US3368319A - Tall column structure of connected sections with warren cross-bracing and legs of channel section - Google Patents

Description

Feb. 13, 1968 w. L. WERNER ET AL 3,368,319

, TALL COLUMN STRUCTURE OF CONNECTED SECTIONS WITH WARREN CROSS-BRACING AND LEGS OF CHANNEL SECTION Filed Aug. 16, 1965 3 Sheets-Sheet 1 FIG.2 j

WILLIAM L.WERNER MAXI ME J. THOMAS fl/ 6. W

ATTORNEY mar 6 INVENTORS Feb. 13, 1968 w. WERNER ET AL 3,

TALL COLUMN STRUCTURE OF CONNECTED SECTIONS WITH WARREN CROSS-BRACING AND LEGS OF CHANNEL SECTION Filed Aug. 16, 1965 3 Sheets-Sheet 2 2z MENTORS WILLIAM L.WERNER MAXIME J.THOMAS F|G.5

ga/5W ATTORNEY Feb. 13, 1968 w. WERNER ET L 3,368,319

TALL COLUMN STRUCTURE OF CONNECTED SECTIONS WITH WARREN CROSSBRACING AND LEGS OF CHANNEL SECTION Filed Aug. 16, 1965 5 Sheets-Sheet 3 FIG.8

INVENTORS WILLIAM L.WERNER MAXIM E J.THOMAS ATTORNEY United States Patent 3,368,319 TALL COLUMN STRUCTURE OF CONNECTED SECTIONS WITH WARREN CROSS-BRACING AND LEGS 0F CHANNEL SECTION William L. Werner, Sunnyvale, and Maxime J. Thomas, Oakland, Calif., assignors to Granger Associates, Palo Alto, Calif., a corporation of California Filed Aug. 16, 1965, Ser. No. 479,705 2 Claims. (Cl. 52-637) ABSTRACT OF THE DISCLOSURE A tall column structure comprising a plurality of aligned sections of triangular truss cross-section, consisting of mutually spaced legs aligned end to end with continuous Warren bracing interconnecting the legs such that for some directions of wind loading said legs are braced at one-half bay height and for other directions of wind loading the legs are braced at full 'bay height, the legs being of modified channel cross-section to provide a two to one ratio of radius of gyration about different axes.

This invention relates generally to tall columns, and the invention has reference, more particularly, to a strong lightweight column suitable for use as a radio or television transmitting tower, as a power line tower, etc.

Heretofore for many years and up to the present, it has been common to employ angles, '1 beams, tubes, channels and similar standard shapes as structural members in building tall towers, such towers being built of vertical spaced legs interconnected by trusses employing suitable bracing such as Z or Wagner bracing, W or Warren bracing, or X-type diagonal bracing. Columns, as thusly constructed, have been relatively inefficient in their utilization of material in that as much as 25% or more material is employed in constructing such prior art towers to produce a given strength requirement than is found necessary when using towers of the present invention.

Applicant, as the result of considerable study and research in tower design and experimentation, has conceived a novel tower structure using tower vertical leg members of non-uniform section properties in a triangular truss arrangement, the said arrangement employing a substantially minimum amount of structural material for any given tower requirement, thereby considerably reducing the Weight and cost of the tower and this is also extremely important where towers are to be shipped for any distance, the said tower structure being suited for prefabrication and shipped in knocked down condition for rapid erection on the site of use.

It is, therefore, the principal object of the present invention to provide a novel tall column structure of triangular configuration employing a minimum amount of material for any given tower specification requirement.

One feature of the present invention is to provide a novel tall column structure of the above character employing vertical leg members of modified channel shape and having non-uniform section properties, disposed in a triangular truss conformation and suitably braced by use of Warren-type bracing.

Another feature of the present invention is to provide a novel tall column structure of the above character which can be built into identical sections of convenient length for assembly.

Another feature of the present invention is to provide tower leg sections and bracing that are pre-cut and holepunched for the necessary bolt or rivet holes so that the tower can be shipped in compact unassembled form, if desired, to the site of use and then there assembled quickly by use of reversible tower sections of uniform lengths,

thereby making assembly easy without the possibility of putting legs in backwards.

Still another feature of the present invention is to provide a novel tower structure of the above character wherein the tower leg splices are located at the one-quarter bay point from the ends of the tower sections so as not to interfere with the cross-bracing which, when the tower is assembled, continues in uninterrupted fashion for the full height of the tower.

