FLOORS AND SCREEDS

(Photo: Mike Amodeo (Contractors) Ltd.)
Floor slabs, lasers and levels
buildings. The Government also outlawed
Modern buildings may have main structural frames of precast or in-situ standard fee scales and promoted fee
concrete, or steel but they almost always have concrete floors. These may competition.
As a consequence, architects and
be cast in-situ on temporary formwork, or concrete topping cast on precast
engineers are often appointed on ‘design
concrete planks, or concrete cast on profiled steel sheeting. They are so only’ contracts where they do not inspect
familiar that it is tempting to take their design and specification for granted. the contractor’s work and only visit site
However, the past 20–30 years have seen revolutions in procurement, when requested. Structural engineers
contracts and construction methods which have combined to create a therefore have fewer opportunities to
situation where standard design assumptions and specifications are no longer visit site and see how their designs are
appropriate and could even be dangerous. Alasdair N Beal of Thomasons constructed.
Contractors have also changed: few now
reports.
carry out site work themselves. Instead
they commonly divide projects into
‘work packages’ and let these to specialist
n the 1970s and early 1980s, when contractor had to make allowances in levels subcontractors, restricting their own

I most current codes of practice were
being written, construction projects
were organised differently from today.
for formwork movement or structural
deflection. Temporary tamping rails were
set up on the soffit shuttering to level the
role to planning and co-ordination. Some
take care to discuss and co-ordinate the
subcontractors’ work carefully but others
Contracts were usually consultant-led – a concrete. Most floors had a tamped surface, simply split the project into packages and
client employed an architect and structural with a screed finish. Power finishing was leave the subcontractors to sort things out
engineer to design the building, obtain generally only applied to ground-bearing on-site.
tenders, appoint the contractor and inspect warehouse slabs. Floors cast on metal
the work on-site, with fees from standard decking were tamped level between screed Performance specifications
RIBA and ACE scales. Reinforced concrete rails set up on steel beam lines. Project specifications have also changed:
was built by the main contractor’s own clients can no longer rely on architect’s
labour, or by subcontract joiners, steelfixers Design-and-build contracts and engineer’s drawings to define their
and concretors. Since the early 1980s, contractor-led design- requirements, so they now rely on
The architect’s and engineer’s drawings and-build contracts have been heavily performance specifications. This has
specified finished structure levels and the promoted and are now used for most new affected floor levels and tolerances; instead

