US2156818A - Fabricated truss - Google Patents

Description

' May 2,1939. F. N. ROPP 2 56,

FABRICATED IfRUSS 7 Filed Sept. 15, 1936 ATTORNEK [NI/ENTOR FRANKLIN N. Rep-P- Patented ay 2, 1-939 FABRIOATED muss Franklin N. Ropp, Los Angeles, Calif" assignor' to Soule Steel Company, Los Angel'es,0alif., a corporation of California Application September 15, 1936, Serial No. 100,881

8 Claims.

My invention. relates to the fabrication of steel structures for buildings, bridges, and the like. with particular reference to trusses.

For-the purpose of illustrating the principles of the invention, the present disclosure will be devoted to a truss in which the top chord supports a substantially uniformly distributed load, such as a roof structure, and the bottom chord also supports a uniformly distributed load, such as a ceiling. It will be understood, however, that the principles so illustrated apply to any type of truss, and that the configurations of the chords and the relative dimensions of the elements of the truss may be Widely varied to meet the requirements of particular installations.

The total or combined stress throughca section oi the upper chord of such a truss is the sum of a compression stress caused by truss ac tion and a bending stress attributable to load- 2@ ing between panel points; the total or combined stress through a section of the lower chord is the sum of a tension stress caused by truss action and a bending stress caused by the ceiling load between the panel points; and the com- 25 bined unit stress through any'section of either chord is the combined stress divided by the area of the section. Because the top chord is in compression, the allowable unit stress is less for the top chord than for the bottom chord. p An important object of, my invention is to proride a novel truss construction whereby the value oi the combined unit stress in either chord taken through consecutive sections throughout the length of the chord will have a minimum varia- 5 tion from a given value, preferably approxi mately the allowable unit stress for each chord. In such a-construction the cross-section of a chord will be non uniform, varying substantially with the combined stress throughthe section. Since the maximum combined unit stress in such a series determines the allowable load of the truss, andsince any understress with respect to such maximum value represents excess material, in achieving this object I approacha maxi- 45 mum value for the ratio of allowable .load to weight of material in the truss, thus providing an emciently designed, light, and economical structure. 1 The preferred formof my invention is char- 5, acterized by the conception that'the two chords of a truss may be shaped and dimensioned in a complementary manner to each other in the sense that corresponding sections of the two chords will vary inversely to each other. In

5 achieving this inverse relationship, I attain oneof the most impcrtant'objects of my invention,

- namely, the provision of a truss structure in which the combined corresponding sections of the two chords throughout their lengths will be constant in total cross-sectional area and in the pattern of that area.

While it will be apparent, in the detailed description to follow, that my truss, considered as a finished product without consideration for the problems of manufacture, is, in itself, novel and represents a substantial advance in the:

art, a great part of the value of my invention resides in the simplification and economy of manufacture made possible by the novel design. The requirements for economical manufacture of steel members conflict directly with the re-' quirements of chord members in an ideal truss, since only members of uniform cross-section may be produced by steel mills at less than prohibitive costs, whereas variation in cross-section is required for the chords of an ideal truss. The importance of the complementary relationship l between the two chords provided by my invention may be understood when it is' pointed out that a pair of such complementary chords may be readily cut from a single uniform member without waste of material. A feature of mylnvention, then, is that I ofier a complete solution for the aforementioned conflict between the requirements for economical manufacture of steel and the requirements for emcient distribushapes may be carried in stock in standard sizes and specialized only as used. Thus, a standard six inch I-laeam in stock may be converted by my process of fabrication into chord members specialized-for any number of particular installations.

In a preferred form of my invention it is my object to sever the uniform structural member along a longitudinal path that is essentially sinusoidal. Also, in the preferred form of my invention it is my object to cut the two complementary chord members from a uniform member having a. bottom flange, such as an I-beam, and to cut one of the chords shorter than the other chord so that said bottom flange will remain a part of the longer chord at each end thereof to serve as supporting surfaces at each end of the truss. By such a procedure I eliminate the necessity for end detail in the construction of the truss.

Further objects and advantages of the invention will be made evident throughout the following part of the specification.

