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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