www.concrete.org.uk SEPTEMBER 2011 concrete 15

If even structural collapse. they are either laid to constant soffit formwork. More recent guides such as Concrete Society so movement during concreting or after Good Concrete Guide 5(4) and SCI Advisory striking formwork is not a problem. Blinding level variations weight caused by steel beam deflection is do not matter as long as the required not mentioned. these are now often the new specification is supposed to be decided by clients or project managers using checked. so there is more to ±10mm. particularly supported on steel beams that sag under the Changing levels on drawings from ‘finished if beam deflections are high. it In Eurocodes. the slab top surface will levels’ to ‘pre-strike levels’ certainly makes method gives a level surface but can lead to also sag. thickness must be right: if it is too thin. However. allowing method. client specifications quote levels unaware of it and in any case they often striking formwork. (Photo: Propex Concrete Systems.2 reduces the allowance formwork or on metal decking and steel steel frame also has a level tolerance of for construction loads from 1.uk . Second. lead to endless arguments about whether discrepancies were caused by construction Laser levels and construction errors or post-strike deflection.5kN/m² to beams that deflect. which then causes further deflection. the 1991-1-6 Table 4. instead strike levels. a vicious circle Instead. and adjust levels so that the finished slab is calculated from BS 5950-4. floor slabs are rarely surface relative to datum. two further factors come the design. To cut cost and weight. the weight of the concrete rigidity and the concrete age when it is defines standard options for constructing 16 concrete SEPTEMBER 2011 www. For following trades such concrete contractors will all say that this Modern specifications are also changing as bricklayers or ceiling fixers the pre-strike is a design issue. consider than just levelling the concrete thicker: 10mm of extra concrete increases In practice.75kN/m²(7). done. As a result. the extra weight could extra concrete to make up the surface the weight of concrete is not mentioned overload the supporting structure.2(6) and BS EN upper floors are either cast on temporary requiring yet more concrete. NSCS Version 3(1) says that to the levels after formwork is removed. To avoid Laser levels have revolutionised warehouse this. First. he simply lays concrete uses laser levelling but the potential extra make the slab too thin and if it is low. The fragmented nature of modern levels. engineers and tolerances are not as important but the slab contractors to work out their own solutions. requiring the possibility of beam deflection increasing and if it is too thick. the to the levels on the drawings and his job is dead load has not been allowed for in the slab will be too thick. the problem is even worse: loses strength.24kN/m². As it is finished floors. the slab will be 0. design. FLOORS AND SCREEDS of the project architect and engineer struck – and the contractor controls these. design and construction makes it a difficult in addition to bringing obvious scope for problem for any one party to solve. the client would have to check pre- floor construction since the 1980s. it should remembered that if the industry fails to find a solution. for deflection of the structure. stating that all levels on drawings should now specify ‘pre-strike’ tolerances and the contractor’s construction drawings should be considered as pre-strike rather than ‘finished’ slab levels. not mentioned in BS 5950-4. However. Desk Note AD344(5) do draw attention to the Upper floors are different. curing protection is removed from the large areas are cast in a single pour. The former method gives surface will also be low and vice versa. Once the technique is is struck. Discussion – slabs on metal decking confusion. tamping rails. so it has to do this without delaying construction. levelled hardened concrete but before the formwork by laser level. stiffness and fire resistance makes decking and beams deflect. the slab try to level the slab leads to overstressing or relative to datum. life easier for the concrete subcontractor: large thickness variations and a dead load A laser-levelled floor is different: its top instead of having to estimate precambers much higher than the theoretical value surface level is set relative to the datum. good control of dead load and structural the soffit is precambered. If these factors were not by BS 5950-4 (tamping from screed rails). Tight level issue but leave it to clients.) specifying these to suit the design and It is not clear how compliance with construction method. so if beams are low. now spread to upper floors. If the levels on drawings are generic specification clauses. the top surface thickness but produces greater surface will follow the same profile and if the slab is Discussion – in-situ concrete slabs undulations than past practice. many are still levels on drawings are finished levels. but for screeded floors and tolerances for the completed structure. this will at the correct level. On a busy site it may be impossible mastered it is faster and cheaper. there For laser-levelled slabs laid on steel beams deflection after casting is not just affected by is a risk that a major accident may occur. presumably because laser- minimum concrete thickness is achieved levelling was not in use when it was written. these can be checked any specifications often require upper floors time after completion but how can pre- in an office to be constructed to the same strike concrete levels be checked? Checking tolerances as a warehouse floor (eg. have no say in two of the most important they are pre-strike levels. and metal decking. However. the weight on the metal decking and beams constructed using the method assumed When the concrete is levelled using by 0. the change creates problems Clients.3. surface after the formwork has been struck would ±15mm relative to datum). which would have to be after of long strips tamped between road forms. these are also often power-finished Composite floors on metal decking instead of screeded.concrete. so the slab has a constant can develop where adding extra concrete to thickness or else the surface is laser-levelled thickness: if the formwork is low. it is also depends on formwork An industry-wide solution is needed that into play. If the contractor Therefore if the shuttering is high. in BS EN 1994-1-1 Cl.org. However. finished levels. the consequences may be disastrous. engineers how floor levels are specified: for directly levels are irrelevant – they have to work cannot solve the problem either. 9. Also level. after Similarly. sheeting manufacturers and In-situ concrete floor specifications for everyone else. these are measured off the allowed for in the design. NSCS Version 4(2) It could be argued that engineers’ issues – the client’s specification of level takes this further. and the concrete is laid on solid ground. The latter weight of concrete. BS 5950-4(3) draws attention to the extra Laser levelling is ideal for constructing concrete dead load caused by decking a ground-bearing warehouse floors with deflection but the possibility of extra a flat surface.

