Building a Truss Bridge

Project Files

file-folder-bridge.pdf

Instructions for Building the Bridge

The United States Military Academy's Designing and Building File-Folder Bridges instructions document by Stephen J. Ressler.

West Point Bridge Testing.pdf

Instructions for Testing the Bridge

Pages 2-11 through 2-25 of the above instructions document.

Corel Draw Files & Labeled Diagrams of the Files

Gusset Plates

Tubes and Bars

Pieces for Testing

Small Epilog Workflow

Small and Large Laser Cutter Workflows

Designing the Project File

To design the files, we opened a new document in Corel Draw and set the page to 18" x 18". We then imported a gusset plate document that we were provided. Next, we made the tubes by creating black boxes in the following quantities and measurements:

Four 21cm x 4.6cm
Four 17cm x 4.6cm
Six 12cm x 3.8cm
Five 10cm x 4.8cm
Seven 9cm x 2.9cm

Next, for the 21cm x 4.6cm and 17cm x 4.6cm boxes, we made red lines 1, 2, 3, and 4 cm away from the top 21cm or 17cm long black line. For the 12cm x 3.8cm boxes, we made red lines 1, 1.6, 2.6, and 3.2 cm away from the top 12cm long black line. For the 10cm x 4.8cm boxes, we made red lines 1.5, 2.1, 3.6, and 4.2 cm away from the top 10cm long black line. Finally, for the 9cm x 2.9cm boxes we made red lines 0.6, 1.2, 1.8, and 2.4 cm away from the top 9cm long black line. We then made 32 10cm x 4mm black boxes.

To create the test pieces, we made black boxes as such:

Three 20cm x 4mm
Three 20cm x 6mm
Three 20cm x 8mm

We then made 10mm x10mm boxes and attactched them to the each end of the above mentioned boxes. Using the knife tool, we removed the nine 4mm lines that connect the 10mm x 10mm boxes to the above mentioned boxes.

Next, we created three black boxes each in the following measurements:

5cm x 4.6cm
10cm x 4.6cm
16cm x 4.6cm
5cm x 3.8cm
10cm x 3.8cm
16cm x 3.8cm

We then made red lines 1, 2, 3, and 4 cm away from the top black line for the 4.6 cm wide boxes and red lines 1, 1.6, 2.6, and 3.2 cm from the top black line for the 3.8 cm wide boxes.

Bridge Tube and Bar Details

Diagram of Tubes from instructions document.

Test Piece Details

Bars and 10mm x 10mm boxes (post testing)

Tubes (post testing)

File to Laser Computer

After designing the files we saved the CorelDraw file and exported it to the engproj file. From there we could then open engproj on the computer attatched to the laser cutter and open the file.

Settings

Once the file was open we imported it to the laser cutter application. We set the lines to vector on the red lines and cut on the black lines and set the material to cardboard for all lines. We then ensured auto-focus was not selected before hitting send.

Laser Setup

On the laser cutter we selected the job with the same name as our file. We then manually focused the laser cutter over our 18' x 18' piece of cardstock before hitting print.

Laser Cutting

Once cutting we watched the laser to ensure no fires started. The digital design and final physical project did not have any differences. An important note to consider when laser cutting is "kerf," the thickness of the laser you are using and how big the cut lines you are cutting will be.

tubes cutting lapse.MOV

Video by Raaj Thakur '26

Part Processing

The cardboard template is set up by printing out the pictured template, taping the paper to the cardboard, and putting wax paper over the cardboard and taping the wax paper in place on the backside of the cardboard.

To verify that our laser cut pieces were the correct size, we lined up the gusset plates, tubes, and bars to the template and made sure they lined up with their corresponding outline.

Processing Pieces & Constructing the Bridge

To start the assembly of the truss bridge, first we aligned the gusset plates in their appropriate places on the template and taped them into place. Next, we added the 4mm bars to their places on the template and trimmed them to fit the desired length according to the template. We then applied super glue to the gusset plates on the spots where the 4mm bar ends would go and, holding the 4mm bar in place, we sprayed accelerator on the super glue. We then repeated this processs with the corresponding bars and tubes until both truss sides and the top were completed. Next, we glued the trusses to the top piece to complete the bridge.

