Mars Hill/Lowell Observatory Truss Bridge
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Bridge Technology
History of Town Lattice Truss Bridges
In the early 19th century, the American population experienced a
large increase, resulting in a migration westward. This created an increasing
need for bridges in the United States.
Bridge builders accommodated this need with covered timber truss bridges.
These bridges were covered to protect the costly structures and to
greatly increase their longevity.
In 1820, Ithiel Town, an architect from Connecticut, patented the Town Lattice
Truss bridge design.
This design
utilizes a lattice framed beam that was unseen in previous bridges.
The Town Lattice Truss is composed entirely of timber.
When compared to other bridges at the time, the Town Lattice was built
out of a larger quantity of shorter and smaller members.
These members were connected by large wooden pegs or treenails.
Advantages of Town Lattice Truss Design
The ingenuity behind Town’s design is that the construction did not require
skilled laborers, though it was very labor intensive.
Plank members were easier to handle and connections only required drilled
holes with inserted pegs.
Other
bridges at the time relied on large heavy timbers with elaborate joinery (CBM
2005).
Design of Town Lattice Trusses
Town Lattice Trusses are comprised of two main
elements: chords and webs.
The
chords are typically longitudinal built-up beams.
The majority of Town Lattice Trusses have four chords, two at the top and
two at the bottom.
The webs are the
individual plank members that make up the lattice structure.
They connect the top and bottom chords diagonally in both directions and
are spaced evenly along the length of the bridge as can be seen in Figure 1.
Figure 1: General Town Lattice Truss Configuration