GB2176818A - Structural member - Google Patents

Abstract

The invention provides a method of forming a structural member such as decking to span between adjacent supports 2 which comprise the steps of casting a first concrete layer 3 eg on a support sheet 1 to a depth of less than the desired final thickness of the member and, when the first layer has hardened, casting a second concrete layer 4 on top of the first layer. Reinforcements 5 may be provided in and project from the first layer. <IMAGE>

Description

SPECIFICATION Structural member This invention relates to structural members such as "decking" which is used to provide a span between supports. "Decking" is used for example for floors, ceilings and permanent shuttering.

It is often the case with structural members which provide a span between supports that by increasing the depth of the member, the allowable span is actually decreased for a given imposed load capability. For example, with composite decking composed of a profiled steel sheet which is erected into position and used as the support for a concrete layer cast in situ, the limitation on spans is generally caused by the deflection of the sheet under construction loading due to the wet concrete. Typically, a larger overall depth of such decking results in a smaller overall span for a given imposed load capability.

Accordingly, the invention provides a method of forming a structural member comprising the steps of casting a first concrete layer to a depth less than the desired final thickness of the member and, when the first layer has hardened, casting a second concrete layer on the top of the first layer.

The invention is particularly applicable to composite "decking" in which the first layer is cast on a support sheet which will form part of the finished member.

At least one reinforcement may be provided in and projecting from the first layer such that the second layer is cast around the reinforcement. Such a reinforcement will thus resist shear at the interface between the concrete layers.

The member may be built up in more than two concrete layers in certain circumstances.

The invention also provides a structural member formed as above.

An example of the invention will now be described with reference to the accompanying drawings in which Figure 1 is a cross-sectional elevation of a composite deck formed according to the invention, and Figure 2 is a cross-sectional elevation through a pre-cast panel corresponding to the first concrete pour of the member of Figure 1.

The decking of Figure 1 comprises a profiled steel sheet 1 having any desired profile.

Alternative profiles are shown at 1 A and 1B which are trapezoidal and re-entrant respectively in shape. As is known in the art, such sheets are erected into position, spanning the gap between supports shown schematically at 2. Concrete is then poured onto the sheet 1 but in the conventional method this is done to the required final thickness of the decking in a single pour. In accordance with the invention however, a first concrete layer 3 is first cast to a depth less than the desired final thickness of the deck. When the first concrete layer 3 has reached an adequate cube strength (it need not be completely set) a second concrete layer 4 is cast on top of the first concrete layer 3 to bring the combined slab up to the desired final structural thickness.

The top of the first concrete layer 3 is left very rough in order to obtain a good bond with the second layer 4 and to resist horizontal shear at the interface between the layers.

To improve shear resistance, reinforcements 5 may be provided in the first concrete layer 3 and projecting upwardly therefrom so that the second concrete layer is cast around these reinforcements. Bar reinforcements 6 may be provided in the channels of the profiled sheeting if required in the design. A mild steel mesh 7 may be incorporated in the second concrete layer 4 for fire protection and distribution of loads.

Instead of pouring the first concrete layer 3 in situ, the first concrete layer may be precast thereby forming a pre-cast panel as shown in Figure 2. Such panels can then be delivered to the site and hoisted into position with the second concrete layer being poured on site. The edge of the steel sheet 1 is turned up at the edge as shown at 8 to form a side of the pre-cast unit.

As an illustration of the improvement which can be effected by the invention, an overall thickness of 200 mm of normal weight concrete cast in a single pour with a typical profiled steel sheet allows a single span of 2.45 m for an imposed loading of 6.0 KN/sq.m.

However, an overall depth of 100 mm of the same concrete in one pour aliows a span of 3.02 m for the same imposed loading. Thus, by pouring the first concrete layer 3 in the embodiment to a total depth of 100 mm, advantage can be taken of the larger span available which is considerably greater than that allowable if the entire concrete thickness were poured in one go. The remaining 100 mm can be made up by the second layer to provide the required total structural depth.

1. A method of forming a structural member comprising the steps of casting a first concrete layer to a depth less than the desired final thickness of the member and, when the first layer has hardened, casting a second concrete layer on top of the first layer.

