A short, square timber post supports a load of 115 kips. If the allowable unit compressive stress is 1000 psi, what nominal size square timber should be used

Question

A short, square timber post supports a load of 115 kips. If the allowable unit compressive stress is 1000 psi, what nominal size square timber should be used.

I don't understand how to get the answer.

Answer 1

Hi Amarya,

The equation for your question is the simple stress equation: σ = F/A. Which means stress is the quotient of force over cross sectional area.

Step #1 - Collect Given Data

1. Force = 115 kips = 115,000 lbs
2. Stress = 1000 psi

Step #2 - Plug in Numbers to find Cross Sectional Area

1. A = F/σ = 115000/1000 = 115 in².

Step #3 - Find Exact and Nominal Dimensions

1. From Step #2 it was calculated that the calculated cross sectional area is 115 in² and we know that it is a square, therefore area = b². Therefore the side length (b) = √A = √115 = 10.72 in. Knowing that dimensional lumber is 1/2" smaller than the stated size, your next nearest size is 12"x12" hence (11.5"x11.5")