Concrete September 2011
Floor slabs, lasers and levels
Alasdair N Beal, Thomasons
Modern buildings may have main structural frames of precast or in-
However, the past 20-
In the 1970s and early 1980s, when most current codes of practice were being written, construction projects were organised differently from today. Contracts were usually consultant-
The architect’s and engineer’s drawings specified finished structure levels and the contractor had to make allowances in levels for formwork movement or structural deflection. Temporary tamping rails were set up on the soffit shuttering to level the concrete. Most floors had a tamped surface, with a screed finish. Power finishing was generally only applied to ground-
Since the early 1980s, contractor-
As a consequence, architects and engineers are often appointed on ‘design only’ contracts where they do not inspect the contractor’s work and only visit site when requested. Structural engineers therefore have fewer opportunities to visit site and see how their designs are constructed.
Contractors have also changed: few now carry out site work themselves. Instead they commonly divide projects into ‘work packages’ and let these to specialist subcontractors, restricting their own role to planning and co-
Project specifications have also changed: clients can no longer rely on architect’s and engineer’s drawings to define their requirements, so they now rely on performance specifications. This has affected floor levels and tolerances; instead of the project architect and engineer specifying these to suit the design and construction method, these are now often decided by clients or project managers using generic specification clauses. As a result, specifications often require upper floors in an office to be constructed to the same tolerances as a warehouse floor (e.g. surface ±15mm relative to datum).
Laser levels and construction
Laser levels have revolutionised warehouse floor construction since the 1980s; instead of long strips tamped between road forms, large areas are cast in a single pour, levelled by laser level. Once the technique is mastered it is faster and cheaper, so it has now spread to upper floors. To cut cost and weight, these are also often power-
Laser levelling is ideal for constructing a ground-
Upper floors are different. Tight level tolerances are not as important but the slab thickness must be right: if it is too thin, it loses strength, stiffness and fire resistance and if it is too thick, the extra weight could overload the supporting structure. Also upper floors are either cast on temporary formwork or on metal decking and steel beams that deflect, so there is more to consider than just levelling the concrete surface relative to datum.
When the concrete is levelled using tamping rails, these are measured off the soffit formwork, so the slab has a constant thickness: if the formwork is low, the slab surface will also be low and vice versa. If the soffit is precambered, the top surface will follow the same profile and if the slab is supported on steel beams that sag under the weight of concrete, the slab top surface will also sag.
Modern specifications are also changing how floor levels are specified: for directly finished floors, NSCS Version 3  says that levels on drawings are finished levels, after striking formwork, but for screeded floors they are pre-
Changing levels on drawings from ‘finished levels’ to ‘pre-
However, the change creates problems for everyone else. For following trades such as bricklayers or ceiling fixers the pre-
It could be argued that engineers’ drawings should now specify ‘pre-
It is not clear how compliance with the new specification is supposed to be checked. If the levels on drawings are finished levels, these can be checked any time after completion but how can prestrike concrete levels be checked? Checking after the formwork has been struck would lead to endless arguments about whether discrepancies were caused by construction errors or post-
Composite floors on metal decking
In Eurocodes, the problem is even worse: the possibility of beam deflection increasing the weight of concrete is not mentioned in BS EN 1994-
In practice, floor slabs are rarely constructed using the method assumed by BS 5950-
Clients, sheeting manufacturers and concrete contractors will all say that this is a design issue. However, engineers cannot solve the problem either. As it is not mentioned in BS 5950-
Proposed specifications and design guidance
Two standard options are proposed for composite concrete slabs cast on metal decking. Each specifies a construction method, specification and design guidance that are compatible and suitable for use together. In both cases, 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.
Option 1: ‘Constant thickness’ specification
This produces a floor slab which has constant thickness along supporting beams and is level between them. This gives the lightest, most economical structure. It is suitable where absolute level is not critical. Alternatively, a screed may be applied to produce a level floor surface within close tolerances.
(a) Construction method: lay concrete to constant thickness, either by tamping from screed rails or by ‘dipping’ to check concrete thickness.
(i) Level of top surface at column positions ±15mm.
(ii) Slab thickness on beam lines ±10mm.
(c) Design guidance
(i) If screed is to be applied, calculated dead load should allow for increased mid-
(ii) Steel beams may be precambered to reduce deflection.
Option 2: ‘Constant level’ specification
Produces a floor slab with a level top surface but close tolerances are difficult to achieve. It results in a heavier, less economical structure than Option 1.
(a) Construction method: check steel beam levels; adjust target slab level to maintain correct thickness around columns; laser level concrete surface.
(b) Specification clauses
(i) Level of slab top surface (all points) ±15mm.
(b) Slab thickness around columns ±10mm.
(c) Design guidance
(i) Floor self-
(ii) design decking and structure for additional 0.6kN/m² dead load to cover concrete ponding caused by beam deflection.
(iii) limit total cumulative dead load deflection of primary and secondary beams to a maximum of 25mm.
A higher deflection limit would increase the weight of concrete on the sheeting and structure. The limit of 25mm is proposed as a reasonable compromise.
These standard options are suitable for most buildings and should be adopted where possible, in the interests of consistent practice and construction safety. Where they are not used, the client, engineer and contractor must agree on a specification, design assumptions and construction method and ensure that these are compatible with one another.
As discussed, the change in NSCS Version 4 to make levels on architects’ and engineers’ drawings ‘pre-
This clause of NSCS Version 4 should be reconsidered. The most practical solution would be to revert to the principle that levels on drawings are required levels of the finished structure, with the contractor being responsible for making the necessary allowances during construction to ensure that these are achieved.
Where precambering is needed to achieve the required levels (e.g. on longer spans), it may be necessary to level the surface or dipping to check concrete thickness instead of laser levelling.
Thanks are due to Barry Watts (Thomasons), Rob Smith (Cidon Construction) and Dan Williams (SMD) for their assistance.
1. CONSTRUCT. National Structural Concrete Specification. Third Edition, The Concrete Society, Camberley, 2004.
2. CONSTRUCT. National Structural Concrete Specification. Fourth Edition, The Concrete Centre, Camberley, 2010.
3. BRITISH STANDARDS INSTITUTION, BS 5950 Structural use of steelwork in building. Part 4 -
4. CONCRETE SOCIETY. Good Concrete Guide 5 -
5. STEEL CONSTRUCTION INSTITUTE, Advisory Desk Note AD344. Levelling techniques for composite floors. New Steel Construction, April 2010, Vol.18, No.4, pp. 36-
6. BRITISH STANDARDS INSTITUTION, BS EN 1994-
7. BRITISH STANDARDS INSTITUTION, BS EN 1991-