US3867803A - Flat joist truss with rounded load-transfer surfaces - Google Patents

Abstract

This invention relates to an improved light metal plateconnected flat joist truss and, more specifically, to a truss of the type aforementioned wherein novel webs are used characterized by rounded load-transfer surfaces on both ends that adjoin one another in a zigzag pattern while mating in strut-forming relation with the parallel chordal elements supported thereby so as to produce uniform essentially line contact between all abutting surfaces.

Description

United States Patent 1191 Platt Feb. 25, 1975 [54] FLAT JOIST TRUSS WITH ROUNDED 3,507,524 4/1970 statue-561161 52/693 x 3,651,612 3/1972 5611mm 52/693 LOAD-TRANSFER SURFACES Bruce W. Platt, Littleton, Colo.

Richardson Lumber Company, Denver, Colo.

Filed: Sept. 27, 1973 Appl. 196.; 401,186

Inventor:

Assignee:

US. Cl. 52/693 Int. Cl. E04c 3/12 Field of Search 52/690, 692, 693, 639,

References Cited UNITED STATES PATENTS 7 Primary ExaminerFrank L. Abbott Assistant Examiner-Carl D. Friedman Attorney, Agent, or FirmEdwards, Spangler, Wymore & Klaas [57] ABSTRACT This invention relates to an improved light metal plate-connected flat joist truss and, more specifically, to a truss of the type aforementioned wherein novel webs are used characterized by rounded load-transfer surfaces on both ends that adjoin one another in a zigzag pattern while mating in strut-forming relation with the parallel chordal elements supported thereby so as to produce uniform essentially line contact between all abutting surfaces.

4 Claims, 4 Drawing Figures FLAT JOIST TRUSS WITH ROUNDED LOAD-TRANSFER SURFACES Wooden truss beams having a pair of chordal elements supported and maintained in fixed spaced parallel relation by a zigzag pattern of strut-forming webs and gusset plates fastened at the intersections are well known in the art. Traditionally, the ends of each web are angle cut to define a pair of parallel horizontallydisposed surfaces intended to fit flush against the opposed parallel surfaces on the inside of the chords when the web is placed in inclined relation therebetween, and a second pair of vertically-disposed parallel end cuts designed to mate with a like cut on the adjacent end of the adjoining web in face-to-face contacting relation. In the United States, the regulatory agency that sets the standards and specifications for flat joist trusses is the Truss Plate institute. Design Specification For Light Metal Plate Connected Trusses: TPI-70 published by this institute states at section 303.3 as follows:

Tight fitting joints carrying compressive stresses may be designed to transmit up to 50% of the compressive force through wood to wood compression in accordance with the provisions of the NDS This means, of course, that while the gusset plates must carry a minimum of 50% of the all compressive loads and 100% of the tensile load, the wood to wood joint must, of necessity, carry the remainder of the compression load. it is in this connection that the difficulty arises with the prior art plate-connected flat joist trusses.

To begin with, it is only fair to point out that when the angle-cut web ends mate properly with one another and the chordal elements, the portion of the compression load they carry is spread over a wider area than with the round-cornered webs of the instant invention which make essentially line, rather than broad area. contact; however, such a condition rarely exists in the prior art angle-cut webs. Instead. errors in assembly, normal dimensional variations in the lumber, shrinkage, warpage and poor saw cuts all contribute to a condition where the planar surfaces on the angle-cut web ends seldom mate with like surfaces on an adjacent web or with the chords in face-to-face contacting relation asthey are intended to do. The net result is, of course, that the expected load-carrying ability of the joist truss is seriously reduced.

Also, just because the webs fit and mate properly in one section of the beam is no assurance that they will do so over its entire length because the aforementioned factors effecting the way they fit are of such a nature that a uniform condition, whether good or bad, becomes highly unlikely. Take the saw cuts, for example. Parallel end cuts adapted to mate with the opposed chordal surfaces will not do so in face-to-face contacting relation even though properly angled if, for instance, the web is cut just a fraction of an inch too long or too short or the lumber is thin even though within acceptable limits. Conversely, webs'cut to the right length will fit no better if one or both of the angle cuts at its opposite ends are off even a few degrees.

Completely apart from the difficulties associated with achieving a proper fit is the problem of determining the proper length and angle of the cuts to be made in the web ends depending upon the inclination thereof relative to one another and to the chordal elements. While many, and perhaps most, truss beams are made with the webs abutted in zigzag fashion at right angles to one another and at a 45 angle to the chords, by no means all trusses are so designed. This means, of course, that not only must the slope of the angle cuts be refigured for each different web inclination but, addition, the saws used to produce same mustbe reset.

Inherent among the problems associated with wooden truss beams having angle-cut webs is that of compounding errors. For instance, a short web will have to be raised into a more nearly upright position in order to reach all the way from one chordal element to the other. As this occurs, its planar faces will no longer mate properly with the chords or the companion webs alongside thereof. What is more, correcting for such errors or attempting to cancel out their effect by cutting complementary surfaces on adjoining webs or varying the length thereof becomes an exceedingly complicated and not always effective remedy.

