Simplified design of structural timber

Abstract

This is an instructional manual on the design of structural timber.

INTRODUCTION. Timber is a material of construction most commonly used, and one of the most important of which buildings and other structures are built. It is found in many regions of the world. Compared with steel and reinforced concrete, timber is lighter, hence it is easy to transport. Timber may be classified as hard, tough, and flexible. Wood has been used by man since time immemorial. It was probably the only material of which Noah's Ark was built that survived the universal Deluge. It has been used to built the humblest home as well as the most beautiful and lavish palaces of monarchs, sovereigns, and potentates. It is a very useful material of construction in time of peace and in time of war. 1.2 TIMBER, WOOD, LUMBER. From Webster’s dictionary ore gathered the meanings of the following: a. TIMBER is wood, whether the tree or cut and seasoned, suitable for use in building, carpentry, etc. b. WOOD is the hard fibrous substance of trees and shrubs beneath the bark. c. LUMBER (chiefly U.S.A. and Canada) is timber, especially sawed or sliced into boards, etc. d. LUMBERING (U.S.A. and Canada) is the business of getting timber or logs from the forest for lumber. e. LOG is bulky length of a tree trunk or of unshaped timber. f. LOGGING is the business of felling trees, cutting them into logs, and transporting the logs to sawmills or to market. 1.3. CHARACTERISTICS. Timbers are of many kinds, varying greatly in their structural characteristics. Certain kinds of wood are more desirable for some purposes than others. Green timber is heavier than dry or seasoned timber. Seasoned timber is that timber which has been subjected to controlled drying, thereby improving its strength. Green timber has a tendency to warp and to shrink. Moist timber is more flexible than dry timber. Dry timber is lighter than green timber, has greater strength, and works better than green timber in sizing, sawing, and in making tight joints and connections. Soft wood is brittle, while most hardwood is flexible. Hardwoods do not split as easily as soft, woods. Any kind of timber shrinks as it dries, and for this reason, shrinkage should be taken into account in the construction of big buildings and bridges. Since shrinkage in timber can cause a great damage or serious defects, it would be best to use seasoned timber whenever possible. 1.4. PRESERVATION OF WOOD. One of the disadvantages of wood as a structural material is that it is subject to decay. However, if used under proper conditions, the wood in a structure will generally outlive the useful life of the structure itself. Among the causes of decay are the following; (1) alternating wetness and dryness, resulting in wet rot, (2) lack of ventilation, resulting in dry ret, and (3) the destructive action of fungi, worms and insects. In most structures, the effects of these causes are readily controlled. When it is known to the designer that the causes of decay cannot be eliminated, two solutions to the problem are available: (1) to allow the decaying process to proceed and to replace decayed timbers when necessary, in which case an average life of about six to ten years can be expected, or (2) to use timber treated with creosote or some other suitable preservative, in which case the average life is ten to fourteen years more, depending upon the degree of exposure to decay. In the creosote treatment, the timber, after having been properly seasoned, is placed in a closed cylindrical chamber, and steam is introduced to soften the wood fibres. Air and moisture in the timber are then removed by a vacuum pump finally, creosote is injected into the cylinder under pressure, resulting in almost complete penetration of the wood fibres by the creosote preservative. Other simpler and less expensive preservative treatments are also being used.