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ments are distributed uniformly over the entire structure. Vertical and horizontal members interact to redis- tribute the bending moments. When, as in this case, adjacent members interact through a rigid junction, the redistribution of bending moments may effect an economy of material running as high as 30 per cent. Con- sidered broadly, the type of joint at the knee of the one-piece rigid frame bridge, shown in the lower part of Fig. 3, is the joint inherent in a properly welded design. _ Two plates can be welded together so that the joints as such vanish. A homogeneous joint, or transition be- tween two pieces of material, is a joint whose elastic behavior cannot be distinguished from that of the same configuration cut from a single piece of material. In the unattainable ideal, the maximum stress would have the same value as the average stress. It is thus indicated that a definite though intangible advantage continu- ally permeates the welded steel struc- ture. Since the joints can be made so that the continuity of rigidity is preserved perfectly in the transition from one member to another, the structure will act as an elastic entity. The fact that this ideal can be ap- proached closely in welded joints has two implications for the structural designer. Inertia Moment Is Lower First, as has been mentioned, is the fact that continuity can be taken into consideration in the design of gross structures such as bridges and build- ings in which the moment of inertia of each member is much less than the corresponding constant for the whole structure. Welded design is eminently adapted to the curvilinear structures which result when continu- ity of elastic action is considered in the design. The second implication is that the behavior of the properly welded structure, under impact and fatigue load, is similar to that of a jointless structure as far as service life is concerned. The entire struc- ture’s resistance to severe loading conditions represents the thoroughly 16 ® IG. 2 — The transition zone between weld met- al and 0.40 per cent carbon base metal. The large grain size indi- cates damage from the high temperature of the arc ® consolidated resistance of its compon- ents. A homogeneous joint has two char- acteristics. First, the material com- prising the joint must have uniform a — es Fig. 3—Sketch indicating two methods of supporting a load over a span. Above is a three-piece truss and abut- ment arrangement; below is a one- piece rigid frame physical properties from point to point; it must not have been damaged in any respect by the fabricating Second, the distribution of process. stress throughout the joint, from point to point, should be of maximum uniformity. A perfect distribution of stress over the joint is also an un- attainable ideal since the joint at its best represents an abrupt change in contour. With a homogeneous joint of the desired characteristics, the two mem- bers are joined in such a manner that neither the elastic action nor the plas- tic action of the composite is dis- tinguishable from the action of a single piece. Again, the joint has van- ished. Three Controlling Factors Three factors control the two de- sired characteristics of the homo- geneous welded joint. First, the par- ent metal is damaged by the extreme heat of the welding operation. Fig. 2 shows the transition zone between weld metal and 0.40 per cent carbon base metal. The large grain size is indicative of the high temperature to which the parent metal has been raised by the heat of the arc. The metallic heat path to the body of cold parent metal forms an excellent ther- mal sink and the resulting rapid heat flow away from the weld thorough- ly quenches the heated zones around the weld. The net result is a zone of highly overheated and quenched material adjacent to the weld. The severity of damage in this zone is a particular function of the carbon content of the parent metal, or more generally, a function of the air-hard- ening ability of the steel. This damaged zone, whatever its ex- vent, represents a discontinuity in physical properties, since the large, overheated grain is decidedly weak in resistance to fatigue and impact. In addition, the hardened microconstitu- ents, due to the quenching action, are very strong, but they lack the duc- tility necessary to compensate for the thermal stresses to which the piece Undercuts and resulting high local stress occur at point of maximum metal damage Tension loading will produce very high local stresses due to internal boundary Metallurgical damage: The zones of overheated and quenched metal represent severe discontinuities in physical properties Residual stresses: Their directions and magnitudes are unknown Fig. 4 (Left)—Photoelastic study of an improperly designed welded butt joint. The two welds, deposited from each side, do not meet at the center. Fig. 5 (Right)—Graphical representation of an improperly executed welded joint— physical properties are not uniform from point to point MARINE REvVIEw—January, 1934