Structural design factors

This section discusses the family home's structure, which is influenced by the plan layout, spatial organization and massing, which should be considered from the inception stage of design.

Prefabricated homes can be considered, especially lightweight timber systems, and they are becoming more common on the local market. However, these follow an individual set of rules, most homes being built as part of a series (plan catalogues). Custom design can be applied, but full understanding of these systems is needed for implementation. Thus, these buildings are usually built as the "in-house" designer suggests. Herein, this chapter concentrates on "traditional" methods of design and construction.

The vertical load-bearing structure of a family home usually consists of load-bearing walls. In single-or two-stsingle-ory homes, these are usually the external walls, generally sufficient to suppsingle-ort flosingle-or slabs. In some situations, or when large spaces are required, frames are used. (Twentieth-century architecture saw concrete framed structures used in villa design by Le Corbusier, steel frames by Mies van der Rohe, etc.).

Most load-bearing walls are constructed from brick or other silicate-based construction blocks. Today the emergence of "ecological architecture" has seen a modern rebirth of earth walls.

Note

Care should be taken with external walls to provide adequate thermal insulation, usually resulting in many layered structures. External brick walls will require an additional 10-15 cm thermal insulation on the external surface, which is finished in plaster or another material. Until now, external brick walls were of 38 cm thickness. Rule of thumb suggests that brick walls should be at least 48 cm thick in standard situations and not less than 30 cm for lightweight construction. These wall dimensions are included when calculating gross plan area. (AN )

External walls may or may not be load-bearing. Care should be taken when placing doors and windows to ensure that the resulting pillar walls are suitably designed, in terms of cross-section area, to carry loads. Closed elevations with fewer openings perform better (especially regarding thermal performance of walls constructed from porous materials). Non-load-bearing walls are preferable for locating openings. Usually it is not enough to depend upon external walls for load-bearing purposes (e.g., single tract buildings). Interior walls also might be structural.

When load-bearing walls run parallel to the longitudinal access of the home, these are called side walls.

When perpendicular, these are called cross walls.

Internal load-bearing walls can be smaller in cross-section. Often steel reinforced piers may be used.

When using internal load-bearing structures, spatial arrangements can be varied. Frequently, beams (cut uncomfortably across rooms) can be seen supporting upper levels below the ceiling plane. Care should be taken to ensure that they do not influence the shaping of interior spaces. (fig. 3.50f)

Figure 3.45. Arrangement of load bearing structures in family homes

Horizontal load-bearing structures (e.g., floors and beams) are usually of reinforced concrete, cast in-situ or from pre-cast monolithic modular manufactured systems. Timber floor slabs are also common.

Pre-cast beams sizes are determined by their length, distance between load-bearing walls and height, which should be calculated prior to production. These are usually supplied as 60 cm modular units.

Pre-cast monolithic slabs systems are suited to situations where two load bearing walls are provided.

In floor plans where walls might be irregular or not right-angled (e.g., converging walls and curved walls), pre-cast systems might be difficult or not suitable. In such situations, in-situ slabs should be constructed.

Monolithic slabs should take into account the location of a site and accessibility for contractors’

technical support. Monolithic slabs are indicated when liquid concrete can be delivered to site, or the contractor has the technology to produce high volumes of concrete onsite. Larger areas need to have pouring formworks erected and onsite manufacture of steel reinforcing. Pre-cast systems are better suited to smaller projects, regular building forms and a smaller labor force, ideal for smaller construction firms.

Economical choice of spans for structures should be considered (correct proportion for cross sections).

Residential buildings usually have maximum spans of 6.00 m (7.20 m pre-cast unit length), and about the same applies as an upper limit to in-situ slabs. (Larger spans require more specialized engineering solutions and are usually thicker and cost more.)

Vertical and horizontal systems of spatial load-bearing have been divided into groups of possible solutions, shown in figure 3.50. When making initial design sketches, it is advisable to follow these guidelines, in order to avoid errors made by less experienced designers.

The choice of roof structure and coverings affects the architectural appearance. Various roof forms are given in figure 3.51. Roofs can be described as flat or pitched. Flat roofs are those that have a slope of 1-3% and might be accessible (roof terraces) or unaccessible. Recently, green roofs have been covered with vegetation for environmentally-friendly reasons. Pitched roofs are often related to methods of covering: a roof of up to 20° might be covered in metal cladding, this being the lowest angle suitable for most roof tiles. (Manufacturers of roof covering state suitable roof pitch angles in their trade literature.) In Hungary roofs had a traditional pitch of 35-45° to suit climatic conditions.

Some homes, usually rural ones, have high pitched roofs (> 45°), but this is not traditional.

Figure 3.46. Commonly used roof structure types in family homes

Flat roofs allow for freer contour planning and covering of plane areas; pitched roofs require an educated understanding of geometry. Figure 3.52 shows the most common roof forms used in housing.

