Benefits of Concrete Frames
Frame cost should not dictate the choice of
frame. Rather it should be just one of a number of issues that should
be considered when making the choice of frame material. Only then can
one be confident that the best and optimum structural solution has been
chosen. The following checklist should assist designers and cost
consultants to achieve the best value solution.
Frame costs
The recent rises in reinforcement and steel prices have increased frame
costs, but concrete frame costs remain significantly lower than those
of steel. Steel prices during recent times have left many in the
construction industry reviewing design solutions that have a heavy
reliance on this material.
Foundation costs
The foundations typically represent approximately 3% of the whole
project initial cost. For the heaviest reinforced concrete solution,
the foundations will be more expensive, but this represents only a
relatively small cost, and can be offset by using post-tensioned slabs
- typically 15% lighter.
Cladding costs
The thinner the overall structural and services zone, the less the
cladding costs. Given that cladding represents up to 25% of the
construction cost, it is worth minimising the cladding area. The
minimum floor-to-floor height is almost always achieved with a
concrete flat slab and separate services zone.
Partitions
Sealing and fire stopping at partition heads is simplest with flat
soffits. Significant savings of up to 10% of the partitions package can
be made compared to the equivalent dry lining package abutting a
profiled soffit with downstands. This can represent up to 4% of the
frame cost.
Air tightness
New Part L of the Building Regulations require pre-completion pressure
testing. Failing these tests means a time-consuming process of
inspecting joints and interfaces, and then resealing them where
necessary. Concrete edge details are simpler to seal and have less risk
of failure. Some contractors have switched to concrete frames on
this criterion alone.
Services
co-ordination/installation/adaptability
The soffit of a concrete flat slab provides a zone for services
distribution free of any downstand beams. This reduces co-ordination
effort for the design team, and therefore, the risk of errors. It
permits flexibility in design and allows co-ordination effort to be
focused elsewhere. Services installation is simplest below a flat
soffit. This permits maximum off site fabrication of services, higher
quality of work and quicker installation.
These advantages should be refelected in cost and value calculations.
Indeed, mechanical and electrical contractors quote an additional cost
of horizontal services distribution, below a profiled slab, of up to
15%.
Flat soffits also allow greater future adaptability. new layouts and
cellular arrangements plus different service requirements are
straightforward.
Fire protection
For concrete structures, fire protection is generally not needed as the
material has inherent fire resistance up to 4 hours. This removes the
time, cost and separate trade required for fire protection.
Acoustics
To meet the more stringent Part E of the Building Regulations,
additional finishings to walls and floors are often required. The
inherent mass of concrete means that these additional finishings are
minimised or even eliminated.
Vibration
The inherent mass of concrete means that concrete floors generally
meet vibration criteria at no extra cost, without any extra stiffening.
For more stringent criteria, such as for laboratories or hospital
operating theatres, the additional cost to meet vibration criteria is
small compared to other structural materials.
Exposed soffit
A concrete structure has a high thermal mass. By exposing the soffits
this can be utilised through fabric energy storage (FES) to reduce
initial plant costs and ongoing operational costs. Furthermore, the
cost of suspended ceilings can be reduced or eliminated.
Programme
Concrete frames have short lead-in times and with modern framework
systems, floor-to-floor construction periods are reduced. Most
CONSTRUCT
members quote 500m2/week/crane on reasonably large and
simple flat slab
projects and more where Hybrid Concrete Construction can be used.
For example, where precast columns are used in conjunction with
post-tensioning, one week cycle times are possible. However, more
important is whole project programme. Concrete provides a safe working
platform and semi-internal conditions, so that services installation
and
follow-on trades can commence early in the programme. Also, concrete
has the flexibility to accommodate design changes later in the process.
Net lettable area
The difference in net lettable area provided by different solutions for
a building can be of significant value. Whilst concrete structures may
have larger columns, finishing is not necessarily required and
typically columns below 0.25m2 are not deducted from net
lettable area.
Reduction in column size can be achieved by the use of high strength
concrete.
Concrete structures have reduced floor-to-floor heights, hence fewer
steps between floors and less plan area. Alongside these, RC shear
walls are narrower than braced steel frames. Therefore, the
stair/stability core area is minimised, freeing up more net lettable
area.
Whole life value
Concrete's range of inherent benefits - fabric energy storage, fire
resistance, sound insulation - means that concrete buildings tend to
have lower operating costs and lower maintenance requirements. This is
an important consideration for owner-occupiers.
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