US 2887762 A
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May 26, 1959 c. DQBELL BETH 0D OF MAKING PRESTRESSED STRUCTURAL MEMBER Original Filed April 3, 1950,
2 Sheets-Sheet 1 INVENTOR ATTORNEY;
y 26, 9 c. DOBELL 2,887,762
METHOD OF MAKING PRESTRESSED STRUCTURAL MEMBER Origihal Filed April :5, 1950 I 2 Sheets-Sheet 2 hya.
INVENTOR METHOD OF MAKING PRESD STRUCTURAL MEMBER @urzou' Dobell, New York, N.Y., assignor to The Preload tCompany, Inc., New York, N.Y., a corporation of Delaware tlllri application April 3, 1950, Serial No. 153,544,.
Divided and this application November 15, 1954, Serial No. 468,838
1 Claim. (Cl. 29-155) This invention relates to a structural metal member and to a method for making a structural metal member of improved strength.
Prior to this invention there existed in this art the problem of producing a structural metal member which could economically support-a greater load without materially increasing the volume of metal employed. Heretofore, attempts were made to meet this problem by resorting to particular materials in the construction of structural metal members. These attempts have manifested limited commercial success, the increase in cost more than offsetting the gain in structural strength.
Further activity was exhibited in the prior art in the form of securing metal bars on rods and turnbuckles to existing structural metal members already under load. Thesedevices are in the nature of reinforcements and are not suitable for use in new work.
It is an object of this invention to provide a simple and expedient method of making a structural metal memher which overcomes the deficiencies of the prior art.
It is another object of this invention to provide an improved structural metal member'which can be more economically produced than was heretofore available.
It is another object of this. invention to provide a structural metal member which contains less metal with respect to load carrying capacity than has heretofore been available.
Other objects and advantages of the present invention will become apparent from a study of the following specification when considered in conjunction with the drawings, in which:
Figure 1 is a perspective view of the structural metal member completely assembled;
Figure 2 is a fragmentary vertical section of the load carrying flange;
Figure 3 is a fragmentary vertical section of the load carrying flange showing the tensioning elements Figure 4 is similar to Figure 3 and shows the load carrying flange after the rolling operation;
Figure 5 is a bottom view of a load carrying flange showing the prestressing element tensioned and secured;
Figure 6 is an end view of the structural metal member showing the rollers in place;
Figure 7 is a vertical section of a structural metal member showing a modified. embodiment of the invention; and
Figure 8 is a side view of a conventional bridge truss showing the modified embodiment of Figure 7 in use.
Before discussing the invention in detail in conjunction with the drawings it is desirable to first present generally the nature and scope of this invention.
The present invention overcomes the inherent defects of the prior art by prestressing the metal members prior to their being utilized as load carrying members. The prestressing is accomplished by the inclusion of prestressing elements such as tensioned wires in the tension surface of the structural steel member.
This invention broadly contemplates the use of prestressing means with a structural metal member to greatly improve its strength and load carrying capacity. By the method of this invention tensioned high strength wire or other suitable prestressing means is embedded in the teusion surface of a structural steel member. This pro" duces tensional forces in the compression surface hereafter called the load bearing surface. It also produces compressional forces in the tension surface hereafter called the load carrying surface. These forces exist While the structural metal'member is in a noload condition.
Upon subjecting the member to a load, the member itself will not bra-stressed until the forces brought about by the prestressing are neutralized. Beyond this new tralization point the load will stress both the member and. the prestressing means in proportion to their strength, weight, and other physical properties.
As an example of this invention, a structural metal member with an allowable structural code tensile stress of 20,000 p.s.i. may be put under an initial compression in the tension or load carrying surface of 20,000 p.s.i. This is accomplished by the inclusion of high strength steel wire tensioned to 120,000 p.s.i. in the load carrying surface. On subjecting the member to a. live load, the load carrying flange may change from 20,000 p.s.i. compression to 20,000 p.s.i. tension, an overall change of 40,000 p.s.i'. The stress in the wire would change correspondingly from 120,000 p.s.i. tension to 160,000 p.s.i. tension. Such a structural metal member would be twice as strong as a member not made by the method of this invention.
The structural metal member of this invention may be made of any of the materials commonly used for this purpose, such assteel or the like. This invention, however, has particular applicability with structural members which are made of light metals such as alloys of aluminum, magnesium, and similar materials. llt is known that structural members constructed of these light metals possess the disadvantage of deflecting much more than steel members for the same cross-section and load due to their lower modulus of elasticity. Although the allowable code stresses for light metal are approxi' equal, if not greater than those for steel, the prob of deflection is important from a design standpoint. However, when a light metal member is'prestressed with steel wires in accordance with this-invention, the deflection disadvantage can be eliminated to a major degree. This is apparent when it is considered that there is an equal stretch of the steel wires and light metal in the tension surface of the prestressed member. However, for the same stretch, there is not an equal change of loading in the light metal and the steel wires. The steel wirm have a greater modulus of elasticity and, therefore, assume the load to a degree proportional to the ratio of elasticity moduli of the steel wires and the light metal respectively.
In addition to the foregoing, the manufacturing processes for light metal members are quite flexible. In view of the properties of the materials involved, it is frequcntly possible to extrude light metal shapes. This being the case, slots or holes in the tension surface for receiving the prestressing elements can be included in the extrusion process thereby eliminating the necessity of treating-the member'once it is fabricated. Also prestressing elements such as wires can be included in the holes or slots during the extrusion process, these wires being in a tensiouedconditiou due to the extrusion pressure.
