US3533203A - Compressed structural members - Google Patents

Description

0d. 13, 1970 H. c. FISCHER Erm.`

COMPRESSED STRUCTURAL MEMBERS In 2 G F .F m 9 1 .iI 4 2 b. a n d e f 1 4 HM a .n 1.\m .no l 1 W. /l 6 /l .+l\l| FIG 4 27 FIG 5 FIG 6 3,533,203 CGMPRESSED STRUCTURAL MEMBERS Herbert Corliss Fischer and Herbert Corliss Fischer, Jr.,

both of 3 Sawyer Road, Wellesley, Mass. 02181 Continuation of application Ser. No. 617,583, Feb. 21,

1967. This application Sept. 4, 1969, Ser. No. 856,901 The portion of the term of the patent subsequent to Mar. 11, 1986, has been disclaimed Tnt. Cl. E04c 3/10, 5/07 U.S. Cl. 52-223 18 Claims ABSTRACT F THE DISCLOSURE A stressed structural element having a relatively rigid, incompressible body combined with one or more internal or external tensioning strands, each comprising a multiplicity of synthetic or organic iibers such as nylon, polyester, polypropylene or the like, having a high recoverable stretch of about to 40 percent with the above mentioned i'bers, the tensioning strands being maintained in intimate association with the rigid body and in highly elongated tensioned condition to compress the body and so stress the structural element.

This is a continuation of Ser. No. `617,583 filed Feb. 21, 1967, and now abandoned.

This application is a continuation-impart of our applications Ser. Nos. 388,416, filed Aug. 10, 1964, now Pat. No. 3,208,838; 464,309, led June 16, 1965, now U.S. Pat. No. 3,431,687, tiled Mar. l1, 1969, and 567,540, tiled July 25, 1966, now abandoned.

The invention relates to novel prestressed structural elements and methods of their manufacture.

As has long been known in the art, prestressing, by maintaining a structural member in a precompressed condition may be utilized to increase its tensile strength and its strength in bending. For example, concrete, a cast stone made primarily of portland cement, aggregate and water, has the property of great strength in compression. Unfortunately, though, it is weak in tension, causing it to be brittle and so subject to cracking and breakage when subjected to unexpected tension stresses, such as frequently occur in handling and assembling precase concrete elements prior to and during building construction.

There have been used two ways of attempting to overcome this weakness of concrete in tension: one is to reinforce; the other is to precompress by prestressing.

Reinforcing, however, has the distinct drawback in that it still permits the concrete to crack under tensions, although the steel reinforcing continues to hold the cracked pieces generally together. This is because the steel reinforcing must elongate as it takes up the load, but, because the elongation is more than the encasing concrete can stand, it cracks under the load.

Prestressing is largely a matter of precompressing the concrete element in those areas where tension stresses are anticipated. If enough precompression is stored up in the concrete element, there will be no tensile stresses under the design load. Under these conditions, the concrete, being under compression, loses its brittleness and behaves as a flexible and resilient material. A prestressed concrete element merely loses some of its precompression as it accepts its load, so that the actual compression stresses are reduced as the load increases, at least up to the design load.

3,533,203 Patented Got. 13, 1970 With other relatively rigid, that is relatively incompressible materials, such as metals, synthetic plastics, wood, etc., precompressing is also known to be highly useful in increasing their tensile strengths, and is particularly useful insofar as bending of structural members is concerned.

In practice, prestressing of concrete, for example, has been accomplished in two ways; post-tensioning and pretensioning. In either case, a steel rod or cable is tensioned to compress the concrete. With post-tensioning, such is accomplished by casting a hole in the concrete element, and, after the concrete has set, placing a steel rod or cable through the hole and tensioning it to compress the concrete, leaving the tensioning elements exposed at the ends of the hole. With pretensioning, a steel cable is tensioned as by hydraulic jacks and end anchors and the concrete poured around the steel while it is maintained in tension. After curing, the concrete will bond to the steel cable by reason of the grooves therein so that the end anchors can be cut 01T, the steel remaining in tension to maintain the surrounding concrete in compression and so prestress it.

Although this latter method of prestressing has met with considerable success over more than a decade, it is still subject to a number of disadvantages.

