The Building Regulations
The Building RegulationsThese are the mass of regulations that cover safety, health, welfare, convenience, energy efficiency etc. in the way buildings are constructed. Not to be confused with Planning consent (which is more to do with whether you can put up the building in the first place). See more on the regulations , part AThe Approved documentsApproved documents (England) are detailed publications which come under the English Building Regulations. They are based on tried and tested methods of building and if you follow them you are assured of complying with the Regs. The equivalents for Scotland are the Technical HandbookUnder the Scottish Building Regulations, the Technical Handbook gives construction principles, which, if you follow them guarantee compliance with the Regulations, for Wales: the Approved documents (Wales), and for N.I. the Technical BookletsUnder the Northern Ireland Building Regulations, the Technical Booklets give construction principles, which, if you follow them guarantee compliance with the Regulations, (England) part A, deals with building structures , cover the structural safety of building and there are basically three ways of approaching structural design:
- using the ‘approved documents’. This method is based on traditional UK building practise for what is known from experience to work. You follow a fairly simple set of rules regarding foundations, walls (basically masonry walls), floors and roofs etc. and it will be automatically approved (providing you follow the rules correctly). You might need a structural engineer to calculate the odd long span or tricky bit. You can see an abridged version of the Approved Documents part A (showing only the parts about house building) It covers Basic requirements for stability, Areas at risk from house longhorn beetle, Thickness of walls in certain small buildings, Proportions for masonry chimneys above the roof surface, Foundations of plain concrete, Wall cladding and Roof covering.
- using elements which manufacturers certify as having certain structural properties. Examples of this are SIPsStructural Insulated Panels - prefabricated (usually in a factory) timber panels often forming part of an integrated building system and aimed at fast site erection. see more on SIPs panels, steel lintels over openings in walls, and timber I beams.
- using a structural engineer to design the structure from scratch (possibly incorporating some of the ‘approved documents’ at the same time). This can be quite a major expense, especially if there are lots of non-standard aspects to the design. It can pay to keep things simple because anything which deviates from the norm needs proving to the building inspector to be structurally sound.
If you are using an architect then they will see to all this for you either by using an in-house engineer or contracting the work out.
The ‘approved documents‘ method is fine but it simply does not cover anything which is non standard such as timber frame, SIPS, in situ reinforced concrete, straw bale etc. because these are not ‘traditional’ in the UK. The most recent version of the regs has even dropped the span tables for floor joists, roof joists, ceiling[for the purposes of part B of the Approved Documents] - A part of a building which encloses and is exposed overhead in a room, protected shaft or circulation space. (The soffit of a rooflight is included as part of the surface of the ceiling, but not the frame. An upstand below a rooflight would be considered as a wall.) joists, purlins etc. (although the archived copies are still available.) They say they intend to issue approved documents on timber construction but so far nothing. The situation is further complicated by the current change from British Standards to Eurocodes which is described on the TRADATimber Research and Development Association A trade association with a strong reputation for research and publication on all things timber web site. More about timber later.
Some basic structural principles
There are various principles which crop up regularly and, while you may need an engineer to do the calculations, it pays to have a basic understanding of what is going on. This is particularly true at the design stage but also during the building process there are two main reasons to understand the subject:
- When a building is partly complete there may be weaknesses to be aware of such as the need for temporary support for parts of a structure
- If changes to parts of the construction need to be suddenly made for some reason (such as certain materials becoming unavailable) then if you understand the structural implications you may be able to get on with making the changes before getting calculations done or getting Building Regulations approval for the changes
Compression / tension
Possibly the most fundamental thing to understand is the difference between compressive and tensile forces. Unreinforced masonry such as stone, concrete, brick, block etc. is considered to have lots of compressive strength but virtually no tensile strength. So they are strong when the bits are simply piled one on top of one another but if any bits are hanging out or trying to span a gap they can simply snap like a biscuit. (In fact they can usually span a small gap such as stone lintels have been used for traditionally. To span larger openings you have to resort to arches which keep the masonry in compression). Domes are a kind of three dimensional arch.