A further feature of the invention is to provide a novel tower structure wherein the sections are provided with pre-punched guy bracket attachment locations at onethird their lengths so that, by reversing the sections, the attachment locations can be placed at the two-thirds section length.

These and other features and advantages of the present invention will become more apparent after a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a view in elevation of the novel tower structure of the present invention;

FIG. 2 is a schematic view in plan of the structure in FIG. 1 taken along line 22 of FIG. 1;

FIG. 3 is an enlarged sectional view taken along line 33 of FIG. 1;

FIG. 4 is an enlarged fragmentary sectional view taken along line 4-4 of FIG. 1, showing the ground guy wire attachments;

FIG. 5 is an enlarged view with parts broken away of the tower shown in FIG. 1;

FIG. 6 is an enlarged fragmentary view taken along the line 66 of FIG. 5;

FIG. 7 is a view similar to FIG. 6 showing the use of tubular cross-bracing instead of angular cross-bracing;

FIG. 8 is an enlarged sectional view taken along line 88 of FIG. 5, showing a splice joint;

FIG. 9 is a view taken along line 99 of FIG. 5, illustrating a guy wire attachment;

FIG. 10 is a schematic view showing the proportions of the leg structure in cross-section, together with reference axes;

FIG. 11 is a graph illustrating a typical allowable stress diagram for varying section dimensions of the leg structure of the present invention; and

FIGS. 12 and 13 are developed schematic views of the Z-type and Warren-type cross-bracing respectively.

Similar characters of reference are used in the above figures to designate corresponding parts.

As a result of study, research and testing, applicants have found that, for a tall column structure of equilateral triangular truss construction, in order to obtain the greatest strength resistance to winds and loading of the structure, vertical leg members should be used wherein the radius of gyration of the cross-section of the leg member taken about an axis passing through the center of gravity of the cross-section and extending parallel to an adjoining face of the tower or to a flange of the leg member is twice the radius of gyration of such leg member cross-section taken about an axis passing through the center of gravity of the cross-section and extending parallel with the opposite face of the tower or to the web of the leg member.

To confirm the practicability of this novel design, the theory of tall columns must be considered, bearing in mind that a tall column fails to take more vertical load long before the yield stress or deformation of the material is reached. Eulers theory of long columns is as follows:

Fa 5 HE A (if 3 wherein Fa is the allowable stress in the material of the tower legs; P is the total load applied to the leg; A is the cross-sectional area of the leg; E is the modulus of the elasticity of the leg; L is the length between support points of the tower leg; and, r is the radius of gyration of the leg cross-section.

From the above formula, it will be noted that in slender columns the load capacity depends essentially on the modulus of elasticity rather than on the strength of the material so long as the yield point is not reached. Thus, efficient tower design uses low L/r ratios for the vertical legs of the tower in order to gain the benefit of high strength steel. For example, a high strength steel of 50,000 p.s.i. yield and a L/r range of 40 to 45 gives great load capability. The cross-bracing members, i.e. the Warren brace, are best made of the least expensive low cost steel since all steels have about the same modulus of elasticity.

In arriving at the optimized cross-section of the leg structure, in order to attain desired maximum strength together with minimum amount of metal used in the column leg structure so as to reduce the overall costs and weight, tubular structures were first considered, but these were found to be objectionable for a number of reasons: firstly, tubing costs about one and one-half times more per unit weight than an angle or channel section; secondly, tubing is more difficult to join than other sections and usually involves expensive welding and/ or flange joints; thirdly, open sections nest and pack more compactly for shipping than tubular sections; and fourthly, tubular sections are not well suited to a bolted design whereas the flanges of an open section provide convenient surfaces for bolting cross-bracing thereto. Therefore, it appears that the open shape is the desirable shape for the cross-section of leg members.

