Truss Design

The final truss design we chose was relatively similar to the previous test bridge’s truss due to the high efficiency we obtained from that test. Although the previous bridge was 10g overweight, we decided to use it and amended the number of fettuccine trusses in the final design. After

determining the final results of the previous test bridge, improvisation to the final design was made through a series of analysis.

7.1.1 Amendments and Modifications to Bridge Design

Our main concern was regarding the weight of the bridge as the brief requirement gave 200g whereas our previous bridge was about 210 g. Based on one of our earliest experiment of prioritizing the members, we understand that the vertical members require more reinforcement compared to the diagonal bracing members. This is because fettuccine is weaker when compressed.

The layers of the diagonal bracings under tension were reduced from 2 to 1 layer.

Figure 7(a): The removal of a layer of fettuccine on tensile members.

Besides that, the dimensions of the bridge have been slightly modified. Firstly being the length which was reduced from 850mm to 820mm. This is to ensure a lower risk of the bridge failing due to the upward force of the table at the edge. The upward force may lift the edges of the

fettuccine bridge and overthrow the equilibrium of the bridge. Furthermore, the spacing between vertical members was fixed so that they are similarly 75mm apart, leaving just 35mm rested on the tabletop.

Figure 7(b): Dimensions of the final truss bridge.

Another change we applied was the center beams which are the points that the load will be latched on to. In our previous bridge, we used beam 1, a sandwiched 3-layer I beam. After the experiment to test the strength of the I-beam versus beam 2, a fettuccine simulation of a RC beam, the second choice was applied into the final design.

Figure 7(c): Section of beam 1 and beam 2 Figure 7(d): Beam type 1 & 2 820mm

750mm

80mm

7.1.2 Fettuccine Bridge Construction

The construction steps can be divided into 4 main steps. It is important to follow a proper sequence so that the main frame is straight and stable, followed by other components.

First, the elevations for the final model are cad and printed out as a guideline. Based on the length of the 1:1 cad drawing, the strongest and longest 4 chords were made. Similar to previous tests, we used the concept of running bond so that no one part of the member is weaker due to more than one joint.

Figure 7(e): Top and bottom chords as main frame.

Second, the vertical members were attached using 3-second glue, using the printed drawing as guideline to ensure they are straight. They are attached on the inside of the top and bottom chords so that force of the load can be directly transferred across them.

Figure 7(f): Adding vertical members.

Next, the diagonal bracing members were attached. We trimmed the edges carefully in order for it to sit on top of the vertical members but within the 4 chords.

Figure 7(g): Adding diagonal members.

When the two side faces of the bridge were completed, we erected it and made sure that the vertical members are perpendicular to the tabletop using a MDF board and set squares. The two faces were stabilized by taping it to the drawing and the table. 50mm horizontal members were placed between the two faces to act as the connecting member of the two faces. The base is lined using plastic sheets to prevent the bridge from sticking to the table.

Finally, when the bridge is able to stand upright, we cross-braced the open ends to ensure the bridge retain its shape. In the center, beam 2 is applied where 2 of it sits perpendicular to the bridge’s frame a on each side of the middle vertical member. They sit on top of the horizontal beam so the force of the load can move downwards onto the horizontal frames.

Figure 7(i): Diagonal bracing. Figure 7(j): Beam 2 located in center.

Figure 7(h): Joining the two faces.

7.1.3 Joints

Joints are important in the way that they have to fit well with one another so that the transfer of loads can be even. Most of our joints were treated by trimming the stacked layers properly so they are flat and able to attach perpendicularly to the top and bottom chords.

The chords are created through many joints of 240mm fettuccines. We based the joint concept on running bonds, so that no one point of the member is weaker due to the presence of more than 1 joint. This is done by separating the 240mm fettuccine into 3 equal parts, the two marks being the points of jointing, where no 2 joints will be present at the same length of the member.

Figure 7(k): Running bond concept of jointing.

The basic joint is to glue two flat surfaces together. This is used for the vertical or horizontal members that we glue in between the 4 chords. Putting them in between allows a better load transfer and also to prevent the bridge from collapsing inwards.

Figure 7(l): Flattened edges are glued together.

The diagonal members were properly trimmed so that not only can their edges stick cleanly to the horizontal members; they still have space to attach to the vertical members. We believe that this method of jointing can help brace not only the vertical members and the horizontal members, but also to maintain the horizontal and vertical members in perpendicular to one another.

Figure 7(m): Trimming diagonal edges to fit.