Spanning Limits of Steel Trusses: Theoretical vs Practical Considerations
The question of the maximum span for steel trusses is an intriguing one that brings to light the complex interplay between theory and practical constraints. In a theoretical context, trusses could indeed be infinitely long, considering only the proportional increase in member thickness and weight. However, in the real world, we must contend with economic trade-offs and practical limitations.
Practical Limitations: Economic Trade-offs
After a certain point, increasing the length of steel trusses becomes economically impractical. The thickness of structural members required to support a progressively longer span leads to increasing costs, making the structure no longer a cost-effective solution. This is where alternative methods come into play, such as using intermediate supports like columns in buildings or piers in bridges.
Structural Supports and Design Alternatives
For example, in a building, it is often less expensive to install a column or a load-bearing wall than to construct a pier in the middle of a river. Additionally, large rivers require navigable space to be left open to accommodate river traffic such as barges. As a result, cable designs may be incorporated, allowing for fewer and more manageable trusses or multiple piers to be used.
Truss Spanning Capabilities
Despite the practical limitations, steel trusses can span up to approximately 90 meters, which is a significant distance. While spans this long can present challenges, such as maintaining structural integrity during installation, once the trusses are properly braced and placed on bearing walls, the system becomes extremely strong.
Optimizing Truss Design for Long Spans
There is no specific limit to the span of a truss, and strategies can be employed to maximize it. By increasing the depth of the truss based on the required deflection, the truss's geometry can be optimized to reduce both weight and the forces within the members. This optimization is crucial, as compression members are significantly more costly compared to tension members. Reducing the number of compression members while designing the geometry of the truss is a good way to mitigate these costs.
Non-Technical Considerations in Truss Design
The decision-making process for truss design is not solely dictated by technical factors. Aesthetics, beauty, transportation logistics, political requirements, and other non-technical inputs play a significant role. For instance, a truss design may have to be adjusted to fit within the overall architectural style of a building or comply with local laws and regulations regarding structural safety.
Ultimately, the choice of a truss span is a balance between maximizing structural efficiency and meeting the practical, economic, and non-technical needs of a project. Understanding these considerations is key to selecting the right truss for any given application.