NONDESTRUCTIVE TESTING OF STRUCTURAL WOOD MEMBERS

Description and use

The use of wood for structural applications requires that, by some means, the strength, stiffness, and related properties of each piece be identified so that it can be relied upon to carry its load. Conventional strength-grading procedures have done a good job in the past in eliminating pieces of low strength. They have not been adequate, however, to classify each piece completely and, as a consequence, many structural elements are capable of carrying loads well beyond those which they are now carrying.

FIGURE 16.-Use of treated timber poles for transmission tower structures

Some more efficient means of strength classification has long been sought, and a nondestructive way of classifying stiffness or strength has been recognized as the most promising approach. The wood industry has, for some years, used a nondestructive means of determining moisture content based on the variation of electrical resistance or dielectric properties of wood with variations in moisture content. Stiffness is an important property in many applications, particularly in such uses as floor joists in frame construction. A lumber producer, Potlatch Forest Industries, Inc., has recognized the need for more accurate evaluation of lumber stiffness and has developed equipment for automatic inspection of each piece of dimension lumber for stiffness. There are indications that it may be possible to relate stiffness and flexural strength with sufficient precision that strength ratings may be established by the use of the relationship between stiffness and strength of a piece. Such inspection equipment thus can mark each piece with a suitable stiffness and possibly strength rating.

The research laboratory of the Western Pine Association has developed equipment for automatic inspection of dimension lumber and automatic rating for strength. While the basic principle used is slightly different from that used by Potlatch, the objective is similar. A commercial version of the machine has been designed and will soon be ready for field testing.

Future potential

Equipment such as that described will result in more accurate evaluation of structural properties. Thus, with more efficient use, material costs will be reduced.

Research is underway to search out other means of nondestructive testing. For example, the speed of sound is closely related to its elastic properties and the manner in which wood damps out vibration may be used as a clue to its strength. Research currently underway at the Forest Products Laboratory and a few other laboratories is providing a basic understanding of the vibration characteristics of wood and their application to nondestructive testing.

With the advent of nuclear physics and further understanding of radioactive isotopes, increased interest has developed in the use of radiation methods for nondestructive evaluation of wood quality. Principal effort is being concentrated on techniques based on transmission or backscatter of gamma radiation, but other research has involved beta radiation also. Such radiation is absorbed or scattered in a manner related to the amount of wood substance present and the amount of moisture in wood, and provides rapid nondestructive measurement of wood density or voids in wood, both of which are related to strength. It may also be possible to measure wood moisture content, a factor relating to many strength and other performance characteristics of wood.

References

1. Youngs, R. L. An Understanding of the Physical and Mechanical Properties of Wood. Forest Products Journal, May 1961.

Description and use

LIGHT FRAME TRUSSES

Builders of houses and other light frame construction have found the prefabricated wood roof truss a strong and economical method of roof construction. Fully half of the houses now built utilize this type of roof. Since a single design of truss may be used in hundreds of houses and thousands of trusses, great attention is being paid to the details of their design. Truss members are commonly of lumber, and joints may be glued plywood or wood plates, or may be any of a great variety of mechanical fastenings including bolts, connectors, nails, and special sheet metal plates. Development of these sheet metal plates has been especially rapid in recent years due to research at the Forest Products Laboratory, the laboratories of the Small Homes Council at the University of Illinois, the Truss Plate Institute, the Douglas Fir Plywood Association, and several university laboratories.

Future potential

A research program on the durability of the new types of truss joints and their behavior under longtime or repeated loading is planned at the Forest Products Laboratory. This research may well lead to an improved design and a wider use of this type of construction. References

1. Luxford, R. F. Light Wood Trusses. American Society of Civil Engineers Proceedings Paper No. 1839, 1958.

HORIZONTAL AND VERTICAL DIAPHRAGMS

Description and use

Recent observations in the Western States and elsewhere have shown that wood structures hold up well under high winds or earthquakes. As a result, engineers and architects have shown increased interest in the use of wood in structures especially designed to resist those forces. Floors and walls are now using materials and details of design that make them essentially structural diaphragms. The diaphragms include conventional materials conventionally used and also a variety of unconventional applications and of the new sheet materials such as plywood, hardboards, or fiberboards. Research has been sponsored to a considerable extent by the Structural Engineers Association of California and the findings are recognized in the Uniform Building Code of the International Conference of Building Officials and other building codes. The Douglas Fir Plywood Association has also carried on research and has prepared recommendations on structural diaphragms.

