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Homework Help: Structural Mechanics - Statics / Truss

  1. Oct 10, 2012 #1
    1. The problem statement, all variables and given/known data

    The simply supported timber foot bridge shown in figure 5 spans 7 metres and, as shown, is carried by two Engineering grade Douglas Fir beams. The uniformly distributed load due to the self weight of the foot bridge and beams is 96 kg/m. The bridge is required to carry a live load of 2.5 kPa on the 1.4 m wide carriageway.

    http://imageshack.us/a/img801/7514/facebook1wz.png [Broken]

    Calculate the dead load per metre of beam based on the above configuration and express your answer in kN/m.
    Calculate the live load per metre of beam based on the above configuration and express your answer in kN/m
    From the loads calculated in (a) and (b) determine the ultimate limit state load
    Assuming b = 100 mm and k8 = 0.75 determine the minimum depth of beam required to met the strength criteria of NZS 3603
    (Beam depth to be to the nearest 50mm)
    Calculate the long term deflection of the beam chosen in (d)
    Briefly comment on the acceptability of the deflection calculated above

    2. Relevant equations

    3. The attempt at a solution

    I am extremely stuck in this. I want to ask the lecturer this question but I don't even know where to start. I went to almost every lecture but it seems I just can't manage to grasp this subject effectively. Consequently I cannot provide a great amount of workings when attempting this question.

    My attempt at working out this question is:

    The width (7000mm) times the width (1400mm)

    and then work out live load .. (the formula I believe is something like -- 1.5 times the load).

    the rest i have no idea.

    can someone please give me the relevant website on the internet where I can access the theory and example questions for this problem?

    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Oct 11, 2012 #2


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    I assume you have taken or are taking a course in Strength of Materials? The dead load is given in mass per meter; convert to weight per meter and each beam sees half of that. After calculating total live load per your formula and using correct units, the live load per meter is that load divided by length, again half of which goes to each beam. Then I am not familiar with your refernced code, but basically, it's calculating max moment for a simply supported beam with appropriate safety factors, and max stress per Mc/I based on the wood allowable stress for DougFir, and deflection based on its properties and appropriate code equations.
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