From the standpoint of quality, particularly as to crack control in precast concrete elements, because of the low elongation of steel (for example, a `1/2 inch diameter ASTM grade 7 wire uncoated will only elongate about 3A inch in 10 feet at 25,000 pounds tension), it is most difiicult to achieve the optimum degree of prestressing, because of slipping of the steel within the concrete element, resulting in crack development at much lower tension stresses than anticipated. Too, since it is necessary to use steel cable of the order of about `1/2 inch in order to achieve satisfactory bonding to the concrete, and tot equate the steel tensionwith respect to the compressive strength of the concrete, distribution of the prestress throughout the cross-sectional area of the element by utilizing multiple steel cables cannot be eliected except in the largest sizes of elements. In addition, the ends of the steel prestressing elements necessarily remain expos-ed on the outer surface of the element.

Furthermore, from the standpoint of usability, such prestressing is restricted to use with relatively high strength concrete, and cannot be used, for example, with light weight, low strength foamed concrete such as is commonly used in building facing elements. This is both because of the low elongation of steel even at high tensile stresses and the impossibility of bonding small diameter steel rod or cable to the foamed concrete element which makes it impossible to provide a steel prestressing element of suitably low tension to equate with low strength concrete.

From the standpoint of manufacturing ditliculties, expensive equipment is needed to create the high steel tension needed, and to maintain it during the time necessary VJfor the concrete to cure. For this reason, heretofore, prestressed concrete elements could be economically manufactured only at central locations :and trucked to the construction site, and their range of sizes and configurations was limited by the cost and availability of such equipment, especially when but a few of a particular concrete element was involved.

`Other materials, such as synthetic organic plastic materials, have similarly been prestressed by incorporating therein stressed, relatively inextensible elements, but all were subject to the Same difficulties enumerated above, namely, the practical difficulty in achieving optimum prestressing because of the low elongation of the prestressing elements, even when they are under high tension and the resulting inability to utilize them, particularly with low strength structural elements, especially of light weight materials such as wood and plastic.

Accordingly, the major object of the present invention is to provide novel prestressed structural elements and methods of manufacturing same not subject to the abovementioned difficulties.

Other objects of the invention are to make possible the prestressing of light weight, low strength structural elements, as well as the distribution of tensioning stresses throughout the element cross section, even with low strength materials.

Another object of the invention is to make possible the concealing of the reinforcing strands within the structural element, so that they are not exposed on the surface thereof.

Another object of the invention is to avoid the use of metal pretensioning elements with their known electrolytic and corrosion problems.

These and still further objects of the invention have been accomplished by our discovery of the unique utility as pretensioning elements of a class of materials long known in other uses but never heretofore considered to be practical as pretensioning elements. These materials comprise well-known synthetic organic plastic polymers, such as nylon, polypropylene, and polyester, preferably in the form of highly elastic multifilament strands, twisted or untwisted, of moderate tensile strength as compared to steel, but uniquely capable of producing high compressive forces due to their high recoverable stretch or elongation, upwards of 10 to 20 percent, providing progressively increasing tension up to their breaking tension at their elastic limit. We have found that such multi-filament strands, such as ropes, when stretched to as much as 80 to 90 percent of their elastic limit of the order of at least 10,000 to 15,000 and up to more than 35,000 p.s.i. at break, may be combined with relatively rigid materials, especially such light weight materials as light metals, plastic, wood, etc., capable of maintaining the tensioned strand at a relatively fixed length, particularly as to further elongation, to provide unusually light weight pretensioned structural elements.

Surprisingly, and contrary to what had been the general belief that the tension forces of such stressed strand needed for producing the required compressive forces would decrease so rapidly with time as to make it useless for a relatively permanent pretensioning element, our tests have established that, after an initial tension force decay of to 10 percent which occurs within a few minutes, further decay is apparently constant at approximately a logarithmic rate with time and occurs very slowly, such that nylon, for example, will apparently retain more than 273 of its initial tension for producing compressive force after 100 years. Thus, by prestressing a relatively rigid material by such a strand maintained at at least about 5 to 10 percent elongation and under tension of at least about to 20 percent of its ultimate breaking strength, and preferably higher, a wide variety of uniquely prestressed structural elements may be provided according to the invention.

For the purpose of more fully explaining further objects and features of preferred embodiments of the invention, reference is now made to the following detailed descriptions thereof, together with the accompanying drawings, wherein:

FIGS. 1 and 2 are, respectively, plan and cross-sectional views of an internally prestressed structural element according to the invention;

FIGS. 3 and 4 are, respectively, side and cross-sectional views of an externally prestressed structural ele ment according to the invention;

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FIG. 5 shows a typical end product utilizing the structural element of FIGS. 3 and 4, and

FIG. 6 shows an internally prestressed tubular structural element.

