Structural Timber

Introduction:

In my report I will discuss the historical and environmental element of timber as a structural frame. As our awareness of our carbon footprint increases so does societies need to be more sustainable. Timber has been around since the beginning of time; it has low embodied energy and absorbs carbon, reducing the environmental impact it has on the earth, thus being one of the most rated building materials.

Historical:

Wood is a natural product because it comes from trees. Different trees and different parts of trees produce different types of woods, for example coniferous trees produce softwoods and broad-leafed trees produce hardwoods. Wood must go through many processes before it can be made into a timber frame, some of which include sawing, seasoning and grading. The first timber framed buildings were pit houses, lean-tos and teepees.

Figure 1: Pit house Figure 2: Lean- to Figure 3: Teepee

(Source: Google Images)

In the middle ages the first braced wall frame of timber was built. This was known as heavy wood framed construction. Since then braced wall frames were very popular and one of the only methods used. When the british carpenters imigrated to North America in the 17th and 18th centuries they were able to share their knowlenge of braced wall frames with the North Americans. They then proceeded to work on buildings framed with hand- hewn wood pieces, which are pieces of wood cut or shaped with hard blows of a heavy cutting instrument like an axe, joined by interlocking wood to wood connections. This was because nails were rare and expensive so they could only be used for door and window constructions.

Figure 4: interlocking hand hewn wood (Source: Google images)

In the 19th century builders realised that the heavy posts of the frame could be removed due to the closely spaced vertical pieces of wood used to fill the walls of the heavy timber frame were strong enough by themselves. This is called light wood frame construction and was only accomplished due to boards, smaller pieces of wood and nails becoming inexpensive and more accessible. The earliest light wood framed construction is the balloon frame, which used full length studs that ran from the foundation to the roof but this was too hard to put upright. Several more frames were developed including the universal standard platform frame. This is constructed by elevating the walls on top of a floor platform, this is repeated if there is a second story and the roof is then built upon the set of wall. Different variations of the platform frame are available to account for the number of stories in the house, to include attics and to even incorporate the use of a concrete slab foundation. Timber structures are an increasingly growing option all around the world; it is well known for its accessibility, cheapness and sustainability factor.

Environmental:

Buildings account for 30 to 40 precent of the world's energy use and associated greenhouse gas emissions (The U.S Green Building Council 2008). To be sustainable, that is to meet the needs of the current generation without compromising the needs of the future generations; we need reduce the amount of energy expended by buildings through design and construction. This is why timber is the perfect building material because not only is it strong so it will last for thousands of years but it is renewable and uses less energy than other materials such as brick, concrete or steel. The table below represents the embodied energy in construction and maintaining buildings with different wall materials. The examples with a timber frame use much less energy than other building materials not only for completing the construction but also after forty years of maintenance.

Type of Construction Energy per unit area of assembly (MJ/m2) Energy used to complete construction (MJ) Energy used in maintenance over 40 years (MJ)

Timber frame, timber clad, painted 188 31020 24750

Timber frame, brick veneer, unpainted 561 92565 0

Double