Why Is Midspan Moment Called Positive Moment?

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Why Is Midspan Moment Called Positive Moment?

Question:
“Midspan: beam sags down → bottom is pulled (tension) → positive moment. Why do we call it positive moment and not negative moment?”

1. What Really Happens at Midspan?

At the midspan of a simply supported beam under gravity load:

  • The beam sags downward
  • The top fibers are in compression
  • The bottom fibers are in tension

This behavior is pure physics. It happens no matter which country, code, or software you use.

2. Then Why Do Engineers Call It “Positive” Moment?

The answer is simple:

Because of sign convention, not because of physics.

In structural engineering, we use an agreed rule:

  • Sagging bending → Positive moment (+M)
  • Hogging bending → Negative moment (−M)

This rule was chosen long ago so that:

  • All engineers speak the same “language”
  • Bending moment diagrams look the same everywhere
  • Design drawings are clear and safe

3. Could We Call Sagging a Negative Moment Instead?

Yes — mathematically, we could.

But if each engineer or software chose different signs:

  • Results would be confusing
  • Design checks could be misread
  • Rebar might be placed in the wrong location

So the profession agreed on one standard convention.

4. Connection Between Moment Sign and Rebar Placement

Moment Type Beam Shape Tension Zone Main Reinforcement
Positive Moment (+M) Sagging Bottom Bottom bars
Negative Moment (−M) Hogging Top Top bars

This is why:

  • Midspan → positive moment → bottom reinforcement
  • Over supports → negative moment → top reinforcement

5. Why This Convention Is Very Practical

This sign convention works perfectly with:

  • Bending moment diagrams (positive drawn downward)
  • RC beam detailing practice
  • Structural software (Robot, ETABS, MIDAS, SAFE)
  • Design codes (ACI, Eurocode, BS)

When an engineer sees:

M+ = 120 kNm

They instantly understand:

  • The beam is sagging
  • Bottom is in tension
  • Bottom steel is required

6. Key Takeaway (Remember This)

Positive moment is not “positive” by physics — it is positive by engineering agreement.

Sagging = Positive moment (+M)
Hogging = Negative moment (−M)

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