In general, the preferred embodiments of the invention provide compressed structural elements such as those shown in the drawings by compressing elements of conventional rigid materials such as metal, plastic, concrete, wood, etc., by means of internal or external fiber tensioning strands capable of dimensional change such as potentially high elastic stretch, of the order of at least about 5 to 10 percent, relatively to that of the material to be prestressed, with such strands being maintained at about 5 to 10 percent elongation in excess of their stabilized relaxed length and under tension of at least about 10 to 20 percent of its ultimate breaking strength. It is vital to the invention that the material to be prestressed be in a substantially rigid, non-yielding condition, that is, be relatively incompressible, when compressive forces are applied to it by the tensioning strand, so that the highly tensioned strand is in intimate contact with the rigid body and is maintained thereby at a relatively xed length so that it continuously maintains its compressive force to prestress the element. The use of the non-metallic ber tensioning strands of the invention avoids the electrolytic and corrosion problems of metal, and has still other advantages as hereinafter set forth.

A wide variety of elastically contractible organic plastic strands may be utilized according to the invention including such organic plastic materials as nylon, polypropylene, and polyester, for example, as well as others, preferably in multi-filament, untwisted or twisted configurations as with rope or rovings. Such strands have a usefully high tensile strength with a great amount of elastically recoverable extension, by which we mean that extension which repeatedly remains after the strand has been stretched a sufficient number of times to achieve a relatively stabilized relaxed length. For example, nylon rope, a preferred tensioning strand according to the invention, has a breaking strength of about 36,000 p.s.i. at about 50 percent elongation beyond its stabilized length, and may be elongated to as much as to 90 percent of its ultimate breaking elongation to provide a tension useful for producing prestressing compressive forces as high as about 30,000 p.s.i. at an elongation of about 40 percent for example. On the basis of our tests and calculations, such a nylon rope, when maintained at a relatively fixed elongation of 40 percent by the rigid element to be prestressed, will still have a prestressing compression force of 20,000 p.s.i. even after years. Such a material is uniquely useful as a prestressing element, especially for prestressing relatively light-weight rigid elements as of wood, aluminum, plastic, etc., which are very difficult to prestress with a material such as steel because the high tensile strength of steel makes it necessary to utilize extremely small prestressing elements which are diflcult to assemble and control. With the much lower tensile strengths of the prestressing materials of the present invention, much larger and hence easier to control prestressing elements may be successfully used, especially since, unlike steel with its maximum elongation of l-Z percent, with the prestressing materials of the present invention, even a l0 percent decrease in their fixed tensioned length either by slippage or deformation of the structural member will result only in about a 10 percent loss in the compressive force applied.

Polyester and polypropylene ropes have somewhat lower breaking strengths, with elongations about half that of nylon, b ut are nevertheless highly useful in the practice of the invention. Other plastic materials with similar physical properties are also useful.

Whatever its exact nature, the potentially contractible tensioning strand of the inventiona as set forth in our earliest application, Ser. No, 388,416, may be incorporatedwith a non-rigid material such as unset concrete or plastic, while the concrete or plastic is in unset, yieldable condition usually by pouring it into or compressing it within a suitable form in which the tensioning strands are supported, after which the non-rigid body is allowed to set to a rigid condition and the tensioning strands then contracted or allowed to contract to compress the element. Or the tensioning strand may be applied in tensioned condition externally of a rigid element to be compressively stressed with the tensioning strand in intimate contact therewith so that the stressed reinforcing element dimensionally changes to compressively stress such element.

The stressing of the tensioning strand after the previously non-rigid body has :sufficiently set may be accomplished on a temperature basis. For example, as conventional practice, the curing of precast concrete elements is carried out by heating the form containing the concrete. Utilizing the invention, a tensioning strand can be selected in which a major proportion of its contraction occurs at a suitable temperature, below which the concrete cure can be initially carried out for substantial setting and bonding to the tensioning strand above which the tensioning strand will shrink to tension it and so compress the concrete element.

The invention may be applied to concrete made of Portland cement, using a wide variety of aggregates, as presently known to the art, not only the usual sand, gravel and stone aggregates, but also light weight aggregates lsuch as burnt clay, expanded blast furnace slag, pumice and expanded vermiculate, as well as air-entrained and foamed concrete.

