In the scheme design of steel beams, the "span ratio" typically refers to the span/depth ratio. This is the proportion of the beam's length (span) to its height (depth). A specific range for this ratio is commonly used in initial design stages for uniformly loaded beams.
Typical Span/Depth Ratio for Steel Beams
According to standard practice in scheme design for uniformly loaded steel beams, sections are typically selected with a span/depth ratio of 18 to 20.
This ratio serves as a useful rule of thumb to determine an approximate depth for a steel beam given its span. Using a beam within this range helps ensure a balance between stiffness, weight, and material efficiency under typical loading conditions without requiring detailed calculations initially.
Understanding the Span/Depth Ratio
The span/depth ratio (L/d) is a fundamental concept in structural engineering:
- Span (L): The distance between the supports of the beam.
- Depth (d): The overall height of the beam section.
A lower ratio (deeper beam for the same span) generally results in a stiffer beam, which helps control deflection. A higher ratio (shallower beam) might be more economical in terms of material but could lead to excessive deflection under load.
Example from Practice
The provided reference illustrates this concept with a practical example:
| Span | Typical Depth (approx.) | Span/Depth Calculation |
|---|---|---|
| 8 meters | 450 mm (0.45 meters) | 8 m / 0.45 m ≈ 17.78 |
As shown in the table, for an 8-meter span, a typical steel beam depth of approximately 450 mm is used in scheme design, resulting in a span/depth ratio of around 17.78, which falls within the 18 to 20 range. This demonstrates how the ratio guides the initial selection of beam dimensions.
- Key Takeaway: While detailed engineering calculations are always required for final design, the 18-20 span/depth ratio is a valuable starting point for architects and engineers during the conceptual or scheme design phase for typical steel beams.