Bearing in mind that it is necessary to maintain a 60 angle between the faces of the triangular tower, it is apparent that 60 angle irons could be used. Thus, in FIG. 12, two faces of a prior art tower unfolded so as to be illustrated in plan, may use 60 angle irons or hollow tubing, for example, with ordinary bracing providing both lateral and transverse support at each bay height, and here the best cross-section is one giving equal stiffness or radius of gyration in all directions such as a tubular section. Nevertheless, this structure is relatively heavy for the strength and rigidity provided the column, and it became apparent to applicants that, in order to reduce weight, a different type of cross-bracing should be used. Thus, on further study it was found that, in order to maintain minimum weight throughout the tower, it is desirable to use the Warren-type cross-bracing. This bracing,

Thus, it will be seen that the forces applied against a leg 1 in FIG. 13 substantially perpendicular to the paper or perpendicular to a leg of the actual tower and bisecting the angle between the two adjacent tower faces will be resisted by braces occurring at half bay points, whereas those forces applied substantially along the surface of the paper such as indicated by the arrow 2, or transversely of the actual tower and along a face thereof, are only resisted by braces occurring at full bay height B. Thus, for winds or forces applied to the actual tower from certain directions, bracing occurs at every half bay height, whereas, for forces applied from other directions, the Warren bracing affords support at alternate points or at full bay height points only so that the L in Eulers formula would have to be equivalent to the bay height B rather than 3/2. It therefore becomes necessary to select a proportionality of a modified channel structure that 4 will give a 2 to 1 ratio of r about axes CC and AA in FIG. 10, the r about axis CC being twice that about axis AA, these axes passing through the center of gravity 3 of the section.

From these considerations it follows that the optimum proportion of the cross-sectional structure shown in FIG. 10 is such that the flanges 4 and 4 which make angles of with the plane of the web 55 are of the same width 7, and their width is made equal to the width w of the web so that we have a non-uniform section of substantial channel shape with each section portion equal in width to the remaining section portions. Such sections are extremely convenient for attaching cross-bracing thereto as they can be hole-punched and also are convenient for shipping as they can be nested together in compact form. Actual tests have shown that in tall columns this non-uniform cross-section shape of the tower leg member took 35% greater load before yield than a 60 angle cross-section using the same amount of metal in the cross-section. Also, tests have shown that the 2 to 1 stiffness ratio used by applicants in the leg members of the tower to optimize vertical load capability by balancing the tendency of the legs to fail in a radial or tangential mode due to forces applied substantially parallel to the faces of the tower, is correct and confirms the design theory. The tests also point out the advantages of using high strength steel in the vertical leg members, and, since such high strength steel is virtually the same price as lower strength steel, considerable additional load capacity is gained without additional costs.

In practice, the tower legs 6, having the shape shown in FIG. 8, are preferably fabricated in pre-determined lengths such as 10 feet and/ or 20 feet lengths. These legs are pro-punched for bolt or rivet holes 45 and all the legs are punched similarly, the holes 45 in the splice joints being punched within one-quarter of a bay height from the ends of each leg, as especially illustrated in FIG. 5. This location of the splice joints within one-fourth of the bay height of a leg enables the cross-bracing 10, which is shown in FIGS. 5 and 6 as of the angle-type and in FIG. 7 as of the tubular-type, to be continuous from the bottom of the tower to the top thereof when the legs are assembled, as is particularly illustrated in FIG. 5, without requiring any additional cross-bracing which would add to the weight and cost of the tower.

While the tubular cross-bracing 10 of FIG. 7 offers less wind resistance when assembled in the tower than does the angle cross-bracing 10 of FIG. 6, nevertheless, the tubular bracing, as heretofore pointed out, is more expensive. The ends of the angle bracing 10 are sheared off substantially to a point 5 when the bolt holes in the angle iron are punched so as to facilitate quick assembly upon the legs 6 by use of bolts or rivets 11. Similarly, the ends of tubes 10 are flattened and rounded when the holes 45 are punched. It will be noted in FIG. 8 that the splice joints comprise splice rails 7 and complementary plates 8 secured to opposite sides of the legs 6 by the bolts 11. The splice rails 7 are shown as split longitudinally to produce, in effect, two rails at each joint, although actually a single unitary rail can be used when desired, the split rail being somewhat easier to assemble, the said splice rails and plates being usually bolted at the site of use to the tower legs 6 by use of the bolts 11 when the tower is shipped knocked down. When the tower is ihilpped assembled, rivets may be preferred in lieu of the As especially shown in FIGS. 5 and 9, the guy bracing comprises bars 12 extending transversely of the faces of the tower. The tower legs 6 are each provided with bolt holes at one-third their lengths from one end for receiving bolts carrying the transverse bars 12, certain of which holes coincide with the bolts holding the cross-bracing 10. The top-most bars 12 are adapted to be attached to holes which ordinarily would be used for the splice joints. This construction makes it possible to brace the tower at its top and at any tower section either one-third from the top thereof or one-third from the bottom thereof, depending on how the legs are assembled, thus providing convenient points of attachment of the bars 12. The bars 12 have slightly bent-up ends 13 carrying sleeved bolts 14 to which bolts the guy clevises 15 are connected, these clevises being connected through insulators 16 to the guy ropes or wires 17, the said guy ropes or wires extending to the ground and being connected through additional insulators 16, clevises 15 and turnbuckles 18 to stabilizer plates 19 that in turn are connected by anchor rods 20 to the ground through use of buried concrete blocks 21, in which blocks the anchor rods are embedded. The tower is adapted to be supported upon a suitable concrete base by means of plates 22 secured to short stud tower legs 6, the plates 22 being bolted to the base by use of bolts 23.