Proportion and slenderness
Imagine a thin stack of perfectly made, absolutely regular identical bricks. You could stack them one on top of another in perfect balance and they would form a perfect column which could carry a huge load (provided there were no lateral forces on the bricks). Because bricks are imperfect and irregular and columns and walls experience all kinds of lateral forces you need to make columns and walls quite thick to average out the imperfections and provide lateral stability.
Historically, learning how thick structures needed to be was a matter of trial, error and experience based on the concept of slenderness ratios, buttressing and the uniformity and strength of the building material. At one end of the scale were rules of thumb and at the other were secret calculations by master masons (yes – those who later became Freemasons).
The current Approved Documents in the Building Regulations Part A1, go a long way to regularising this historical experience of compressive structures by stating limits to height/width proportions of buildings, thicknesses and heights of walls, chimneys etc. and when they need buttressing or lateral support.
Allied to slenderness is lateral support where a buttressA masonry support to a wall used to prevent lateral movement or pierPart of a wall, in the form of a thickened section a bit like a built-in column, usually rectangular in plan, to give extra support to the wall or chimney helps to ‘prop up’ an adjoining wall. Where walls meet at a corner they effectively prop each other up. There are strict limits to how long unsupported walls can be and to the size of openings in buttresses.
There are situations covered in the Building Regulations where upper floors should be used to transfer lateral forces from walls to buttresses, chimneys etc. This is a way of tying the various parts of a building together so that the whole is stronger than any of its constituent parts.
Spans for beams and joists
The strength of solid timber beams and joists is proportional to the square of the depth so for instance a 200mm deep by 50mm wide joist is four times as strong as a 100mm deep by 50mm joist.
However, the critical structural issue with Building Regulations usually concerns stiffness rather than strength. There are strict limits on the deflection under load for elements such as floors, ceilings and roofs. Stiffness is proportional to the cube of the depth of a member so for instance a 200mm deep by 50mm wide joist is eight times as stiff as a 100mm deep by 50mm joist.
Most of the strength of a structural member such as a joist is in the top and bottom layers: compression in the top and tension in the bottom. This is why I beamsprefabricated timber beams with a web of OSBOriented Strand Board and softwood flanges. They are very light, efficient and regular are so efficient. They concentrate the timber at the top and bottom and use the minimum web possible.
Triangulation / racking
Most frame structures, particularly timber ones, are low on stiffness at the joints between horizontal and vertical members.
This is termed racking strength. To remedy this there are two approaches which both basically introduce diagonal structural strength by use of triangles. The most obvious examples are the diagonal struts in roof trusses. This fink truss is actually formed of eight triangles and is therefore very strong for its small timber sections and weight.
The other example is the fixing of a sheet material over the whole of a wall frame which effectively creates an infinite number of triangles in all directions. The advantage of this approach is that the the sheet, usually ply or OSB, is easier than struts to fix to the frame and needs very little thickness. This is the advantage with SIPs where all the elements, walls, floors and roofs are made of timber frame with sheet on both sides. It is sometimes known as stressed skin construction. It is important to be aware that this type of construction is carefully engineered and if anyone makes future changes they need to have it recalculated to avoid structural problems.
Take the example of a two storey[for the purposes of part B (fire) of the Approved Documents to the Building Regulations] this includes - (a) any gallery[for the purposes of part B of the Approved Documents] - A raised area or platform around the sides or at the back of a room which provides extra space. if its area is more than half that of the space into which it projects; and (b) a roof, unless it is accessible only for maintenance and repair. external wall in diagram A. Although it will probably be manufactured in smaller panels for ease of transportation, when they are assembled on site the overall strength and stiffness is mainly provided by the areas shown with diagonal lines which stretch unbroken from one side of the panel to the other. There is increased rigidity where they provide triangulation.