Schedule of Truss Members

The members are names by the two letters corresponding to its endpoints.

Schedule of Gusset Plates

Details the gusset plate that will be used for each connection in the bridge. The "Gusset Plate" column's numbers correspond to the earlier pictured gusset plate labeled diagram, the "Connections" column corresponds to the structure template.

Problems I encountered during this process were primarily getting super glue on my fingers. To remedy this, I sandpapered the super glue off my fingers and wore gloves and used greater caution thereafter.

The Final Product

Although our bridge had a unique flair, it certainly does have structural shortcomings. These include not sitting flat on the ground and, overall, looking dilapidated. This could have been prevented with more dilligent construction and better techniques when aligning and securing the two trusses and the top piece.

Testing the Bridge Components

Using a wood see-saw clamped in place on a table, a bucket, sand, a cup, a scale, and the bars and tubes detailed in the table to the left, we were able to test the compression and tension strength of the bridge components.

To test the components we would clamp the piece in place on the left side of the see-saw for tension or put the piece under the wood on the right side for compression. We would then, with a bucket hung on the see-saw where the triangle bit is missing from the wood, slowly add sand to the bucket until the piece was crushed, snapped, or deemed to have failed (example of a failed piece: a piece that is still in the same placement under compression but has been smushed has technically not completely failed, however, would not be considered reliable to hold up a bridge with living people on it, thus, failing).

bucket, scale, and cup full of sand

wooden see-saw clamped down onto table

Tesnion vs Compression

Video of Testing Compression

Copy of IMG_8086.MOV

Video of Testing Tension

Copy of IMG_8085.MOV

Bridge Spreadsheet

For the Bars Under Tension table, the T1 group had smaller tension values due to T1 having the smallest width, 4mm. Consequently, the T3 group had the greatest tension values because they had the largest width, 8mm. Falling in perfect analogy as the T1 and T3 groups, group T2 had tension values in between the T1 and T3 groups because it had a width between the other two groups, 6mm.

For the Tubes Under Compression table, the values were fairly skewed among all the test groups. This is mainly because some tubes were not as strong as others due to inconsistiencies in gluing the pieces together (ex. more or less glue used per piece dependening on who assembled the piece in this group project). The values also were a bit too subject to personal opinion in that, in stopping measurements when we had deemed a piece to have 'failed,' we created inconsistencies in what constintited a 'failure' among the group members who were pouring the sand and observing the damage. Thus, the Tubes Under Compression data is too inconsistent to make conclusions from.

Bars and Tubes Post-Testing

By observing where the bars most commonly broke, we can see that the end points (in their connection with potential gusset plates) were the weakest areas of the bars.

Observing the tubes allows us to see that the ends of the tubes were where most failures occured (crushing, crumpling, breaking, etc.).

It is most likely to fail on the connections from the bars to the gusset plates due to tension and the connections from the tubes to gusset plates due to compression.

Testing The Bridge

To test the bridge, we first put the bridge ends on two tables. We then took six 4x2 legos and put them on joints B, C, and D on both trusses. Next, we added two books of weight 3,500 grams and then two more books for a total of 5,000 grams. Each load needed to successfully be supported by the bridge for three seconds.

Copy of IMG_1532.pdf

IMG_9042.MOV

Video of the Bridge Testing

Final Conclusion

To make the truss bridge we used cardboard, super glue, accelerator, the laser cutter, and Corel Draw. To test the bridge and bridge parts we used a wood see-saw, a bucket, sand/gravel, clamps, a scale, a spreadsheet, six 4x2 legos, and four books. Testing for tension and compression forces on the bridge parts allowed us to ensure the saftey of the bridge. Similarly, the bridge testing process.

This project taught me about the forces on a bridge and how all the parts come together to create a stable bridge. It is important to account for both compression and tension when designing a bridge because they are forces that both affect the stability and safety of the bridge. Possible ways this bridge could successfully hold more weight would be changing to a more sturdy material, more carefully putting the bridge together, and ensuring the bridge integrity every step of the assembly process.

Back

Report abuse