Economic sense

Reinforced concrete can be up to 5% cheaper than steel frames for commercial buildings. This is the finding of an independent study commissioned by The Concrete Centre and carried out by architect Allies and Morrison, engineer Arup, QS Davis Langdon and consultant Mace. This advantage was compounded by a lead time of only four to six weeks for reinforced concrete compared with up to 18 weeks for steel.

The study compared the costs of constructing three- and six-storey commercial buildings using a variety of short-span and long-span reinforced concrete-framed and steel-framed options in two different locations, taking into account construction, category A fit-out and the effect of programme times on cost. The independence of the study was underlined by the structural design for all options being carried out by Arup.

Designs were commissioned for a three-storey 4,650m2 office building in an out-of-town business park situation in the South-east (building A) and a six-storey 16,500m2 office building located in central London (building B). The buildings were based on commonly used structural grids, using pad or piled foundations and with specifications suited to contemporary market conditions. Building A is an air-conditioned, L-shaped building with curtain walling and some natural ventilation, and building B is an air-conditioned rectangular building with central atrium and curtain walling.

The designs were taken to normal outline design stage, the only differences being directly attributable to the structural frame material. Budget costings were assigned to all elements of construction, from substructure, superstructure and external envelope through to preliminaries, with the exception of external works, which were considered to be too highly site-specific to permit accurate costing.

For building A, three concrete- and three steel-framed options were developed, all six of which were short-span situations (7.5m). For building B, three concrete- and three steel-framed options were developed for short-span situations (7.5m) and one concrete and one steel option for a long-span situation (15m), resulting in eight options.

As well as identifying the variation in the costs of frames, the study considers the effects that the choice of framing material and method of construction have on the rest of the building. The study found that, in terms of overall construction cost for building A, the most economic concrete-framed option, the steel- framed options were between 0.6% and 5.1% more expensive than flat slab, after adjusting time-related preliminaries for construction programme difference.

With regard to speed of construction, for building A the construction programmes for the concrete-framed options were between two and four weeks longer than for the steel-framed options, all of which have identical 48 week durations.

In terms of overall construction cost for building B, for the short-span situation the most economic concrete-framed option, the steel-framed options were found to be between 1.5% and 5.5% more expensive than the concrete flat slab after adjusting time-related preliminaries. For the long-span situation, there was no discernible difference between the costs of the concrete- and steel-framed solutions.

With regard to speed of construction, for the short-span options in building B, the construction programmes for concrete flat slab construction and steel-framed composite construction were identical at 67 weeks, with 66 weeks for a post-tensioned flat slab construction, 65 weeks for both steel-framed construction with precast hollowcore floors and Slimdek construction, and 70 weeks for a hybrid concrete construction.

The most significant differential for both buildings occurred using a Slimdek steel-framed option, for which the overall construction costs were found to be between 5.1% and 5.5% more expensive than a concrete flat slab solution, after adjusting time-related preliminaries. When only the costs of the frame are considered, the concrete flat slab option was found to be an average of 38% less expensive than the Slimdek option.

When procurement and lead times are taken into account as well as construction time (based on a 10-week procurement programme and contractor lead times of four to six weeks for concrete-framed construction and 12-18 weeks for steel-framed construction) the overall programme for the concrete options is shorter.

The main conclusion to be drawn from the study is that, for a range of structural options commonly used in the construction of modern offices, using concrete frames is more economic than using steel. Choosing concrete rather than steel produced savings of up to 5% in construction costs.

Differences in cost

Foundations for the concrete options cost more, but account for a relatively small proportion of the overall cost, the difference between the foundations for concrete and steel equating to less than 0.3% of the overall costs.

The main source of savings between concrete and steel lies in the superstructure, when the frame, cladding and internal planning are all taken into account. There are minimal differences in the finishes, other than those caused by variations in storey heights depending on the structural solution adopted.