In FIGS. l and 2 is shown a standard concrete test element (ASTM) generally designated constructed according to the invention. Specifically, such a test specimen has a six inch square cross section and is twenty-one inches longa and was produced by incorporating tensioning elements 12, 14, 16, 18 with a normal Portland cement concrete having a strength in the range of 5,000- 6,000 p.s.i. compressive strength, as is known in the art and described, for example, in Design and Control of Concrete Mixtures, 10th ed., published by Portland Cement Association (1952) A specifically shown in FIGS. 1 and 2, the tensioning elements l2, 14, 16, 18 are multi-filament, heat-shrinkable polyester fibers having a recoverable stretch of about 10 percent, in which about a 10 percent shrinkage occurs upon heating to a temperature of about 300 degrees.

This may be done by incorporating such fibers in the form of rovings, appropriately arranged in the unset cement body and then setting the cement at a lower temperature than that effective to cause substantial shrinkage of the polyester fibers. Thereafter, the concrete body is heated to a temperature sufficiently high to cause the polyester fibers to contract. A test specimen so made had a breaking strength approximately twice that of a control specimen without such fibers and exhibited most desirable crack control characteristics, in that cracks appeared well in advance of breakage, which cracks could be made to disappear by unloading the specimen. Other contractible fibers than polyester can be used as well, and such fibers may be` arranged in the from of rope as well as rovings.

Precompressed structural elements may also be produced by using organic plastic multi-fiber strands, such as ropes, of the type described above which are capable of at least about 5' to 10 percent elongation and elastic recovery under tension of at least about 10` to 20 percent of their ultimate breaking strength. This may be accomplished by tensioning one or more such ropes for example of nylon, and holding each by appropriate clamps 20, such as are shown and described `with reference to FIG. 6, pouring the concrete or other unset plastic body around the tensioned ropes, and allowing it to set while maintaining the ropes undertenson. Afterthe concrete has set, the tension on the ropes may be released to compress the concrete and the ends of the ropes may be cut off.

Nylon ropes are particularly desirable in such application, not only because of their great elongation and the high compressive forces produced thereby, but also because they appear to react chemically with the concrete for firm bonding thereto.

As still another modification of the internally tensioned compressed structural element of FIGS. 1 and 2, the body 10, regardless of the rigid material of which it is cornposed may be stressed according to the invention by passing ropes 12, 14, 16, 18 through appropriate holes therein, tensioning said ropes to a condition equivalent to at least about 5 to 10 percent in excess of their stabilized relaxed length and under tension of at least about 10 to 20 percent of their ultimate breaking strength, and so maintaining them at a fixed length relatively to body 10 as by clamps 20.

In FIGS. 3 and 4 is shown an externally stressed structural member according to the invention in which a rigid body 2S, as of metal, wood or plastic, is stressed by a tensionedstrand 27 wrapped therearound, which may be in the form of, for example, an endless rope, roving or multiple turn-wound multifilamefnt strand. As before, strand 27 is of an extensible material pretensioned to at least about 5 to l0 percent in excess of its stable relaxed length and to a tension of at least about l0 to 20 percent of its breaking tension. As required by it use, strand 27 need not wrap the body 25, but may extend along one or both sides thereof and be attached adjacent its end by any suitable means such as clamps 20.

In FIG. 5 is shown a typical end product using the structural elements of FIGS. 3 and 4 as the side rails 32, 34 of an otherwise conventional ladder 30 having rungs 36. The lader material is of hardwood although aluminum could be used as well. Strand 27 is of nylon rope of 1A; inch diameter, extended to 40 percent of its stabilized relaxed length to develop a residual compressive force of about 700 pounds, about 1/3 of its ultimate breaking strength and is retained in a groove about the periphery of the side rails. By thus` prestressing with about 2 ounces of nylon on each side rail, a 12 foot length of ladder broke, when horizontally supported at its ends and loaded at its center, at 1280 pounds, as compared to a similar ladder without a compressing strand 27 which broke at 580 pounds.

In FIG. 6 is shown another structural member in the form of an aluminum tube 30 having a strand 32 elongated and stressed according to the invention passing through its center and held at its ends by known clamps, generally designated 20, having a conical body 22 for receiving the opened end of the multi-fiber strand 32 and clamping it by cooperation with a conical plug 23 held in position in body 22 by screw threads 24. The internally stressed, tubular structural member of FIG. 6 is useful in a wide variety of end products, including supporting poles of many types including boat masts and ski poles, as well as for such highly stressed and flexible elements as helicopter rotor blades.

