A rolled ISMB 350 section (depth = 350 mm, width of flange = 140 mm, thickness of web = 8.1 mm, thickness of flange = 14.2 mm) is used as a compression member. Considering the buckling about Y-Y axis as per IS 800 : 2007, the corresponding buckling class to which the beam belongs to is:
b
Buckling Class of ISMB 350 Section per IS 800:2007
When a steel section is used as a compression member, its capacity is limited by buckling. The buckling strength depends on the slenderness ratio and the buckling class of the section. The buckling class categorizes steel sections based on their cross-sectional shape and dimensions, which influences how they buckle.
According to IS 800:2007, the buckling class is determined using Table 6. This table provides guidelines for assigning buckling classes (a, b, c, or d) based on:
- Type of section (e.g., rolled I, welded I, hollow section, angle, channel).
- Buckling axis (major axis z-z or minor axis y-y).
- Cross-section dimensions, specifically ratios like h/bf (depth to flange width) and thickness of flange (tf).
Analyzing the ISMB 350 Section Properties
We are given the following dimensions for the ISMB 350 section:
- Depth (h) = 350 mm
- Width of flange (bf) = 140 mm
- Thickness of web (tw) = 8.1 mm
- Thickness of flange (tf) = 14.2 mm
The question asks for the buckling class considering buckling about the Y-Y axis, which is the minor axis for an I-section like ISMB 350.
Determining Buckling Class about Y-Y Axis
For hot-rolled I, H, and T sections buckling about the y-y axis (minor axis), IS 800:2007 Table 6 uses the following criteria:
We need to calculate the ratio of overall depth (h) to the width of flange (bf):
$$ \frac{h}{b_f} = \frac{350 \, \text{mm}}{140 \, \text{mm}} = 2.5 $$
We also need the thickness of the flange (tf), which is given as 14.2 mm.
Now, we consult the relevant portion of IS 800:2007 Table 6 for hot-rolled sections buckling about the y-y axis:
| Section Type | Buckling about axis | Limits | Buckling Class |
|---|---|---|---|
| Hot-rolled I, H, and T sections | y-y (minor axis) | h/bf > 1.2 and tf < 40 mm | b |
| h/bf > 1.2 and 40 mm ≤ tf < 100 mm | c |
From our calculations:
- h/bf = 2.5
- tf = 14.2 mm
Comparing these values with the criteria in the table:
- The condition h/bf > 1.2 is satisfied (2.5 > 1.2).
- The condition tf < 40 mm is satisfied (14.2 mm < 40 mm).
Based on IS 800:2007 Table 6, a hot-rolled section with h/bf > 1.2 and tf < 40 mm, when buckling about the y-y axis, belongs to buckling class 'b'.
Therefore, the corresponding buckling class for the ISMB 350 section considering buckling about the Y-Y axis as per IS 800:2007 is 'b'.
Important Questions from Beams
For a simply supported beam or slab, the effective span is calculated as:
Which of the following is CORRECT for indeterminate beam condition?
A cantilever beam is one which is -
In case of deep beam or in thin webbed R.C.C members, the first crack formed is-
In case of web crippling, the dispersion of load from bearing plate takes place at:
