Description This system consists of rolled steel beams supporting precast concrete units.
The precast units may be supported on the top flange of the steel beams, or, to reduce construction depth, supported on ‘shelf’ angles. Shelf angles are bolted or welded to the beam web, with an outstand leg long enough to provide adequate bearing of the precast units and to aid positioning of the units during erection. Precast concrete units are generally grouted in position. The units may have a screed (which may be structural), or may have a raised floor. The precast units are either hollow core, normally 150-260 mm deep, or they are solid planks of 75 mm to 100 mm depth.
Temporary lateral bracing is often required to limit the effective length for lateral torsional buckling of the beam during the construction stage when only one side is loaded.
In order to meet robustness requirements, mesh and a structural topping may be required, or reinforcement concreted into hollow cores and passed through holes in the steel beam web. Tying may also be required between the concrete units and the edge beams.
Typical beam
span range 6 m and 7.5 m grids are common for both beams and precast units.
Main design considerations for the floor layout
Construction stage loading (planks on one side only) must be considered.
Temporary bracing may be required.
Beams loaded on one side only in the permanent condition should either be avoided or designed for the applied torsion.
Central spine beams are common, with smaller edge beams. If edge beams carry significant cladding loads, or support inflexible cladding, deflection may be critical.
Advantages Fewer secondary beams, due to long-span precast units.
A simple solution involving basic member design.
Disadvantages The beams are subject to torsion and may need stabilising during the construction stage.
More individual lifting operations compared with the erection of decking, and the erection sequence requires access for installation of the concrete units.
Services
integration Main service ducts are located below the beams with larger equipment located between beams.
Governing design criteria for beams
Flange width or shelf angle width for bearing and erection access. A bearing of 75 mm is recommended (50 mm minimum). To allow for tolerances in the precast units and the erected steelwork, a gap of 30 mm between units is usually provided. When the top flange of a beam supports precast planks, the minimum flange width is therefore 178 mm.
Shelf angles should project at least 50 mm beyond the beam flange. When shelf angels are provided, 25 mm clearance is required between the end of the concrete unit and the beam flange, as shown in Figure 5.18.
The critical beam criteria are often torsional resistance and twist, or combined torsion and lateral torsional buckling resistance (LTB) in the construction condition (with loading on one side only).
(a) Units sitting on shelf angles (b) Units sitting on top of downstand beam Figure 5.16 Floor construction with precast concrete units in
non-composite construction
Figure 5.17 Precast concrete units on steelwork
75 min.
25 min.
30 nominal 180 min.
75 min.
50 50 min.
Figure 5.18 Bearing and clearance requirements for precast units Governing
design criteria for precast units
Bending resistance.
Shear resistance of precast units.
Typical section
sizes Beams:
When supporting precast planks on the top flange, the minimum rolled serial size is 406 × 178 UB.
Precast units: (approximate)
150 mm deep, 6 m span @ 2.5 kN/m2 200 mm deep, 7.5 m span @ 3.0 kN/m2 250 mm deep, 9 m span @ 5.0 kN/m2 Design approach 1. Try 6 m or 7.5 m grid.
2. Choose precast concrete planks from manufacturer’s data. Ensure these meet the required fire resistance.
3. Design the steel beams, using software, or by simple manual calculation of the bending moment and deflection, with member resistances taken from the P363[48]
4. Check the temporary construction condition, and consider temporary bracing as part of the erection method.
Grade of steel S275 or S355
Overall floor zone Approximately 800 mm including ceiling (7.5 m grid)
75 75 topping
535 beam
150 ceiling
800 mm (7.5 m grid) 75
50
75 50
200
Figure 5.19 Beam and precast concrete unit – typical cross sections Fire protection Spray, board or intumescent coating to beam.
For shelf angle beams, 30 or 60 minutes fire resistance may be possible without applied fire protection.
Connections Full depth end plate connections (welded to the beam flanges) are common, as the beams usually carry torsional loading in the construction condition.
For beam design; P364[49]
Design guidance
For precast units; manufacturer’s design tables.
Software Beam design – BDES software, available from Corus, can be used to design non-composite beams. BDES is available from www.corusconstruction.com