Preliminaries are very similar, other than time-related aspects, although individual projects may have logistical difficulties, site constraints, access, adjacent buildings, etc that are particular to that project and will affect the preliminaries. Such aspects are intrinsically project specific and are therefore beyond the scope of the study.

There are no differences in the design or specification of the mechanical and electrical services as a result of the structural designs selected; however, those designs involving downstand beams incur a cost premium as a result of the added complexity of installing the services around such projections.

Foundations

Concrete construction is generally heavier than steel-framed construction, and this is reflected in the higher cost of foundations to the concrete-framed options. To some extent this cost premium can be offset by adopting post-tensioned slabs, which are typically some 15% lighter. In the case of building B, the foundations to the post-tensioned options are about 4% less expensive than those on the flat slab option.

Frame and upper floors

On a like-for-like comparison, the concrete frames and upper floors have been shown to be less expensive than their steel counterparts. The steel structures were between 1% and 54% more expensive than the concrete flat slab solution for building A and between 4% and 66% more expensive than the concrete flat slab solution for building B.

Cladding

The thinner the overall structural and services zone, the lower the cladding cost. Given that the cladding on the building in the study represents between 14% and 18% of the construction cost, minimising the cladding area represents considerable value to the client. Also of increasing importance is the potential benefit that a reduced cladding area has on the building's energy use. The minimum floor-to-floor height is almost always achieved with a flat slab and separate services zone, offering the potential for additional storeys in high-rise buildings and thus improved rental or sales return.

Internal planning

It should be noted that a premium is incurred on the steel-framed options in sealing and fire stopping at partition heads against the irregular soffits of the steel decking and around irregularly shaped intersecting frame members. Failure to consider this aspect can result in expensive and time-consuming remedial work late in the construction programme.

Mechanical and electrical services

General

Mechanical and electrical services represent a large proportion of the overall construction costs of the buildings, averaging 35% for building A and 33% for building B. The design team was briefed not to design the services in detail, nor to take into account any benefits associated with the concrete frame’s greater potential for fabric energy storage or reduced solar gain because of lower storey heights. Nonetheless, it should be noted that the removal of suspended ceilings in order to benefit from the thermal mass of the concrete would reduce the overall capital project costs for all concrete options by approximately 2% for building A and by approximately 3% for building B.

The most significant differential for both buildings occurred using a Slimdek steel-framed option, for which the overall construction costs were found to be between 5.1% and 5.5% more expensive than a concrete flat slab solution

Types of ventilation

Both buildings have been assumed as fully air-conditioned but, although natural ventilation can be used to eliminate air conditioning, it is not generally current practice for these depths at floor-plate and thus was not included in this study.

Flexibility

A flat soffit provides a clear zone for services distribution, free of any downstand beams. This reduces co-ordination effort for the design team and therefore the risk of errors, permits flexibility in design and allows coordination effort to be focused elsewhere. Services installation is simplest below a flat soffit, permitting maximum off-site fabrication of services, higher quality of work and quicker installation.

These advantages can typically produce cost savings on initial services installation costs but, more importantly, because they facilitate the

use of prefabricated services equipment packages, they can offer reduced installation programmes, together with cost-in-use benefits in the form of reduced maintenance downtime due to ease of equipment change-out, greater flexibility and less disruption to an occupier’s business operations.

Flat soffits also allow greater future adaptability for building refurbishment, new layouts and cellular arrangements; in addition, different service requirements are straightforward and more easily accommodated.

These benefits are the reasons for the development of Slimdek; however, this study shows the significant cost premium incurred with this option and shows how a reinforced concrete flat slab or post-tensioned flat slab are the cheapest ways of getting a clear, flat soffit.

Net lettable area

Differences in nett lettable area resulting from the different structural options adopted have not been considered in the study. However, it should be noted that there are two main areas in which such differences are found: stairs and core areas.

Typically, stairs are re-sized as a result of the reduced storey height module, producing slightly increased net lettable areas.

