LEARNING ACTIVITY #1
1-13
The
Learning
Activity
| The Need | Design Requirements | The Design | Structural Drawing |
| Typical Connections | Scheldule of truss members | Full-scale shop drawing | Schedule of gusset plates |
The Problem
Just outside the small town of Hauptville, New York, Grant
Road crosses Union Creek via a beautiful old 19th Century Pratt
truss bridge similar to the one shown here. Recently, the Town
Engineer determined that the structure is no longer safe for
modern truck traffic and must be replaced. Because of its
historic value, the old bridge will be disassembled, moved to a
nearby public park, and rebuilt as a pedestrian bridge. A new
highway bridge for Grant Road must be built on the existing site.
The Owner for this project is the Town of Hauptville. Several months ago, the Town Council selected
Thayer Associates, a respected local engineering firm, as the Design Professional for this project. The Hauptville
Town Engineer worked closely with civil engineers from Thayer Associates to develop three functional requirements
for the bridge:
n The new bridge must be constructed on the abutments from the old structure.
These existing supports are 24 meters apart.
[Our 1/40 scale model bridge will actually have a span of 60 centimeters.]
n The bridge must carry two lanes of traffic.
[Our model bridge must have a roadway width of at least 9 centimeters and at least 9 centimeters
of overhead clearance above the deck.]
n The bridge must meet the structural safety requirements of the AASHTO bridge design code.6
[Our model bridge must carry a “traffic load” consisting of a 5 kilogram mass placed on the structure
at mid-span.]
The Town Council also added an important aesthetic requirement. To preserve the town’s historical character,
the new Grant Road Bridge should look similar to the old one—a Pratt through truss. The old bridge was
made of wrought iron, but the Town Engineer has decided that the new structure will be safer and more practical
if it is made of steel.
[For our model, steel will be represented by cardboard from manila file folders.]
Based on these design requirements, a team of engineers from Thayer Associates has developed plans and
specifications for the new Grant Road Bridge over Union Creek. The plans and specifications include a structural
drawing, isometric drawings of two typical connections, a schedule of truss members, a schedule of
connections, and full-scale shop drawings of the structure.
6 AASHTO is the American Association of State Highway and Transportation Officials, the organization
that develops and publishes standard design specifications for bridges in the United States.
The structural drawing of the new Grant Road Bridge is designated as Drawing S-1 and is provided on page
1-16. The drawing includes a side elevation, a front elevation, and a plan view. Note that every connection in
the structure is designated with a letter—A through N for one main truss and A’ through N’ for the other. These
letters are used to identify the members and gusset plates.
The two isometric drawings below are typical gusset-plate connections found at the top and bottom chords
of the main trusses. These drawings illustrate the types of structural members used throughout the Grant Road
bridge—hollow tubes for the top chords and verticals; doubled bars for the bottom chords and diagonals. The
drawings also show how two gusset plates are used at each connection to hold all of the structural members
together. The drawings do not show lateral bracing, struts, or floor beams, which have been omitted for clarity.
The Schedule of Truss Members identifies every member required to build the bridge. Note that each
member is identified by the two letters corresponding to its endpoints. For example, Member AD is a segment
of the bottom chord that goes from Connection A to Connection D.
For this model, a member designated as a 4mm bar is actually a 4mm-wide strip of cardboard. The designation
6mm x 10mm refers to a hollow tube measuring 6mm by 10mm. The lengths shown in the schedule are
approximate.
A full-scale layout drawing of the main trusses (Drawing SD-1) is provided along with this book. This drawing
will be used to assemble the main trusses. A full-scale layout drawing of the gusset plates (Drawing SD-2) is
also provided on page 1-17. This drawing shows exactly half of the gusset plates required for the bridge.
The Schedule of Gusset Plates designates the gusset plates that will be used for each truss connection in the
bridge. The numbers in the “Gusset Plate” column are from Drawing SD-2. The letters in the “Connections”
column are from the structural drawing, Drawing S-1.
