SELECTING THE STRUCTURAL FRAME FOR A BUILDING

The frame is a key element of any building. A structural frame is typically defined as

―the load-bearing assembly of beams, columns and other structural members connected together and to a foundation to make up a structure‖ (Blockley, 2005).

According to Soetanto et al. (2006a), the structural frame is the skeleton that defines and holds the whole building together. This section describes the structural frame selection process along with the major structural frame materials, i.e. concrete and steel. It aims to identify the principal criteria for project team members in their choice

of whether to select concrete or another frame option, such as steel, timber, etc.

Concrete frames and steel frames are also described in this section.

2.4.1 The Process of Structural Frame Selection

The choice of the primary structure of a building has a major influence on the value of the building to the client, because it provides both the functionality and future flexibility, and largely determines the speed with which the construction process can be executed (SCI, 2000). Furthermore, the choice of structural frame is of particular significance since it interfaces with many of the other elements of the building, thereby influencing their specification and buildability (Soetanto et al., 2006b). Frame choice can have a huge impact on both the short and long-term performance of the completed building. In the short term the frame must give its client the satisfaction of his/her needs, such as construction being completed on time and to budget, it must also satisfy future changes in functional requirements of the building in the long term (Soetanto et al., 2006a).

There is a wide choice of structural frame materials for building projects. There are four basic materials available: concrete, steel, timber, or masonry. Although many options are available, these tend to be based on structural steel or reinforced concrete for the simplest buildings (Soetanto et al., 2007). Bibby (2006) indicated that the choice of whether to go for a concrete or steel frame is still mainly dependent on building type and site-specific constraints. Although the choice of frame is heavily influenced by the issues specific to that project there are a number of issues that are commonly considered by project team members (Soetanto et al., 2006b). The choice of primary structure is generally determined by cost with less regard to functionality and performance characteristics (SCI, 2000). This is further corroborated by Idrus and

Newman (2003) who state that frame selection criteria often focus on cost and time requirements and a previous survey by Soetanto et al., (2006b) identified 31 issues perceived to be important in influencing the structural frame decision making process.

These are shown in Table 2.1 below:

Table 2.1 Criteria for assessing the potential performance of structural frames (Soetanto et al., 2006b)

No Performance Criteria No Performance Criteria

1 The layout, structure and engineering

systems are well integrated 17 The disposal (i.e. demolition and site clearance) costs can be minimized

2 The layout and size work well 18

The building minimises environmental impacts (in terms of energy/resource consumptions and waste).

3 The circulation works well 19

The building enhances the team/client‘s confidence (in the selected structural frame)

4 The building has sufficient floor to ceiling

clear height 20 The design costs can be minimised

5 The building provides appropriate lettable

area/spans 21 The building is perceived to be simple to

8 The building can be quickly constructed 24 The building overall meets the perceived needs

9 The construction costs can be minimised 25 The colour and texture of materials enhance enjoyment of the building 10 The building has been designed so it can

be safely constructed and maintained 26 The quality and presentation of finishes are good

11 The overall risk is perceived to be low 27 The building overall looks durable 12 The building is designed for demolition

and recyclability 28 The connections between components are well designed and buildable

13 The building is adaptable to changing

needs 29 The tolerances of the components are

realistic

14 The finishes are durable and maintainable 30 The building provides best value 15 The form and materials optimise the use of

thermal mass 31 The client is satisfied with the finished product

16 The facility management (i.e. O & M,

replacement) costs can be minimized

Cost model studies published by The Concrete Centre (UK) revealed that the structural frame comprises between 7-12% of the final cost of a building in relation to the type of the building (Ryder, 2007). So, The Concrete Centre (2004) suggests that frame cost should not solely dictate the choice of frame. Indeed many other issues should also be taken into consideration when selecting the optimal frame solution

such as programme, health and safety, environmental performance, etc. This was the subject of Paper 1 (Appendix A) and Paper 2 (Appendix B) which found that although the choice of frame is heavily influenced by the issues specific to the project in hand, ten key issues were identified that are particularly important during early design.

These key issues are presented in section 4.2.2.

