Systems

Load bearing capacity

1750 kg i

_oad bearing capacity

100 4

5210 kg

5360 kg

6400 fcQ

I Tm

Oh Ai

Steei piMars wlin the same amount of material (0013m1) and the same lenght {5m). but with diHerence In load bearing capacity

13.54

Material use in steel pillars. Source: Reitzel, 1975.

'10600 kg

13.55

Material use in steel beams. Source: Reitzel, 1975.

13.55

Material use in steel beams. Source: Reitzel, 1975.

Every structural system has its own specific use of material, depending upon its strength. Solid structures of brick and concrete are highly intensive in their use of material, whereas timber and steel are usually more economical, but each material can also have different structural methods using different amounts of material.

Figures 13.54 and 13.55 show structural alternatives to pillars and beams. These examples deal with steel components, but the same principles apply for timber. One important aspect of structures with economical materials use is that they are often more labour intensive. The lattice beam with its many joints costs more to produce than the equivalent laminated timber beam, even if the use of material can be twenty times less. In some cases, however, the extra cost of transport and more intensive use of raw materials in the laminated alternative can change the economic equation quite drastically. It is also quite important that use of lightweight structures enables one to reduce foundations considerably (see Table 13.6). In cases where the soil is clay, weak or swampy this can be a particular advantage.

The energy use in the production of a structure is dependent upon the quantity of material used as well as the type of material. A comparison of the energy use for producing structural systems in different materials is given in Figure 13.56. In conclusion, a timber lattice beam is definitively the most energy-efficient alternative.

The embodied energy and sources of energy used will be directly reflected in the climate footprint of a construction. For concrete, steel and aluminium one must add the materials related chemical emissions during production. Eventual re-absorption of carbon dioxide into concrete must be subtracted, and in the case of timber the carbon bound can be subtracted too. Ultimately there is little doubt that timber constructions come out very favourably (Goverse et al., 2001; Petersen etal, 2002; Kram etal., 2001; Graubner, 1992, etc). However, consideration relating to recyclability, thermal stabilizing properties (see Table 13.7), and durability will naturally also come into play, and perhaps even be of prime importance.

Comparing lightweight and massive timber construction is quite a complex issue. If carbon storage is important then this gives an added

|Table 13.6 Foundation requirements for different constructions and soils

CO

Construction weight

Marsh to soft clay (kg concrete/m2)

Firm clay to sand & gravel (kg concrete/m2)

DC

Light (wood and steel)

150

100

< รป.

Heavy (brick and concrete)

250

100

(Source: Glelen, 1997).

13.56

Comparative calculation of energy needed to produce different structures. The aluminium alternative is produced from ore. Source: NTI, 1990.

13.56

Comparative calculation of energy needed to produce different structures. The aluminium alternative is produced from ore. Source: NTI, 1990.

advantage to the latter. On the other hand long transport distances will weigh against massive timber.

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