ENG1001-Assignment-1.docx

Check for determinacy number of members m31 number of

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Check for determinacy: Number of members, m=31 Number of reaction, r=3 Number of joints, j=17 If m +r =2j, the truss is determinant 31+3=34, therefore determinant Self weight of member: 0.147kg Failure weight: 1.027kg 7.0 Design 7. 1 Material properties of spaghetti 3
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ENG1001 Report- Bridge$ Single Strand 2 strands 3 strands side by side 3 strands in triangle orientation Density 1408.9 Modulus of elasticity, E(Mpa) 3980.78 Ultimate tensile strength (Mpa) 29.043 Second moment of area, I (mm^4) 0.7402 1.39 2.23 5.55 7.2 Test for failure against tension Highest Tension Force members Member Member Force, P (N) Member cross sectional area, A x (mm 2 ) acting = P/A x Safety Factor / acting Single strand member with highest force IK 10.278 3.179 3.233 29.043/3.23 =8.99 7.3 Test for failure against yielding Highest Yielding Force members Member Member Force, P Member cross sectional area, A x acting = P/A x Safety Factor / acting Single strand member with highest force LO 1.153 3.179 0.363 29.043/.363 =80.0 Double strand member with highest force LM 6.89 6.358 1.08 29.043/1.08 =26.9 Triple strand member with highest force HJ 9.95 9.537 1.04 29.043/1.04 =27.95 7.4 Test for failure against buckling Highest Compression Force members Member Member Length, L (mm) Membe r Force, P (N) Second Moment of Area, (I) (mm 4 ) Buckling Strength, P e (N) Safety Factor P e / P acting 4
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ENG1001 Report- Bridge$ Single strand member with highest force LO 125 1.15 0.740 3.8 3.3 Double strand member with highest force LM 125 6.89 1.391 7.13 1.03*** Triple strand member with highest force HJ 125 9.95 5.549 28.48 2.86 **Factor of safety less than one. Changed member size for the construction day to accommodate for this to make the factor greater than one as testing indicated it was a common failure point. The above tables indicate that the safety factor of all the strands of different sizes are above the factor of safety of 1. There is one exception in these calculations which was not found during the trialing of the bridge design that has a factor safety of 1.03 and to compensate, a triple strand member has been used in this location. The factor of safety for tension and yielding indicates that there is a low chance of the truss failing due to these forces however, the compressive forces have a factor of safety closer to one and appear to be the reason that the truss would fail. The force in the member were calculated using an applied load of 1027g (10.27N) so the bridge should theoretically be able to hold more than this once the member that has a factor of safety below 1 is changed. 8.0 Discussion 8.1 Design 1 was an early stage product, which was erected purely to familiarise the group with the materials and tools used to later construct bridges that meet the specifications required of the build. Being that the bridge has member lengths below the minimum allowed and were not reinforced enough to hold a relatively large weight. 8.2 Design 2 was constructed to reach the 200mm height hurdle with ease, constructing the bridge with 2 main trusses that are orthogonal to one another. This design had potential but due to there not being a supporting strut at the point where the two trusses meet, there was a large amount of deflection resulting in the bridge’s failure.
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