Referring to the drawing, which is for illustrative purposes only,

Fig. 1 is an elevational view of a portion of a ber by diagonal struts I3, individually indicated as Ila, lib, I30, and lid. Prior to being severed from the upper chord member I I, the lower chord member I2 existed as a unitary part of the I-beam III, as indicated by dotted lines I211. The fabricated beam shown in Fig. l is divided longitudinally into panels A, and extending longitudinally of the web I! of the I-beam I0 I have shown a sinuous path indicated by a dotted line I4 which undulates from a distance D from the lower face II of the beam I0 to a distance D from the upper face I6 of the beam It, the valleys of this sinuous path I4 being centralized 'on the line L of division of the panels A. The undulating curve or path I4 is an elongated sine curve wherein one panel A represents a revolution of 360, the curve being of shallow character as shown and being formed in centralized relation to an axis X-X which is closer to the lower edge of the I-beam ID with the result that the dimension D is greater than the dimension D. Following and approximating the sine curve I4, I show a line or path of cut I8 on which the beam III is actually severed by suitable means such as an oxyacetylene cutting torch. It will be noted that the line of cut I 8 does not follow the sine curve I4 to the extreme ends of the beams I0 but swerves downwardly at I8 so that the cut will run out the lower edge or face ii of the beam I0, thereby leaving a flange portion 2I at each end of the upper portion or'chord member II, and at the same time producing a lower chord member I2 havlngits ends 22 foreshortened with respect to the ends of the upper chord member I I. It will be perceived that the cutting of the I-beam In in the mannershown produces therefrom two members II and I2 which have T cross section, but which members II and I2 respectively have webs "and 24 which are complementary in character in that the cutting line I8 which defines the lower edge of the web 22 of the upper chord 7 member II matches or is complementary to the upper undulating edge, Ila of the web 24 of the lower chord member I2. By severing the beam IIi along the undulating path described, I have provided in the web 23 of the upper chord member I I a plurality of widened portions P forming plates to which the upper ends of the diagonal struts I3 may be secured as by use of rivets '25, and at the same time I have produced in the web 24 of the lower chord member I2 widened portions P disposed in vertical planes intermediate planes indicated by the lines L to which the lower ends of the struts I3 may be connected by means of rivets 25. It will be noted that the line of cut I8 crosses the sine curve I4 at points 28 so that the line of cut, instead of being a true sine curve, merely follows the sine curve and deviates therefrom in a manner to produce shoulders-2'1 on the plate portions P of the webs 23 and 24.

In a chord of uniform section the combined unit stresses would be widely divergent and the at the panel point by deepening it, very materially r increases its moment of inertia, and, furthermore, reduces the unit stress in bending, since the unit stress is an inverse function of the moment of the inertia. The reverse holds for the center or reduced section, except that the increase of the moment of inertia for the increased section I is much greater relatively than the reduction of the moment of inertia for the reduced section. By properly varying the sections of the top chord and the bottom chord, I achieve in each chord member substantially uniform combined stress throughout each panel of the finished truss, and, by forming the two chord members by a cut that favors the upper member, I approach relatively closely the maximum allowable unit stress in each chord without exceeding said allowable stress.

By staggering the upper and lower panel points, it is possible to choose a beam section such that the depth of the beam is equal to the required depth for the top chord section between upper panel points, plus the required depth for the lower chord section at the lower panel points, or the required depth for the upper chord section at the upper panel points, plus the required depth for the lower chord section between lower panel points, whichever may control.

In Fig. 3 I show a graph of moments in -one ofthe panels A of the upper chord member II, specifically the rightward panel A of Fig. l, which is representative of the remaining panels formed throughout the length of the truss. In that portion of the chord member I I lying in the panel A or intersected by two vertical planes indicated by adjacent lines L, there exists a structural member S having fixed ends e owing to the fact that these ends 6 are connected rigidly to the adjacent portions of the upper chord member I I. The section S is subject to bending stresses due to a load superimposed thereon and is likewise subject to column or compressive stresses due to the tend,

ency for the loads on the truss to deflect the truss downwardly from end to end thereof. 'Ihe'structural member 8, owing to its having fixed ends, resists the superimposed forces applied thereto in the manner of a pair of cantilevers c facing inwardly from the ends e and being joined by a central beam section b. In Fig. 3 I have shown a graph of moments from superimposed loads applied to the structural member S, wherein the base of moments III is moved up relative to the moment curve 3| in accordance with the character of stresses to which the structural member slight reduction in cross section in the plates P due to the formation of the, rivet holes therein,

and to compensate for the reduction in section 'modulus in the plates P, I prefer to deviate the line of cutting Hi from the sine curve It so as to provide the shoulders 21 or additional metal in tened in character, which areled to by sloping inclines 2', and which are joined with adjacent valleys 1; through sloping declines d. This con dition or essential characteristic is found where the line of cut'coincides with the sine curve it, where the line of cut deviates or is exaggerated from the sine curve it as shown by the actual line of cut it, or further as indicated by the dotted line 35 shown in the lower right-hand corner-of Fig. l, which dotted line 35 indicates an indulating line of cut having essentially flat peaks 1) and complementary valleys 11' connected by approximately straight inclines and declines t and d with shoulders 21! which are comparatively sharp.

Owing to the fact that'the axis X-X is located below the longitudinal axis of the I-beam til, the

width D" of the sections b of the upper chord member i I is greater than the width D of the sectiO s b of the lower chord member it. This is another characteristic of the preferred practice of my invention contributing toward maximum economy in a fabricated truss of this character. By following this procedure, the upper sections b are given a greater section modulus than the lower sections b, which is' in agreement with the requirement that a larger section is required in the upper chord elements in view of the fact that they are subjected both to flexure and compression stresses, whereas the lower chord elements are subjected principally to tension, although they may be required to carry a ceiling load.

