Loading

Buildings need to accommodate the loads and forces acting on them if they are to resist collapse. One of the most important considerations is how forces are transferred within the structure. Buildings are subject to three types of loading:

1. Dead loads. Dead loads remain relatively constant throughout the life of a building, unless it is remodelled at a future date. These loads comprise the combined weight of the materials used to construct the building. Loads are transferred to the ground via the foundations. Because the weight of individual components is known, the dead load can be easily calculated.
2. Live loads. Unlike dead loads, the live loads acting on a building will vary. Live loads comprise the weight of people using the building, the weight of furniture and equipment, etc. Seasonal changes will result in (temporary) live loading from rainfall and snow. Structural design calculations assume an average maximum live load based on the use of the building (plus a safety factor). If the building use changes, then it will be necessary to check the anticipated live loading against that used at the design stage.
3. Wind loads. All buildings are subject to wind loading. Maximum wind loads (gusts) are determined by considering the maximum recorded wind speed in a particular location and adding a safety factor. Wind loading is an important consideration for both permanent and temporary structures. It is also an important consideration when designing and installing temporary weather protection to protect building workers and work in progress from the elements.

When the total loading has been calculated for the proposed building, it is then possible to design the building structure (the structural frame) and the foundations. This needs to be done in conjunction with the design of the building envelope.

Structural frames

Timber, steel and reinforced concrete are the main materials used for structural frames (Photograph 1.1). In some cases, it is common to use one material only for the structural frame (e.g. timber). In other situations, it may be beneficial to use a composite frame construction (e.g. concrete and steel). Combining two or more materials is known as hybrid construction. The benefits of one material over another need to be considered against a wide variety of design and performance parameters, such as the following:

• Extent of clear span required
• Height of the building
• Extent of anticipated loading
• Fire resistance and protection
• Embodied energy and associated environmental impact
• Ease of fixing the fabric to the frame (constructability)
• Availability of materials and labour skills
• Extent of prefabrication desired
• Site access (restrictions)
• Erection programme and sequence
• Maintenance and ease of adaptability
• Ease of disassembly and reuse of materials
• Life cycle costs

Dimensional stability

Stability of the building as a whole will be determined by the independent movement of different materials and components within the structure over time – a complex interaction determined by the dimensional variation of individual components when subjected to changes in moisture content, changes in temperature and not forgetting changes in loading:

• Moisture movement. Dimensional variation will occur in porous materials as they take up or, conversely, lose moisture through evaporation. Seasonal variations in temperature will occur in temperate climates and affect many building materials. Indoor temperature variations should also be considered.
• Thermal movement. All building materials exhibit some amount of thermal movement because of seasonal changes in temperature and (often rapid) diurnal fluctuations. Dimensional variation is usually linear. The extent of movement will be determined by the temperature range the material is subjected to, its coefficient of expansion, its size and its colour. These factors are influenced by the material’s degree of exposure, and care is required to allow for adequate expansion and contraction through the use of control joints.
• Loading. Dimensional variation will occur in materials that are subjected to load. Deformation under load may be permanent; however, some materials will return to their natural state when the load is removed. Thus live and wind loads need to be considered too.

Understanding the different physical properties of materials will help in detailing the junctions between materials and with the design, positioning and size of control joints. Movement in materials can be substantial and involve large forces. If materials are restrained in such a way that they cannot move, then these forces may exceed the strength of the material and result in some form of failure. Control joints, sometimes described as ‘movement joints’ or ‘expansion joints’, are an effective way of accommodating movement and associated stresses.

Designers and builders must understand the nature of the materials and products they are specifying and building with. These include the materials’ scientific properties, structural properties, characteristics when subjected to fire; interaction with other materials, anticipated durability for a given situation, life cycle cost, service life, maintenance requirements, recycling potential, environmental characteristics such as embodied energy, health and safety characteristics, and, last but not least, their aesthetic properties if they are to be seen when the building is complete. With such a long list of considerations, it is essential that designers and builders work closely with manufacturers and consult independent technical reports. A thorough understanding of materials is fundamental to ensuring feasible constructability and disassembly strategies. Consideration should be given to the service life of materials and manufactured products, since any assembly is only as durable as the shortest service life of its component parts.

