For the purposes of this Guide, long-span floor systems are generally spanning greater than six metres for reinforced concrete systems or eight metres for prestressed systems. Some systems are effective below these arbitrary limits and their full range is included herein for completeness.
The aim of this Guide is to provide designers with an appreciation of the factors that should be taken into
account in selecting a floor system for a particular building.
It is emphasised that the graphs are not design charts but aids to enable designers to quickly identify
appropriate floor systems to carry the applied loading for the desired span, and thus provide approximate
dimensions for the preliminary design.
A serviceable long-span floor is one that has sufficient strength to carry the permanent and imposed actions
as well having adequate stiffness to limit deflection and vibrations.
An economical long-span floor is one that optimises the material and labour costs. Minimum weight does
not necessarily result in the lowest cost. Structural designers should review the design progressively as it
proceeds to ensure the structural design is integrated with other aspects of the design of the building, eg
building services, and no significant errors have been made. The accuracy of structural design theories,
applied actions and material properties is such that there is no point in determining actions or member
sizes to an excessive degree of accuracy.
Formwork for insitu concrete and propping of precast is a major consideration as it can directly affect the
architectural appearance of the project and the construction cost and schedule. Formwork can be
50–75% of the direct cost of a floor. Generally, a specialist designer, employed by the contractor, will
be responsible for formwork design. It will take into account the requirements of the specification and the
construction process, eg construction loads, stripping, back propping, and specified finishes and formwork
To simplify formwork, member sizes should be rationalised consistent with structural economy, eg
band-beam widths of 1200, 1800 and 2400 mm will enable standard plywood sheets to be used. Similarly,
drop panels in flat slabs should be dimensioned to suit plywood and timber sizes.
Because of the high cost of site labour, making soffit formwork on site is often avoided. Formwork is usually
designed to have a number of reuses and to facilitate easy stripping and re-erection to keep the overall floor
construction period as short as possible. Sophisticated table and flying forms have been used on multistorey
projects, while, to minimise costs, precast floor units are sometimes preferred
Reinforcement details should be as simple as possible to facilitate fixing, reduce the risk of errors
and to simplify checking. Cogs and hooks should be limited to those that are essential. Mesh should be
used where possible, while consideration should be given to using stock length bars for larger projects,
and to using alternative bar sizes to distinguish column strips from middle strips.
Ribbed floors consisting of equally spaced ribs are usually supported directly by columns Figure 12.
They are either one-way spanning systems known as ribbed slab or a two-way ribbed system known as a
waffle slab. This form of construction is not very common because of the formwork costs and the low
fire rating. A 120-mm-thick slab with a minimum rib thickness of 125 mm for continuous ribs is required to
achieve a 2-hour fire rating. A rib thickness of greater than 125 mm is usually required to accommodate
tensile and shear reinforcement. Ribbed slabs are suitable for medium to heavy loads, can span
reasonable distances, are very stiff and particularly suitable where the soffit is exposed.
Slab depths typically vary from 75 to 125 mm and rib widths from 125 to 200 mm. Rib spacing of 600 to
1500 mm can be used. The overall depth of the floor typically varies from 300 to 600 mm with overall spans
of up to 15 m if reinforced, longer if post-tensioned. The use of ribs to the soffit of the slab reduces the
quantity of concrete and reinforcement and also the weight of the floor. The saving of materials will be
offset by the complication in formwork and placing of reinforcement. However, formwork complication is
minimised by use of standard, modular, reusable formwork, usually made from polypropylene or
fibreglass and with tapered sides to allow stripping. For ribs at 1200-mm centres (to suit standard forms)
the economical reinforced concrete floor span 'L' is approximately D x 15 for a single span and D x 22 for
a multi-span, where D is the overall floor depth.
Precasting offers the advantages of off-site
manufacture under factory conditions and fast
erection on site. When combined with prestressing,
additional benefits of long span and high load-capacity
can be obtained. A number of different systems are
■ Hollowcore planks, either with or without a
■ Composite flooring using precast permanent
■ Composite flooring using precast beam and infill
■ Solid slabs either reinforced or prestressed
■ Single and double T-beams.