These machines Pipe threading machines are supplied in a number of sizes and have a wide range of capacity. are driven by belts from gasoline engines, or direct connection, or by belt from an electric motor. obtained using either a solid die up to about 2-in. size of pipe, or with separate jaws of 2 or 4 pieces for larger sizes of pipe. Some of these machines are equipped with a patent release which will open the jaws or reverse the machine when the desired length of thread has been cut. As pipe threads are cut to a taper to insure tight joints, it is unnecessary to cut the thread much over the length of the die. Bar threading machines are similar to those used for pipe threading, the difference being in the size and shape of the teeth of the die. Bar dies also are cut without taper threads. Bolts and bars up to about 4 in. can be threaded in these machines and either right or left-hand threads cut. On bars above 4 in. in diameter and where special threads are required, the cutting is done in a lathe. 40. Cotton, Manila, and Wire Rope.-Two kinds of rope are used in building construction, -one made from hemp, cotton, or manila fiber, and the other of wire. The latter is the kind most employed as it will withstand harder usage, handle larger loads, and has a longer life than the others. Cotton rope is not much used as it is not suitable for loads of any amount. Its principal use is in braided rope for sash cords as this rope is very flexible and may be run over small sheaves. Hemp and manila fiber rope are used to a large extent where intermittent service and portability are required. They will give good service where the loads are not excessive and where lightness is desired. Hemp and manila rope are made up from threads twisted to form strands and the strands twisted together to form the rope. Due to the internal chafing between the fibers and the stresses due to bending when passing over sheaves, this type of rope should not be used in elevators and hoist service as it rapidly depreciates in strength value. MANILA ROPE. ULTIMATE STRENGTH, WEIGHT AND WORKING STRESS OF BEST MANILA ROPE (From Ketchum's Structural Engineers' Handbook, p. 443) Wire rope is made up of a number of small wires composing a strand and several strands twisted or braided to form a rope. The material used for the wires may be wrought iron, cast steel, or plough steel, each being suited to special needs. Wire rope is used for hoisting and elevator service as its depreciation from the action of bending over sheaves and on drums is less than is the case with manila or hemp ropes. The strength of wire rope is about 4 times that of manila rope and the weight per foot 8 times. Wire rope is made for various other services, such as transmission, and also in round and flat or ribbon form. The chier advantage of the latter type is that in the larger sizes it is more flexible and can be run over smaller sheaves. Wire rope made of 18 strands, the 6 inner ones being laid in the reverse direction to the 12 outer ones, forms a non-twisting rope and one that is very efficient where long lengths are to be used, as in hoisting materials several stories with a derrick. CRUCIBLE STEEL HOISTING ROPE. WEIGHT, ULTIMATE STRENGTH, AND WORKING LOADS OF WIRE ROPE COMPOSED OF 6 STRANDS AND A HEMP CENTER, 19 WIRES TO THE STRAND (From Ketchum's Structural Engineers' Handbook, p. 444) PLOUGH STEEL HOISTING ROPE. WEIGHT, ULTIMATE STRENGTH, AND WORKING LOADS OF WIRE ROPE Composed of 6 STRANDS AND A HEMP CENTER, 19 WIRES TO THE STRAND (From Ketchum's Structural Engineers' Handbook, p. 446) 41. Chains and Chain Tackle.-Essential items of hoisting and erecting equipment are tackle chains. In Fig. 80 are shown three standard types (taken from Ketchum's "Structural Engineers' Handbook," p. 451) and in the accompanying table are given data on the strength of chains (furnished by the American Bridge Co.). SECTION 7 BUILDING MATERIALS TIMBER BY HENRY D. DEWELL Trees may be divided into two main divisions-exogen and endogen. The first division comprises those trees in which the wood is arranged in concentric bands or layers. This construction results from each year's new growth of the tree forming an approximate cylinder of new wood outside of the previous old growth cylinder. The second class of trees, the endogen type, embraces the palms, bamboos, and other trees in which the wood is not arranged in bands or rings. All timbers used in building construction in North America may be said to fall under the exogen class. Two subdivisions of the exogen trees are recognized―viz., (1) conifers, sometimes called softwoods, and (2) non-coniferous, dictoyledons, or hardwoods. The terms "softwood" and "hardwood" are not correct, as many of the coniferous timbers are hard, and some of the non-coniferous woods are comparatively soft. In the class of coniferous trees falls the larger portion of the structural timbers, the longleaf yellow pine, shortleaf yellow pine, Douglas fir, white pine, spruce, hemlock, cypress, redwood, etc. Under the non-coniferous, or hardwoods, are classed the oak, ash, elm, maple, wainut, hickory, etc. 1. General Characteristics of Timber.-The cross section of a tree of the exogen type shows on the outside the bark, and, in the interior, a series of concentric rings, those next to the bark being lighter in color than the interior. This outside zone of rings is termed the sapwood, as contrasted with the darker interior portion, which is called the heartwood. The sapwood is the living portion of the tree. The heartwood, on the other hand, has ceased to grow, and is only of structural importance to the tree. Sapwood is, however, structurally as strong as heartwood, except in the case of old, overmature trees. season. The rings of growth are termed annual rings, since a new layer of wood is formed each Each annual ring is divided into two parts, the inner ring being softer and lighter in color than the outer ring. The inner ring is termed the spring wood, being formed in the spring of the year, while the outer, harder, and darker ring is the summer wood. The annual rings thus form a record of the age of the tree. The general structure of wood is cellular, the cells, or wood elements being in the nature of minute tubes, with interior cavities called lumina. These elements vary in size and shape; and different arrangements of cells characterize different trees. Upon the character