Timber's
strength, light weight, and energy-absorbing properties furnish
features desirable for bridge construction. Timber is capable of
supporting short-term overloads without adverse effects. Contrary to
popular belief, large wood members provide good fire resistance
qualities that meet or exceed those of other materials in severe fire
exposures. From an economic standpoint, wood is competitive with other
materials on a first-cost basis and shows advantages when life cycle
costs are compared. Timber bridges can be constructed in virtually any
weather conditions, without detriment to the material. Wood is not
damaged by continuous freezing and thawing and resists harmful effects
of de-icing agents, which cause deterioration in other bridge materials.
Timber bridges do not require special equipment for installation and
can normally be constructed without highly skilled labor. They also
present a natural and aesthetically pleasing appearance, particularly in
natural surroundings. The misconception that wood provides a short
service life has plagued timber as a construction material. Although
wood is susceptible to decay or insect attack under specific conditions,
it is inherently a very durable material when protected from moisture.
Many covered bridges built during the 19th century have lasted over 100
years because they were protected from direct exposure to the elements.
In modem applications, it is seldom practical or economical to cover
bridges; however, the use of wood preservatives has extended the life of
wood used in exposed bridge applications. Using modem application
techniques and preservative chemicals, wood can now be effectively
protected from deterioration for periods of 50 years or longer. In
addition, wood treated with preservatives requires little maintenance
and no painting. Another misconception about wood as a bridge material
is that its use is limited to minor structures of no appreciable size.
This belief is probably based on the fact that trees for commercial
timber are limited in size and are normally harvested before they reach
maximum size. Although tree diameter limits the size of sawn lumber, the
advent of glued-laminated timber (glulam) some 40 years ago provided
designers with several compensating alternatives. Glulam, which is the
most widely used modem timber bridge material, is manufactured by
bonding sawn lumber laminations together with waterproof structural
adhesives. Thus, glulam members are virtually unlimited in depth, width,
and length and can be manufactured in a wide range of shapes. Glulam
provides higher design strengths than sawn lumber and provides better
utilization of the available timber resource by permitting the
manufacture of large wood structural elements from smaller lumber sizes.
Technological advances in laminating over the past four decades have
further increased the suitability and performance of wood for modern
highway bridge applications.
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