Why on earth do home builders continue to ignore the more rational approaches to structure? They ignore the "skeleton of the house" and go all-out for affectation and trimmings. Take frame houses, for instance. Conventional stud-walls are inefficient; while overdesigned they retain weak joints; both erection labor and materials are wasted; they offer maximum fire hazard; erection is slow and thus vulnerable to bad weather.
Extensive studies into the structural engineering of houses have been made by various private and government agencies. One government report, Strength of Houses,
Houses have never been designed like engineering structures. Since prehistoric times, safe house construction has been found by the tedious and wasteful method of trial and error. If the modern research that has proven so successful in the solution of other problems had been applied to houses, not only would homes be more satisfactory as dwellings but, much more important, the cost would be much less. This would be an outstanding contribution to the problem of providing acceptable houses for the low-income groups in this country.
Note the lack of engineering approach in house-building practice today. It is customary to assume that walls, floor, and roof contribute nothing to the strength of the building! All loads are supposed to be carried independently by the frame. And all stresses and loads are analyzed separately for each structural component! Design loads are calculated for compression, traverse, impact, and racking on walls, floor, and roof. Dead loads (gravity weight of construction) are added to live loads (objects or persons on the floor). To these the builder must add calculated wind, water, and snow loads. Then, to complicate matters more, the builder must be acquainted with the strength of various materials and fastening methods; the modules of elasticity, stress, shear, and deflection for each independent member, as well as for the effective cross-sectional area, grade, and species of each variety of material used!
In the engineering approach the usual building practice of analyzing each structural component separately is replaced by a consideration of integrated structural effect; and a deliberate attempt is made to have the foundation and roof function as extensions of the wall — to eliminate the separation of function between wall and roof, floor and foundation.
A structural system has only four basic forces to overcome; compression, tension, bending, and shear. Bending occurs when a weight or force is placed at a distance from a support. Shear, in mechanics, means a thrust outward at right angles to the stress. With these basic structural reactions in mind, various framing systems can be evaluated and compared in relation to strength per unit of material and of time expended in fabrication. Wood was inefficiently used, of course, by early settlers in building cabins of massive logs cut from trees. Later, with the advent of power-driven sawmills, wood-frames having lighter members were developed and less wood was required. The post-and-girder structural system provided a transition between the log cabin and the currently employed vertical-stud-wall system of construction, which in some ways, however, marks a decline from the post-and-girder design.
The Balloon Frame which employs vertical studs in an old-fashioned box-type house is still often used in a 2-story building. With ceiling joists supported on a ribband board let into the studs, it becomes possible to extend the studs unbroken to full building height. Platform-framing has almost entirely replaced balloon-framing on single story structures, especially a platform consisting of subflooring over joists supporting a stud wall. The wall, in turn, carries the roof-and-ceiling construction. Platform-framing lends itself especially well to panelized and tilt-up construction. In the tilt-up system the wall may be framed to any degree desirable on the ground and then tilted upward into place, braced, and fastened.
A major disadvantage of stud-wall construction is that it is prone to fire, since the rate of burning depends on the ratio of surface area to volume of timber. The use of many wood members makes stud construction fast-burning. The type of wood-structure that uses heavier and fewer pieces is better from a fire safety standpoint.
The Plank and Beam structural system is becoming more and more popular with home builders. It has many advantages, including resistance to fire. The National Lumber Manufacturers Association made a detailed study comparing relative costs of plank-and-beam construction with conventional construction, since there are fewer pieces to handle and the members used combine structural function with finish.
Beams covered with standard 2-inch decking may be spaced as far as 8 feet apart. A structural feature worth mentioning here is that, for the same evenly distributed load, the plank that is continuous over two spans is nearly two and one-half times as stiff (or rigid) as the plank that extends over a single span.
Pole-type wall construction is logical in conjunction with plank-and-beam work. Barked poles, penta-treated under pressure, have a guaranteed life of from 50 to 75 years. Spaced on 8-foot centers, poles serve the triple function of foundation support, bracing and framework to which floor, wall, and roof members are fastened. Labor, time and materials are all saved in pole framing. Since lateral girts replace wall studs, and since fewer and longer pieces of lumber are used, the actual framework can be erected rapidly (a bad weather advantage, especially, as the building can be quickly placed under cover). Four-foot-deep hand-dug postholes form the extent of pole-frame excavation, which makes this construction adaptable to hillside building sites. By placing a pole four feet in the ground (or five feet if you need double rigidity), a rigid or bracing effect is achieved. The same pole placed on a pier or foundation above ground reacts as a hinge and does not furnish rigidity to the structure. The lateral stability of conventional construction has to come by second intention, interior partitions, or from the end walls of the building. In this sense conventional framing is not structurally complete, since a direct lateral force, such as strong wind, might fell the building if it were not for the bracing supplied by the nonframing members added later.
The Rigid Frame is another structurally complete framing system. Like pole framing, however, it is seldom employed in residential construction. The rigid frame is made transversely stable by designing the wall-to-roof joints to remain rigid, whereas the pole-frame system supplies rigidity at the base of the support columns.
