For the beam shown above with length L = 7 m with the applied uniformly distributed load of w = 18 kN/m, what is the maximum bending stress if the beam has a cross section with the following dimensions:

b = 100 mm

d = 250 mm

tf = 20 mm

tw = 15 mm

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= ____MPa Round to one decimal place

Question

For the beam shown above with length L = 7 m with the applied uniformly distributed load of w = 18 kN/m, what is the maximum bending stress if the beam has a cross section with the following dimensions:

b = 100 mm

d = 250 mm

tf = 20 mm

tw = 15 mm

𝜎
𝑏
𝑒
𝑛
𝑑
𝑖
𝑛
𝑔
 = ____MPa Round to one decimal place

BlackTom AI · Accepted Answer

The prompt asks for the maximum bending stress for a simply supported beam under a uniform distributed load with a given I-beam cross-section. Since there are no multiple-choice options provided, I will first outline the necessary steps and then perform the calculation to obtain the maximum stress.

Step 1: Identify the support condition and maximum bending moment.
- A simply supported beam with a uniform distributed load w over length L has a maximum bending moment at midspan given by M_max = w L^2 / 8.
- Here: w = 18 kN/m and L = 7 m.
- Convert to consistent units: M_max = 18 × 7^2 / 8 kN·m = 18 × 49 / 8 = 882 / 8 = 110.25 kN·m = 110.25 × 10^3 N·m = 110,250,000 N·mm.

Step 2: Compute the second moment of area I for the given I-beam cross-section.
- Cross-section details: flange width b = 100 mm, flange thickness tf = 20 mm, web thickness tw = 15 mm, overall depth d = 250 mm.
- Web height: h_w = d − 2tf = 250 − 40 = 210 mm.
- Moment of inertia contributions:
  - Web about the strong axis: I_web = tw · h_w^3 / 12 = 15 · 210^3 / 12.
    210^3 = 9,261,000; I_web = 15 × 9,261,000 / 12 = 11,576,250 mm^4.
  - Each flange: I_flange_centroid = b · tf^3 / 12 = 100 · 20^3 / 12 = 100 · 8000 / 12 = 66,666.67 mm^4.
  - Distance from centroid to flange centroid: y_f = d/2 − tf/2 = 125 − 10 = 115 mm.
  - Flange area: A_f = b · tf = 100 × 20 = 2000 mm^2.
  - Flange contribution with parallel axis: 2 × (I_flange_centroid + A_f · y_f^2) = 2 × (66,666.67 + 2000 × 115^2).
  - 115^2 = 13225, so A_f · y_f^2 = 2000 × 13225 = 26,450,000.
  - Sum for both flanges: 2 × (66,666.67 + 26,450,000) = 2 × 26,516,666.67 ≈ 53,033,333.34 mm^4.
- Total I: I_total = I_web + flange_contributions ≈ 11,576,250 + 53,033,333.34 ≈ 64,609,583.34 mm^4.
- For clarity, I ≈ 6.46096 × 10^7 mm^4.

Step 3: Determine the distance to the extreme fiber.
- The neutral axis is at mid-depth for this symmetric I-beam, so the distance to the extreme fiber y_max = d/2 = 250/2 = 125 mm.

Step 4: Compute the maximum bending stress.
- Stress formula: σ_max = M_max · y_max / I_total.
- M_max = 110,250,000 N·mm, y_max = 125 mm, I_total ≈ 64,609,583.34 mm^4.
- σ_max ≈ (110,250,000 × 125) / 64,609,583.34 ≈ 13,781,250,000 / 64,609,583.34 ≈ 213.3 MPa.

Result interpretation:
- The calculated maximum bending stress using the given cross-section and loading is approximately 213.3 MPa, which matches the expected value to one decimal place when rounded.

Notes on the missing options:
- The provided data includes an answer value of 213.3 but no answer_options array. If multiple-choice options were present, each option would be evaluated against the computed σ_max to determine which is correct, while explaining why the others are invalid. In this case, the calculation confirms the numeric result independently of options.

STRCTENG 302 Steel Quiz No 1: Basic Knowledge Mechanics and Structural Steel Review

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A square section of side length [math: b] is subjected to a bending stress of [math: σ] \sigma = 5.4 MPa under a bending moment of [math: M]=6.8 kNm. Find the side length [math: b] of the square section. Provide your answer in mm, rounded to two decimal places.

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