Composite slabs are a critical component in the construction of multistory steel structures, providing a solid and durable floor system. These slabs are subjected to various construction loads that must be carefully evaluated and managed to ensure structural integrity during the construction phase. Failure to properly assess and distribute these loads can lead to significant structural damage and even catastrophic failures. This article will explore the types of construction loads imposed on composite slabs, their importance, and the considerations that must be taken into account during the design and construction phases.
Why It Is Important to Evaluate Loads on Composite Slabs
The evaluation of construction loads imposed on composite slabs is paramount to preventing slab failure during construction. Improper handling or excessive loads on a slab can cause deflection, cracking, or even collapse, resulting in safety hazards for workers and significant financial costs. Structural failures during construction not only risk the lives of workers but can also cause delays, increase material costs, and require expensive repairs. Therefore, understanding the types of loads and their magnitudes is crucial for ensuring that composite slabs remain stable throughout the construction process.
Types of Construction Loads on Composite Slabs
Construction loads on composite slabs can be categorized into two major types: construction loads imposed during concreting and construction loads exerted after concreting. Each of these types requires careful consideration to ensure the stability of the slab at different stages of the construction process.
A. Construction Loads Imposed During Concreting
The loads imposed on composite slabs during the concreting phase consist of several key factors. These include the weight of workers, the fresh concrete itself, formworks, pipelines, small equipment, and impact forces. The structural designer must account for these loads to prevent overloading and damage to the slab during the early stages of construction.
- Weight of Workers The weight of workers involved in the concreting process must be considered when assessing load distribution. Typically, no more than six workers should be present on the slab at one time, with no more than four workers around the outlet of the pipeline. The weight of workers, when distributed appropriately, does not present a significant risk to the slab’s structural integrity.
- Weight of Fresh Concrete During the concreting process, the weight of the fresh concrete is one of the most significant loads imposed on the slab. The height of the concrete poured should not exceed knee level above the decking, and the structural designer should account for this load when calculating the slab’s capacity. This ensures that the weight of the concrete does not exceed the slab’s ability to support it, reducing the risk of deflection or failure.
- Formworks and Small Equipment Loads The weight of formworks and small equipment used in the concreting process must be calculated and considered in the overall load calculations. These elements can be easily quantified and added to the total load on the slab, ensuring that the slab’s design accommodates them without overloading.
- Weight of Pipeline and Impact Loads The weight of the pipeline used to transport concrete must also be considered in the design. Typically, a 150mm pipeline filled with concrete will exert significant weight. To prevent localized damage to the decking, the weight of the pipeline must be distributed over a larger surface area, often using timber or other materials. Additionally, impact forces from the placement of concrete should be minimized to avoid sudden stresses on the slab.
- Heaped Concrete During the concreting process, concrete may sometimes accumulate in piles, especially at the outlet of the pipeline. Structural designers should anticipate this and account for the load of any heaped concrete. A cone of heaped concrete with a base of 100cm and a height of 20cm is one such consideration. To avoid excessive accumulation, the outlet of the pipeline should be moved regularly.
- Additional Loads During Concreting In certain cases, additional concrete may be required if the slab and steel beams experience deflection, particularly if the slab needs to be finished at a specific level. Any extra concrete should only be placed after consulting with the structural designer to ensure that the additional load is within permissible limits.
B. Construction Loads Exerted After Concreting
Once the concrete has been placed and the slab has set, additional loads can be imposed on the composite slab during the remainder of the construction process. These loads typically include pallets of blocks, bags of fire protection material, skips of debris, and other construction equipment. It is essential to carefully assess these loads to ensure that they do not compromise the strength of the concrete before it has fully cured.
- Examples of Post-Concreting Loads Common construction items that exert loads after concreting include pallets of blocks, bricks, bags of fire protection material, and bags of cement. Each of these items can impose significant weight on the slab, and structural designers must account for them in the load calculations.
- Magnitude of Loads According to British Standards, the allowable load on the composite slab after concreting is 1.5 kN/m². If the imposed load exceeds this threshold, the slab may be at risk of damage, particularly before the concrete has reached sufficient strength. Therefore, it is essential to consider the strength of the concrete before imposing any additional loads.
- Concrete Strength Considerations Before imposing loads greater than 1.5 kN/m², the concrete’s strength should be evaluated. Typically, concrete should not be subjected to such loads until it has achieved 75% of its strength, which usually occurs within the first 28 days. Structural designers should test the concrete’s strength by using cylinder or cube samples to ensure that the slab can withstand the imposed loads.
- Examples of Construction Loads After Concreting In practice, the loads on the composite slab after concreting may come from various items such as concrete blocks (up to 10 kN/m²), pallets of bricks (up to 15 kN/m²), or bags of fire protection material (up to 2.5 kN/m²). These loads must be carefully accounted for, as excessive weight can impair the newly placed concrete.
C. Heavy Construction Loads After Concreting
In addition to typical loads, certain heavy construction loads can be imposed on composite slabs after concreting. These heavy loads require special consideration due to their potential to cause significant stress on the slab.
- Generators Large equipment, such as welding generators, can exert a load of up to 50 kN. This load must be carefully assessed to ensure that the slab can support it without failing.
- Forklift Trucks Forklift trucks are another source of heavy load. They can impose loads of up to 100 kN, excluding the live load of the vehicle. The placement of forklift trucks on composite slabs should be carefully managed to prevent overstressing the structure.
- Crane Counterweights The counterweights used in cranes are often heavy and can exert substantial loads on the slab. Each counterweight should be clearly marked with its weight, and the load distribution must be carefully considered.
- Mobile Access Platforms Mobile access platforms used to install services or finishes also impose loads on the composite slab. Structural designers must check the potential loading from these platforms to ensure that the slab can support the additional weight.
Conclusion
Understanding the various construction loads imposed on composite slabs during both the concreting and post-concreting phases is essential to ensure the structural safety and integrity of the building. By evaluating and properly distributing these loads, structural designers can prevent damage, enhance the durability of the slab, and ensure the safety of workers during the construction process. Proper load management is crucial not only for the success of the current project but also for minimizing future maintenance costs and structural failures.