In multi-storey steel structure construction, the selection of the floor system plays a critical role in ensuring the overall efficiency, safety, and cost-effectiveness of the building. A wide variety of floor systems are available, each designed to meet different load-bearing, span, and functionality requirements. This article explores common types of floor systems used in steel buildings, their advantages, disadvantages, and the criteria that guide the selection of the appropriate system for a given project.
1. Short-Span Composite Beams and Composite Slabs with Metal Decking
This is one of the most commonly used floor systems in multi-storey steel structures. It features steel beams with shear studs welded to their top flanges, creating composite action with the concrete slab.
Span Ranges:
- Primary beams range from 6m to 9m.
- Secondary beams span between 6m and 7.5m.
Advantages:
- Cost-effective: This system is generally more affordable compared to other complex floor systems.
- Lightweight: The design reduces the overall weight of the structure.
- Thinner Beams: It allows for thinner beams compared to non-composite systems, reducing material costs and structural bulk.
Disadvantages:
- Fire Protection: The beams require fire protection to meet safety standards.
- Column Requirements: The need for a greater number of columns can increase the structural complexity.
- Increased Floor Thickness: Compared to other systems, the composite floor system results in a thicker floor, which might not be desirable in certain architectural designs.
2. Slimdek Floor System
The Slimdek floor system uses asymmetric steel beams paired with a concrete slab, offering a more compact design.
Span Ranges:
- Ranges from 6.5m to 7.5m.
Advantages:
- Leveled Soffit: One of the key advantages of the Slimdek system is that it provides a nearly flat soffit, making it ideal for simplifying utility installations, such as air conditioning ducts and electrical wiring.
- Reduced Floor Thickness: The system is designed to be thinner, which can reduce the overall height of the building.
Disadvantages:
- Special Connection Detailing: The differing widths of the top and bottom flanges of the beams require special connection detailing, adding to the complexity of the design.
- Increased Steel Weight: Compared to other floor systems, the Slimdek system tends to use more steel, which can increase the cost and overall weight.
3. Cellular Composite Beams with Composite Slabs and Steel Decking
This floor system is designed with cellular beams that support composite slabs, offering a more flexible and efficient design with a larger span.
Span Ranges:
- Beams range from 10m to 18m.
Advantages:
- Large Clear Span: Cellular beams create large, clear spaces without the need for additional support columns, which is ideal for open-plan office spaces or large halls.
- Lightweight: Cellular beams are typically lighter than other systems with similar spans, reducing the load on the overall structure.
- Cost-Effective: Despite the complexity of the beam fabrication, the system is relatively affordable compared to other long-span systems.
Disadvantages:
- High Fabrication Cost: The intricate design of the cellular beams results in higher initial fabrication costs.
4. Slimflor Beams with Precast Concrete Units
The Slimflor system consists of slim steel beams supporting precast concrete units, which are reinforced with concrete to join the units together.
Span Ranges:
- Typically ranges from 4.5m to 7.5m, with occasional spans reaching up to 10m.
Advantages:
- Fire Protection: This system does not require fire protection for beams up to 60 minutes.
- Smaller Floor Thickness: The system reduces the overall floor thickness, offering height savings in multi-storey buildings.
- Offsite Operations: Shear studs can be welded offsite, reducing the work required on site.
Disadvantages:
- Heavy Steel Work: Compared to other systems, Slimflor systems use more steel, which can increase the weight and cost.
- Extra Fabrication: The system requires extra fabrication work and more detailed connections between the steel beams and precast units.
5. Long-Span Composite Beams and Composite Slabs with Metal Decking
In this system, primary and secondary steel beams support composite slabs. This system can be arranged in two primary configurations, either with short span primary beams and long span secondary beams or vice versa.
Span Ranges:
- Long-span primary beams range from 10m to 15m.
- Secondary beams typically span from 3m to 9m.
Advantages:
- Large Clear Areas: The system provides large, open spaces without requiring additional columns, which is ideal for commercial or industrial buildings.
- Efficient for Long Spans: Perfect for buildings that require long, uninterrupted spans.
Disadvantages:
- Large Floor Thickness: The need for long spans results in larger floor thicknesses.
- Fire Protection and Heavy Steel Work: Fire protection is required, and there is an increased amount of steelwork, both of which add to the cost.
6. Composite Beams with Precast Concrete Units
This system combines hollow or solid precast concrete units placed on steel beams, with shear studs welded to the beams for stability.
Span Ranges:
- Beams and precast units typically range from 6m to 9m.
Advantages:
- Fewer Secondary Beams: Fewer secondary beams are needed due to the large spans of the precast concrete units.
- Reduced Onsite Work: Shear connectors can be welded offsite, reducing the labor required during the construction process.
Disadvantages:
- Lateral Buckling Risk: The beams are more susceptible to lateral tensional buckling during construction.
- Installation Complexity: The installation of precast concrete units is more complex, requiring careful planning and additional resources.
7. Non-Composite Beams with Precast Concrete Units
This system uses non-composite beams, with precast concrete units placed on the flanges of the beams. The units are secured by bolting or welding to the web of the beams.
Span Ranges:
- Beams and precast units typically range from 6m to 7.5m.
Advantages:
- Reduced Number of Secondary Beams: This system requires fewer secondary beams due to the large spans of the precast concrete units.
Disadvantages:
- Need for Temporary Supports: Lateral temporary supports are required to prevent buckling of the beams during construction.
- Complex Installation: The installation process for precast concrete units can be time-consuming and labor-intensive.
8. Criteria for Choosing a Floor System
Selecting the right floor system depends on various factors:
- Span Length and Layout: The required span length determines which system is most suitable, with long-span systems ideal for larger open spaces.
- Load-Bearing Capacity: The type of building and expected load (e.g., office, residential, industrial) affects the system choice.
- Cost and Efficiency: Budget constraints and the need for a cost-effective solution often dictate the floor system selection.
- Safety and Fire Protection: Systems requiring fire protection or higher safety standards may influence the choice.
- Installation Complexity: Systems that reduce on-site labor or allow off-site fabrication may be preferred for time-sensitive projects.
Conclusion
In the construction of multi-storey steel structures, the floor system plays a vital role in ensuring the building’s stability, functionality, and efficiency. Whether it’s a cost-effective composite system or a long-span cellular beam solution, the right choice depends on the specific requirements of the building project. Careful consideration of span ranges, material weight, fire protection, and overall cost can help engineers and architects choose the most suitable system for their design needs. Consulting with structural engineers early in the design process can lead to optimized solutions that meet both technical and budgetary constraints.