It has now been found in accordance with the teaching of the instant invention that these and other shortcomings of the prior art light metal plate-connected flat joist trusses can be eliminated by the simple, yet unobvious, expedient of rounding the load-bearing surfaces of the webs so that they engage one another and the opposed surfaces of the chords with broad essentially uniform line without reference to their angle of inclination or other error-producing factors that would result in a mismatch of the conventional angle-cut webs. The resulting truss is much easier to fabricate and assemble and, while tolerances remain significant, the failure to maintain tight specifications is far less critical than in the prior art units of the same type. A truss fabricated in accordance with the teaching of the present invention can, with only minor modifications, be designed to provide the same load-carrying capabilities as the perfectly fitted angle-cut web truss while exceeding those of the poorly titted one. Whats more. even though such a truss might require a bit more material for equivalent design strength, the attendant cost increase would be more than offset by an accompanying reduction in labor costs.

Accordingly, it is the principal object of the present invention to provide a novel and improved light plateconnected flat joist truss.

A second objective is the provision of a unit of the 'type aforementioned wherein the load-bearing corners at opposite ends of each web are rounded.

Still another object of the within described invention is to provide a wooden truss beam wherein essentially uniform broad line contact is maintained between all mating surfaces of the webs and chordal elements notwithstanding their dimensional variances.

An additional objective of the invention herein jdisclosed and claimed is to provide a flat joist truss wherein all of the web ends are made alike regardless of their intended angularity with respect to one another in the zigzag strut pattern.

Further objects are to provide a wooden truss which is strong, reliable, inexpensive to fabricate, remarkably uniform, compact, readily adaptable for any application where conventional wooden trusses can be used, and one that is much easier to assemble than the prior art trusses of the same general type.

Other objects will be in part apparent and in part pointed out specifically hereinafter in connection with the description of the drawings that follows, and in which:

FIG. 1 is a perspective view looking down and to the left upon a light plate-connected flat joist truss constructed in accordance with the teaching of the present invention as seen from a vantage point thereabove;

FIG. 2 is a fragmentary front elevation showing some of the gusset plates removed to better reveal the roundcornered webs;

FIG..3 is a greatly enlarged fragmentary detail show ing the butt joints formed by adjacent round-cornered webs and the chordal element in supporting relation therebeneath; and,

FIG. 4 is a fragmentary detail to the same scale as FIG. 3 showing a slightly modified form of web in which the entire ends are rounded not just the loadbearing corners.

Referring next to the drawings for a detailed description of the present invention and, initially, to FIG. 1 for this purpose, reference numeral has been chosen to broadly designate the improved light metal plateconnected wood truss forming the subject matter hereof which will be seen to include a lower chord l2 atop which rests a zigzag assembly of strut-forming webs 14 that support upper chordal element 16 in fixed spaced parallel relation. In the particular form shown, this zigzag assembly is divided into two sections by upright struts 18 that span the gap between the chordal elements 12 and 16 while cooperating therewith to frame a rectangular opening 20 intermediate the ends. Similar, but somewhat shorter, mansards 22 are located at the ends so as to extend vertically from the lower chord 12 up to the horizontal member 24 that doubles up" the upper chord 16 for a short distance at each end. Such features as these doubled-up end sections, the overhang 26 of the latter, the center uprights 18 and the opening 20 through the middle, while commonplace, are by no means found in all such wooden trusses nor is any novelty predicated thereon. Doubling up of the chords and, in some instances the webs, is quite generally done at the bearing points and the location of these points varies considerably depending upon the building design. Instead, the truss illustrated embodying these features is intended as being merely representative of one such truss that can be fabricated using the round-cornered and round-ended webs 14 that cooperate with the chordal elements to produce the novel truss of the instant invention.

In like manner, while the truss shown is formed from l X 4s as the majority of them are, dimension lumber of other sizes can be substituted for that shown without the exercise of invention. The gusset plates 28 are, likewise, of standard design and, as previously mentioned, are responsible for carrying a minimum of 50% of all compressive forces as well as l00% of all tensile forces.

Suitable plates are widely available commercially and no patentability is predicated upon them alone, although they constitute an integral part of the novel truss that also includes the chords and round-cornered webs.