Future potential

Continued research on improved diaphragm design, improved use of materials, improved fastening methods, and further studies of the actual forces that buildings must withstand will lead to improved and more economical design methods.

References

1. Doyle, D. V. Diaphragm Action of Diagonally Sheathed Wood Panels. Forest Products Laboratory Report No. 2082, 1957.

HURRICANE-RESISTANT CONSTRUCTION

Description and use

Observations of houses damaged by high winds, waves, and floods have indicated certain weaknesses in the methods of construction, use of materials, and the fastenings of the materials and parts. Wellconstructed wood structures withstood these storms remarkably well. Whi e it is probable that no type of construction can be expected to be immune to the full force of these elements, the investigation has shown that much of the damage inflicted by severe storms could be prevented, or at least minimized, by employing the known basic principles of good construction.

Future potential

The use of basic principles of good construction, choice of materials, and methods of anchoring and fastening the parts and materials of structures will minimize losses in severe storms.

References

1. Luxford, R. F., and Smith, W. R. Observations of Damage to Houses by High Winds, Waves, and Floods, and Some Construction Precautions. Forest Products Laboratory Report No. 2095, 1957.

NEW COMPOSITE WOOD PRODUCTS

Description and use

In recent years, plywood has become an accepted wood product on the market. More recently, and particularly since World War II, a great variety of other wood products have become available. These are used in panel materials by themselves, or in combination with

plywood or other materials. For example, in addition to plywood we now have fiberboard or insulation board, hardboard, particle board, and laminated wood. Through adhesive processes we can combine these panels with themselves and with such other materials as paper, plastics, low-density core materials, and even nonwood materials, to produce what are known as composites. Perhaps the most common composite material is overlaid plywood, such as is used in tabletops, where the base material may be a thick plywood panel, and the surfacing material a high-density decorative paper-plastic laminate. Much research has gone into study of the properties and performance of such materials, their bonding and further fabrication, their relative characteristics with respect to each other, and their specific performance under certain end uses. Through development of such a variety of products. it is now possible to choose and design materials to meet rather exacting end-use requirements.

An example of a fabricating problem involved in the manufacture of a composite material is the importance of balanced construction in plastic-faced wood panels. Decorative plastic materials have been found to have certain dimensional movements when subjected to different atmospheric humidity conditions and to heat. These movements have been measured in the laboratory. Along with them the dimensional properties of the base panel materials, such as plywood, lumber, or particle board, have been measured. The stresses developed when these materials tend to move, measured in terms of modulus of elasticity, have also been determined.

When several materials having different properties are glued together, the resulting composite may exhibit unusual properties that may result in warping of the composite. It has been found that a knowledge of the basic properties of the materials involved, such as the unrestrained dimensional movement, the modulus of elasticity, the vapor transmission, the rate of dimensional change with change in moisture content, and the creep properties of the adhesives used to bond them together, can be used to predict how the final composite will perform in use. With this knowledge it is then possible to design composite panels for various uses. Such knowledge is particularly useful when we think in terms of designing composite panels for building construction. A knowledge of the dimensional performance of the proposed composite is essential in order to get satisfactory performance in the final building or structure.

References

1. Brouse, Don. Composite Wood-Trends in the United States. Composite Woods, volume 4, pages 37-44, January-March 1957. 2. Heebink, B. G. Importance of Balanced Construction in PlasticFaced Wood Panels. Forest Products Laboratory Report No. 2197,

Overlaid lumber is a lumber product surfaced on one or both sides with a thin sheet material called an overlay. The overlay may be made of paper, paper plastic, or other thin synthetic sheet materials. The most common overlay in use at the present time is the decorative plastic laminate made of paper and phenolic resin. This is the sur