The surest method of achieving rigidity in a joint is through glue-nailed gusset plates—that is, a member (usually plywood) which is fastened to one or both sides of an angled connection. A rigid frame is in effect an arch, and as such it develops considerable outward thrust at the base, which then makes a well-designed foundation mandatory. Rigid frames have an added labor-saving advantage when they can be pre-assembled (prefabricated) on the ground and then tilted into position. In any kind of framing operation it is far more efficient to work on the ground than in the air. (Simple jig tables or contraptions can be used to advantage against errors and misfits where complex assemblies are indicated.) It is the cutting, fitting, and waste inherent in using extremely small components that inflate framing construction costs.
In pre-assembling wall units the owner-builder may capitalize on some of the many money-saving building techniques developed by the highly competitive pre-fabricated home industry. The Lu-Re-Co panel system, for instance, is fully adaptable, and offers a 30% labor and 10% framing saving over standard construction. Both interior and exterior panels, measuring 4 feet by 8 feet or less (depending upon window and door sizes), are framed in a special jib, and each panel (including sheathing, siding, window, door and even a prime coat of paint) is assembled on the ground.
Stressed Skin panels offer all the advantages of modern prefabricated panel construction along with greatly improved structural qualities. This type of panel consists of a frame with a continuous plywood skin — glue-nailed on each side of the frame. As the loads are partly carried by the skin, the framing members can be lighter and fewer in number than in the standard framing type of panel. Strength and rigidity are both increased by this combination of frame and skin.
A stressed skin panel is similar in structural action to a wide-flange steel beam; the top face carries compressive stress and the bottom face tension. The joist acts as the steel web. High shearing stresses that exist between the plywood skin and framing members are transmitted via glue-nailed joints. There are advantages of using plywood over ordinary laminated wood; the layers of wood are placed with their gains running at right angles to each other, thus eliminating shrinkage and increasing strength.
The 1959 National Association of Home Builders Research House at Knoxville has thin hardboard stressed skin panels on exterior and interior walls and on floor, roof, and ceiling. All panels are 8 feet high and vary in width from 1 to 4 feet, making this system completely modular in planning and details.
If maximum benefit from panelized construction is to be gained, all components must conform to a common unit of measurement, called a module. This will permit efficient assembly and eliminate all waste, fitting, and cutting operations on the job.
From the approach of ultimate structural efficiency, the stressed skin principle shows how ridiculous the framework- and -covering, studs- and -sheathing approach to structure really is. The skin-and-bones concept of structure is both wasteful and inefficient. Studs, rafters, and joists are first erected, and sheathing then applied. The sheathing itself is a structural liability, a dead weight, a parasitic covering.
The greatest structural potential in wood construction today seems to lie in the use of Curved Skin enclosures. The principle of curved skin is based on the engineering concept that all material in a structure should contribute directly to its strength. It is a concept of integral structure which offers exciting prospects to any wood-orientated owner-builder.
The Lamella system, first developed in Germany in 1923, takes full advantage of the high strength of compression in wood parallel to the grain. Loads are spread evenly over the entire network and are resisted by bolt-ties at each diamond-shaped diagonal. Lamella is essentially an arch composed of many short pieces of wood. Buckminster Fuller, too, employs a small-component triangulated system for the enclosure of his hemispherical domes of space. The Fuller dome develops an extremely high structural efficiency and strength-to-weight ratio.
Some builders have gone back to the log cabin design and developed a Solid Bearing line, with good results. A modern form of "log cabin" construction consists of 4 by 8 inch tongue and groove sawn logs. Some varieties use 1/2-inch steel rods spaced every four feet, through which the logs are threaded. Another log manufacturer (National Log Construction Co., Thompson Falls, Mont.) has patented a system for boring out the heart of the entire log and forming a tongue and groove weatherproofing. The "air lock" cavity allows the log to season evenly, thereby minimizing cracking and checking.
Solid-bearing log members can be laid vertically as well as horizontally. Square-edge 1-inch thick boards, with exterior battens and interior horizontal 1 inch ties, were commonly used in California forty years ago. More recently, 2 inch thick tongue and groove planking is nailed together vertically, sometimes with the planks staggered. A more striking resemblance to the palisade of pioneer days has been achieved by some inventive home builders, using peeled cedar posts vertically.
Early settlers in the Ottawa Valley discovered a method of building with wood which requires minimum labor and material cost. Dried cedar firewood logs, ten inches long and averaging five inches in diameter, are bedded in cement as though stacking firewood neatly along a wall. The outside and inside of the walls are then cement-plastered. A builder in Florida has constructed a number of such "cord wood" houses, using 7-inch thick palm tree sections. His four-room bungalow requires only $28 worth of palm logs.
The range of wood structures—between cemented stove wood at one end and stress-skin arches at the other—gives the wood-oriented owner-builder a wide choice.
BIBLIOGRAPHY (books listed in order of importance)
Plank and Beam System: House and Home Finance Agency, Washington, D. C.
Strength of Houses: Building Materials and Structures report 109, Bureau of Standards, Washington, D.C.
Material and Labor Analysis—House Framing Systems: House and Home Finance Agency, Washington, D. C.
Small Homes Council, University of Illinois, Urbana.
Lumber Dealers Research Council, Washington, D. C.
Doane Agricultural Service, St. Louis, Mo. (pole-frame building information).