Next, with reference to FIGS. 2 and 3, it will be seen that the load- transfer surfaces 30 and 32 on the upper end U and the lower end L of each web 14 are rounded so that they rest in essentially tangent relation against some abutment and maintain tight line contact therewith throughout the entire width thereof. One such rounded load-transfer surface 30" customarily rests against the adjacent inside planar surface 34 of a chord while the other one 32 butts up against a similarly rounded corner of the web lying next thereto. Obviously, the rounded load-transfer surface 30 of some of the webs may not rest directly against a surface of one of the full length chordal elements but, instead, bear against the relatively shorter reinforcing member 24 as shown at the ends in FIG. 1. Likewise, load-transfer surfaces 32 will, in some instances, butt up against a vertical planar surface of an upright or mansard rather than another curved surface of an adjoining web. Be that as it may, essentially uniform line contact exists between adjacent abutting surfaces under all these various conditions, the load-transmitting effect of which is both predictable and reliable. While the term line contact as used herein to describe the relationship that exists between abutting surfaces is essentially correct, it is not completely so due to the compressible nature of the wooden chords and web ends when subjected to this type and magnitude of loading. In other words, while line contact would most accurately describe the relationship between tangent surfaces of non-compressible elements, as used herein it is intended to define not only the foregoing relationship but also those in which tangent curved surfaces or one curved and one flat surface of compressible wooden members engage one another under compression loads of sufficient force to slightly flatten, indent or otherwise deform the interface therebetween. Such deformation, slight though it may be, effectively spreads the load over a larger area thus reducing the per unit loading. The extent to which this load-spreading deformation occurs is, of course, a function of the density, hardness, moisture content, grain structure, etc. of the particular wood used as well as the magnitude of the compression load to which the abutting elements are subjected.

Now, looking at FIG. 3, it will become readily apparent that the same type of essentially uniform line contact between the abutting surfaces is maintained irrespective of the angle of inclination of the webs to the horizontal or to one another as exemplified by the solid and broken line positions. In fact, the righthand web is shown in broken lines to be somewhat more steeply inclined than the lefthand one, a condition which could occur with the former web slightly shorter than the latter. Nevertheless, the desired firm line contact therebetween remains effectively the same.

his important to note that the integrity of the truss is dependent upon two main factors, namely: (1) the load-transfer area, and (2) the tightness of the joint. As has already been pointed out, the instant truss design assures a substantially constant load-transfer area between abutting elements regardless of how they contact one another. Equally significant is the uniformly tight joints one is able to achieve by merely applying pressure to the zigzag strut assembly in a direction to close same accordian fashion. When this is done, each web in the chain thereof finds its own position relative to those alongside it necessary to maintain a series of tight essentially uniform load-transfer joints. In other words, if one web is a little shorter than the others, it will merely assume a more upright position while maintaining firm line contact with its neighbors. The net results, of course, will be a slight foreshortening or elongation of the truss depending upon the summation of the particular irregularities present in the wooden elements of which it is made; however, such is of little consequence ,cornered one shown in FIGS. l3, inclusive, has been the round-ended version. Since the slightly greater contact area realized by the round-ended version is less significant than the cost saving in reducing the waste from thousands of such webs, the round-cornered version is slightly preferred over the round-ended one.

What is claimed is:

1. A plate-connected flat joist truss which comprises: a first elongate timber defining a lower chord member with an upwardly-facing planar bearing surface; a second elongate timber defining an upper chord member with a downwardly-facing planar bearing surface; a plurality of relatively shorter web-forming timbers arranged in end-to-end oppositely-inclining relation between the upper and lower chord members cooperating to define a zigzag strut subassembly, said webs each having the surfaces on both ends thereof in contact with one another and with opposed bearing surfaces of the chord members rounded to establish essentially line contact therewith irrespective of the resultant angle of.

inclination each assumes within the strut subassembly; and, a pair of plates fastened on opposite sides of each joint thus formed effective to maintain same in assembled relation.

2. The flat joist truss as set forth in claim 1 in which: the rounded contacting surfaces are cylindrical and lie tangent to the surface in mated engagement therewith along essentially the entire length thereof.

3. The flat joist truss as set forth in claim 1 in which: the corners on both ends of each web member are rounded about spaced substantially parallel axes to define separate quarter rounded cylindrical surfaces.

4. The flat joist truss as set forth in claim 1 in which: both ends of each web member are rounded about a pair of longitudinally-spaced parallel axes to define semicylindrical surfaces.

Claims (4)

1. A plate-connected flat joist truss which comprises: a first elongate timber defining a lower chord member with an upwardlyfacing planar bearing surface; a second elongate timber defining an upper chord member with a downwardly-facing planar bearing surface; a plurality of relatively shorter web-forming timbers arranged in end-to-end oppositely-inclining relation between the upper and lower chord members cooperating to define a zigzag strut subassembly, said webs each having the surfaces on both ends thereof in contact with one another and with opposed bearing surfaces of the chord members rounded to establish essentially line contact therewith irrespective of the resultant angle of inclination each assumes within the strut subassembly; and, a pair of plates fastened on opposite sides of each joint thus formed effective to maintain same in assembled relation.

2. The flat joist truss as set forth in claim 1 in which: the rounded contacting surfaces are cylindrical and lie tangent to the surface in mated engagement therewith along essentially the entire length thereof.

3. The flat joist truss as set forth in claim 1 in which: the corners on both ends of each web member are rounded about spaced substantially parallel axes to define separate quarter rounded cylindrical surfaces.

4. The flat joist truss as set forth in claim 1 in which: both ends of each web member are rounded about a pair of longitudinally-spaced parallel axes to define semicylindrical surfaces.

Images