Trussed beams are an essential structural element used in construction, particularly in applications where large spans are required without the use of intermediate supports. These beams, often consisting of steel sections or wooden beams and struts, offer a combination of strength, efficiency, and economic benefits. The design of trussed beams involves careful consideration of geometry, material selection, and the methods used for connection and spacing. This article explores the design principles, structural components, and practical applications of trussed beams in modern construction.
Introduction
A trussed beam is a type of beam that is stiffened by a system of braces, forming a truss, where the beam itself acts as the chord. These beams are primarily used in situations where large weights need to be supported across wide spaces without the need for supports underneath, such as in industrial buildings or large roof structures. They consist of various components, including steel or wooden beams for the chord and steel rods or other materials for the struts.
Trussed beams are particularly useful in applications requiring a large, open space. For example, in industrial settings where unencumbered floor space is vital, trussed beams allow for the construction of large roof spans without support columns beneath. The spans of trussed beams can range from 10 meters to 100 meters, depending on the building requirements, and they offer an economical solution for spans exceeding 25 meters.
Trussed Beam Design Principles
The design of trussed beams is governed by several key principles that ensure the structure performs efficiently, economically, and safely. These principles guide the geometry, material selection, connection types, and spacing of the trussed beam.
2.1 Trussed Beam Geometry
The geometric configuration of a trussed beam is crucial to its structural performance. The ratio of the span to the truss depth plays a significant role in determining the beam’s ability to handle loads. A span-to-depth ratio in the range of 10 to 15 is ideal for optimal structural performance.
Architectural factors, such as the design of the building and the slope of the roof, control the external geometry of the trussed beam. The design of the truss should aim to provide an efficient layout for the truss members, particularly the diagonal braces. It is recommended that the diagonal members of the truss be inclined at an angle of 35° to 55°, with the longest members placed under tension. Additionally, loads should be applied at the nodes (joints) of the truss for better distribution and structural integrity.
2.2 Trussed Beam Sections
The choice of section for the members of a trussed beam affects both its performance and economy. For structural efficiency, the sections should be symmetrical to resist bending out of the vertical plane of the truss. In members that experience compression, the buckling resistance should be similar in both the vertical and horizontal planes.
For large member forces, it is advisable to use sections such as IPE (European I-beams), HEA or HEB sections, or a combination of two UPE channels for the chords. Diagonal members can be formed using battened angles, which provide the necessary strength and rigidity. For further efficiency, hollow sections are recommended for the chords and internal members of the truss. The top chord (rafter) is typically divided into sections to accommodate the roof covering sheets, ensuring smooth integration into the building’s overall design.
2.3 Types of Connections
Connections are essential components in trussed beams, as they join the various parts of the truss together. There are two primary types of connections used in trussed beams: welding and bolted connections.
- Welding Connections: These connections involve joining parts of the trussed beam using heat to fuse the materials. Welding is typically used for permanent, strong connections where flexibility or ease of disassembly is not required.
- Bolted Connections: Bolted connections use fasteners to join members. They are easier to fabricate and assemble compared to welded connections, offering flexibility and ease of maintenance or modification.
2.4 Spacing of Trussed Beams
The spacing of trussed beams is a critical factor in ensuring both structural efficiency and cost-effectiveness. For spans up to 20 meters, the typical spacing between trusses is around 4 meters, which is approximately 1/5 of the span. In terms of roof trusses, a slope of 22 degrees is commonly used for corrugated steel or asbestos roofing sheets.
To achieve an economical spacing, the cost of the truss should be equal to twice the cost of the purlins and roof covering. For trusses with a span up to 15 meters, a spacing of 1/4 of the span is recommended. For spans between 15 and 30 meters, a spacing of 1/5 of the span is ideal. This balance of spacing ensures that the trussed beams are both structurally sound and cost-effective.
Design Procedure for Trussed Beam Structure
The design of a trussed beam involves several essential steps that ensure the final structure is both safe and economical. The typical procedure includes:
- Selection of Structure Type and Layout: Choose the appropriate trussed beam layout and structural design based on the building’s requirements.
- Determination of Loads: Calculate the loads that the trussed beam will bear, including dead loads (weight of the structure itself) and live loads (temporary loads such as people, equipment, or snow).
- Evaluation of Internal Forces and Moments: Analyze the forces and moments acting on each member of the truss to ensure that each component will perform as expected under load.
- Selection of Materials and Proportioning of Members: Choose suitable materials for the trussed beam members and proportion them to handle the expected forces safely and efficiently.
- Performance Checking and Final Review: Verify the structure’s performance under service conditions to ensure it meets safety and regulatory requirements before final approval.
Design Formula for Trussed Beams
Design formulas for trussed beams provide a mathematical approach to determine the forces in the members, based on the type of load and the configuration of the truss. For example, for a uniformly distributed load, the tension in a rod of a single strut is given by the formula 0.312Wh/r, while for a double strut, it is Wh/3r. Similarly, formulas for compression in the struts and beams can be calculated based on the load applied.
Example Design: Single Strut vs. Double Strut Trussed Beams
- Single Strut Trussed Beam: In this configuration, the tension in the rod and compression in the strut are calculated using specific formulas based on the applied load.
- Double Strut Trussed Beam: This design offers additional stability and is used when larger loads or greater spans are involved. The forces in the rods and beams are adjusted accordingly.
Fabrication and Erection of Trussed Beams
The fabrication and erection of trussed beams play a crucial role in the overall efficiency and economy of the project. The ease of fabrication is influenced by the complexity of the design, while the erection process can affect both time and cost.
For small to medium trusses, the typical approach involves lifting the trusses at the ridge during the erection process. To prevent buckling of the bottom chord during hoisting, it is important to design it to carry compressive stresses effectively. An empirical relation for proportioning the width of the bottom chord is given by b/L = 1/125, where b is the width of the bottom chord at its center, and L is the span length of the truss.
Conclusion
Trussed beams provide an effective and economical solution for spanning large spaces in modern construction. The careful design of trussed beams—considering geometry, section selection, connections, spacing, and fabrication—ensures that these structures are both strong and cost-efficient. Whether used for industrial buildings, large roofs, or other applications requiring wide spans, trussed beams remain a critical component in the engineering and architectural landscape. By adhering to established design principles and formulas, engineers can create safe, efficient, and sustainable structures that meet the needs of the modern world.