Referring now to the drawings, Figure 1 shows the completed structural metal member 20 in perspective. The member consists generally of an upper load bearing flange 21, a lower load carrying or tensioning flange 22, and an intermediate body portion 23 which joins the two flanges together. The load bearing flange 21 is provided with an upper surface 25 upon which a load is placed.
asavgrea The lower load carrying flange 22 is adapted to receive and engagingly embed tensioning elements 24 in the manner described hereinafter.
Figures 2, 3, 4, and 6 show the method by which the improved structural metal member is made.
The first step is to form or cut by suitable means slots 26 extending lengthwise in the lower surface 28 of the load carrying flange 22. Although the slots 26 are shown as rectangular in section and extending parallel to the long axis of the I beam, they may be of any shape and they may run in any direction which will produce a major force component parallel to the longitudinal axis of the member 20.
When the formation of the slots 26 is complete, prestressing elements 24 are placed in them. The prestressing elements are of a special character and should be capable of being tensioned to 120,000 p.s.i. or greater, i.e., high strength wire or other equally appropriate material. The prestressing elements 24 while retained in the slots 26 are tensioned to the proper degree by conventional means (not shown). As illustrated by Figure 5, a single piece of high strength wire 24 is placed in the slots 26 by laying it back and forth in the slots 26, looping it about the end30 of the load carrying flange 22 in the manner indicated at 32. The high strength wire 24 is then tensioned by any conventional means (not shown) and the ends 33 are secured by means of a clamp It is to be understood, however, that while only a single length of high strength wire is shown, it is nevertheless within the scope of this invention to employ a prestressing element for each slot or group of slots and to maintain the prestressing element or elements in tensioned fashion by clamps or other desirable means.
After the prestressing element 24 has been tensioned and secured by clamp 31, the structural metal member is rolled in the manner indicated in Figure 6. During this operation roller 34 is brought to bear on the lower surface 28 of the load carrying flange 22, and rollers 35 and 36 are provided to support the load carrying flange 22 of the structural metal member. In the course of this operation the material of the lower flange will be forced around the tensioned prestressing element 24 to hold it securely, and the slots 26 will be closed as indicated at 29 in Figure 3.
Although the tensioned prestressing element 24 may be untreated, it is preferred that the element 24 be first coated with an abrasive or other material such as carborundum as indicated at 27 of Figure 3. The element 24 is coated so that it will be more securely embedded in the load carrying flange 22 at the conclusion of the rolling operation.
The clamp 31 may now be released and the structural metal member 20 treated by a grinding method to remove the loops 32 which jut out over the ends 30 of the load carrying flange 22.
In the modified embodiment shown in Figures 7 and 8 the structural metal member 20 is provided with prestressing elements 24, which have been coated with an abrasive as shown at 27, in the load carrying flange 22, the intermediate body portion 23, and the load bearing flange 21. The modification is used in a conventional bridge truss and forms the lower girder. When employed in such a structure the modified member acts as a tension member with the prestressing allowing the member to take twice the load or greater. This is in view of the fact that members subject to pure tension and not subject to bending moments can be made stronger if they are first put in compression by prestressing. In this manner a bridge truss is developed which is capable of sup- 4 porting a greater load per unit volume of steel or other material than has previously been available.
While the invention has been described in conjunc on with an I beam, it is of course understood that c \er types of beams or girders may be employed. These other types include L beams, T beams, built-up beams, box girders and the like. In the case of box girders, which are frequently fabricated from fiat strips by welding or riveting, prestressed flat bars would be required in the fabrication for the tension face of the girder. These flat bars appearing in the tension face of the box girder would probably have uniformly distributed prcstressing. However, the flat bars acting as webs and appearing on the sides of the box girder would probably require nonuniform prestressing with greater concentration on the tension half of the web. This would also apply to any other built-up beam section.
This invention is also applicable to the prestressing of columns and piles to counteract certain loading, especially where such loadings take place eccentrically. The prestressing in constructions of this nature is arranged to present tensional forces at points where compressional forces will exist due to the loading of the pile or column. The prestressing will also present compressional forces at points where tensional forces are set up in the pile or column due to the loading thereof. In this manner piles and columns can be constructed which will be adapted to support greater loads and be of improved strength.
What has been described heretofore is a specific embodirnent of the present invention. Other and further embodiments obvious to one skilled in the art from this description are within the contemplation and spirit of this invention.
This application is a division of my application Serial No. 153,544, filed April 3, 1950, now abandoned.
A method of reinforcing an elongated metal structural member having a lower flange and an upper flange connected by a web that comprises forming a plurality of slots in the surface of the lower flange, said slots extending along the longitudinal axis of the flange, positioning an elongated high tensile strength metal reinf ing element in each slot, tensioning said reinforcing ment and pressing the lower flange to close said slots 4 said reinforcing element while it is under tension to cause the sides of the slots to frictionally engage said element to maintain the tension thereon and place the lower flange in compression, therebyplacing the upper flange in tension.
References Cited in the file of this patent UNITED STATES PATENTS 1,148,223 Feeser July 27, 1915 2,039,398 Dye May 5, 1936 2,244,109 Klein June 9, 1941 2,375,068 Bennett May 1, 1945 2,463,580 Warshyk May 10, 1946 2,607,109 Reynolds Aug. 19, 1952 2,609,586 Parry Sept. 9, 1952 2,627,649 Matthyse Feb. 10, 1953 FOREIGN PATENTS 5,537 Great Britain Feb. 1, 1890 556,572 Great Britain Oct. 11, 1943 OTHER REFERENCES Pages 178182 of the Railway Gazette, August 17, 1951.