Hollow core planks are widely used in the construction industry, especially for floors and ceilings in large-scale commercial and residential projects. These precast concrete elements are prized for their strength, lightweight composition and cost-effectiveness. Their design incorporates longitudinal voids that run through the length of the planks, reducing the overall weight while maintaining structural integrity. However, these same characteristics present unique challenges when it comes to fixing into hollow core planks, necessitating careful consideration to avoid compromising their structural performance.
Key Characteristics of Hollow Core Planks
Hollow core planks are typically manufactured in 1200mm-wide sections, designed with a low span-to-depth ratio. This design is achieved through the use of prestressed reinforcement, which enables the planks to span greater distances while maintaining a relatively shallow depth. The prestressed concrete provides additional strength, allowing the planks to resist bending and deflection under load. Hollow core planks are generally supported on steel beams, though they can also be integrated into cast in-situ concrete frames, making them highly versatile in various structural configurations.
Use in Concrete or Steel Frame Buildings
In many projects, particularly where a combination of materials is used, hollow core slabs are incorporated into buildings that feature a concrete or steel frame. For instance, it is common to see a 300mm to 500mm thick concrete transfer slab at the first floor or podium level. This transfer slab is often used to support a lighter steel frame structure combined with hollow core concrete slabs or reduced-depth concrete slabs on the upper floors.
Masonry support in these configurations is frequently located at the first-floor level, where the thick concrete slab provides a solid base for supporting heavy loads, such as brickwork. The thickness of the transfer slab enables it to bear substantial masonry loads, which often eliminates the need for additional fixings to the upper-level slabs, simplifying the overall design and reducing installation complexity.
Considerations for Fixing into Hollow Core Slabs
Fixing into hollow core slabs requires special attention due to the presence of voids within the planks. For instance, when mounting brackets or other elements to the edge of a hollow core slab, it is advisable to fill the end cores of the slab with concrete. This practice enhances the stability and strength of the fixing points. Additionally, surface-mounted brackets are often used in conjunction with these filled cores to provide a secure base for the load-bearing elements.
The choice of fixing type is another crucial factor. While expansion anchors are commonly used in solid concrete applications, they may not be suitable for hollow core planks due to the voids. In such cases, resin anchors or screwbolt-type anchors are often preferred, as they provide better performance in hollow sections by ensuring a stronger grip within the material.
When surface-mounting bracketry onto hollow core slabs, it is important to carefully plan the location of the fixing points in relation to the solid concrete sections between the hollow cores. This consideration is especially important when installing elements like windposts or balustrades, where the precise location of the fixing points can impact the overall stability of the installation. The use of slotted holes in the base fixing plate can provide additional flexibility during installation, allowing adjustments to be made on-site to accommodate the positioning of the hollow cores.
Fixing to Steel Beams Supporting Hollow Core Slabs
In cases where hollow core slabs are supported on steel beams, the method of fixing masonry supports changes. It is common practice to attach masonry supports to web plates that are welded to the steel beams. This provides a very secure and reliable fixing point, ensuring that the masonry load is adequately supported. However, early engagement with the design team and coordination with the client is critical to ensure that these web plates are pre-welded to the steel frame before installation.
If these critical fixing points are not coordinated early in the design and construction phases, it can lead to delays on-site. Retroactively welding web plates or performing other hot works once the steel frame is already in place can be time-consuming and costly. Early design coordination mitigates these risks, ensuring that the project progresses smoothly and on schedule.
In conclusion, while hollow core planks offer many advantages in construction, their unique structure requires special considerations during installation, particularly when fixing supports or brackets. By taking these factors into account, construction teams can ensure that the integrity of the structure is maintained while maximizing the efficiency and cost-effectiveness of hollow core plank systems.
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