Flexibility and adaptability

Designing and detailing a building to be flexible and adaptable in use presents a number of challenges, some of which may be known and foreseen at the briefing stage, but many of which cannot be predicted. Thought should be given to the manner in which internal, non‐loadbearing walls are constructed and their ease of disassembly and reuse (repositioning). Similarly, the position of services and the manner in which they are fixed to the building fabric need careful thought at the design and detailing stage. For example, a flexible house design would have a structural shell with non‐loadbearing internal walls (movable partitions, folding walls, etc.), zoned underfloor space heating (allowing for flexible use of space) and carefully positioned wet and electrical service runs (in a designated service zone or service wall).

Open building

The open building concept aims to provide buildings that are relatively easy to adapt to changing needs, with minimum waste of materials and little inconvenience to building users. The main concept is based on taking the entire life cycle of a building and the different service lives of the building’s individual components into account. Since an assembly of components is dependent upon the service life of its shortest‐living element, it may be useful to view the building as a system of time‐dependent levels. Terminology varies a little, but the use of a three‐level system, primary, secondary and tertiary, is common. Described in more detail, the levels are:

• The primary system. Service life of approximately 50 − 100 years. This comprises the main building elements, such as the loadbearing walls and roof or the structural frame and floors and roof. The primary system is a long‐term investment and is difficult to change without considerable cost and disruption.
• The secondary system. Service life of approximately 15 − 50 years. This comprises elements such as internal walls, floor and ceiling finishes, building services installations, doors and vertical circulation systems such as lifts and escalators. The secondary system is a medium‐term investment and should be capable of replacement or adaptation through disassembly and reassembly. The shorter the service life of components, the greater the need for replacement, hence the need for easy and safe access.
• The tertiary system. Service life of approximately 5 − 15 years. This comprises elements such as fittings and furniture and equipment associated with the building use (e.g. office equipment). The tertiary system is a short‐term investment and elements should be capable of being changed without any major building work.

Applying this strategy to a development of, for example, apartments, the structure and external fabric would be the primary system. The secondary system would include kitchens, bathrooms and services. The tertiary system would cover items such as the furniture and household appliances. If a discrete, modular system is used, then it is relatively easy to replace the kitchen or bathroom without major disruption and to recycle the materials. This ‘plug‐in’ approach is certainly not a new concept but has started to become a more realistic option as the sector has started to adopt off‐site production (see also Chapter 2).

Passive security measures

A passive approach to security is based on the concept of inherent security measures, where careful consideration at the design and detailing phase can make a major difference to the security of the building and its contents. Building layout and the positioning of, for example, doors and windows to benefit from natural surveillance need to be combined with the specification of materials and components that match the necessary functional requirements. The main structural materials and the method of construction will have a significant impact on the resistance of the structure to forced entry. For example, consideration should be given to the ease with which external cladding may be removed and/or broken through, and depending on the estimated risk, an alternative form of construction may be more appropriate. Unlawful entry through roofs and rooflights is also a potential risk. Building designers must consider the security of all building elements.

Ram raiding, the act of driving a vehicle through the external fabric of the building to create an unauthorised means of access and egress for the purposes of theft, has become a significant problem for the owners of commercial and industrial premises. Concrete and steel bollards, set in robust foundations and spaced at close centres around the perimeter of the building, are one means of providing some security against ram raiding, especially where it is inappropriate to construct a secure perimeter fence.

Active security measures

Active security measures, such as alarms and monitoring devices, may be deployed in lieu of passive measures or in addition to inherent security features. For new buildings, active measures should be considered at the design stage to ensure a good match between passive and active security. Integration of cables and mounting and installation of equipment should also be considered early in the detailed design stage. Likewise, when applying active security measures to existing buildings, care should be taken to analyse and utilise any inherent features. Some of the active measures include:

• Intruder alarm systems
• Entrance control systems in foyers/entrance lobbies
• Coded door access
• CCTV monitoring
• Security personnel patrols

Specifying quality

Trying to define quality is a real challenge when it comes to construction, partly because of the complex nature of building activity and partly because of the number of actors who have a stake in achieving quality. The term quality tends to be used in a subjective manner and, of course, is negotiable between the project stakeholders. In terms of the written specification, quality can be defined through the quality of materials and the quality of workmanship. Designers can define the quality of materials they require through their choice of proprietary products or through the use of performance parameters and appropriate reference to standards and codes. Designers do not tell the builder how to construct the building; this is the contractor’s responsibility, hence the need for method statements. The specification will set out the appropriate levels of workmanship, again by reference to codes and standards, but it is the people doing the work, and to a certain extent the quality of supervision, that determines the quality of the finished building.