FIG. 11 illustrates the benefits to be derived by the use of a web to flanges or W/ ratio of one to one in the nonuniform cross-section of the leg structure. It will be noted that, at this value of the ratio, the leg member has its maximum allowable stress and that at other values of this ratio, the allowable stresses on the leg member falls 01f rapidly. The sample of which this ratio was plotted was an aluminum leg and wherein L in the column formula would be a whole bay height or B.

As thusly constructed, the novel tower of this invention is substantially 25% or more lighter in weight than towers of the prior art carrying the same stresses and loads. Since the legs of the tower can be shipped in knocked-down condition and nested together and as the tower cross-bracing can also be shipped nested together, an extremely compact bundle for shipping results. Owing to the definite placement of the various pro-punched holes in the legs, it is a simple matter to attach the Warren cross-bracing to the legs in continuous, unbroken zigzag form from the bottom to the top of the tower without any additional transverse cross-bracing being required. With splices being located within one-fourth bay points from the ends of the tower sections, no interference with the cross-bracing obtains. Also, with the guy attachments at one-third the leg section lengths, no interference in the continuous bracing or in the splicing is necessary. Since the legs are all identical and reversible, there is no possibility of improper assembly on the site of use, resulting in rapid assembly and economical installation.

Since many changes could be made in the above construction of the novel tower structure of this invention and many apparently widely difierent embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. For example, the novel feature of employing a leg section of non-uniform characteristics in combination with Warren-type bracing could be applied to elliptical, flatoval or other shapes wherein the proportions are such that a two to one ratio of radius of gyration is obtained.

What is claimed is:

1. A tower structure comprising a plurality of similar connected tower sections, said tower sections being of substantially equilateral triangular cross-section having three legs interconnected by continuous W cross-bracing extending from the bottom to the top of the tower, each of said sections being interconnected to an adjoining section by use of splice joints confined within one-quarter bay height from the ends of said sections so as not to interfere with said continuous W bracing, said tower legs being of non-uniform cross-section having a central web and flanges projecting from the side edges of said web and extending at sixty degree angles therewith, said flanges being of the same width and equal to the width of said web, the radius of gyration of said leg cross-section taken about an axis passing through the center of gravity of the section and extending parallel to one of said flanges being twice that taken about an axis passing through the center of gravity of the section and extending parallel to the web of said section, whereby said legs have twice the stifiness in one direction wherein the cross-bracing occurs at bay height than in another direction wherein the cross-bracing occurs at one-half bay height.

2. A tower structure as defined in claim 1 wherein said tower legs are all identical, with identically punched holes for attachment of said cross-bracing and splices, said legs being punched at one-third their heights for receiving fasteners for attaching guy members as well as cross-bracing, whereby reversal of said legs provides for guy member attachments at two-thirds leg height, if desired.

References Cited UNITED STATES PATENTS 2,074,548 3/1937 Humphrey 52637 2,583,287 1/1952 Andrews 52148 X 2,808,912 10/1957 Clark 52148 3,020,985 2/1962 Turner 52-448 FOREIGN PATENTS 262,653 2/ 1929 Italy.

OTHER REFERENCES Scientific American, Tl.s5, p. 576, Dec. 23, 1916.

FRANK L. ABBOTT, Primary Examiner.

ALFRED C. PERHA-M, Examiner.

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