Composite sheet metal slabs
Composite sheet slabs are a type of composite slab that introduces a corrugated galvanized steel sheet and a compression layer of concrete. This sheet acts by avoiding the use of propping in the floors, since they are capable of fully supporting the concrete while it sets. In addition, this sheet has a load-bearing function, so both materials collaborate structurally. The system is usually completed with a distribution reinforcement inside the concrete in order to prevent cracking. Therefore, both materials work in solidarity.
The great advantage of this type of forged.
Among the advantages of composite sheet metal slabs we find that it acts as a work platform during construction, providing safety against falling objects. The corrugations provide adherence to the sheet with the concrete. In addition, the useful edge of the slab is reduced with the consequent weight reduction in the structure.
Usually the ratio between strength and weight is higher than in a conventional concrete floor, so the transport and assembly costs are lower. In addition, the materials are lighter and require less space. As a disadvantage we find that the sheet must be treated if necessary with a fire protection layer, to prevent it from being exposed to a hypothetical fire. This is because, being metallic, it would lose its resistance suddenly as the temperature of the metal rises.
Materials of the example
Profiled sheet: Steel S280 GD according to standard EN 10326 and equivalent. For Fe 360 type mechanical performance according to UNI-CNR 10022 standard, the total tension of the profile must not exceed 180 N/mm².
Concrete: The use of concrete of the fck class 250 N/cm² with a tension of 85 N/cm² is foreseen as indicated in DM 14/2/92.
Negative reinforcement: We recommend the use of Fe B 38K type construction rods, with an admissible tension of 215 N/mm².
Static characteristics of the profiled sheet: The moments of inertia and the resistance modules are calculated considering the reduction of the compressed elements both due to the effect of the positive and negative moments according to the UNI CNR 10022 standard.
Characteristics of the slabs: The calculations of the sections are referred to the theory of reinforced concrete where a relationship between the modulus of elasticity of steel and concrete is considered in n=15.
History of composite plate slabs
The first patent for this type of slab emerged in the United States in 1950. Initially, the connection between the concrete and the steel was made by means of galvanized steel plates welded to the ridge of the sheet. Later they were replaced by a welded mesh, also in the upper part of the plate.
Subsequent research developed new ways to guarantee a perfect combination between both materials by introducing ribs on the side faces.
In Europe, the introduction of collaborating sheet metal slabs dates back to the end of the 1960s. The investigations to solve the main problem of the collaborative floor, the shear stress, continued to develop and around the mid-eighties the American Society of Civil Engineers published “Specifications for the Design and Construction of Composite Slabs”. In turn, the British Standard BS-5950-82 is published. Both have been and continue to be a reference for this type of system.
The materials of a composite plate floor are arranged in such a way that when the floor is put into service the steel plate resists the tensile stresses. While the concrete resists compressive stresses, so both materials work optimally and jointly.
It is usual to insert a corrugated steel mesh into the concrete in order to prevent cracking, due to the shrinkage it experiences during the setting process.
During the assembly process, the metal sheet must be capable of supporting the weight of the fresh concrete by itself while it sets. When this is not possible, the slabs must be supported while the concrete sets. Once the concrete has set, both materials are united in such a way that they behave in solidarity.
From the point of view of its structural operation, a collaborating sheet slab is a unidirectional slab, since the ribs of the sheet are parallel to one direction and therefore must rest on the edges perpendicular to them. Given the arrangement of the materials, this system works best when it is arranged through isolated spans, that is, they do not have continuity beyond their supports and therefore form an isostatic structure. When several consecutive spans are arranged and it is desired that they work jointly, tensile stresses appear on their upper face. Which means that reinforcements must be added to the compression reinforcement.
Composite sheet slabs are mainly used in combination with metal structures. However, they can be used in concrete, wood or masonry structures as long as the fixing and overlapping conditions are met. In addition, this system allows connecting metal beams to the concrete of the floor.
The packages of metal profiles will be distributed according to the layout of the work and will be made to coincide on the main beams of the structure. The ribs of the profiles will always go on the support beams.
The fixing of the metal profiles on the metal beams can be done directly using both screws and nails. When the beams are made of concrete or of a nature other than metal, it is advisable to anchor metal plates on said beams. All metal profiles must be left on all supports with a minimum density of two fixings per support and profile. The profiles will be fixed as they are installed on the support beams.
Before proceeding with concreting, the profiles must be clean of dirt, grease, etc. To do this, if necessary, proceed to wash and degrease them.
The pouring of concrete is easier if it is done by pumping, being the fastest solution in execution. Said discharge will be carried out on the areas of support beams, avoiding the concentration of concrete and reducing the height of the discharge from the pipe to the slab to no more than 40 centimeters.
Composite floors or composite slabs that are supported by steel beams and concrete must have a minimum support of 75 mm. With a minimum external support of 50 mm for metal profiles.
For composite floors or composite slabs supported by beams of other materials, these values must be increased to a minimum of 100mm and 70mm respectively.
For overlapping plates and continuous plates that are supported by steel and concrete, the minimum support must be 75 mm. And for other materials of 100 mm.
The calculation of HI-BOND collaborating slabs is based on the following regulations:
MD of 9/1/96: Technical standards for the calculation, execution and testing of reinforced concrete works, prestressed and metal structures.
UNI-CNR 10022/84. Cold-formed profiles: Instructions for their use in construction.
EN 10326 Steel sheets and strips for structural uses, continuously hot-dip galvanized.
CEN European Committee for Standardization, Eurocode 4; Design of composite steel and concrete structures.
Aippeg instructions for the calculation of slabs with corrugated profile and collaborating concrete.