In the case of ceilings in steel compositeconstruction, the advantages of steel and reinforced concrete are optimally exploited. Both materials are connected to each other in a force-fit manner so that they act statically as one component. By exploiting the good tensile strength of the steel in the (lower) tensile zone and the compressive strength of the concrete in the (upper) compression zone, it is possible to keep beam heights and slab thicknesses low even under high loads, while enabling large spans.
Composite beam slabs and composite slabs are used in building construction primarily in industrial and commercial buildings, often in conjunction with an overall steel structure. Composite ceilings in particular also play a role in the renovation of old buildings.
Sketch: Composite beams combined with composite sheet and cast-in-place concrete - bauwion
This article focuses in particular on the planning of composite structures in the form of composite beams and composite slabs. All reinforced concrete slabs commonly used in multi-storey construction can also be used as pressure belts for composite girders. In practice, steel composite beams are not only combined with composite slabs, but also with precast slabs, precast reinforced concrete slabs or in-situ concrete slabs.
of slabs in steel composite construction:
Steel composite construction offers the possibility of large spans with low construction heights and low material consumption. It is therefore a particularly economical form of construction in multi-storey construction, especially in commercial and industrial construction.
Compared to slabs in pure reinforced concrete construction, slabs in composite construction have a lower dead weight and slimmer cross-sections with the same load-bearing capacity. Larger spans are possible. Due to the high degree of prefabrication, slabs in steel composite construction can also achieve shorter construction times, depending on the respective type. This advantage exists in particular when composite beams with prefabricated slab elements are compared to a conventional in-situ concrete slab .
Compared to pure steel structures, ceilings in steel composite construction have significantly improved sound insulation and a higher heat storage capacity due to the high proportion of concrete. However, the advantages and possibilities in the field of fire protection are unmistakable, as concrete is classified as a non-combustible building material.
Structural design:
Structural safety calculations of composite components are carried out by the structural engineer in accordance with Eurocode 4, in particular in accordance with DIN EN 1994-1-1. If it is a composite slab that has a prefabricated slab as a component, the construction rules of this type of slab must also be taken into account. [...]
Fire protection:
In composite construction , the respective advantages of the building materials steel and reinforced concrete are optimally exploited, and this applies in particular to the fire behaviour. The basic fire protection design rules for this are contained in DIN EN 1994-1-2.
The material steel heats up quickly in the event of a fire due to its good thermal conductivity and loses strength in the process. However, if the steel is protected by concrete in the composite component, its rapid heating is significantly reduced in the event of a fire.
In addition, in composite construction , both components, steel and reinforced concrete, jointly ensure load transfer, so that if the steel fails, the reinforced concrete takes over the load shares of the steel. For example, additional reinforcement in the chambered concrete of a composite beam alone can increase the fire resistance class according to DIN 4102-4 with the same slab thickness. In this way, ceilings in steel composite construction can achieve a required fire resistance duration even without additional external measures.
Steel composite components with visible steel parts (e.g. composite sheets in composite ceilings or visible steel beams) can in principle be protected against excessive heating in the event of a fire in various ways:
Sound insulation:
The protection goals in terms of sound insulation must be defined in advance between the planner and the building owner, as several sets of regulations exist side by side. Especially in buildings with several units, high requirements apply according to the state of the art, which in particular concern the sound transmission between the units, i.e. also the transmission via false ceilings. DIN 4109 regulates the absolute minimum standard, which is considered outdated today. Planners should use the increased values according to DIN 4109 Supplement 2 or VDI Guideline 4100 as the absolute minimum standard. The Dega Recommendation 103 of the German Society for Acoustics can also be helpful in this respect.
The protection against airborne noise increases with an increase in the area-related mass of the ceiling tile, which is determined by the thickness and bulk density of a component. In principle, concrete as a heavy building material provides very good conditions for the containment of airborne noise. For example, ceilings in composite construction with a high proportion of concrete have advantages in terms of their sound insulation effect.
