1368 A NATIONAL PLAN FOR AMERICAN FORESTRY 



form of construction that is cheaper and approximates the strength 

 of solid timber. At present, effort is directed toward the production 

 of a glued laminated beam that will require high-grade lumber only 

 at the top and bottom, where stresses are highest, and can utilize 

 low grade and short-length material to fill in. Accomplishment of 

 this objective will make available beams of better and more uniform 

 quality and will lead to closer design, lower costs, and increased 

 markets for low grades of lumber. 



Conventional joints and fastenings in heavy timber construction are 

 inefficient. Modern engineering efficiency and high costs of material 

 will no longer permit lavish use of material to obtain strength and 

 rigidity. Methods must be improved so that joints of greater du- 

 rability and reliability can be made at less cost and with more efficient 

 utilization of the strength of the wood. Marked progress has re- 

 cently been made in determining strength values for nailed and bolted 

 joints and correcting previous handbook figures that varied as much 

 as 600 percent. Further work for research lies in determining the 

 holding power of screws in different woods and in developing metal 

 jointings in the nature of dowels or keys to supplement bolt bearings 

 in structural members. Experience abroad indicates that wood 

 adequately jointed may successfully contest the market in large 

 and increasingly important uses, such as radio masts, transmission- 

 line towers, and higheay bridges. 



The basic design factors of wood structural members are not 

 sufficiently know T n. Timber structural design at present is a process 

 of approximation. In the average wooden structure there are parts 

 vastly oversized for the strength required and others inadequate to 

 resist racking, bending, compression, and other live-load effects. 

 Since we must consider three axial directions in wood there are 3 

 Young's moduli, 3 shear moduli, and 6 Poisson's ratios, or 12 elastic 

 constants to be taken into account. To compute rightly the elastic 

 behavior of wooden members under stresses requires experimental 

 determinations and the development of engineering formulas far in 

 advance of those now available. 



An example of the practical benefits to be gained today by a revalua- 

 tion of design factors in wood members is the more satisfactory and 

 economical design of bridge beams in shear advocated by the Forest 

 Products Laboratory and recently adopted by engineering profes- 

 sional bodies. Acceptance of the new rule means that railway and 

 highway bridge stringers can be sized to meet actual shearing stresses 

 rather than the stress figures formerly used, which often exaggerated 

 the actual condition by 50 to 100 percent. Wooden bridge design 

 has been made more exact and the sizes of beams have been brought 

 more closely within the range of commercial production, so that 

 there is less reason to turn to more expensive material. 



Greater attention must be paid to designing wooden structures 

 for appearance. The very great economy of treated wood for high- 

 way bridges and similar purposes is frequently ignored because 

 other structural materials are believed to produce more pleasing or 

 imposing structures. Introducing art into wood-bridge design will 

 serve the double purpose of expanding the markets for wood and 

 getting larger returns on public expenditures. 