and arrangement of the wood cells, and the nature and quantities of the compounds associated with them depend the physical qualities of the timber. Wood elements are classified as wood fibers, tracheids, vessels, pith ray cells, and wood-parenchyma fibers. In the walls of the wood elements are openings, covered by thin membranes, known as "pits," these pits being further classified as "simple pits" and "bordered pits." The pits serve to transmit water between the wood cells. In the coniferous or softwood trees, the tracheids (and in the non-coniferous or hardwood trees the wood-fibers) are the elements of most importance; these give the mechanical strength of the tree. The tracheids and wood fibers run parallel to the length of the tree, so that they appear in section in a cross section of the tree. Pith rays are cells that lie in horizontal planes, and extend radially from the center of the tree to the outside, connecting the vertical elements. These pith rays, also called "medullary rays, "have as their main function the transmission and storage of food. Pith rays are plainly visibly in many timbers, as for example, oak. In this latter wood, they give the pleasing appearance in "quartered" or "quarter-sawn' oak. The mechanical properties of timber are affected vitally by the arrangement, as well as by the character, of the wood elements. Since the wood elements, as the tracheids and the wood-fibers, are vertical, most woods are comparatively easy to split. On the other hand, the pith rays tend to hold the vertical wood elements together and thus lessen the tendency to split. When the annual rings are wide, the timber is said to be "coarse-grained;" and conversely, when these rings are narrow, the timber is said to be “fine-grained." Normally, the fibers and tracheids are parallel to the axis of the trunk or limb of the tree. Such a condition gives "straight-grained" timber. In many cases, the fibers may be twisted or they may run in a spiral direction giving the condition termed "cross-grained" timber. The junction of the limb and stem (or trunk) forms the knots found in all structural timber. "Dead" or "loose" knots are formed by the stubs of broken or decayed limbs, the growth of the tree eventually covering these stubs. 2. Effect of Composition on Mechanical Properties of Timbers.—The chemical composition of wood consists mainly of cellulose and other materials designated as lignin. There are also present certain other substances, as water and resin. The weight of wood depends: (1) on the amount of wood substance in the cell walls, and (2) on the amount of water contained in the wood. In green wood the second factor is of the greater importance. The water is contained in the substance of living cells, saturates the walls of all cells, and more or less fills the cavities of all lifeless cells, fibers, and vessels. Sapwood contains the most water. The rate at which the water evaporates from timber depends upon the structure of the wood, and also upon the size and shape of the stick. High temperatures accelerate the drying of timber, even in humid atmosphere. When timber dries, the cell walls of the wood-elements shrink. The wood-elements decrease in cross section, but remain approximately of the same length. This phenomenon explains the shrinkage in cross section of any piece of unseasoned or green timber, and the approximate absence of shrinkage lengthwise. Since the wood cells in the same tree vary in thickness, unequal shrinkage takes place, resulting in strains which tend to split and warp the timber. Again, the ends of a stick of green timber dry faster than the interior portions, producing cracks in the ends of such timbers. Such tendency to crack is known as "checking." It is customary in lumber yards to nail strips of wood across the ends of wide planks or boards, in order to prevent the ends from checking. The same tendency to check exists in the sides of timbers, due to the exposed surfaces drying faster than the interior. The shrinkage of the pith rays is one of the causes of the longitudinal shrinkage of timber, and the greater the number of pith rays, the greater the longitudinal shrinkage. A log shrinks tangentially to the annual rings much more than radially, since in a tangential direction the cells of the summer wood which are subject to greater shrinkage than the spring wood, are continuous across the width of board, but are interrupted in a radial direction by the rings of the spring wood. This tangential shrinkage leads to permanent checks. The difference in shrinkage, as measured tangentially and radially to the annual rings, explains the different behavior of lumber cut tangentially, radially, quartered, etc. The following table gives the approximate shrinkage in width of timber, drying in open air: (1) All light conifers (soft pine, spruce, cedar, cypress).. (2) Heavy conifers (hard pine, tamarack, yew), honey-locust, box elder, wood of old oaks. Percentage of width 3 4 5 6 Up to 10 Shrinkage of timber is a very troublesome factor to deal with in the design and construction of details. Fully seasoned timber, unless kiln-dried lumber is purchased, is almost impossible to obtain, and consequently some shrinkage is almost certain to occur. This shrinkage will cause joints to open, washers under bolts and nut heads to become loose, settlement of floors, etc. It is exceedingly important to recognize the probability of shrinkage and to design details that will be as free as possible from the effects of such shrinkage. Season checks are always unsightly but in interior work they are not generally serious from a structural standpoint except in the case of beams and girders where the unit longitudinal shearing stress is high. In such a case, season checks near the ends of the beam reduce the effective area to resist longitudinal shear. In construction exposed to the weather, season checks permit moisture to collect resulting in decay of the timber. 3. Effect of Seasoning on Strength of Timber.-The general effect of seasoning timber, if such seasoning is properly done, is to increase the strength of the timber. This statement applies especially to the strength of the timber in bearing across the grain. Forest Service Bulletin No. 88 gives the results of bending tests on green and air-dried halves of ten 8 × 16-in. × 32-ft. stringers-that is to say, ten green stringers, 8 × 16 in.,1 See also Appendix F. |