When designing a building, one of the most important decisions an engineer or architect will make is choosing the appropriate structural system. Among the various structural systems available, steel frames are commonly used due to their strength, durability, and versatility. However, not all steel frame systems are created equal. Some are more economical and less complex than others. This article will provide an overview of the most common types of economical steel frame systems and offer guidance on how to choose the most suitable option for your specific building project.
Types of Economical Steel Frame Systems
There are several types of steel frame systems, each with its own advantages and disadvantages. The most commonly used and economical steel frame systems are:
- Braced Frames in Simple Construction
- Braced Frames with Continuous Construction
- Unbraced Sway Frames
- Discrete Stability Frames
Each of these systems has varying levels of complexity, cost, and performance. Understanding these differences will help designers select the most efficient and cost-effective system based on the specific requirements of the building.
1. Braced Frames in Simple Construction
Braced frames in simple construction are among the most economical and least complex types of steel frames. This system consists of simple beam-to-column connections, where the beams are designed to resist only sagging moments. The columns are typically small in size and mass, which helps reduce material costs.
Advantages:
- Simple Analysis and Optimization: Since the structure is determinate, it can be analyzed with straightforward methods. This simplicity allows for easy optimization of both beams and columns.
- Cost-Effective Beam-to-Column Connections: The beam-to-column connections in this system are relatively simple and economical.
- Small Columns: The columns in braced frames are smaller and lighter compared to more complex systems, resulting in cost savings.
Disadvantages:
- Beam Costs in Serviceability: If serviceability requirements govern the design (e.g., deflection limits), the beam costs may increase, which can make the frame less economical.
- Not Suitable for Small-Scale Structures Focused on Strength: Braced frames in simple construction are not ideal for small structures that are primarily strength-driven.
- Increased Bracing Element Size for Taller Buildings: As the height and number of stories increase, the bracing elements become larger and more costly. This can make the bracing elements uneconomical for taller structures, requiring careful proportioning.
2. Braced Frames with Continuous Construction
Braced frames with continuous construction are a step up in complexity and cost compared to simple braced frames. In this system, the beam-to-column connections are continuous, providing improved stiffness to the overall structure. This type of frame is more suitable when the stiffness of the floor system is a major factor in the design.
Advantages:
- Improved Stiffness: The continuous beam-to-column connections enhance the overall stiffness of the floor system, making it more resistant to deflections.
Disadvantages:
- Higher Costs: Continuous beam-to-column connections are more costly than simple connections. This can increase both the material and labor costs for the project.
- Large External Columns: To resist the imposed moments, the external columns must be much larger. This requires more material, driving up costs.
- Complex Analysis: The continuous connections complicate the analysis of the structure. As a result, beams and columns cannot be optimized as easily, making the system less economical than simpler braced frames.
3. Unbraced Sway Frames
Unbraced sway frames are used when bracing cannot be incorporated into the design due to architectural constraints. These systems rely on either partially rigid or continuous steel frame connections to resist horizontal forces. While unbraced sway frames offer flexibility in architectural design, they are more complex and expensive than both simple and continuous braced frames.
Advantages:
- Architectural Flexibility: Since there is no need for bracing elements, unbraced sway frames allow for more flexible and open architectural designs. This is particularly useful when the building layout requires large, open spaces.
Disadvantages:
- Higher Construction Costs: Unbraced sway frames are more expensive than braced frames because the beam-to-column connections are more complicated and the external columns must be large to resist the applied moments.
- Complexity in Sizing Structural Elements: The analysis of unbraced sway frames is significantly more complicated, making it difficult to optimize the sizes of structural elements. This adds to the cost and complexity of the design.
- Non-Economical Beam-to-Column Connections: Unlike simple braced frames, the connections between beams and columns in unbraced sway frames are not simple or economical, further increasing costs.
4. Discrete Stability Frames
Discrete stability frames are considered the most complex and uneconomical steel frame system. These frames are designed to provide high levels of stability and resistance to horizontal forces. In a discrete stability frame, the structural elements (beams and columns) must be much stronger to provide adequate sway stiffness and resistance.
Advantages:
- Stability and Resistance: Discrete stability frames are ideal for providing maximum stability and horizontal force resistance, which makes them suitable for buildings in areas prone to high winds or seismic activity.
- Architectural Flexibility: The absence of triangulated bracing elements allows for greater architectural freedom in the design of the building.
Disadvantages:
- High Material Costs: Due to the need for strong beams and columns, discrete stability frames require more material than other systems, making them the most expensive option.
- Complex Beam-to-Column Connections: The connections between beams and columns in discrete stability frames are more complex and costly compared to other systems.
- Difficult Element Sizing: The complexity of the analysis makes it challenging to optimize the sizes of the structural elements. This adds to both the cost and difficulty of the design process.
Conclusion
When selecting a steel frame system for a building project, the choice will largely depend on the specific requirements of the structure, including height, load-bearing capacity, and architectural constraints. Braced frames in simple construction are typically the most economical and straightforward choice, particularly for buildings with moderate heights. However, if increased stiffness is required, or if bracing is not feasible due to architectural reasons, more complex systems like braced frames with continuous construction, unbraced sway frames, or discrete stability frames may be considered.