Why and how to choose the right cross-sections?
Hollow-core technology became popular in the 1970's and has been evolving ever since. At first the most important goal was to make the slabs as lightweight as possible, and to use concrete only where it is actually needed. The less concrete is consumed, the more profitable and the more environmentally friendly the slabs are. The cross section of a slab may contain less than 50 per cent of concrete.
Modern steel frames and structures are often accompanied by hollow-core slabs as floor and roof slabs. This has brought new requirements for the slabs.
”It has been established that the supports on which the slabs are placed bend over time. This applies not only to steel structures but also to the beams of concrete structures”, says Senior Advisor Lassi Järvinen from Elematic.
The concrete-filled pillars in the cross section between the voids are called webs. The shear capacity of the webs can drop more than 50 per cent because of the bending.
”This is an alarming number and needs to be addressed in designing the cross sections. Our standard cross sections have been redesigned together with the best experts to keep the shear capacity as stable as possible from top to bottom.”
Calculating the shear stress can be complicated. This has been researched by VTT, the technical research centre of Finland. Since the early seventies, VTT has performed more than one thousand load tests on single prestressed hollow-core slabs.
In 2005, Dr. Matti Pajari at VTT published a report called Resistance of prestressed hollow-core slabs against web shear failure. He found that the traditional method for shear calculation was not working very well when the slabs have noncircular voids. In recent years attention has been paid to make the calculations more accurate.
The size and the shape
The most common width of a hollow-core slab is 1200mm. The height varies typically from 150mm to 500mm. The most common shapes of the tubular voids are round or elliptic – or have been up to now.
”The focus on shear capacity has led to skull-shaped voids, rather than round voids. This way the shear capacity of the webs can be maintained a lot better”, Järvinen says.
Shaping the hollows like a skull, wide on top and narrow on bottom, is a good idea in many ways. It makes it possible to install the reinforcement wires higher into the webs of the slab.
The wires are at the bottom of the webs. They must not be too close to the hollows, but the higher the wire is, the better fire-resistance rating the slab can have. Skull-shaped hollows have more room for the wires.
For example, in hospitals and nursing homes in Finland the fire-resistance rating must be 3 hours (REI-180). Choosing the right cross section enables this without making the bottom of the slab thicker.
Naturally, the local building codes must be met as well.
The number of webs is a major factor when the slab is penetrated. It is strongly recommended to have an odd number of hollows in the cross section.
”With five hollows there is one additional web compared to four hollows. This means more reinforcement wires. Odd numbers are definitely recommended”, Järvinen says.
The slabs often need to be cut in half. If there is an even number of hollows, the saw needs to go through concrete all the way. With an odd number of hollows, the slab can be cut in half through a hollow.
More attention should be paid to the design of the cross sections, rather than the weight of the slab.
”Only looking at the weight is narrow thinking. The thickness of the slab, the number and the shape of the hollows, there are many things to consider to be able to achieve the best results.”