The area occupied by concrete cores tends to be slightly smaller than that needed for steel cores, due to the allowance for steel bracing zones and the structural concrete walls serving a dual function as partitions.

A previous study by the concrete industry found that, on an overall basis, the difference can be as much as 1.5% extra net lettable area.

Programmes – general conclusions

Concrete-frame options offer a significant advantage in lead times over steel-frame options.

For building A, during the eight-week saving in lead time, nearly 90% of the flat slab concrete frame could be constructed, and the 50 weeks construction programme for the concrete flat slab option is only marginally longer than the 48 weeks for the steel composite solution.

For building B, the ten-week saving in lead time equates to the period required to construct the flat slab concrete frame up to fourth-floor level and commence the walls and columns from the fourth to fifth floor – approximately 60% of the complete frame. The construction programmes for these options are identical at 67 weeks for the concrete flat slab and the steel composite options.

Construction programmes

The programmes reflect a pragmatic contractor’s approach to the construction process. Inevitably, different planners would produce slightly different programmes based on a considerable number of variable factors. Overall project programmes are highly influenced by the procurement route and type of contract adopted, and alternative procurement approaches such as construction management or design and build would no doubt produce different results. For example, construction management and design-and-build approaches lend themselves to concrete construction, where the ability to accommodate late information and variations are particularly beneficial as the work can be let before the design of following packages has been finalised.

The programmes prepared for this study reflect one procurement approach but, in practice, contractors are more likely to programme to a pre-set completion date in the knowledge of the type of contract, their projected costs, the risk profile of the project, their knowledge of and relationship with the client and design team, their supply chain and their exposure to both liquidated damages and to market forces in play at the time of the project.

A practical view had to be taken of such factors as logistics, site access, boundary constraints, cranage, etc, which are essentially site-specific. It could be argued that the steelwork could have started on-site sooner, with earlier subcontract award or longer periods for design, package tendering, mobilisation or foundations making the steelwork lead time less critical or even non-critical. Conversely, the use of a purely domestic subcontract, without the ability to pre-order, would push the programme back.

Whereas fire protection used to be a critical activity, modern details such as site-applied intumescent coating have removed fireproofing from the critical path altogether. However, although not on the critical path, the fireproofing activity requires a greater level of detailing and causes disruption that can adversely affect other trades, such as difficulties caused by fixings penetrating through fire-proofing and damage needing rectification.

Off-site intumescent coatings have been introduced in an effort to reduce the construction time, but these can suffer from damage in transit, requiring site remedial work which can eliminate the original saving.

The durations of first fix, second fix and M&E installations are essentially the same, with slight differences in quantities appearing to make little difference to the programmes. However, it is becoming increasingly common to use prefabrication for the M&E services distribution, which can offer programme advantages when used in conjunction with the open flat soffits provided by the flat slab, PT flat slab and Slimdek options. Prefabrication of sections of the M&E installations also offers advantages in subsequent maintenance and refurbishment of the building. No account is taken within the programmes of any construction time savings resulting from such prefabrication.

Finance costs

Although the reported costings excluded the effects of finance costs, if finance costs were to be considered, they should not be limited to the construction period alone as, in most cases, a significant feature of the financing will relate to the duration for which the land is held.

It is not possible to examine the entire project from inception to completion, as the duration prior to the commencement of procurement cannot be defined on a generic basis. However, consideration of the periods that have been identified in the programmes for procurement, lead time and construction would produce the following comparison, assuming a rate of 7.25% a year (base rate plus 2%) and comparing the programme programme extra or saving against the flat slab option.

This comparison takes no account of differences in cumulative finance costs arising from the different cash flow profiles experienced with the differing forms of construction. For example, the steel-framed options require greater expenditure early on than the concrete-framed options, where concrete’s “pay as you pour” principle works in the client’s favour. A more comprehensive analysis of the construction cash flow profiles would be required in order to present a detailed comparison of these effects on finance costs.