Complex contours used in pitched roof design can often necessitate multiple planes, which are often ugly and expensive. (fig. 3.52.1) At the sketch design stage, roof forms should be examined.

Pitched roofs can offer the advantage of providing a "buffer zone" function. In the winter, it can assist in reducing heat loss; and in the summer, a vented attic can help prevent overheating of living spaces.

Single pitched roofs can also function as the closing slab in addition to their function as roof covering.

(fig. 3.51i and fig. 3.51j)

Pitched roofs often have built-in attic rooms. This can be done even at a later date if allowed for.

(fig. 3.51f and 3.53) Roof structures are generally lightweight with low thermal inertia. High risk of summer overheating also reduced levels of sound insulation. This is why reinforced concrete slabs are often used (coffin roof). Attic spaces costs about the same as a conventional building story and therefore offer little in terms of economical benefit. This solution is usually used as a response to building height rules.

Figure 3.47. Structural arrangement for inhabited roofs

To construct an attic space, roofs with a pitch steeper than 35° are preferred, as lower-pitched roofs will have a reduced useable area of less than 1.90 m headroom. The roof frame itself should be designed to ensure that a minimum amount of "space" is left clear of horizontal and vertical structural elements.

Windows to the roof space can be placed in the gable walls or on the roof itself. Windows placed in gable walls need no special treatment. Roof windows can be built above the roof plane or on the roof plane. (fig. 2.54a and fig. 2.54b) Windows built above the roof plane require adaptation of the roof structure. Windows built into the roof plane are more expensive, but due to the fact the structure needs little or no adaptation, they might be more economical.

For those living in a roof space, it is important that roof windows are placed in positions that allow views out. (If the sill or the skylight window is too high, a feeling of claustrophobia is caused.) The side wall, often found in roof structures, is for the most part relatively high. This, in turn, raises the height of roof windows, upsetting the building proportions.

Flat roofs are often adapted to serve as terraces adjacent to living spaces. When attempting this, make sure that the structure supporting the upper level is supported below the floor slab (e.g., by a wall or pillars). Non-load-bearing walls are well-placed on the slab if not too heavy (lightweight walls and areas of glazing). Ensure that the terrace slab is not the same thickness as the interior slab, as no room will be left to provide waterproofing, thermal insulation or a suitable drainage slope.

Interior stairs must be considered in all homes with two or more stories. Where several apartments are located in one building, provide a common stair. Steps for comfortable use should have an angle within the range of the following calculation:

2R + G = 60-64 cm (R = Vertical Rise, G = Going or Horizontal Depth)

When stairs are curved in the plan, the central line of each step should be used for this calculation.

Building codes allow for steps to have a rise (R) of 20cm, but this can be uncomfortable and best suited to less frequently used places (e.g., cellar or roof). The latter can be accessed (if not inhabited) by a special attic ladder or fixed ladder. Attic space is usual accessed by a service hatch which conceals a collapsible ladder. If the attic is to be used for storage or clothes drying, then provide access stairs.

A rise of 18 cm is comfortable in most areas. (fig. 3.55) Figure 3.57 shows a comfortable, economical stair. (Special stairs should be designed for larger homes.) A curved stair should be calculated to ensure that there is no discomfort for the user, even though step widths narrow towards the centre. The handrail should also rises in a gentle arc. (fig. 3.56) Curved staircases should be used only when enough space is available to use them comfortably. Narrow spiral staircases should only be used to access gallery spaces as they can not be used to transport larger pieces of furniture or, in emergencies, stretchers.

Figure 3.48. Interior stair dimensions, construction

Figure 3.49. Sweeping stair construction diagram

Figure 3.50. Various interior stair types

An opening should be provided in the floor slab to allow for the location of stairs. This applies to precast and in-situ slabs, and it should be suited to accommodate the stairs’ load and geometry.

Single-flight, straight stairs are best when placed against a load-bearing wall. Fold-back stairs, being almost square in plan, require a support beam at the edge of floor slab. Try to locate this within the floor’s thickness and avoid beams that project below the ceiling plane. Stairs are always easier to construct when placed beside load-bearing walls. (fig. 3.58)

Figure 3.51. Relationship between interior stairs and floor slabs

Stairs to upper levels and basements should be placed together in order to save space. (fig. 3.59a) Consider that spaces below stairs can be hard to access and keep clean. (They are rather useful as storage areas.)

Staircases are an important design element. They should be attractive and treated with correct architectural gravity. Stairs might also provide an attractive way to separate spaces (fig. 3.59a) Stairs are not pleasing when accessed from narrow corridors or when straight flights of stairs are built as a corridor.

Figure 3.52. Spatial arrangement of interior stairs