DEFLECTIONS AT CONSTRUCTION STAGE
Metal decking is designed to deflect under the weight of wet concrete as it is placed, in accordance with BS5950 Parts 4 & 6 or Eurocodes 3 and 4. The decking is designed for the slab thickness specified, based on constant thickness. No allowance is made for any additional loading due to excessive concrete thickness as a consequence of deflection of the structural steel frame during construction. This must be considered by the designer when specifying the slab surface tolerance required, to avoid experiencing deflections far greater than that designed.
For best practice guidance for concreting refer (The Concrete Society – Good Concrete Guide 5: Composite concrete slabs on steel decking) should be followed to avoid excessive deflection.
During the Composite, or Normal Stage, the composite slab is checked for super-imposed Dead (Permanent) and Imposed Live (Variable) loads as specified by the client / engineer. Composite slabs are usually designed as simply supported and in accordance with BS5950 Part 4 or Eurocode 4.
Concentrated loads (i.e. line loads from walls) should be checked separately to ensure the specified slab criteria is adequate for the required loadings. Specific checks for concentrated loads can be carried out using SMD Elements design software.
Consideration should be given to any loadings that may be applied to the slab during construction (i.e. from plant or material storage), as these may be more onerous than the design loadings for the intended building use.
The recommended minimum natural frequency for a composite floor plate (consisting of both the composite slab and composite beams) is 5Hz when used in office or domestic type applications. This limit should be increased to 8.4Hz for floors subjected to rhythmic activities such as gyms, dance studios or even plant areas supporting machinery. Using SMD Elements® software, the dynamic deflection of the composite slab is calculated in accordance with SCI Publication P-354: Design of Floors for Vibration – A New Approach. Using the guidance and calculation method contained in P-354, this deflection can then be added to that for the composite beams enabling the Natural Frequency of the floor plate to be determined. Refer SCI Publications P076: Design guide on the vibration of floors and P354: Design of floors for vibration – A New Approach
The fire design of a composite slab uses either the Simplified Method to BS5950, Eurocode NCCI Method or Fire Engineering Approach (Refer SCI Publication P056 or Eurocode NCCI PN005-GB for further information). The Simplified and Eurocode NCCI Methods are based on mesh only, with the mesh continuous over at least one internal support. This will usually give the most economic design.
The Fire Engineering method uses additional reinforcement in the troughs of the decking to achieve the required fire rating. The SMD Elements Design Software enables the user to check designs using any of these methods to suit the design standard adopted, BS5950 or Eurocodes.
The recommended top cover to the mesh reinforcement is a minimum of 15mm and a maximum of 45mm. Generally, minimum laps should be 300mm for A142 and 400mm for A193, A252 and A393. The mesh must satisfy the elongation requirements of BS4449, for more specific guidance refer to SCI Publication P300 – Composite Slabs and Beams using Steel Decking: Best Practice for Design and Construction.
In addition to the requirements of BS5950 Part 4 or Eurocode 4 with regard to structural behaviour under normal design loads, the slab must also meet the minimum insulation requirement specified in BS5950 Part 8, Eurocode 4 or Eurocode NCCI PN005-GB.
The composite slab and mesh reinforcement (not necessarily the metal deck) should be continuous over one or more internal supports. Continuity is taken to include all end bay conditions. The fire resistance of a single span in isolation is taken as 30 minutes, unless otherwise demonstrated by fire engineering or by a fire test.
Composite Beam Design
Composite beams with steel decking should be designed in accordance with BS5950 Part 3: Section 3.1 or Eurocode 4. Thru’ deck welded shear studs are commonly used to transfer horizontal shear forces between the steel beam and concrete slab, as required in the relevant design standard. These studs are welded to the supporting beams through the troughs in the decking, therefore it is essential that the decking and beam geometries are considered by the structural engineer when specifying stud quantities on beams running perpendicular to the decking span.
The resistance of shear studs in solid concrete is outlined in BS5950 Part 3: Section 3.1 and Eurocode 4. When used in composite decked slabs, these stud resistances may need to be reduced due to the decking geometry and/or orientation. For calculation of shear stud reduction factors, refer to BS5950 Part 3: Section 3.1 or Eurocode 4.
Transverse reinforcement is required in the concrete flange of composite beams to resist splitting forces. This will usually be in the form of mesh and/or additional bars running perpendicular to the beam centre line. In locations where the decking spans perpendicular to the beam centre line, the deck can also be considered, providing it is either continuous across the beam flange or securely anchored to the beam flange with thru’ deck welded studs at butt joints. Where perimeter beams are designed as composite, additional ‘U’ bars may be required depending on edge dimensions, refer SCI Publication P300: Composite slabs and beams using steel decking: Best practice for design and construction.
Lateral Restraint and Diaphragm Action
Metal deck may also be used as lateral restraint to stabilise the beams against lateral torsional buckling during construction (where the deck spans perpendicular to the beam) and, through diaphragm action, to stabilise the building as a whole by transferring wind loads back to the walls or columns (where designed by the structural engineer).
Deck is typically fixed to the beam flange using either powder (‘shot-fired’) or gas-actuated nails. Where fixings are required to resist lateral forces in accordance with BS EN 1993 or BS5950-1, the more robust Hilti X-ENP19 shot-fired nail (or similar approved) is recommended.
The safe working shear resistances (per nail) are indicated in the tables below.
Note: In some instances the value differs depending on the decking gauge used.
Safe Working Shear Resistances
Figures for each profile are based on maximum fixing spacing over intermediate beams as mentioned in TGN Section 4.6 – Fixings.