Hollow Core Slab Cross Section

[Sophie Reynolds]

Thetest panels for the fire test consisted of a main Deltabeam with a span width of 3,915 mm.The span width of the slabs was 2,350 mm between the main beam and the two edge beams as shown in Fig. 1. The cross sections of the Deltabeams and the hollow-core slabs are shown in Figs. 2, 3, and 4.

The design and documentation of the bearing capacity of the Deltabeam was carried out by Peikko. The degree of utilization of the main Deltabeam and the two edge beams was practically equal in order to obtain unique deflections during the fire tests. The steel in the beams was S355J2+N in...

The Deltabeam composite beam is the most significant single product of Peikko Group. Deltabeams have been used in over 10,000 buildings worldwide, including in Europe, North America and Asia.During...

Hi, I think this was discussed on the forum a few times in the past, but I wonder if anyone has any other way to show cores in hollowcore floor slabs other than manually adding these as detail items (repeating detail) or using precast extension. I still think that there should be a way to do that considering one can show metal deck profiles in section.

Ahmed Muharram, B.Sc, AEE, ACI, ACP

BIM Manager

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Hollow-core slabs are constantly evolving. What are the key features that need to be considered when making choices? Why are certain cross sections recommended? The answers come from experience and expertise.

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 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.

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.

The idea to reduce the self-weight of concrete slabs by putting voids in the centre of the cross-section, dates to the beginning of the previous century. Several inventors from different countries applied for patents on various systems. The present article is mainly based on an analysis of patents published during the first half of the 20th century, and personal experiences from 1960 on. Patents usually offer a complex description of inventions (claims). Reconstructing the history of hollow core slabs based on these patents is a laborious but fascinating exercise. This article aims to give a general overview and is not meant to be exhaustive.

In general, these manufacturing methods can be used for the production of reinforced slabs as well as for the production of pre-stressed slabs. They are mostly in normal dense concrete, but there are also examples of structural light weight concrete.

In the early days, hollow core slabs were manufactured eitherin a plant, or on site. Often individual moulds were used and sometimes even long line beds, but in a discontinuous way. The compaction of the concrete was mostly carried out by tamping the fresh concrete. Here also a patent study could bring more insight, but it is not the main subject of the present article.

The most characteristic feature in the development of hollow core slabs was that they deviated strongly from the at that time existing design principles of reinforced and prestressed concrete, by which compression is taken up by concrete and tension by reinforcement. Indeed, in most cases, the developed manufacturing technique was only possible under the following conditions:

As a consequence, the tensile capacity of the concrete had to be taken into account in the design and new techniques for connections had to be developed. This was new especially with regard to transfer of forces at the support, shear capacity of the units, diaphragm action of the floors, transversal load distribution among adjacent units, non-rigid supports, floor openings, fire resistance, etc.

With regard to prestressed hollow core slabs, the fib Commission on Prefabrication played a crucial role in the development of the design. Extensive research and intensive field experience gathered from all over the world, learned that hollow core floors are perfectly able to fulfil all the needed structural functions, on condition that some elementary design principles are met. In 1988 the FIP Commission on Prefabrication published Recommendations for the design of prestressed hollow core floors. They have been used as a basis for national and international standards, for example the Eurocode 2 and the European CEN Product Standard EN 1168. An updated version of the FIB Recommendations 1988 will be published this year.

Wilhelm Siegler (Germany, 1906) can probably claim the first application of longitudinal void formers in concrete slabs [3]. His system to realize cores was based on prefabricated short moulding tubes in hardened mortar or another material, which were positioned on a scaffolding (Fig. 2). The length of the slabs was arbitrary. The tubes had lateral lugs at the bottom, serving as mould for the webs. They were placed either continuously in the longitudinal direction, or with short inter-distances at certain places to form transversal ribs. The longitudinal and transversal webs were reinforced in the classical way.

The question could be raised about the distinction between hollow core elements and box elements. The above variants still correspond to the definition of hollow core slabs given afore, but from a certain thickness on, they are to be classified as box slabs or beams. By the way, the inventors of the solutions of Table 1 are in the first place claiming for floor slabs, although they do not exclude in the patent description the applicability for box beams or even walls.

Today, this production technique is rather rare but is still used. After pouring a bottom layer, prismatic void formers, usually in polystyrene, are installed. Afterwards, a second layer of concrete is poured to shape the webs and the top layer.

In 1930, a patent is granted to the Belgian inventor Jules Heyneman for a precast floor slab with longitudinal voids [10]. These voids are formed by means of elastic moulds made of e.g. steel and held in place by wedges. When these wedges are removed, the cross-section of this mould is reduced, and the mould can be removed from the hollows in the beam without difficulty. Unfortunately, the drawings of the patent contain no details about these void formers. The number of voids in the cross-section could be modified. The floor units were in reinforced concrete. The patent describes mainly the product itself, without any detail about the manufacturing. The longitudinal joints between the units are indented and provided with transversal reinforcing stirrups. They were filled on site with mortar.

The inconvenience of the solution was of course the weakness of the flexible steel pipes. In 1939 a solution with pneumatic expansible and collapsible rubber core forms was patented by Walter H. Cobi (US) [11]. Fig. 4 shows a longitudinal

and transversal section of the system.

Charles Lethbridge (GB) [12] presented in 1940 an improved method with removable steel tubes of uniform cross-section extending longitudinally through the whole mould and conforming in shape to the cross-sectional form of the hollow core unit. After positioning of the desired reinforcement bars, the concrete was cast, and the mould vibrated as a whole. At the same time the core tubes were slightly moved relatively to the mould. When the concrete was sufficiently compacted to maintain its shape, the tubes were withdrawn via the end of the mould and the concrete was left to harden. By the employment of metal core members with a smooth surface and maintaining them in motion, the concrete was prevented from adhering to the tubes and the latter could be removed without difficulty. Preferably and for simplicity the core tubes were of circular cross-section which allowed for a rotary motion during casting.

In France in 1952, STUP Freyssinet [13] applied for a patent for the manufacture of prestressed hollow core elements on long steel beds. The invention was meant for floors of buildings. The units were in prestressed concrete, with a length equal to the floor span without intermediate supports, and a variable width in function of the needed slab thickness and the possibilities of handling. The elements had longitudinal voids over the whole length, with a circular shape. The vertical edges were profiled and filled with mortar after erection, to enable the transmission of vertical loads from one element to the others. The elements were cast on long line steel moulds. Transversal mould plates could be placed at any place to realise the length of the units. The longitudinal voids were moulded with long tubes in reinforced rubber, inflated with a liquid under pressure before and during casting. After compaction of the concrete, the pressure was released, and the tubes removed.

In March 1931, the German Wilhelm Schfer [14] applied for a patent to make precast reinforced and prestressed hollow core slabs on long line beds in stacks one line above the other. His purpose was to improve an already at that time existing production system (patent not available) based on a kind of slip form technique with movable cores and side plates, in which the different production steps were executed one after the other. His patent describes how to make the production in an automatic continuous way. We could consider it as a precursor of the slip form system. Patents were granted in Germany, in UK, in the USA and in Switzerland, all in 1933.

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