Less is more with hollow core slabs

The good old hollow core slab has managed to remain one of the most common floor and roof slabs over the years. We asked structural engineering professional Lasse Rajala, Business Unit Director at Sweco, to explain the secret behind its success.

Versatile product for many purposes

Hollow core slabs are precast slabs of pre-stressed concrete typically used in the construction of floors in multi-storey residential, commercial, office, and industrial buildings. It is also possible to use hollow core slabs in vertical or horizontal installations as walls or noise barriers.The slab has been especially popular in Northern Europe, where the emphasis of home construction has been on precast concrete. There are different types of hollow-core slabs. Usually, the standard width is 1,200 mm.

Savings in concrete

The highly optimized and economical use of material makes hollow core slabs one of the most sustainable products in construction.

The precast concrete slab has tubular voids running the full length of the slab, making the slab much lighter than a massive solid concrete floor slab of equal thickness or strength.

In the cross section of hollow core slabs, concrete is used only where it is actually needed. Areas where concrete acts only as ballast are replaced with hollows. For instance, in 200-mm hollow-core slabs, 49.9 percent of the cross section consists of voids. In 400-mm hollow core slabs, this percentage may be as high as 55.6. This brings savings in concrete material costs, as well savings in vertical structures, foundations and reinforcement.

Long-lasting slab

Pre-stressed hollow core slabs will not crack under service loads. This reduces deflections compared to structures with reinforced concrete, because the entire hollow core slab section contributes to resisting the loads. When cracking is eliminated, reinforcement will be better protected from corrosion, enabling a longer life span for the structure.

Freedom for individual design

When a building with hollow core slabs is designed, the lightweight, long-span solution offers more possibilities compared to traditional massive short-span slabs. When hollow core slabs are used in residential buildings, the partition walls inside flats can usually be non-load bearing. This gives freedom for the individual design of flats, as well as for modifications during the building’s lifetime.

In commercial and public buildings, long-span hollow core slabs have enabled comfortable car parks to be built without pillars, with quick and easy access and exit.

Sound insulation for demanding requirements

There are very high requirements for sound insulation in modern multi-storey residential buildings in many countries. Hollow core slabs meet this requirement well, especially against airborne sound transmission. With standard hollow core slab solutions, the requirement R’w ≥ 55 dB against airborne sound transmission can be easily achieved.

The most common thicknesses with corresponding spans:

The most common thicknesses of hollow core slabs with corresponding spans

370-mm hollow core slabs have been specially developed for residential buildings to fulfill sound insulation requirements without additional topping concrete.

Challenging structural design in Oslo

The award-winning DNB Bank Headquarters in Oslo counts among Lasse Rajala’s most challenging works in recent years. The imposing building was awarded in the European Steel Design Awards 2015 for its multifaceted, demanding structure and innovative implementation.

The main designer of the building, which was completed in 2012, was a famous Dutch architectural office MVRVD. Steel structures were delivered and installed by Ruukki Construction. Sweco was responsible for the structural design of the building’s steel frame.

The building consists of cube-like six-by-six-meter projections from the core of the structure. Hollow core slabs are used for all intermediate floors of the 17-storey building. There are numerous suspended structures and projections connected to the glass facade in the complex ensemble.

“This makes the structural design of the building very challenging – the degree of complexity is in a class of its own,” Rajala says.

In addition to the numerous details, the building had to be designed to meet earthquake-resistance standards due to the quality of the soil. “Hollow core slabs are ideal for seismic areas.

As the own weight of the hollow core slab is much less than a corresponding solid slab, the lateral forces during seismic action will be reduced accordingly,” Rajala says.

Lasse Rajala works at Sweco Finland Ltd., specialized in the built environment and industry. Sweco operates in the fi elds of structural, industrial, environmental and municipal engineering, as well as building service systems and architecture.