Construction
The Comparison between Allowable Stress Design (ASD) and Load and Resistance Factor Design (LRFD)
| Aspect | Allowable Stress Design (ASD) | Load and Resistance Factor Design (LRFD) |
|---|---|---|
| Design Philosophy | Based on elastic behavior; stresses are kept within the elastic limit of the material. | Based on both elastic and plastic behavior; factors in the probability of load exceeding the strength. |
| Safety Factor | Uses a single factor of safety. | Uses multiple load and resistance factors for different types of loads and material properties. |
| Calculation Method | Stresses are calculated and compared with allowable stresses. | Uses factored loads and resistances; designs for the worst-case scenario using a combination of factors. |
| Usage | Traditional, simpler method; commonly used in the past. | Modern, more complex method; widely accepted in contemporary engineering practice. |
| Precision | Less precise, as it doesn’t consider variability in loads and material strengths. | More precise, as it accounts for different load combinations and uncertainties. |
| Industry Preference | Still used in some applications, especially in renovation of older structures. | Preferred in new construction, especially for critical structures where safety is paramount. |
| Force Consideration | Forces are calculated based on actual loads without any factors. | Forces are amplified by load factors to ensure safety under worst-case scenarios. |
| Design Load | Uses nominal loads directly in design. | Applies load factors to nominal loads, resulting in higher design loads. |
| Resistance Factor | Not applicable as it uses factor of safety applied to material strength directly. | Uses resistance factors to reduce the nominal strength, accounting for material variability and uncertainties. |
| Load Combinations | Simpler, often based on single load case consideration. | More complex, involves multiple load combinations considering various load cases. |
| Design Ratios | Actual stress divided by allowable stress. | Factored load divided by factored resistance. |
| Flexibility in Design | Less flexible due to uniform safety factor. | More flexible, allows for optimization based on varying load and resistance factors. |
The Steel Design Collection contains many Excel Sheets that comply with AISC Standards.