Bending Moment

As an 18 year old sophomore architecture student at Montana State University, I had my very first exposure to moments.

I was working full time daily until 9 p.m., which meant homework got done after I got off work….therefore, the 8 a.m. engineering class and my lack of sleep fog, did not go well together.  I had a real difficult time understanding bending moments.

Years later, I found out moments were not as tough as I tried to make them.

Somehow it all was analogous to me getting a C in 8th grade Algebra, and an A+ in high school honors Algebra. I guess it just took me a practice run.

wood bending momentA bending moment exists in a structural element (think – beam, wall column, roof purlin) when a force (think – wind and snow) is applied to the element so the element bends.

Moments are measured as a force multiplied by a distance so they have as a unit: foot-pounds (ft-lb)

The concept of bending moment is very important in the engineering of pole buildings. Our design programs utilize these calculations repeatedly.

Stresses increase proportionally with bending moment, but are also dependent on the cross-section (Section Modulus or Sm) of the structural element. Failure in bending will occur when the bending moment is sufficient to induce tensile stresses (forces) greater than the yield stress (strength) of the material throughout the entire cross-section. It is possible a failure of a structural element in shear (exactly like it sounds) may occur before failure in bending; however the mechanics of failure in shear and in bending are different.

The bending moment at a section through a structural element may be defined as “the sum of the moments about that section of all external forces acting to one side of that section”. The forces and moments on either side of the section must be equal in order to counteract each other and maintain a state of equilibrium so the same bending moment will result from summing the moments, regardless of which side of the section is selected. I know it’s not an easy concept, but is important to the design of any building which has a beam in it…whether it’s your local Home Depot store, or your kids’ backyard swing-set.

Lost? Puts my first engineering class struggles into perspective.

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