Understanding Trussed Bridge Spans

A trussed bridge is a bridge whose load-bearing superstructure is composed of a truss, which is a structure of connected elements forming triangular units. These triangular units distribute tension and compression forces efficiently throughout the structure, making them suitable for supporting significant loads over varying distances.

Typical Spans for Different Bridge Types

Bridges come in many types, each suited for different span lengths and load requirements. The span refers to the distance between two supports of the bridge. Different bridge types have different economic and structural limits for their maximum span.

• Beam Bridges: Typically used for short spans.
• Arch Bridges: Can cover moderate to long spans.
• Suspension Bridges: Ideal for very long spans.
• Cable-Stayed Bridges: Also suitable for long spans.
• Trussed Bridges: Known for covering moderate to long spans effectively.

Span Limit of Trussed Bridges

Trussed bridges are particularly efficient for spans longer than simple beam bridges but generally shorter than suspension or cable-stayed bridges. The maximum economical and practical limit for the span of a trussed bridge typically falls within a specific range.

Based on common engineering practices and historical bridge construction, the typical maximum span limit for a trussed bridge is in the range of 30 to 80 meters. While some historic or specialized trussed bridges might slightly exceed this, this range represents the most common and efficient application for this bridge type.

Let's look at the given options in the context of typical bridge spans:

• 50 to 150 m: This range is generally too long for the most economical application of standard trussed bridges, often leaning towards cable-stayed or suspension bridge ranges for the upper end.
• 3 to 7.5 m: This is a very short span, more characteristic of simple beam or slab bridges rather than complex trussed structures.
• 30 to 80 m: This range aligns well with the typical economical and practical span limits where trussed bridges are widely used and efficient.
• 18 to 30 m: While trussed bridges can be used for spans starting around 18-30m, the question asks for the "maximum limit," and the range extends further.

Therefore, the range of 30 to 80 meters represents a typical maximum limit for the span of a trussed bridge.

Revision Table: Bridge Types and Typical Spans

Additional Information on Bridge Spans

The maximum span of a bridge isn't just about the structural type; several factors influence the choice and feasibility of a particular span:

• Materials: The strength and weight of construction materials (steel, concrete) play a crucial role. High-strength materials allow for longer spans.
• Loads: The expected traffic load (vehicles, trains, pedestrians) impacts the design and required strength.
• Site Conditions: The geology of the site, foundation conditions, environmental factors (wind, seismic activity), and river/waterway requirements affect design and possible span lengths.
• Economy: While technically possible to build very long spans with certain types, the cost increases significantly with span length. The chosen span is often the most economical option that meets structural and functional requirements.
• Construction Technology: Advanced construction methods and equipment allow for the building of longer and more complex spans.

Trussed bridges offer a good balance of strength, material efficiency, and construction feasibility for moderate to long spans, making the 30 to 80-meter range a common application.