Practice Question: Find Crane outrigger Base Spreader thickness
NCEES Civil PE Specification Means and Methods: Construction Methods
Problem
You are setting up a crane equipped with 2 ft by 2 ft outriggers that can take a maximum force of 20 kips.
You learn that the site has weak soil and you will have to pour a concrete block to act as a spreader for the load.
What is the required thickness of the reinforced concrete spreader based on the soil bearing capacity?
Consider a soil bearing capacity of 1500 psf.
Consider a vertical stress distribution of 1:1.
Ignore Factor of Safety in your calculation.
Select an Answer:
CORRECT 😃
INCORRECT 😟
% Students Correctly Answered:
63.8%
Solution
Answer: C) 10 inch
The spreader RC block shall work to spread the area of the pressure to be on a larger area, so the final pressure on the soil will be less than the bearing capacity.
The minimum required area of the pressure: \[= \frac{Applied\ load}{Bearing\ capacity }= \frac{20 \ kip \times1000 }{1500 \ psf} = 13.33 ft^2\]
Minimum width of the pressure:
\[= \sqrt{13.33} = 3.65 ft = 43.8\ inch\]
Vertical Pressure Distribution = 1:1 so every 1 ft in thickness will increase the width by 2 ft.
The required minimum thickness:
\[= \frac{(43.8-24)}{2} = 9.9\ inch\]
So, minimum thickness is most nearly = 10 inch
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