logbook week 2
DESCRIPTION
ÂTRANSCRIPT
Due to the material provided to us – thin strips of balsa wood – a frame structural system was the most logical choice. When planning our structure’s design we ran through a variety of sketches (as displayed left) but when faced with actually building the tower we found that many of the designs were not applicable due to lack of material. We thus opted for a column structure which aimed to maximise height while limiting the use of materials. In our initial sketches we mere mindful of the lateral force and compressive force the tower was predicted to be subject to, therefore many feature forms of bracing in an attempt to minimise movement.
The initial plan for the structure was to create triangular prism columns which were then to be stacked to add height (pictured right). While this design concept was initially sound, once the structure grew beyond two columns in height structural deficiencies came through. We underestimated the force of the dead load (a static load imposed by the self weight of the structure.) This force caused the long beams to buckle, deflect outward and bend, so it became clear to us that shorter columns needed to be used to reduce the effect of the dead load.
W02 Studio Activity:
FRAME Photograph: initial column design (photographer: Jasmin Goldberg)
In order to accommodate and reduce the bending of the column elements we added horizontal braces half way up the height of the tower. In halving the length of the tower we halved the load that the vertical column was undertaking, thus reducing buckling of the structure and increasing structure stability. (Horizontal braces highlighted in photograph – left – using red)
As the tower got higher we also encountered problems with the design of the base. It was found that the surface area of the base was too small, causing the tower to lean in one direction and then topple over. To account for this lean we extended one of the supporting vertical beams in the opposite direction to the lean in an attempt to counterbalance the structure; this was successful. We also added lateral bracing to the base column in order to prevent/minimising the bending of the vertical beams which were taking the most force, being at the bottom of the structure.
Photograph: addition of horizontal bracing (photographer: Jasmin Goldberg)
Photograph: revised base (photographer: Jasmin Goldberg)
In the ‘deconstructing’ phase we were interested to find out that the brackets deemed necessary when constructing the tower were not actually essential to the stability. The tower remained stable when the horizontal, half column braces were snapped. The critical collapse point of the structure was indeed the lateral bracing in the base. This makes sense considering the base is the foundation of the structure and is bearing the greatest load due to the self-‐weight of the structure.
Photograph: final structure, full length (photographer: Jasmin Goldberg)
W02 Lecture Activity
In this week’s lecture we took part in a water tower exercise using straws as support columns and pins as joints. Initially we did a test to find out how much load the columns on their own could take and the findings were as follows: One straw = 500g Four straws (bunch) = 2000g Four straw structure = 200g (pictured right) These results indicated that the structural members, when held together, are the strongest, as their combination protects the supporting column against deflection while combining the individual load capacity of each straw. However, when the four are separated into the four straw structure (pictured right) it is unable to accommodate even the load of one straw. This was because the height of the pin joins allowed for movement of the structural members, causing deflection under the applied load and collapse of the structure.
Alternate structures were then tested to determine whether deflection could be protected against. Firstly shorter columns were tested with double pin joints (pictured left). These additional, lower, pin joints restricted the movement of the supporting members, thus reducing deflection and ensuring their straightness which was found to be much more stable than angled columns which were prone to collapse. It was also found that the shorter structural columns were able to take more load and less likely to bend under the applied load.
The structure that was found to be the most successful was this one (pictured right). The straws were folded in half, shortening their length and thus increasing the load they were able to take. They were then arranged to create the angular supporting members shown, secured by pin joints. This angular arrangement of the straws further reduced movement of members, decreasing likelihood of deflection. Furthermore, the triangular arrangement ensured a balanced load path through the structure to prevent lean or the collapse of a single side or column.
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W02 – Key terms glossary Bracing Column (ENVS10003, March 17)
• A column is a compression element, also known as a strut • A bracing column is such a structural element used to increase the stability of a structure and assist in managing the load of forces.
Frame (ENVS10003, 2014, March 9)
o Structural system consisting of beams connected by joints o Efficient in transferring loads to the ground
Stability
• Resistance to change, displacement or collapse Structural Joint (Ching, 2008; ENVS10003, 2014, March 9)
• Three types of structural joints: pin, roller and fixed • Roller Joint
o Loads transferred in one direction – vertically downward o Load in any other direction causes movement of roller joint o Allow rotation but resist translation in any direction perpendicular into or away
from their faces o Useful when a joint must allow the expansion and contraction of a structural
element • Pin Joint
o Found within a truss system o Modes of action can be in two directions – planar o Theoretically allow rotation but resist translation in any direction Representation of joints
(ENVS 10003, 2014, March 9)
• Fixed joints
o Maintain angular relationship between the joined elements o Restrain rotation and translation in any direction o Provide both force and movement resistance o Bending can occur if a load occurs in one member connected by the joint
Tension (Newton, 2014)
• Basic structural force • Occurs when an external load pulls on a structural member • When subject to tension forces, particles of a material move apart • Causes material to stretch and elongate • Acts opposite to compression forces (see W01) • Amount of elongation of a material subject to tension forces depends on the material’s stiffness, cross sectional area and the
magnitude of the load.
Glossary Reference List: Ching, F. D. K., (2008). Building construction illustrated (fourth edition). New Jersey: John Wiley & Sons. 2.30. ENVS10003, (2014, March 9). W02 s1 Structural Systems. Retrieved March 16, 2014, from: https://www.youtube.com/watch?v=l-‐-‐JtPpI8uw&feature=youtu.be ENVS10003, (2014, March 17). W03_s1 Structural Elements. Retrieved March 18, 2014, from: https://www.youtube.com/watch?v=wQIa1O6fp98&feature=youtu.be ENVS10003, (2014, March 9). W02 s2 Structural Joints. Retrieved March 16, 2014, from: http://www.youtube.com/watch?v=kxRdY0jSoJo&feature=youtu.be ENVS10003, (2014, March 17). W03_s1 Structural Elements. Retrieved March 18, 2014, from: https://www.youtube.com/watch?v=wQIa1O6fp98&feature=youtu.be Newton, C., (2014). Basic Structural Forces (I). Retrieved March 15, 2014, from: https://app.lms.unimelb.edu.au/bbcswebdav/courses/ENVS10003_2014_SM1/WEEK%2001/Basic%20Structural%20Forces%201.pdf Knowledge Map (W02) Reference List: Ching, F. D. K., (2008). Building construction illustrated (fourth edition). New Jersey: John Wiley & Sons. 2.02-‐2.04. ENVS10003, (2014, March 9). W02 c1 Construction Systems. Retrieved March 14, 2014, from: https://www.youtube.com/watch?v=8zTarEeGXOo&feature=youtu.be ENVS10003, (2014, March 9). ESD and Selecting Materials. Retrieved March 14, 2014, from: https://www.youtube.com/watch?v=luxirHHxjIY&feature=youtu.be ENVS10003, (2014, March 9). W02 s1 Structural Systems. Retrieved March 16, 2014, from: https://www.youtube.com/watch?v=l-‐-‐JtPpI8uw&feature=youtu.be
