Mechanical Fixings

Even though forged iron has been known in Northern Europe since 1000 AD, neither iron nor steel were extensively used as a building material until the industrial revolution. Houses were built in earth, stone, brick and timber. The three first materials were fastened together with simple mortars, whereas timber components which were to be lengthened, strengthened or connected were joined together with locking joints.

A common quality of locking joints is that they reduce the strength of the timber as little as possible. Certain joints are used to preserve the timber's tensile and bending strength, others to preserve the compressive strength (see Figure 13.28). Wooden plugs used to be an integral part of locking joints, often incorporated with the locks, but their most important role was as fixings for both structure and claddings. These days, nails and screws in steel are the sole components used for the vast majority of mechanical fixings in timber building. Steel bolts are used in buildings with large structural elements. Fixing products are also made of aluminium, copper, bronze and stainless steel.

A normal-sized timber house will contain about 100 to 150 kg of nails, screws and bolts. Steel structures are joined mainly through welding,

17.1.1 Timber

17.1

Timber bolts.

17.1

Timber bolts.

but bolts can also be used, as was the case during the nineteenth century, before mobile welding became commonplace.

Joints. Timber joint technology is particularly well-developed in countries like Japan, with a choice of some 600 joint types. In Scandinavia there is a tradition of log construction with 10 to 20 different jointing techniques. In some types of structural framework, such as in the stave churches, grooves are often used to fix external panelling. Nails are not necessary in this form of construction, and where the fastening is part and parcel of the whole structural system it is known as a 'macro-joint'.

Pins and bolts. The use of timber pins and bolts is particularly widespread in areas with early forms of timber frame tradition (Figure 17.1). Pins of juniper, oak and maple are considered the best, although other types of wood can be used. The pins in timber framing are often 1530 cm long, while pins for fixing and cladding are smaller. Dowels can be executed in such a way that they are slightly larger in diameter than the holes. If they are also drier than the recipient timber component when they are inserted, they will expand later thus locking into place. They are often dipped in water or milk before being rammed in. Alternatively, they can be glued.

There is a lack of steel in India, and wooden bolts are often used as a fixing component in timber structures. The Forest Research Institute in Dehra Dun has researched the strength of wooden bolts and found that they were consistently about 68% as strong as steel of the same size. The timber bolts used in the research had nuts 12 mm in diameter and 100 mm in length. The timber was taken from various trees of normal strengths (Masani et al., 1972). However, timber is simply not as homogeneous as steel and its strength properties are less standardized and difficult to assess. Timber plugs disappeared from the market in Europe during the middle of the nineteenth century as a result of new standards specifying strength properties.

To a certain extent, the use of timber plugs is on its way back into building; for example, in military radar stations where metal components would disturb radio signals. Several massive timber construction systems also use timber dowels. There are guidelines for their production and dimensions. Industrial production of timber bolts and pins is not necessarily less efficient or expensive than for the equivalent steel products (Kessel et al., 1994).

Fixings made of timber are based purely on renewable resources. The energy consumption and pollution in the production phase are low. The quality of timber used for jointing, pins and bolts is normally so good that no impregnation is needed.

Durability of the products is also very good. Whilst connections in steel in certain situations can lead to condensation and decay of the adjacent timber, timber fixing components are neutral and stable.

Wooden plugs can easily be sawn off or drilled out for re-use of the structures. Pure timber waste can be burned for energy recovery or composted.

17.1.2 Metal

17.2

Standard nails.

17.2

Standard nails.

17.3

Standard wood screws.

17.3

Standard wood screws.

Nails. There are two main groups of nails: cut nails and wire nails. Cut nails are the oldest and original type and usually have a slight wedge form. They were used in all situations until the end of the nineteenth century, when the manufacture of wire nails began. Wire nails are ubiquitous nowadays (Figure 17.2). In the UK they are round or oblong; in Scandinavia they usually have a square cross-section with a pyramidal tip. Galvanized nails are used on external surfaces to cope with recurring dampness. They are also used internally, where galvanizing is usually unnecessary.

Gangnailplates are made for fixing larger components together, such as the timbers within a roof truss. The gangnailplate is a galvanized steel sheet punched to form many nails, which makes a good fastening and prevents the timber from splitting.

Screws draw themselves into the timber as they are turned, and are used in finer joinery work, ironmongery and internal detailing (Figure 17.3). The work is more demanding than nailing, but screws damage the timber far less.

Bolts. Metal bolts are used in connections where strong forces are to be transferred. Toothplate timber connectors are often laid between structural parts to increase the capacity of a bolt to transfer loads. These connectors have spikes that are pressed into the timber so that the forces are transferred to the surface of friction between the two parts. The bolt's task is thereby reduced to simply holding the two structural parts together.

Generally speaking, metals have limited reserves. In certain cases scrap metal is used. Energy demand in production is high, and serious pollutants are emitted from the processes. It must be considered as an overinvestment of quality when using galvanized steel products in dry, indoor environments. Untreated steel products have a far better environmental profile.

Metal products do not cause environmental problems in buildings. In a fire, however, they will quickly become red hot and burn through adjacent timber.

The durability of metal products is generally good. If a metal component is exposed to great variations in temperature, condensation can form on it. This has a deteriorating effect on the adjacent timber through electrolytic activity. If timber is damp when a metal component is added, the same effect could occur. Timber impregnated with salt can also corrode metal.

Nails and nailplates have no re-use value, and will probably not be saved for material recycling. Exceptions can occur when demolition material is burnt and metals are cleaned from the ashes. High quality screws and bolts can be retained and re-used or recycled. Use of screwed and bolted connections also means that materials they join together can be easily dismantled and re-used. This is, however, not the case if screw heads are damaged or blocked with filler and paint or if bolts are seized up. Under such circumstances dismantling of nailed constructions can prove to be an easier task. Metals that cannot be recycled should be deposited at special tips.

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