2.4.2 Concrete Frames

The concrete sector is worth about £5bn a year, with up to 120 million tonnes of concrete being used in UK construction projects every year. The UK‘s tradition of using in-situ concrete construction has meant that for many years in-situ concrete frame construction dominated the structural (skeletal) frame market. However, industry reports such as that compiled by the BRE (2005) have revealed that concrete lost significant market share to structural steel in the framed structure market in the UK in the 1980s and 1990s, although it still appears to perform well in commercial and residential applications. In addition, like the construction industry as a whole, concrete construction has been criticized for its poor productivity (e.g. Latham, 1994;

Egan, 1998), but research and development has helped to improve various aspects of construction (e.g. Gray, 1995; Nolan, 2005) and is continuing. For instance, The Reading Production Engineering Group (Gray, 1995) identified the barriers to in-situ concrete frame construction process and produced a strategy which would remove these barriers in order to deliver improved and consistently efficient concrete construction. The main recommendation of Gray‘s report (1995) was that the concrete frame contractors and their suppliers should develop a definitive and straightforward specification so as to overcome the problem of great complexity in the production process of concrete frames. Also, BRE (2000) reported from the results of the European Concrete Building Project at Cardington that the improvements

implemented as part of the research should produce time savings of over 30% and man hour savings of over 45% compared with the then current practice.

Concrete‘s range of structural frame solutions, its thermal efficiency, inherent fire resistance, acoustic and vibration performance, durability and low maintenance ensure that it performs well in a number of UK markets such as commercial and residential buildings (TCC, 2006). New cost model studies and research now add cost-effective construction and sustainability to that list (Ryder, 2007; Nolan and Rupasinghe, 2007). As stated by Stefanou (2004) high-rise residential apartment blocks often utilize the additional mass a concrete frame has for improved acoustic insulation, improved energy consumption from its increased thermal mass and a high quality finish from exposed concrete. Furthermore, Eustace (2008) stated that with the advent of new construction techniques and the desire to build larger and taller buildings, concrete has arguably become the construction material of choice. This is further corroborated by Korista (2009) ―Not long ago, most high rise structures were built with structural steel, such as the Sears Tower Chicago, which is the tallest building in the United States. However advancements in the concrete industry over the past few years have made the current trend toward concrete possible‖. In addition, Feenan (2007) pointed out that from hotels to arenas, car parks to shopping centres, apartments to dock leveller pits, and even a digester tower, structural concrete is used throughout the construction industry.

Designers have a wide choice of structural systems for concrete frame buildings. They can choose from three basic types: in-situ, precast or hybrid constructions, which are described below.

In-situ concrete solutions: a form of construction where concrete is poured into

forms for building elements, i.e. columns, beams, floors, walls, stairs or balconies at the building site. The main benefits of using in-situ construction are flexibility and economy.

Precast concrete solutions: In this form of construction, concrete is cast and cured in

a controlled environment which is then transported to the construction site. These types of solutions could all be used to reduce or eliminate formwork, increase the speed of erection, provide high quality finishes, reduce snagging, increase certainty, reduce complexity on site etc. The biggest benefits usually come from repetition.

Precast products are made to consistently high quality standards using a combination of skilled labour or automated processes (Holton, 2009). Precast construction is virtually unlimited in its application and is suitable for single and multi-storey construction.

Hybrid solutions: Hybrid Concrete Construction (HCC) is a method of construction

which integrates precast concrete and cast in-situ concrete to take best advantage of their different inherent qualities (TCC, 2005). These types of solutions use standard precast members and in-situ elements. Hybrid concrete technology is used primarily to achieve fast and cost effective construction by removing labour-intensive operations on-site and replacing them with mechanized production in precasting yards and factories (Goodchild and Glass, 2004). Nevertheless it is felt that these solutions will still struggle to match the speed of erection of steel for most frame applications (BRE, 2005).

2.4.3 Steel Frames

In the UK, steel is the dominant framing material for multi-storey buildings with the latest figures showing a market share at nearly 70% (SCN, 2002). Structural steel‘s low cost, strength, durability, design flexibility, adaptability and recyclability are said to make it the material of choice in building construction (BCSA, 2009). Structural steel framing solutions have been confirmed as faster and cheaper to build than reinforced concrete alternatives in the latest update of a cost comparison study that dates back to 1993 (Barrett, 2007). Speed of construction remains the principal reason for choosing steel, with "lowest overall cost" coming second. Bartley (2009) indicated that ―Our team changed the design to steel for cost and speed of construction‖ as steel generally lends itself to a faster construction programme.

Furthermore, the sustainability case for steel is arguably now a key factor in favour of steel (BCSA, 2009); the recycling and reuse rate for steel construction products in the UK is 94% and as high as 99% for structural steelwork (NSC, 2008). The British Constructional Steelwork Association (BCSA) was the first construction material sector to launch a Sustainability Charter which it did in November 2005. The objective of the Steel Construction Sustainability Charter is ―To develop steel as a sustainable form of construction in terms of economic viability, social progress and environmental responsibility‖ (BCSA, 2007). Also, all steel construction products are manufactured off site and frames can be procured via all building procurement routes.

Indeed steel framed buildings have been a successful choice for a wide range of building types procured in fast track, management packages as well as traditional methods (SCI, 2000).