Structural materials

A building structure usually consists of the following parts:

• The foundation: transfers the weight of the building and other loads to the ground, usually below ground level. In swamps and other areas with no loadbearing capacity, the load must be spread onto rafts or piles going down to a solid base.

• The wall structure: carries the floor and roof and resists wind loads. The walls can be replaced by freestanding columns.

• The floor structure: carries the weight of the people in the building and other loads such as furniture and machinery.

• The roof structure: carries the weight of the roof and resists wind and snow loads.

In theory, these elements can be separated, but in practice the different functions usually have no clear boundaries. The structural elements have an intricate interaction in relation to the bracing of a building; for example, a particular wall structure can be dependent upon a specific floor structure for its structural integrity. Many parts also fulfil multiple building needs, such as thermal insulation, heat storage and sound proofing.

Structural materials are principally defined in terms of bending strength, compressive strength, tensile strength and elasticity. These properties give an idea of the ability of the material to cope with different forces. How this happens depends upon the design and dimension of the structure. A steel cable has its strength in its capacity to take up tensile forces - stretching - such as in a suspension bridge. A brick, however, almost entirely lacks any such stretching properties and must be used in a way that uses its compressive strength. Structures such as in brick that are in a state of static equilibrium tend to have a longer lifespan than those exposed to tensile loads, which in the long run are subjected to material fatigue.

The proportion of structural materials in a building may be 70 to 90% of the weight; a timber and steel building has the lowest percentage, and brick and concrete have the highest.

Structural materials usually provide less negative environmental effects per unit of weight than other building materials. They are usually based on renewable resources such as timber, or on materials with rich resource reserves such as clay, lime or stone. Production is preferably local or regional. The amount of energy consumed in production and

Table 13.1 Structural materials in foundation,

wall, floor and roof

1

Material

Foundation

Wall

Floor

Roof

DC

Steel

Gu

Gu

Gu

Gu

< Û.

Aluminium

Nu

Lu

Lu

Lu

Concretes with air- curing binder

Nu

Nu

Nu

Concretes with hydraulic cement

Gu

Gu

Gu

Gu

Stone

Lu

Lu

Bricks, well-fired

Lu

Gu

Lu1

Lu1

Bricks, low-fired

Lu

Lu1

Lu1

Compressed earth

Lu

Lu2

Lu2

Softwood

Lu3

Gu

Gu

Gu

Hardwood

Lu4

Lu

Lu

Lu

Turf

Nu

Empty spaces indicate not relevant.

Abbreviations: Gu: In general use; Lu: In limited use; Nu: Not in use.

1 As special structural elements or as vaults.

2 As vaults.

3 Pine below the water table.

4 Alder below the water table.

Empty spaces indicate not relevant.

Abbreviations: Gu: In general use; Lu: In limited use; Nu: Not in use.

1 As special structural elements or as vaults.

2 As vaults.

3 Pine below the water table.

4 Alder below the water table.

transport is approximately 30 to 40% of the complete house. Pollution due to greenhouse gases and acidifying sulphur dioxide will vary from 35 to 70%. The level of environmental poisons will probably be lower, and as waste products the majority of structural materials are not a problem. As these materials are relatively simple combinations of elements with large dimensions, they are well suited for recycling, but the type and quantity of binders and the size of the units are decisive factors.

Despite their relatively good environmental profile, the choice of structural materials is a decisive factor in a building's environmental profile because of their large volume and weight. If concrete and bricks are substituted by wood constructions, the emissions of greenhouse gases in material production are reduced by approximately 1 ton of CO2-equivalents per ton of wood used (Kram et al., 2001). This coefficient is based on a weight substitution ratio of concrete and bricks by wood of 5:1 to 10:1. If such substitution were applied widely in Europe, greenhouse gas reductions could be achieved corresponding to Norway's annual emissions of 50 million tons of CO2-equivalents. In addition, the timber should be credited as medium term storage of carbon corresponding to 0.8-0.9 tons of CO2 per ton of wood used (50 years). For Western Europe alone this would be of the order of 3040 million tons of CO2. However, as an important part of the calculations the role of operational energy use should also be taken into consideration. Here constructions based on brick and concrete will constitute additional thermal mass, which can reduce heating and cooling energy demand.

0 0

Post a comment