In the case of steel composite ceilings, 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.
This article deals in particular with 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.
Advantages 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 engineering: 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. See bauwion page ►110 | Reinforced concrete ceilings.
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 e.V. 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 acoustically decoupled from the ceiling and wall construction by impact sound insulation and edge insulation strips, see bauwion page ► 400 | Construction site screeds.
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 car park). 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 create 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.
Installation of composite sheets: The sheets are laid according to laying or execution plans, which show in particular the exact position, dimensions and clamping directions of the profiled sheets, including their profile designation. Information on recesses, connecting elements, expansion joints and the design of the supports, including the associated details, should also be included.
In the run-up to laying, the position of the individual sheet metal elements on the substructure is marked. The structural engineer specifies how, at what intervals and in what way the sheets are attached. The specifications may vary depending on the sheet geometry and situation.
In general, the plates are fixed to the substructure in the area of the corrugations in order to achieve a certain degree of tightness during concreting. Longitudinal joints are overlapped in the specified width and screwed or riveted together at certain intervals.
Open corrugations at transverse joints in the longitudinal direction are delimited and sealed with assembly foam, sealing tapes, angular profiles or with sealing profiles (profile fillers) adapted by the manufacturer to the respective sheet metal profile. Depending on the desired tightness, the longitudinal joints, supports or ceiling openings must also be sealed. Cleaning of the soffit of the ceiling after the concreting process may be necessary despite all sealing measures.
Composite sheets usually require assembly support for the concreting process. The structural engineer specifies the span up to which it can be dispensed with, and he also determines the position and spans of the assembly support.
Installation of reinforcement: When installing the reinforcement on site, care must be taken to ensure compliance with the required concrete covers. Otherwise, the reinforcement can corrode over the years and, in extreme cases, the structure can no longer meet its static requirements. Reinforcement for concrete layers, but possibly also for joints to be poured, must be carried out strictly according to the reinforcement plan. In the case of slab constructions of which a precast slab is a component, the reinforcement of the elements is already installed and monitored in the precast plant. However, reinforcement loops protruding from the precast elements have to be placed over the head bolts of the girders, which makes precise adjustment of the ceiling elements necessary. The same applies to the lower shell of a prefabricated ceiling.
Concrete paving: Concrete for ceilings delivered or produced on site must in principle be installed as quickly as possible. This must prevent cavities from forming in the component. This is prevented by shaking, stomping or poking. However, if this is done for too long, there is a risk of segregation. This is shown by the formation of an aqueous sludge layer on the surface. Concrete must always be placed in layers and should not be brought in from a drop height of more than two metres. It is influenced by external conditions during setting. In extreme climatic conditions such as heat (over 30°C) or frost (below -5°C), concrete may not be poured without suitable additional measures. In particular, there can be an increased risk of frost in the case of composite sheets after the concreting process, as the shielding formwork panels are missing on the underside.
Post-treatment of concrete: The drying process of concrete is called hydration. This leads to drying out and hardening of the concrete component. Concrete components must be treated by suitable measures during the setting period. Otherwise, the concrete will set unevenly quickly as a result of solar radiation or wind, so that cracks can occur. After 28 days, the concrete component is usually fully hardened and hydration is complete. Thorough and careful follow-up treatment is expressly required in DIN 1045-2. The following measures are available for the post-treatment of in-situ concrete components:
The type and duration of the post-treatment are regulated in DIN 1045-3. The Cement Leaflet B8, published by the Association of German Cement Works (see Standards and Literature), is also helpful in this context.
DIN 1045-2, Structures made of concrete, reinforced concrete and prestressed concrete - Part 2: Concrete - Specification, properties, production and conformity - Rules of application for DIN EN 206-1
DIN 4102-4, Fire behaviour of building materials and components - Part 4: Composition and application of classified building materials, components and special components
DIN 4109, sound insulation in building construction; Requirements and verifications
DIN 4109 Supplement 2, Sound insulation in building construction; Instructions for planning and execution; proposals for increased sound insulation; Recommendations for sound insulation in one's own living or working area
DIN EN 206, Concrete - Specification, properties, production and conformity
DIN EN 1994-1-1, Eurocode 4: Design and construction of composite structures made of steel and concrete - Part 1-1: General design rules and application rules for building construction
DIN EN 1994-1-1/ NA, National Annex – Nationally defined parameters - Eurocode 4: Design and construction of composite structures made of steel and concrete - Part 1-1: General design rules and application rules for building construction
DIN EN 1994-1-2, Eurocode 4: Design and construction of composite structures made of steel and concrete - Part 1-2: General rules - Structural design in the event of fire
DIN EN 1994-1-2/A1, Eurocode 4: Design and construction of composite structures of steel and concrete - Part 1-2: General rules - Structural design in the event of fire - German version EN 1994-1-2/A1
DIN EN 1994-1-2/ NA, National Annex - Nationally determined parameters - Eurocode 4: Design and construction of composite structures of steel and concrete - Part 1-2: General rules - Structural design in the event of fire
DIN EN 13501-1, Classification of construction products and construction methods with regard to their reaction to fire, Part 1: Classification with the results of the tests on the reaction to fire of construction products
DIN EN ISO 1461, Zinc coatings applied to steel by hot-dip galvanizing (piece galvanizing) - Requirements and tests
DIN EN ISO 12944-2, Corrosion protection of steel structures by coating systems - Part 2: Classification of ambient conditions
DIN EN ISO 12944-5, Coating materials - Corrosion protection of steel structures by coating systems - Part 5: Coating systems
►Bauforumstahl 2.7 Composite construction aid
►DBV leaflet "Concrete formwork and stripping deadlines"
►Cement leaflet B8, Technical information on the post-treatment of concrete components, Publisher: Verein Deutscher Zementwerke
Source: bauwion
