Prestressed Filigree Slab vs Hollowcore

As far as precast/prestressed spanning elements go, hollowcore is the dominant product in the building industry and has enjoyed significant market growth in North America since the 1970s. Compared to other spanning systems hollowcore offers numerous benefits such as:

  • Economical span/depth ratio
  • Rapid speed of construction
  • High quality manufactured production
  • Superior thermal and acoustic properties

However, as designs have started to push the envelope as far as the use of hollowcore, the product has evolved away from the commodity status of years past, in favor of a more customized designs. Producers are spending large sums on labor to make cutouts, notches and to install special inserts, core fills etc during production in the plant which can be tedious given the nature of the production methods used.

However, there is another alternative that could offer a higher level of flexibility: the filigree slab.

Filigree slabs are also known as lattice girder slabs due to the special lattice girder reinforcement which serves as support during handling and also as a composite tie between the topping and the main slab. They are supplied to the site ‘un-topped’ but serve as an immediate working surface, just like hollowcore. They can be prestressed or not, depending on the requirements, and can be supplied with foam or block infills to create the voids in the slab. the main advantage of filigree slabs is the fact that their structure is ‘open’ during construction to allow for the passage of services, special connections and other integrated work which is best suited to the field.

Another major advantage is the weight of the system at the time of erection, as half slabs can be up to 40% lighter than hollowcore at that point. Further, the widths of half slabs are flexible, and can be in width ranges typically up to 8’ (2.4m) or even larger, similar to double tees. This means that more square feet can be placed per ton lifted compared to hollowcore, and the number of visible joints in the soffit can be reduced compared to standard 4’ wide hollowcore plank.

Further advantages are:

  • Monolithic connections allow for two-way spanning
  • Highly flexible shear capacity
  • Strong seismic and diaphragm action
  • Very easy to create openings

Unlike hollowcore however, filigree slabs require support during erection and for a period after the topping is cast to support against deflection as the moment of inertia of the system is low until composite action is achieved; which can be viewed as a logistical and economical drawback of the system.

Further, the technology to weld the lattice girders is expensive and not readily available in North America, and investment in such equipment would need to be justified by high volumes of production which do not exist in the market. Thus, perhaps why this particular system is uncommon in North America.

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