New Guide explains why structural solution should be optimal not familiar

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New Guide explains why structural solution should be optimal not familiar

The correct choice of structural frame is critical. Such a decision should be based on examining a range of potential solutions and not just

A forthcoming design and construction guide, Concrete Framed Buildings, is the latest in the range of resources from The Concrete Centre to enable full and effective consideration of a wide variety of structural solutions in concrete.

At the start of each project, a decision is made concerning the form and material of the structural frame. It is a decision often influenced initially by cost, or perhaps by the frame type previously specified by the client, designer, engineer or contractor. However, keeping with the familiar may not always offer the best structural solution for that particular project. The frame is the key structural element. The choice of type and design has a significant impact on the performance of the final building. Therefore, it is important to base the choice of structural frame on what offers the optimum solution rather than simply what was specified before.

Design tools

The Concrete Centre has developed a number of design tools to enable fast and effective examination of a wide range of concrete structural options. These include Concept.xls, an invaluable design tool that can produce 13 reinforced concrete conceptual designs within five minutes, RC design spreadsheets that allow the rapid production of clear and accurate calculations and a number of design and best practice guides. All of these enable the best structural solution in concrete, rather than just the most familiar, to be examined and developed. For further information visit www.concretecentre.com

Concrete benefits

However, in addition to the most efficient structural design, there are other matters to be considered. A major issue, particularly for owneroccupiers and private finance initiative (PFI) consortia, is whole-life value. Here, the inherent benefits of concrete, such as fabric energy storage, fire resistance and sound insulation, mean that concrete buildings tend to have lower operating costs and lower maintenance requirements.

Construction costs and programme are also important. Currently, insitu concrete-framed buildings need take no longer to construct than steel-framed buildings. Indeed, they can be even faster. Rationalising reinforcement, designing and detailing for pr?fabrication, precasting or part-precasting and modern quick-release formwork methods can further enhance the speed of concrete construction on site. Importantly, concrete construction lends itself to a fast and safe programme. It provides a safe working platform and gives semi-internal conditions that allow services installation and follow-on trades to start early. Furthermore, design changes can be incorporated late in the programme without a negative impact.

In terms of costs, the difference between steel and concrete frames is insignificant. Although foundations for the heavier concrete solution will be more expensive, the difference still only represents a small percentage of the whole project costs and can easily be offset by using post-tensioned slabs that are typically 15% lighter. Using concrete can also reduce the cladding costs - the thinner the overall structural and service zones, the less cladding is required. Cladding can represent up to 25% of the construction costs so it is worth minimising the cladding area. Indeed, with the engineer's approval, the concrete cladding could even double up as the structure, representing further savings.

Concrete solutions

In addition to estimating first costs, best value calculations should include the structure's ability to meet Standards without excessive additional finishing. Concrete's inherent fire resistance, sound insulation and vibration resistance mean that concrete does not need any additional proofing, finishing or damping to meet building regulation requirements. Furthermore, concrete lends itself to reduced long-term operational and maintenance costs. Its thermal mass reduces the need for heating and air-conditioning; its robustness is resistant to accidental damage and vandalism.

Initial construction costs are, therefore, not the only issue to be considered. All of the above needs to be taken into consideration as a whole package to ensure that the structural design looks beyond the familiar and provides the optimum solution. There is a wide range of concrete structural solutions that should be considered, including the following:

* Flat slabs. Highly versatile and widely used structural elements that provide minimum depth, fast construction and allow flexible colu....

* Ribbed and waffle slabs. Providing a lighter and suffer slab ... equivalent flat slab, thereby reducing the foundation requireme... ? ? prove extremely effective at meeting strict vibration criteria for ... and laboratories.

* Beams and slabs. Involving the use of one- or two-way spanning onto beams in one or two directions. Can be wide and flat or deep and narrow. Commonly used for irregular grids and long spans and also for transferring columns, walls or heavy point loads to columns or walls below.

* Post-tensioned (PT) slabs. PT slabs offer the thinnest slab type. They can use bonded or unbonded tendons. Longer spans can be achieved by prestressing, which can counteract possible deflections. This is a fast and cost-effective structural approach.

* Hybrid concrete construction. Combining both precast and in-situ concrete and thereby offering the benefits of both. Many different forms of hybrid concrete construction are possible as different parts of the structure can be precast.

* Precast concrete. Precast concrete can form all types of structures from crosswall to 'stick' frame with columns beams and slabs. Precast is well suited to projects where speed on site or a fair-faced concrete finish is required.

* Tunnelform. Used to form cellular construction and recognised as a modern method of construction, tunnelform offers excellent productivity and quality benefits with speeds of 300m^sup 2^ of floor per day being possible.

Concluding remarks

The availability of new, innovative, high-performance and highstrength concretes, self-compacting concretes and early-strength testing enhances still further the benefits of modern structural concrete.

To obtain a copy of Concrete Framed Buildings, Concept.xls, RC design spreadsheets, design guidance or best practice guides, contact The Concrete Centre on www.concretecentre.com