Week 2 logbook

W02 - STUDIO REPORT FRAME BUILDING EXERCISE In this tutorial we further explored the concepts behind the practical application of load paths. The exercise was to create a bridge that spanned 150cm, this was to be done using only a single sheet of balsa wood, approximately 60 x 20cm, and a range of adhesive and fastening materials (i.e. pins, glue, blue tack and tape). With such a finite amount of building materials our structure had to be not only capable of bearing load but also make economic use of the materials available.

The first step in creating our bridge was to consider a variety of designs that allowed the bridge to transfer the load to the two tables at either end. In order to plan this we had to include load path diagrams in each of the designs.

Some of the initial plans included frames stretching across the bridge intended to carry the load of the bridge, however with such limited resources we were reluctant to cut our plank so thin. Simply to span the gap we calculated that we would need 3 lengths of the board, these planks would

also need to be large enough to support the load placed on them. We finally decided to cut the timber into quarters lengthwise. This was would have 3 planks for the surface of the bridge and then one to divide up as support.

The first plan was to simply support the bridge with diagonal beams stretching out from the table, the aim of these would be to carry the load of the bridge from points closer to the centre essentially shortening the gap.

STUDIO REPORT This worked in terms of making the outer points of the bridge quite sturdy but the centre of the bridge and the joints between the planks were quite weak. This meant that the design had to be improved. We were finding it difficult to develop ideas for framework so we turned our attention to strengthening the surface of the bridge to reinforce it and prevent it buckling or collapsing.

To do this we decided to cut the outer planks (the load of which was mostly carried by the lower supports) in order to gain more usable pieces of timber. It was then decided that a second plank would be run underneath the central one but instead of laying flat it would instead be connected vertically to the lower face by its edge. Our tutor informed us that this was a technique used in construction and was called an ‘I’ beam. Of course an I-

beam normally has a secondary horizontal ‘flange’ pieced below so the vertical ‘web’ acts as the space in between the two. This meant that ours was more ‘half an Ibeam’. The theory behind this was that the vertical ‘web’ plank would reinforce the upper plank as though a thick piece of timber had be attached underneath, this would also make the balsa wood plank, which is generally extremely flexible, quite rigid.

This created a new problem, whilst we had solved the issue of the weak central plank we then found that the joins between it and the outer planks were quite unstable.

To rectify this a second set of diagonal base supports were added to once again carry the load from points further out on the bridge.

The bridge was tested with small wooden blocks.

STUDIO REPORT OTHER GROUPS

This group also tried to use an I-beam, reinforced by secondary web panels which were spaced along its length. This would have been quite sound over a shorter length but it did not cope well with carrying the weight across the whole gap and was one of the first to buckle as pressure was applied.

The next group also showed similarities to our design in the diagonal supports extending from the table legs, the difference was that theirs was trussed which i would argue made it more stable.

However the centre of their bridge was very weak and dipped alarmingly when load was placed in the centre. Whilst the frame underneath may have offered some support the connection between the centre and exterior sections was too weak to bear the load.

The last group was the group I was most impressed by, their structure was the only one that utilised tension in the form of sticky tape cables. Due to the rigid from created by the tension of the framework the bridge did not bend when load was applied. This bridge was able to carry the most load but what was interesting was that after a large amount of weight was applied the bridge sunck slightly transferring the load bearing capacity from the frame to the cables.