The Waddell A-Truss Bridge

Designing and Building File-Folder Bridges as an Introduction to Engineering

COL Stephen Ressler, P.E., Ph.D.Department of Civil & Mechanical EngineeringU.S. Military Academy, West Point

Why File Folders? Inexpensive.Easy to cut, bend, and glue.Surprisingly predictable structural

behavior.Can be used to build:

Tubes and bars. Connections that are stronger than the

attached structural members.

Let’s build this bridge...

Waddel “A Truss” Bridge over Lin Branch CreekTrimble, MO http://structurae.net/structures/data/index.cfm?id=s0001386

The Design

10 mm x 10 mm Tube

Doubled 4 mm Bar

Doubled 2 mm Bar

Design Requirements: Span–30 cm Loading–5 kg

(at midspan)

Profile ViewProfile View

Our A-Truss Bridge

Materials & Equipment

File foldersYellow carpenter’s glueBuilding board (Styrofoam or cork)PinsScissorsRulerDrawings – side and top view

Prefabrication of Members

Cut out bars Cut out and assemble

tubes Cut out gusset plates Trim bars and tubes to

length

Gluing Flap

Rubber Cement

Gluing Flap

Rubber Cement

Part 1 - Prefabrication

Prefab 10mm X 10mm ColumnsPrefab Double 4 mm BarsPrefab Double 2mm BarsPrefab Gusset Plates – 3 stylesPrefab Middle Beam

Part 2 – Truss Construction

Trim Bars and Tubes to Length

Bottom Chords(2 per team)

Trim Bars and Tubes to Length

Bottom Chords (2 per team)

Trim Bars and Tubes to Length

Verticals (2 per team)

Trim Bars and Tubes to Length

Verticals (2 per team)

Trim Bars and Tubes to Length

End Posts (2 per team)

Trim Bars and Tubes to Length

End Posts (2 per team)

Set up the Building Board

Place the layout drawing on your building board.Each Team Member:

Set up the Building Board

Place a sheet of plastic wrap over the layout drawing.

Add Gusset Plates Place Gusset Plate A at its correct location on the

layout drawings. Hold it in place with two pins.

Add Gusset Plates Repeat the process for Gusset Plates B, C, and D.

Add Bars Apply a line of glue along the bottom edge of Gusset Plates

A, B, and C. Place a 2 mm bar in position as the bottom chord AC. Stretch tight and hold in place with two pins.

Add Bars Apply glue to Gusset Plates B and D. Place a 4 mm bar in position as the vertical member BD. Stretch tight and hold in place with your fingers.

Each team should now have two of these subassemblies —the lower half and the upper half of one truss.

Add Tubes

Apply glue to Gusset Plates A and D. Place a 10mm x 10mm tube in position as end post AD. Hold in place for a minute until the glue sets.

For the bottom half of the truss (one per team):

Add Tubes Apply glue to Gusset Plates C and D. Place a 10 mm x 10 mm tube in position as end post AD. Hold in place for a minute until the glue sets.

Add Tubes Cut a 2 cm length of 10 mm x 10 mm tube. Apply glue to Gusset Plate B. Place the tube vertically on the gusset plate. Hold in place for a minute until the glue sets.

The Finished Half-Truss

Allow all glue joints to dry.

Assemble the Two Halves

Pull out all of the pins on both halves of the truss. Carefully separate the upper half of the truss from the

plastic wrap. Keep the lower half of the truss on the building board.

Assemble the Two Halves

Put glue on the tubes at A, B, C, and D. Place the upper half onto the lower half. Stretch the bars tight and hold until the glue has set.

Assemble the Two Halves

Allow all glue joints on the completed truss to dry.

Finish the Truss

Trim off the excess length on both bottom chords (AC) .

Place the Structure into Service

The completed bridge

Load test with 5 kg of sandsuspended from midspan