# Compression problem

1. Nov 10, 2004

### FarazAli

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 $$4 cm \cdot 9 cm = 3.6 \cdot 10^{-3} m^2$$
and that the total force of the roof is it's own weight $$F = 12600 kg \cdot 9.80 \frac{m}{s^2} = 1.23 \cdot 10^5 N$$.

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

What should I do next (The Compressive strength is given in the book for wood; $$35 \cdot 10^6 \frac{N}{m^2}$$ parallel to grain and $$10 \cdot 10^6 \frac{N}{m^2}$$ perpindicular to grain)

2. Nov 11, 2004

### FarazAli

..........no?

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