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Structural principles

The Building Regulations

The Building Regulations, part A, 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 foundationswalls (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 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.
These Segal system timber framed houses required dozens of pages of engineer’s calculations and drawings

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 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 TRADA 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, brickblock 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.

Timbersteelaluminium, reinforced concrete etc. on the other hand are considered reliably strong in both compression and tension so they can span large openings and can be cantilevered out.

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 wallschimneys etc. and when they need buttressing or lateral support.

buttresses

buttresses act as props to walls

Lateral Support

Allied to slenderness is lateral support where a buttress or pier 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 beams 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 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) 

Reducing spans

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.

jettying

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 permission (because of overlooking neighbours) or get an otherwise impossible view (say up and down the street).

The jettied area on this Passivhaus 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 and in the Building Regulations part A

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.

Fire

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 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.

Thermal insulation

Given that much of the structure is part of external walls it is necessary to make sure that 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.

Sound insulation

Structure can readily carry sound, both airborne and impact. The Building Regulations Approved Document part 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:

There are two basic approaches to complying with the requirements for sound insulation: either Pre-completion testing or the use of Robust Details.

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. Most houses undergo successive extensions, improvements and changes and some building materials are better at this than others.

See Building Materials

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