If at a later date someone decides to make a few changes by moving one door and blocking off another (but without reconnecting the sheathing around it) and also moving a window, then the wall will have lost much of its racking strength. The other areas of ply do contribute somewhat to stiffness but it is the main continuous overlapping diagonals which do most of the work. This is a good reason for keeping a building log book (Building Regulations requirement anyway with respect to Approved Document L, parts L1A and L1B)
The fink truss above is also an excellent example of reducing spans. Not only do the four inner members triangulate the main outer frame to stiffen it, they also support the long spans (mid way for the two sloping members and third way for the horizontal one) so these members can be much thinner than if they were not supported.
Overhangs / jettying / cantilevers
With structural sections such as timber and steel it is possible to create overhangs of various types. In this country it first became popular to jetty out on the first floor in mediaeval times. With masonry it is possible to corbel out very slightly which can create a good visual effect.
There are several possible advantages to doing this:
- walls, windows etc. below are better protected from rain and snow
- better use is made of living space above whereas outdoor space is retained below
- the stepping out of the walls can be visually attractive
- It may be possible to project a window area outwards and provide lateral views which otherwise might not be granted planning permissionthe legal basis for being allowed to do some form of development such as building a house. (not to be confused with Building Regulations which are all about whether the building is properly constructed). see more on Planning the legal basis for being allowed to do some form of development such as building a house. (not to be confused with Building Regulations which is all about whether the building is properly constructed). see more on Planning (because of overlooking neighbours) or get an otherwise impossible view (say up and down the street).
The jettied area on this PassivhausSee more on the Passivhaus standard. The PassivHaus Institute has pioneered a standard for low energy buildings. It includes very low energy usage and ways of achieving this. The word is derived from the idea of buildings which are fundamentally low energy and passive solar heated rather than using extra gadgets to heat them. See Passivhaus for the UK branch of the organisation. in Birmingham allows lateral views and extra space
The extent to which you can cantilever out needs calculating by an engineer because it is not covered in the span tables.
Strength of foundations
This is covered here in the Building Regulations
Fire, thermal insulation and sound
To varying extents, the structure must be protected from fire and it must be taken into account regarding thermal and sound insulation.
The Building Regulations, Approved Document section B3 page 31-40 is where the requirements for the fire resisting construction of houses can be found. It is quite complicated, as the degree of fire resistance[for the purposes of part B of the Approved Documents] - The ability of a component or construction of a building to satisfy, for a stated period of time, some or all of the appropriate criteria specified in the relevant part of BSBritish Standard 476. depends partly on the location of the structural elements.
The fire resistance of structure can be increased in various ways such as cladding it with a certain amount of non combustible material fixed in an approved way, or it may be possible to simply add to the thickness of the structural members. For instance steel structural members are susceptible to weakening at high temperatures and may need surrounding with plasterboard. Timber chars at a certain rate in fire (about 25mm per hour) so some members may need to be made thicker to allow for this.
Given that much of the structure is part of external walls it is necessary to make sure that cold bridgingthis is a pathway where heat can easily escape (or get in) through some part of the structure. It is usually caused by some element of structure such as a steel lintel or wooden studwork. Also known as a thermal bridge. see more on cold bridging via the structure does not compromise the insulation. While it is generally appreciated that cavity walls require insulation it is not always obvious how steel or timber members may cause a cold bridge between the inside and outside surfaces. Even with timber framework or studs in an external wall there may be a significant problem. For more on thermal bridging see Floors and Ceilings, and Main Structure.
Structure can readily carry sound, both airborne and impact. The Building Regulations Approved Document E covers the requirements for sound insulation. It includes not only insulation between neighbouring dwellings with a party wall, and between flats but also within a dwelling:
Structure and Life Cycle Analysis
The type of structure can have an important bearing on how recyclable or reusable the materials are. For instance timber tends to get reused (albeit for lower quality work) as does steel, whereas bricks might only be good for hardcore or paviours and concrete is simply a disposal problem.
There is also an issue with Lifetime Homes"Lifetime Homes make life as easy as possible for as long as possible because they are thoughtfully designed. They provide accessible and adaptable accommodation for everyone, from young families to older people and individuals with a temporary or permanent physical impairment". See the Lifetime Homes page and the Lifetime Homes Foundation web site. Most houses undergo successive extensions, improvements and changes and some building materials are better at this than others.