Analysis of the activity and links to the weekly subject content
Frame is a important techniques in terms of structures. It has a really different structural attributes comparing to compression. This week’s activity is to use the pieces of Balsa wood we prepared to construct a truss that is sustainable and base on this making the framework as high as possible. After the construction is finished, we need to stree the structure to collapse in order to observe how things such as dimensions, joints and bracing change to deeply understand the frame structure.
Photographic sequence of process
We started with the the triangular shape as the base of the structure. Then we found that the prepared sticks were too long which made it not stable and took us longer time to finish so we decided to cut those pieces in half. We made two triangles and used sticks to connect corners of two triangles vertically. To make it stabler, we also connected the corners diagonally.
We lay the structure down so that it was more convenient to stick everything together.
Since the structure was still not as stable as we thought it would be. At midpoint of each vertical pieces of Basal, we diagnoally connect it to the midpoint of the opposite vertical stick which is one level up.
Detachment of midpoint bracing and horizontal stick
After we structured the truss, we pushed it from the top. When we tried to stress the construction to collapse, the problems of the structure were clearly showed.
Description and analysis of the ‘construction system’ employed
As the processes above show, our idea is to start with a triangle shape as a base to structure up because triangle is a relatively stable shape. Then three sticks were vertically put up on each corner of the triangle for another triangle to attach on. After these steps were completed, on each vertical surface, a stick was used to diagonally connect the corner of two triangle. They worked as bracings. We keep copying these steps to form a basic structure. During the cosntructing period, we found that the structure was not strong enough. In order to make each section connect as a whole, we diagonally connect the midpoints between opposed vertical stick. This ‘midpoint’ bracing was parallel to the ‘corner’ bracing. The points where the midpoint bracings and the horizontal sticks encountered were also glued. Stressing the finished truss to collapse was a significant part of this activity. When a force was applied from the top of the structure, due to the way we construct it, it was not rapidly collapse. Firstly, a midpoint bracing was buckling because of the detachment of the point where the horizontal stick and midpoint bracing encountered. As soon as this part broke, the balance broke. Most of the vertical parts started to buckle and twist and break.
Efficiency of material
Firstly, the glue was too weak to stick those pieces together. Also the Basal pieces did not really have a strong compression bearing capacity since it was long and thin which made it easy to buckle and deform. However, this attributes also make it easy to reach higher in a short period of time with less materials. Therefore the way to construct a frame is really important to sustain the truss.
Sketches of deformation and stability during the different stages of the construction-stressing process
From the figure, it is clear that the weakness part where the midpoint bracing and the horizontal stick encountered was unglued started to buckle first. As soon as it buckle but it still carrying part of the force, other glued parts started to twist. Since the force depleted during the process, the force went down to the bottom two layers were bearable so that they were not really affected by the stress.
As soon as the encounter point broke, the torsion of the upper two layers became stronger and some parts started to break as well. At that time, the broken parts were not carrying the force anymore so the force went down to the bottom two layers and they buckle.
Published on Aug 15, 2013