• (Edited )

I hope this is helpful to others as well:

Moment is always measured in a unit of load times unit of distance; for example, in pound-feet (pound x feet, as in multiplied). It is defined as the tendency of an object to rotate about a point. Part of this definition is that the distance is always measured perpendicularly from this point to the point of concentrated load.

Soil loads (pressures) are usually expressed in pounds per linear foot or pounds per square foot, or similar. That is because they don't act as concentrated loads; they are distributed. For a retaining wall moment problem we would need to (1) translate this load to an equivalent total concentrated load, in pounds and (2) determine the distance, in feet, from the base (point about which the wall would rotate).

In order to do this, it is important to understand that the pressure, or lateral load, behind the wall increases as it moves down in a manner that is proportionate to its height (triangular shape). For this reason we also know the equivalent concentrated load is located 1/3 of the height above the triangle's base (location of its centroid); which is (2), the moment distance.

To find the total concentrated load, we need to multiply the pressure expressed in pounds per square feet by the height of the wall to find the pounds per linear foot, in the event that value was not already given. Note that this value represents the load as though it were evenly distributed along the base of the wall. However, since the equivalent concentrated point load is located above the base, we must adjust for the uneven triangular distribution. To calculate for the equivalent concentrated load (in pounds) we must therefore divide this value (pounds per linear foot) in half before multiplying my the height. This will result in (1) the total concentrated moment load.

Multiply (1) and (2) together and you will have your overturning moment, expressed as pound-feet.

• awesome. look forward to studying this!
the 30#\’ for the soul in the example i saw to arrive at the overturning force - this is a given and not some kind of memorization item?

also i didn’t see the units cancel out somewhere for some weird reason. but i need to take another look...

• I'd focus on understanding, not memorizing! Very important.