An improvement in impact sound insulation is hardly achieved by increasing the area-related mass. A double-shell construction is far more effective in this respect. Floating screed is particularly effective as a second shell. It is sound-technically decoupled from the ceiling and wall construction by impact sound insulation and edge insulation strips [...]
Thermal insulation:
Concrete, like steel, has very good thermal conductivity, so that both building materials have almost no thermal insulation effect.
Thermal insulation can be dispensed with in well-ventilated rooms above and below the ceiling (e.g. open underground car park) and in ceilings that only separate heated rooms from each other.
Ceilings in composite construction that separate heated interiors from outdoor spaces (e.g. passageways, flat roofs) must be insulated in accordance with the Energy Saving Ordinance (EnEV). Ceilings against rooms that are included in the calculation as unheated or are not within the system boundary of the heated building volume (e.g. unheated basement or attic) must also be insulated, otherwise condensation cannot be ruled out.
Corrosion protection:
In order to achieve effective corrosion protection of the exposed steel components, it is advisable to first determine the corrosivity category and the expected protection period.
When it comes toprotective measures against corrosion in steel components, a basic distinction is made between
Concrete selection:
The compressive strength class (e.g. C25/30) and the consistency class (e.g. F3) must be defined in the planning, as well as the exposure class (e.g. XF 2), especially if a component comes into contact with the outside air (e.g. as a ceiling soffit in an open parking garage). These specifications must be made by the structural engineer in consultation with the client and the architect.
Composite sheets:
Sheets suitable for the construction of composite ceilings differ from conventional sheets made for roof or ceiling structures without a composite effect .
In constructions without a composite effect , a sheet under a reinforced concrete layer either acts exclusively as lost formwork, so that after the concrete has hardened, the reinforced concrete layer takes over all static forces. Or the concrete layer above the trapezoidal sheet metal only serves as an additional load on the sheet, e.g. to improve storage capacity and sound insulation. In this case, the sheet metal alone takes over the forces in the appropriate dimensioning and shape.
However, in order to create a two-dimensional composite effect between the steel sheet and the reinforced concrete above, a displacement of the two layers among each other must be ruled out. The layers must interlock and interlock in order to act statically as a single component.
In order to create the surface bond with a reinforced concrete layer, the composite sheets are specially shaped in their geometry, in most cases in the form of trapezoidal sheets with undercut geometry, in order to achieve the greatest possible friction bond. In addition, grooves, knobs or the like are embossed or rivets are applied to the sheet metal surface for the mechanical bond.
In addition, in most cases, some form of end anchoring is necessary. These are usually welded-on head bolt anchors. In the case of sheet metal with undercut geometry, the end anchorage can also be realized in the form of sheet metal deformation at the end of the profiled sheet. For composite sheets, the minimum thickness of 0.7 mm applies in Germany. At the customer's request, they are also cut to size in the factory to deviate from standard dimensions. To thread head bolts , it is possible to have holes punched in the sheet metal (especially in the case of multi-field action).
Recesses, slab openings:
In order to realize recesses in composite slabs, the structural engineer calculates the necessary reinforcement around the area to be recessed, depending on the situation. Usually, the recesses are cut into the sheet metal on site before concreting and then edged with special edge profiles by the respective sheet metal manufacturer.
Suspended ceilings and installations:
In combined systems, suspended ceilings are fixed via anchors in the reinforced concrete ceiling layer. It is recommended to involve the structural engineer in the planning of the suspended ceiling at an early stage in order to avoid damaging reinforcing steel. In the case of prefabricated slabs consisting of a prestressed concrete structure, particular caution is advised, as prestressing steels must not be cut or injured by drilling under any circumstances. The position of the fastening anchors must be coordinated with the structural engineer or the manufacturing plant.
For composite ceilings, the manufacturers of trapezoidal sheet metal offer their own options for anchoring suspended ceilings and installations. In the case of undercut sheets, whose folds taper downwards, suspended ceilings and pipes can be hung without drilling.
Laying plumbing in the reinforced concrete layer of a composite ceiling is not common.
Source: bauwion
