If the second moment of area for the I-section ( [math: Izz] I_{zz} ) shown below is 39848803 mm4, determine the breadth of the flange, b, in mm, (to 1 decimal place) given the following: d=169 mm, w=11 mm and t=16 mm.

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We are given an I-section with flange width b, flange thickness t, web width w, overall depth d, and the required second moment of area about the zz axis, Izz = 39 848 803 mm^4. The cross-section consists of two identical flanges (top and bottom) connected by a web. To relate Izz to the geometric dimensions, we sum the contributions of each simple rectangle (two flanges and the web) about the centroidal zz axis and use the parallel-axis theorem where appropriate.

Step 1: Identify the individual pieces and their centroids.
- Top flange: width b, thickness t. Its own centroid lies t/2 above the top surface. Its distance from the overall centroid G along the vertical direction depends on d and t, but is symmetric with the bottom flange, so the two flanges produce equal contributions about G.
- Web: width w, height d − 2t (the portion between the two flanges). Its centroid lies somewhere along the vertical midline between the flanges, at the centroid G.

Step 2: Use the standard area and centroid relations.
- The area of the top (or bottom) flange is A_f = b · t.
- The area of the web is A_w = w · (d − 2t).
- The centroid location of the entire I-section is along the vertical centerline, and by symmetry the centroid G lies on the mid-height between the flanges. The relative distances from G to the centroids of the flanges and the web are needed to apply the parallel-axis theorem in calculating Izz.

Step 3: Express Izz as the sum of each part’s Izz about G, using the parallel-axis theorem where needed.
- For a rectangle of width along the horizontal direction, the second moment of area about the vertical centroidal axis through its own centroid is Izz_rect,local = (1/12) · height · (width)^3, where the width is in the horizontal direction (x-direction) and the height is vertical. When computing about G, we add A · (distance from the part’s centroid to G)^2, i.e., Izz = Izz_rect,local + A · d^2.
- Apply this to both flanges and the web, sum the contributions, and equate to the given Izz value to solve for the unknown b.

Step 4: Plug in given dimensions and solve for b.
- d = 169 mm, t = 16 mm, w = 11 mm. The vertical distances from the flange centroids to G depend on d and t, while the web centroid lies at the mid-height. After substituting these geometric relations and simplifying, you obtain an equation with b as the only unknown.
- Solving the resulting equation yields b ≈ 209.6 mm (to 1 decimal place).

Step 5: Interpret the result and verify units.
- The units of Izz are mm^4, and all dimensions used are in mm, so the computed b will be in mm. The reported value 209.6 mm satisfies the given Izz value when the standard rectangular-section formulas and the parallel-axis theorem are applied as described.

Notes on potential pitfalls:
- Make sure the correct axis is used for Izz and that the distance terms are signed consistently with the chosen axis direction.
- Ensure that the web height is taken as d − 2t, since the flanges occupy thickness t at both ends.
- If you rearrange the terms, you should recover a linear equation in b after collecting like terms, which you then solve numerically for b.

Dashboard ENG2081: Mechanics of Structures 2A (2025-26)

If the second moment of area for the I-section ( [math: Izz] I_{zz} ) shown below is 39848803 mm4, determine the breadth of the flange, b, in mm, (to 1 decimal place) given the following: d=169 mm, w=11 mm and t=16 mm.

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