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

The Waddell A-Truss Bridge

COL Stephen Ressler, P.E., Ph.D.

Department of Civil & Mechanical Engineering U.S. Military Academy, West Point

Objectives  Learn about structural engineering:  Through a hands-on bridge-building project.

 Through the use of free computer software.

 Learn about the ongoing West Point Bridge Design Contest.

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.

What is a Truss?

 A structure composed of members connected together to form a rigid framework.

 Usually composed of interconnected triangles.

 Members carry load in tension or compression .

Component Parts

Support (Abutment)

Standard Truss Configurations

Types of Structural Members

These shapes are called cross-sections .

Types of Truss Connections

Pinned Connection Gusset Plate Connection Most modern bridges use gusset plate connections

Let’s build this bridge...

Waddel “A Truss” Bridge over Lin Branch Creek Trimble, MO

The Design  Design Requirements:   Span–30 cm Loading–5 kg ( at midspan) 10 mm x 10 mm Tube Doubled 4 mm Bar Doubled 2 mm Bar

We’ll talk about how it was designed later...

Our A-Truss Bridge

Materials & Equipment  File folders  Yellow carpenter’s glue  Building board (Styrofoam or cork)  Pins  Scissors  Metal ruler *  Hobby knife or single-edge razor blade *  Rubber cement * *Required only for prefabrication of structural members

Prefabrication of Members  Cut out bars   Cut out and assemble tubes Cut out gusset plates  Trim bars and tubes to length

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

Each Team Member:

 Place the layout drawing on your building board.

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 the lower half and the two of these subassemblies — upper half of one truss.

Add Tubes

For the bottom half



of the truss (one per team):

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.

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.

Forces, Loads, & Reactions  Force – A push or pull.

 Load – A force applied to a structure.

Self-weight of structure, weight of vehicles, pedestrians, snow, wind, etc.

 Reaction – A force developed at the support of a structure to keep that structure in equilibrium.

Forces are represented mathematically as VECTORS.

Assemble the Two Halves    Pull out all of the pins on both halves of the truss.

Carefully wrap.

separate the upper half of the truss from the plastic 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.

Model the Structure 15 cm 15 cm D A B mass=5 kg =2.5 kg per truss C 15 cm

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 sand suspended from midspan

Summary  File-folder bridges:  Accurate representation of real bridges   Vehicle for learning engineering concepts.

Design based on authentic applications of math, science, and computer technology.

 The West Point Bridge Designer:   Experience the engineering design process.

Free!

 The West Point Bridge Design Contest:  Please help us make it successful!