Advice Sheets. a screed may be applied to concrete ponding caused by beam the required levels (eg. 7. Levelling techniques for composite (a) Construction method: check steel following trades. mechanised plant should not be used to lay the concrete unless the contractor has checked that the decking and supporting structure can safely support its weight. FLOORS AND SCREEDS these floors. Structural use of steelwork in building. suitable where absolute level is not critical. (iii) limit total cumulative dead load dipping to check concrete thickness instead (a) Construction method: lay concrete to deflection of primary and secondary of laser levelling.com www. Camberley. Good Concrete Guide surface but close tolerances are difficult to to make levels on architects’ and engineers’ 5 – Composite concrete slabs on steel decking. reduce deflection. National Structural Concrete (i) If screed is to be applied. produce a level floor surface within close deflection. It results in a heavier. calculated practice and construction safety. it is incompatible with floors. Proposed specifications and design guidance Two standard options are proposed for composite concrete slabs cast on metal decking. the change in NSCS Version 4 4. compatible with one another. contractor must agree on a specification. with the contractor constant thickness along supporting beams allowance for additional concrete being responsible for making the necessary and is level between them. London. inappropriate and unenforceable. Part 4 – Option 2: ‘Constant level’ Code of practice for design of composite slabs with profiled steel sheeting. April 2010. CONCRETE SOCIETY. Actions on structures. BS EN 1994- 1-1. The most practical solution Publications. reconsidered. Where Specification. most buildings and should be adopted References (c) Design guidance where possible. Technical Reports and more: Visit: www. 1994. engineer and Society.4. Camberley. STEEL CONSTRUCTION INSTITUTE. Rob (b) Specification the weight of concrete on the sheeting and Smith (Cidon Construction) and Dan Williams (i) Level of top surface at column structure.uk SEPTEMBER 2011 concrete 17 . to maintain correct thickness around generally specify level tolerances for the 6. BSI. from screed rails or by ‘dipping’ to check Acknowledgements: concrete thickness. a reasonable compromise. on longer spans). (b) Slab thickness around columns This clause of NSCS Version 4 should be London. amended 2008. Third Edition. (b) clarify issues for engineers and promote consistent design approaches and (c) ensure that contractors’ construction methods are compatible with the design and specification. finished structure) and it makes checking concrete structures. adjust target slab level standard project specifications (which Vol. (ii) Slab thickness on beam lines These standard options are suitable for ±10mm. New Steel Construction. Advisory Desk economical structure than Option 1. 2010. Each specifies a construction method. The Concrete dead load should allow for increased they are not used. National Structural Concrete Specification. 2. either by tamping beams to a maximum of 25mm. Option 1: ‘Constant thickness’ (c) Design guidance would be to revert to the principle that specification (i) Floor self-weight quoted in levels on drawings are required levels of This produces a floor slab which has manufacturers’ literature must include the finished structure. most economical structure. beam levels. in the interests of consistent 1. additional 0.org.concretebookshop. It is (ii) design decking and structure for that these are achieved. BRITISH STANDARDS INSTITUTION. The Concrete (ii) Steel beams may be precambered to design assumptions and construction Centre. No. the client. specification In-situ concrete floors withdrawn.6kN/m² dead load to cover Where precambering is needed to achieve Alternatively. 1-1. BRITISH STANDARDS INSTITUTION. each with compatible standard (Photo: SMD Ltd.) specification clauses and design guidance. is ill-conceived: it causes problems for Note AD344. BSI. achieve. In both cases. Part 1 – General rules and (b) Specification clauses compliance with the specification rules for buildings. Part 6 – Actions during execution. pp. Design of composite steel and columns.36–38. specification and design guidance that are compatible and suitable for use together. 5.18. General actions. ● constant thickness. mid-span screed thickness. Defining industry-standard options would (a) encourage a more rational approach to specifications by clients and their advisers. London. less drawings ‘pre-strike’ rather than ‘finished’ Concrete Society. amended 2009. 2004. A higher deflection limit would increase Thanks are due to Barry Watts (Thomasons). BS EN 1991- ±15mm. The limit of 25mm is proposed as (SMD) for their assistance. This gives the thickness caused by sheeting deflection. (i) Level of slab top surface (all points) impossible. method and ensure that these are 3. It is therefore impractical. it may be necessary to level the surface or tolerances. BRITISH STANDARDS INSTITUTION. 2008. Camberley. Produces a floor slab with a level top As discussed. BSI.concrete. Fourth Edition. ±10mm. laser level concrete surface. Eurocode 1. positions ±15mm. Eurocode 4. allowances during construction to ensure lightest. CONSTRUCT. CONSTRUCT. BS 5950.

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