. It will be noted that the two points 26 in a panel are 180 apart as measured in. the revolution of the curve between panel points, each of the points 26 being at a quarter point or 90.3 from the nearest panel point. It will be apparent that each point 26 is approximately at the middle of the transition from the heavy section required at the panel point to the lighter section in the middle of the'panel, the transition being smooth and gradual by virtue of the inclination of the cut. Since the quarter point transition is approximately at the intersection of the undulating line with its own longitudinal axis, whether the cut is a sine curve, a modified sine curve, or is angular in configuration, it may be regarded as characteristic of the preferredforms of my invention.

Although I have herein shown and described my invention in simple and practical form, it is recognized that certain parts or elements thereof are representative of other parts, elements, or

mechanisms which may be used in substantially I the same manner to accomplish substantially the same results; therefore, it is to be understood that theinvention ,is not to be limited to the details 'set forth herein but is to be accorded the full scope of the following claims. I claim as my invention:

1. In a fabricated truss of the character described, the combination of: an upper chord member; a lower chord member in spaced relation thereto, said chord members being complementary in form as by the cutting of a plate along a wavy or undulating line extending longitudinally thereof so as to form alternate enlarged portions and narrow portions on said chord members while retaining substantially all of the original metal of said web in said chord members, said lower chord member being disposed with its enlarged portions staggered with respect to the enlarged portions of the upper chord member; and diagonal struts connecting the enlarged portions of the upper chord member with the wide portions of the lower chord member.

2. In a fabricated truss of the character described, the combination of: an upper chord member; a lower chord member in spaced rela-,

tion thereto, said chord members being complementary in form as by the cutting of a plate along an undulating line of a substantially sinusoidal character extending longitudinally thereof so as to form alternate enlarged portions and narrow portions on said chord members while retaining substantially all of the original metal of said plate in said chord members, said lower chord member being disposed with its enlarged portions staggered with respect to the enlarged portions of the upper chord member; and

diagonal struts connecting the enlarged portions of the upper chord member with the enlarged portions of the lower chord member.

'3. In a fabricated truss of the character described, the combination of: 7 an upper chord member; a lower chord member in spaced relation thereto, said chord members being formed by cutting the web of a structural shape long i* tudinally along an undulating line to form wide portions and narrow portionsr on said chord members connected by gradual lines of transi- 1 tion and whereby substantially all of the original metal of said web is retained in said chord members, said lower chord member being shorter than the upper chord member and being disposed with its wide portions staggered with respect to the wide portions of the upper chord member; and 'diagonal struts connecting the wide portions of connected by sloping inclines and declines, said web of said lower chord member having its upper edge defined by an unlulating line compledam.

mentary to that of said upper chord member forming spaced peaks and vallays connected by sloping inclines and declines, the wide portions of the web of the lower chord member being staggered with the wide portions of the-web of the upper chord member; and a single zigzag series of diagonal struts interconnecting the wide portions of the web of the upper chord member with the wide portions of the web of the lower chord member.

5. In a fabricated truss of the character described, the combination of: an upper chord member; a lower chord member in spaced relation thereto, said chord members being formed by cutting the body of a structural I-beam longitudinally along a single wavy or undulating line, to thereby form T-shaped chord members having webs of varying width facing each other while at the same time retaining substantially all of the original metal of the body of said 1- beam in the webs of said chord members and disposed with the wide portions of the web of one member staggered with the wide portions of the web of the other member; and diagonal struts extending from the middle of the wide portions of the web of one chord member to the middle of the wide portions of the web of the other chord member.

. 6. In a' fabricated truss of the character described, the combination of: an upper chord member; a lower chord member in spaced relation thereto, said chord members being formed by cutting the web of a structural shape longitudinally along a single wavy or undulating line centralized on a longitudinal axis disposed closer to the lower edge of the structural shape than to the upper edge thereof, thereby forming wide and narrow portions on said chord members and retaining substantially all of the original .metal of said web in said chord members; and diagonal struts connecting the wide portions of the web of said upper chord member with the wide portions of the ,web of the lower chord member.

7. In a fabricated truss of the character described, the combination of: an upper chord member; a lower chord member in spaced rela:

' web in said chord members the wide portions of the web of the upper chord member being staggered with the wide portions ofthe lower chord member; and diagonal struts connecting the wide portions of the web of the upper chord member with the wide portions of the web of the lower chord member.

8. In a fabricated truss of the character described, the combination of: an upper chord member; a lower chord member of shorter length than said upper chord member arranged in spaced relation thereto, said chord members being formed from an I-beam byacutting the web thereof longitudinally alongtan undulating line.

and terminating said line by| extending it downwardly through a lower flange adjacent at least one end of said I-beam, whereby a portion of said flange remains as a footing; and diagonal struts connecting said chord members together in said spaced. relation.

. FRANKLIN N. ROPP.

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