week 5 Constructing Environments India McKenzie 639 234
Principles, planning and construction
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This screenshot is from this weeks E-learning content and described the various timber beam structures. These examples are markedly different from our design which emphasised the design faults in ours which will be discussed in the next section.
The brief: The task we were given was to construct a beam to span one metre that was to be subjected to extreme compressive forces. Each group was given different combinations of two types of timber; bracing ply 1200mm x 4mm and radiator pine (NZ) 42mm x 19mm. We were also given access to a hammer, a drill and various sizes of common bolts and nails. Our group was given 3 lengths of the pine and one length of the ply. First of all, we assessed the compressive and tensile strength of the two timber types. The ply was much thinner and more flexible thus making it an excellent tensile plank. Conversely, the pine was much more solid and stiff and was less effective in coping with tension. Image courtesy of R. Zuzek
Our finished beam
For this reason, we chose to cut one of our pine lengths into 6 equal parts and stack to create a block like structure. Our aim was to bolster the horizontal pine lengths with stabilising blocks. The ply wood was nailed to the bottom of the span as the bottom rail would be under the most tension as the top rail would be under compression once the force was applied.
LOAD
COMPRESSION
TENSION
In our planning stages, we consulted Ching (2011) for ideas, primarily looking through the chapters on floor, wall and roof systems. On page 4.35, “Wood Beams�, the various styles of beam construction are illustrated and describes the forces working on solid sawn timber beams. Ching, F.D.K, 2011, Building Construction Illustrated, 4th Edition, John Wiley & Sons. Diagram of sketch design
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Testing process Our beam was spanned between two chocks that propped up the beam. A metal plate was then placed inbetween the beam and the load bearing needle so that the needle would not burrow into the beam as had been an issue in the past. The needle was then wound down through a threaded spindle axis. The exerted force was measured in kilograms that was displayed on the screen as seen on this page.
The beam withstood over 100kg of pressure before serious stress began to show. The ruler as pictured below allowed us to measure the degree of deflection as the beam bowed under the concentrated load of the machine. From the neutral origin, the beam deflected over 5cm before it began to splinter. The first thing we noticed was a slippage of the blocks that we had stacked inbetween the top and bottom members. The reason for this was partly to do with our selection of fixtures. Our choice of nails proved to be ineffective as they can resist shear but not axial pressure as was being exerted on the deflecting beam. The fastener (nail) dislodged from its place and the blocks began to slip laterally as the top lateral member exerted considerable compressive force on the blocks below. The ruler illustrating delection. Image courtesy of R. Zuzek
Our beam tolerated a maximum load of approx. 250kg before splintering entirely. Interestingly, the ply board had not fully fractured which emohasises the tensile capabilities of the thin bracing ply as opposed to the relative stiffness of the thicker pine. It also suggests that the ply has a much higher ultimate stress point than the pine. This image shows that the blocks we placed in between the members proved to be ineffective at providing support. Rather infact, they created a detrimental weakness along the edges of the blocks as can be seen by the splintered breakage in the image to the right. From the Interactive Structures module on “Connections and Structural Systems�, we were instructed to consider the timber species, angle of load in relation to the grain, the wood moisture content and the number, spacing and position of fasteners. The latter point would have been of most influence in our design as more tactical placement of fasteners and support structures could have increased the ultimate stress point of our beam.
WK 5_JOURNAL The E-learning video this week that focuses on the material properties of timber proved to be rather telling when evaluating the effectiveness of our beam design. Both Hamish in the workshop and Claire on the video emphasised the importance of grain direction and imperfections in the wood in order to ensure that each length of timber is strong. This screen shot on the left shows the irregularities in timber that can cause fractures and splintering when put under considerable tension. Being able to identify these weaknesses in the wood remains a critical part of our material selection.
The image on the right showed the bowing of the ply in a lateral direction. The principles of this traditional box-beam are sound how Glenferrie Rd (Hawthorn) ever the construction process created weaknesses in the wood. The sheer number of nails and bolts in the elements of the beam created weak point at every disruption of the grain and thus, when placed under even just 80kg, the vertical ply wood that enclosed the beam deflected laterally. This began the discussion of fixtures, connections and construction methods and Hamish suggested that perhaps a strong adhesive would have been a more suitable fixture for this design. Althought we were restricted to just the two fastening options (nails and bolts), we were able to acknowledge the intent of their design as well as recognise the inherent weakness of nails and bolts as a timber fastener when placed under extreme compressive loads.
Image courtesy of R. Zuzek
The most successful beam was this semi-box beam construction as seen in the picture above. Bearing up to 550kg, this design proved to be the most efficient at coping with a concentrated load. The weakness in this design was once again the nail fixtures as they were unable to cope with the axial load. Interetingly, the irregular number and placement of the fixtures on each of the panels allowed us to identify the most effective strategy for fixing the two elements together. The panels that connected to the lateral beams with a one-nail connection allowed the top and bottom beams to deflect in opposite directions whilst the panels that were fixed at all four corners were less effective in that they restrained the flexing of the top and bottom rails and eventually disconnected from the main members.
Image courtesy of R. Zuzek
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Working Drawings
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Working Drawings
WK 5_JOURNAL NOTE: I’m very sorry. I don’t know what happened to this page. I will rescan for the interim submission 2.
Working Drawings
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Working Drawings