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Homework Help: Compression problem

  1. Nov 10, 2004 #1
    Problem from book
    "the roof of a 9m x 10m school has a total mass of 12600 kg. The roof is to be supported byt "2 x 4s" (actually about 4cm X 9cm) along the 10m sides. How many supports are required on each side and how far apart mush they be? Consider only compression and assume a safety factor of 12"

    Well, I figured that the area of one support is [tex]4 cm \cdot 9 cm = 3.6 \cdot 10^{-3} m^2[/tex]
    and that the total force of the roof is it's own weight [tex]F = 12600 kg \cdot 9.80 \frac{m}{s^2} = 1.23 \cdot 10^5 N[/tex].

    The stress is given as [tex]stress = \frac{F}{A}[/tex], so the stress on one support should be [tex]\frac{stress}{x}[/tex] where x is the number of supports.

    What should I do next (The Compressive strength is given in the book for wood; [tex]35 \cdot 10^6 \frac{N}{m^2}[/tex] parallel to grain and [tex]10 \cdot 10^6 \frac{N}{m^2}[/tex] perpindicular to grain)
  2. jcsd
  3. Nov 11, 2004 #2
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