivers. Piers
are needed for continuous spans over highways, valleys, or wide rivers. The minimum single
span for a bridge is 20 feet, below which a culvert is normally used. Pedestrian bridges with
lighter live loads can be smaller in length than 20 feet.