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- Title
DOES INCREASING STRUCTURAL MEMBER LENGTH INCREASE TENSILE STRENGTH?
- Authors
Mayo, Sarah K.
- Abstract
A Pratt Through Truss bridge was constructed out of cardboard file folders following detailed specifications. Built out of solid bars and hollow tubes, the bridge was load-tested to determine how successfully the structure worked. It was observed that an external load applied to the bridge structure resulted in internal forces - tension or compression - developing within each structural member. Focusing on tension forces in the solid bars, the hypothesis was that increasing the length of the structural members would not increase tensile strength. Test specimens were twelve bars-four each of three different lengths (17.7cm, 20cm, 22cm) and all of the same width (6mm). Experimental testing was conducted using a simple, home-built wooden lever machine. Specimens were individually clamped to the lever machine, and load (sand) was added to a bucket until the specimen failed. The weight of the sand was then charted. The mass of the bucket/sand that caused each specimen to fail was converted to a force using the equation W (weight of the object) = mass (mass in kg) X g (acceleration of gravity, 9.81 meters/sec²). The mass was expressed in kilograms, therefore, the weight (W) was in newtons. The principle of the lever was applied using the equation T (unknown internal force/tensile strength) = W (weight of the bucket and sand) L2/L1 (L2 and L1 being measured directly from testing machine) to determine the force in each test specimen at failure. Data entered on a spreadsheet program revealed the tensile strength of the 22cm bars to be 28.28N (newtons), 27.02N, 30.86N, and 26.97N. The tensile strength of the 20cm bars was 30.40N, 26.92N, 28.74N, and 26.36N. The tensile strength of the 17.7cm bars was 30.25N, 31.21N, 31.56N, and 30.10N. An x-y scatter plot graph showing tensile strength vs. member length was then created. Experimental results proved that there was no relationship between member length and tensile strength. This experiment is important because in order to evaluate a structure's load-carrying ability, an engineer must be able to determine the strengths of the structural members that comprise it.
- Subjects
TRUSS bridges; CARDBOARD; TESTING; STRENGTH of materials; MECHANICAL loads
- Publication
Ohio Journal of Science, 2008, Vol 108, Issue 1, pA-33
- ISSN
0030-0950
- Publication type
Article