House Structure

3.3.1 House Structure

A shelter or house should be a barrier from the weather, with barrier surfaces that are supported by a structure, strong enough to withstand anticipated forces. Some

105 shelters have slanted surfaces that serve as both roof and wall, such as a tent by

3.3 HOUSE

tying together a number of slanted poles rising to a single peak and covering the sides with hide or cloth. In order to cover a larger area with more headroom, most shelter structures are designed to be a set of rectangles that join to form cubic or elongated boxes with vertical walls and with horizontal or pitched roofs. The famil- iar basic structure is horizontal beams resting on vertical posts, such as the trilithon structure from Stonehenge that dates from 2500 BCE. Let us consider a typical structure made of two posts or pillars supporting a beam or lintel, with a slanted roof with trusses or rafters. The roof can be left flat in very dry regions, such as the Mesopotamia and Egypt, or pitched at an angle to shed rain and snow in northern climates. The roof also needs to be covered by a material that is water resistant.

A house is usually built from abundant local material, which would be clay in Mesopotamia, stones in Egypt, and wood in China and northern Europe. Vertical posts are expected to support a great deal of weight, including the structure and possible snow load. Many natural materials are good at resisting compression, such as clay, brick, and stone. The usual building material in ancient Mesopotamia was clay, which was mixed with water, straw, and dung and then formed into rectangular bricks and left to dry. These sun-dried mud bricks were somewhat durable in semi- arid places. It was expensive to produce the more durable kiln-dried bricks that required burning fuel in kilns and were reserved for palaces, temples, and elite homes.

The vertical post is subject to compression forces in holding up the weight of the structure. Figure 3.4 shows that in addition to compression, the horizontal beam is also subject to a bending force so that the center will sag under; the top surface would be under compression and the bottom surface will be under tension that can cause the beam to fracture and fail. When the length is doubled, the amount of sagging is not simply doubled but increased by the cube of 2, or 8 times! The slanted roof rafter also experiences bending, especially if the roof is heavy and the pitch angle is low. The materials clay, brick and stone are not good at resisting bending. So the Egyptian solution for horizontal spanning was either to make the stone beam extremely thick, which was expensive, or to decrease the distance between the posts, which decreased the clear space under the roof. Limestone

FIGURE 3.4 Bending of Compression

beam with compression on top and stretching at

Tension bottom.

106 CHAPTER 3 DOMESTIC LIFE: FOOD, CLOTHES, AND HOUSE

beams were used to span no more than 3 m, and the stronger sandstone sometimes spanned as much as 10 m. Another solution was to use wood beams, particularly cedar from wooded Lebanon, which has strong and flexible fibers that are much better at resisting bending.

It is relatively easy to find trees that yield useful beams as long as 8 m, and there are some English cathedrals that have oak beams that are 17 m long, with a

beams, and rafters made of wood, but they collapsed from flammability and rotting after many centuries. That is why modern tourists can see numerous Egyptian and Greek temples with standing columns and no roofs, but the original ancient Chinese palaces and temples are no longer standing.

The problem of spanning a roof over large spaces for palaces and temples was solved later with the invention of arches and domes, where only the bricks and stones were subject to compression. Even later came the invention of steel beams that are much better at resisting bending. When a steel beam is suspended from a cable, it can span extraordinary lengths, such as the Verrazano Narrows Bridge with

a span of 1300 m and the Akashi Kaikyo Bridge with a span of 2000 m. The house also needs a number of larger openings for entry and exit and smaller openings for light and air as well as for the escape of smoke from fireplaces. For the comfort of the occupants, the openings of a house for light and air benefit from windows that can be closed against the wind and rain; for the security of the occupants, the entry needs fastening and locking to avoid the intrusion of humans or animals. Primitive windows were simply holes in the wall, but later they were shut- tered with hide, cloth, or wood. The ancient Chinese used translucent paper windows, which admitted light but were not clear enough to see images. The Romans were among the first to use transparent glass windows, which admit both light and sight.