2. A method as claimed in claim 1 wherein the first layer is cast on a support sheet which will form part of the member.

3. A method as claimed in either preceding claim comprising the further step of providing at least one reinforcement in and projecting from the first layer such that the second layer is cast around the reinforcement.

4. A method of forming composite decking substantially as hereinbefore described with

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Structural member This invention relates to structural members such as "decking" which is used to provide a span between supports. "Decking" is used for example for floors, ceilings and permanent shuttering. It is often the case with structural members which provide a span between supports that by increasing the depth of the member, the allowable span is actually decreased for a given imposed load capability. For example, with composite decking composed of a profiled steel sheet which is erected into position and used as the support for a concrete layer cast in situ, the limitation on spans is generally caused by the deflection of the sheet under construction loading due to the wet concrete. Typically, a larger overall depth of such decking results in a smaller overall span for a given imposed load capability. Accordingly, the invention provides a method of forming a structural member comprising the steps of casting a first concrete layer to a depth less than the desired final thickness of the member and, when the first layer has hardened, casting a second concrete layer on the top of the first layer. The invention is particularly applicable to composite "decking" in which the first layer is cast on a support sheet which will form part of the finished member. At least one reinforcement may be provided in and projecting from the first layer such that the second layer is cast around the reinforcement. Such a reinforcement will thus resist shear at the interface between the concrete layers. The member may be built up in more than two concrete layers in certain circumstances. The invention also provides a structural member formed as above. An example of the invention will now be described with reference to the accompanying drawings in which Figure 1 is a cross-sectional elevation of a composite deck formed according to the invention, and Figure 2 is a cross-sectional elevation through a pre-cast panel corresponding to the first concrete pour of the member of Figure 1. The decking of Figure 1 comprises a profiled steel sheet 1 having any desired profile. Alternative profiles are shown at 1 A and 1B which are trapezoidal and re-entrant respectively in shape. As is known in the art, such sheets are erected into position, spanning the gap between supports shown schematically at 2. Concrete is then poured onto the sheet 1 but in the conventional method this is done to the required final thickness of the decking in a single pour. In accordance with the invention however, a first concrete layer 3 is first cast to a depth less than the desired final thickness of the deck. When the first concrete layer 3 has reached an adequate cube strength (it need not be completely set) a second concrete layer 4 is cast on top of the first concrete layer 3 to bring the combined slab up to the desired final structural thickness. The top of the first concrete layer 3 is left very rough in order to obtain a good bond with the second layer 4 and to resist horizontal shear at the interface between the layers. To improve shear resistance, reinforcements 5 may be provided in the first concrete layer 3 and projecting upwardly therefrom so that the second concrete layer is cast around these reinforcements. Bar reinforcements 6 may be provided in the channels of the profiled sheeting if required in the design. A mild steel mesh 7 may be incorporated in the second concrete layer 4 for fire protection and distribution of loads. Instead of pouring the first concrete layer 3 in situ, the first concrete layer may be precast thereby forming a pre-cast panel as shown in Figure 2. Such panels can then be delivered to the site and hoisted into position with the second concrete layer being poured on site. The edge of the steel sheet 1 is turned up at the edge as shown at 8 to form a side of the pre-cast unit. As an illustration of the improvement which can be effected by the invention, an overall thickness of 200 mm of normal weight concrete cast in a single pour with a typical profiled steel sheet allows a single span of 2.45 m for an imposed loading of 6.0 KN/sq.m. However, an overall depth of 100 mm of the same concrete in one pour aliows a span of 3.02 m for the same imposed loading. Thus, by pouring the first concrete layer 3 in the embodiment to a total depth of 100 mm, advantage can be taken of the larger span available which is considerably greater than that allowable if the entire concrete thickness were poured in one go. The remaining 100 mm can be made up by the second layer to provide the required total structural depth. CLAIMS

1. A method of forming a structural member comprising the steps of casting a first concrete layer to a depth less than the desired final thickness of the member and, when the first layer has hardened, casting a second concrete layer on top of the first layer.

2. A method as claimed in claim 1 wherein the first layer is cast on a support sheet which will form part of the member.

3. A method as claimed in either preceding claim comprising the further step of providing at least one reinforcement in and projecting from the first layer such that the second layer is cast around the reinforcement.

4. A method of forming composite decking substantially as hereinbefore described with reference to and as shown in the accompanying drawings.