Courtesy of Architectural Graphics Standards, 5th Ed.
The most common materials used for residential construction are wood and concrete. Most homes have a combination of both. In this section, we will look at some of the details of wood frame construction. Concrete will be dealt with in the following section.
Although there has a tendency in the Caribbean to favor concrete over wood for homes since the recent spate of major hurricanes, a properly designed and constructed wood frame house can withstand major hurricanes and earthquakes. Construction costs and insurance considerations certainly play a role in these decisions. But these factors vary considerably throughout the Caribbean and such issues are left to the student to consider as a Special Projects topic.
There are many framing styles used for residential homes, but the names for many of the various structural components are essentially the same. Below are diagrams of two standard framing styles with the components labeled. Below the diagram, the structural components are listed with descriptions and comments. We will spend time in class discussing the primary types of stress to which these components are subjected.
If you know of a house that is in the framing stage of construction, try to visit the site and see how many of the structural components you can identify. Of course, be sure to obtain permission from the owners/builders.
Rafters support the roofing material. Large rafters, which can be spaced further apart, are often called beams. Roof beams are typically used for aesthetic purposes when the interior of the roof is to be exposed. A beam also refers to any large structural element that spans long distances and supports significant loads. The beams that support the floors are referred to as joists. But the horizontal members that connect to the rafters at the wall, labeled "ties" in the first diagram, are called joists as well. In your third laboratory, you will learn that the structural load that a beam can support increases linearly with the thickness (t) but as the cube of the height (h). A 2x8 beam can support 8 times more weight than a 2x4, so 2x8 beams can be spaced 8 times further apart for the same load.
The post and beam style of construction uses large vertical posts and horizontal beams to provide the main skeletal structure of the house. The posts and beams are usually tied together with bolts or metal plates. Floors, walls, and the roof are essentially added to the skeletal structure. (A good example of this type of construction can be found at the Shibui complex on St. Thomas.)
Plates are used to cap the studs (and are often called caps) at the top and bottom of the wall. If the plate also rests on the foundation, it is called a sill.
Often a "header", a 2 x 8 or 2 x 10, is placed on edge between the foundation sill and the floor plate. In this case, the floor joists are connected to the header, rather than resting on the sill.
Some Interesting Notes
Diagonal braces can be placed in walls for a similar purpose. This provides rigidity against the shearing motion caused by horizontal winds and P-waves from earthquakes. Plywood sheathing can also be used to provide this rigidity. In general, diagonal elements increase rigidity because they introduce triangles into the structure. This is explained in more detail in the section below on trusses.
A simply survey of houses in the Caribbean indicates a large variety of roof shapes. Indeed, you can construct a roof of nearly any shape. A few common designs are shown below. The gambrel is often used for storage buildings where vertically space is needed. It is also found in homes in which the area under the roof is used as a living space. The lean-to or shed design is the simplist to construct, but provides little or no natural ventilation space. It isa used primarily for storage buildings. The most popular design found in the Caribbean is the hip roof. Although similar to the gable, the hip roof can be more strongly anchored into the walls and offers a more aerodynamic profile to wind, an important consideration in hurricane proned areas.
Trussing
If a roof is constructed with 2x4 rafters, rather than heavier beams, trusses are usually installed for additional structural support. Trusses are elements that connect the rafters to joists and/or other rafters. The diagram below shows a variety of truss designs for different roof shapes. You may occassionally find trusses in open ceiling designs, but usually it is used when there is an attic and the structure is hidden by a ceiling attached to the roof joists.
An important characteristic that all the designs have in common, is the use of the triangle. As you may have learned as early as grade school, triangular structures are rigid because the shape of a triangle cannot change without changing the length of its sides. Consider the two frames shown below. For simplicity, assume the structural components (struts) are connected at each vertex with a single nail. When external forces are applied in any direction to the triangular structure, the forces generated at the vertices will be shear forces. Provided the members themselves are sufficiently strong, the nails would have to be sheared for the structure to fail.
In contrast, the rectangular structure can change shape without changing the lengths of its sides. External forces can alter the shape of the structure by introducing only a net rotational torque about the nails; shearing of the nails is not required.
The rectangular frame above could represent a standard wall structure simply by adding more vertical studs. This would not change the fact that the wall could distort under external forces without shearing the connecting nails. A diagonal brace would effectively introduce a triangle into the structure and greatly increase the rigidity of the wall. The addition of a diagonal brace is highly advisable if the wall sheathing consists of horizontal or vertical boards. It is not necessary if the boards a placed on the diagonal or if plywood sheets are used in the wall sheathing.
Next, let's look at Concrete Construction.
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