Rotational movement in bridges is a direct result of live load deflection and occurs with greatest magnitude at the bridge supports.

Rotational movement in bridges is a direct result of live load deflection and occurs with greatest magnitude at the bridge supports.

• A. True
• B. False
• C. Not determinable
• D. It depends on span length

Answer: (A)

Rotational movement is the slope of the bridge deck as it deflects under live load. When live loads cause bending, the deck takes on a curved shape. The supports set the boundary conditions: vertical deflection is restrained to zero at the supports, but rotation is not forced to zero. That combination makes the deflected shape change most steeply right at the ends, so the angle the deck rotates through—the rotation—is largest near the bridge supports. At midspan the deflection is greater in amount, but the slope there tends to be less steep, so the angular movement is not as large as at the supports. So the statement is true: live-load deflection produces rotational movement, with the greatest magnitude occurring at the supports.

Rotational movement is the slope of the bridge deck as it deflects under live load. When live loads cause bending, the deck takes on a curved shape. The supports set the boundary conditions: vertical deflection is restrained to zero at the supports, but rotation is not forced to zero. That combination makes the deflected shape change most steeply right at the ends, so the angle the deck rotates through—the rotation—is largest near the bridge supports. At midspan the deflection is greater in amount, but the slope there tends to be less steep, so the angular movement is not as large as at the supports. So the statement is true: live-load deflection produces rotational movement, with the greatest magnitude occurring at the supports.