High School

A rectangular beam 100 mm wide and 380 mm deep is subjected to a shear force of 190 kN. Calculate the shear stress in the beam at:

i) Mid-depth
ii) 80 mm below the top of the beam

Answer :

i) The shear stress at mid-depth of the beam is 5 MPa.

ii) The shear stress 80 mm below the top of the beam is 6.33 MPa.

i) To calculate the shear stress at the mid-depth of the beam, we need to determine the cross-sectional area of the beam and divide the shear force by that area.

Cross-sectional area of the beam: A = width × depth

A = 100 mm × 380 mm = 38,000 mm² = 0.038 m²

Shear stress at mid-depth: τ = Shear force / Cross-sectional area

τ = 190 kN / 0.038 m² = 5,000 kPa or 5 MPa

ii) To calculate the shear stress 80 mm below the top of the beam, we need to consider the reduced effective depth of the section.

Effective depth = Total depth - Distance from the top

Effective depth = 380 mm - 80 mm = 300 mm

Cross-sectional area at 80 mm below the top: A' = width × effective depth

A' = 100 mm × 300 mm = 30,000 mm² = 0.03 m²

Shear stress at 80 mm below the top: τ' = Shear force / Cross-sectional area

τ' = 190 kN / 0.03 m² = 6,333.33 kPa or 6.33 MPa

Therefore, the shear stress at the mid-depth is 5 MPa, and at 80 mm below the top, it is 6.33 MPa.

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