Specification methods

There are a number of methods available for specifying. Some methods allow the contractor some latitude for choice and therefore an element of competition in the tendering process, while others are deliberately restrictive. The four specification methods are:

1. Descriptive specifying: where exact properties of materials and methods of installation are described in detail. Proprietary names are not used; hence this method is not restrictive.
2. Reference standard specifying: where reference is made to established standards to which processes and products must comply, e.g. a national or international standard. This is also non‐restrictive.
3. Proprietary specifying: where manufacturers’ brand names are stated in the written specification. Here the contractor is restricted to using the specified product unless the specification is written in such a way to allow substitution of an equivalent. Proprietary specification is the most popular method where the designer produces the design requirements and specifies in detail the materials to be used (listing proprietary products), methods and standard of workmanship required.
4. Performance specifying: where the required outcomes are specified together with the criteria by which the chosen solution will be evaluated. This is non‐restrictive and the contractor is free to use any product that meets the specified performance criteria. Performance specification is where the designer describes the material and workmanship attributes required, leaving the decision about specific products and standards of workmanship to the contractor.

The task is to select the most appropriate method for a particular situation and project context. The type of funding arrangement for the project and client preferences usually influences this decision. Typically, projects funded with public funds will have to allow for competition, so proprietary specifying is not usually possible. Projects funded from private sources may have no restrictions, unless the client has a preference or policy of using a particular approach. Obviously the client’s requirements need to be considered alongside the method best suited to clearly describe the design intent and the required quality, while also considering which method will help to get the best price for the work and, if desired, allow for innovation. In some respects, this also concerns the level of detail required for a project or particular elements of that project. Although one method is usually dominant for a project, it is not uncommon to use a mix of methods for different items in the same document.

It has been argued that performance specifications encourage innovation, although it is hard to find much evidence to support such a view. The performance approach allows, in theory at least, a degree of choice and hence competition. The advantage of one approach over another is largely a matter of circumstance and personal preference. However, it is common for performance and prescriptive specifications to be used on the same project for different elements of the building.

National Building Specification

Standard formats provide a useful template for specifiers and help to ensure a degree of consistency, as well as saving time. In the UK, the National Building Specification (NBS) and the National Engineering Specification (NES) are widely used. This commercially available suite of specification formats includes NBS Building, NBS Engineering Services and NBS Landscape. Available as computer software, it helps to make the writing of specifications relatively straightforward, because prompts are given to assist the writer’s memory. Despite the name, the NBS is not a national specification in the sense that it must be used; many design offices use their own particular hybrid specifications that suit them and their type of work.

NBS Building is available in three different formats to suit the size of a particular project, ranging from Minor Works (small projects) to Intermediate and Standard (large projects). It is an extensive document containing a library of clauses. These clauses are selected and/or deleted by the specifier, and information is added at the appropriate prompt to suit a particular project.

With the uptake of BIM many specification decisions are tied to the product libraries held with the digital model(s) used by designers and specialist subcontractors.

Green specifications

The National Green Specification (NGS) is an independent organisation, partnered by the Building Research Establishment (BRE), to host an Internet‐based resource for specifiers. It provides building product information plus work sections and clauses written in a format suitable for importing into the NBS, thus helping to promote the specification of green products.

Coordinated Project Information

Coordinated Project Information (CPI) is a system that categorises drawings and written information (specifications). CPI is used in British Standards and in the measurement of building works, the Standard Method of Measurement (SMM7). This relates directly to the classification system used in the NBS.

One of the conventions of CPI and Uniclass is the ‘Common Arrangement of Work Sections’ (CAWS). CAWS lists around 300 different classes of work according to the operatives who will do the work; indeed the system was designed to assist the dissemination of information to subcontractors. This allows bills of quantities to be arranged according to CAWS. The system also makes it easy to refer items coded on drawings, in schedules and in bills of quantities back to the written specification. The main categories are shown in Table 1.1 (note there is no ‘I’, ‘O’ or ‘Y’). The main sections are further divided into sub‐sections.

The recent introduction of the New Rules of Measurement (NRM) has brought a move away from CAWS to a new indexing system that aims to better reflect developments in building technologies (e.g. offsite and recycling). This numbered system contains 41 sections and no longer makes reference to CPI, as shown in Table 1.2.