The great strength and recoverable elongation of strands and ropes of synthetic organic plastic fibers, over 50 percent before 'breakage for nylon and about 20 percent for polypropylene and for polyester such as Dacron, their great contractive force of at least 10 to 20 percent of their breaking tension even :at such relatively low elongations of about l0 precent in excess of their stabilized relaxed length, (and what has proved to be their unexpectedly low rate of decay of their stretched compressive force over a period of many years) make them uniquely suitable for use as pretensioning elements, since tehir uniquely high elongations are achieved at far lower tensile stresses than steel or glass fiber strands. For example, a 1/2 inch nylon or Dacron rope can be so elongated to about 10 percent in excess of its stabilized relaxed length with about a 400G-500() pound force as compared to a force five times higher needed to stretch at 1/2 inch steel cable about 1 percent. This not only makes it possible to use simpler and cheaper prestressing machinery, but also at the same time to produce a more uniformly stressed product even of light weight concrete, Wood or aluminum, for example, and to distribute the stress throughout the element cross section by utilizing multiple strands as desired. This is because, with the high degree of elongation provided by such organic plastic fiber ropes, slippage of the tensioning element relatively to the rigid body to a minor degree is not a problem, whereas, at the low elongations of steel, even minor slippage will destroy the tensioning effect of the steel. Any required amount of pretensioning may be provided according to the invention by increasing the number of organic plastic fiber strands, and slippage may be virtually eliminated by coating the strands with a suitable adhesive such as an epoxy resin to provide any desired degree of bonding to maintain the tensioning strands in intimate contact with the body.

Various other modifications of the invention within the spirit thereof and the scope of the appended claims will occur to those skilled in the art.

We claim:

1. A stressed structural element composed at least in part of concrete comprising portland cement and aggregate and having integral non-metallic tensioning means in intimate contact with said element, said tensioning means comprising a multiplicity of organic plastic fibers having a recoverable stretch of at least 10 percent and having a tensile strength of at least about 10,000 p.s.i. with a decay factor of no more than about 1/3 over a period of years and having a plurality of non-terminated, curved portions permanently maintained in tensioned condition at a relatively fixed length and in initimate contact With said concrete at at least l percent elongation and under tension of at least percent of its ultimate breaking strength, providing concrete element containing a substantial proportion of non-terminated, curved stressed fibers effective to stress said element.

2. A stressed structural element composed at least in part of concrete comprising portland cement and aggregate and having integral non-metallic tensioning means in intimate contact with said element, said tensioning means comprising a multiplicity of organic plastic fibers having a recoverable stretch of at least 10 percent and having a tensile strength of at least about 10,000 p.s.i. with a decay factor of no more than about 1/3 over a period of years and having a plurality of portions permanently maintained in tensioned condition at a relatively iixed length and in initimate contact with said concrete at at least 10 percent elongation and under tension of at least 10 percent of its ultimate breaking strength, providing concrete element containing a substantial proportion of stressed fibers effective to stress said element.

3. A reinforced element including a rigid, non-yielding solid body having non-terminated reinforcing strand means composed of a multiplicity of synthetic organic plastic fibers having a recoverable stretch of at least l0 percent and having a tensile strength of at least about 10,000 p.s.i. extending continuously throughout a major portion of the length of said body concealed within the substance of said body including its ends and maintaining said body therebetween in compressed condition, said reinforcing strand means being permanently maintained at a relatively fixed length and in intimate association with said body at at least 10 percent elongation and under tension of at least 10 percent of its ultimate breaking strength to compress said body, providing an element reinforced with a substantial proportion of stressed fibers.

4. A reinforced element including a rigid, non-yielding solid body having reinforcing strand means composed of a multiplicity of synthetic organic plastic fibers having a recoverable stretch of at least 10 percent and having a tensile strength of at least about 10,000 p.s.i. extending continuously in a closed curve encircling said body and maintaining said body therewithin in compressed condi- Cil tion, the bers of said reinforcing strand means being permanently maintained at a relatively fixed length and in intimate association with said body at at least 10 percent elongation and under tension of at least l0 percent of its ultimate breaking strength to compress said body therewithin, providing an element reinforced with a substantial proportion of stressed fibers.

5. A reinforced element including a rigid, non-yielding solid body having reinforcing strand means composed of a multiplicity of synthetic organic plastic fibers having a high coefficient of contraction of at least 10 percent relatively to the coefficient of contraction of said body and maintaining said body therewithin in compressed condition, said reinforcing strand means being permanently maintained at a relatively fixed length and in intimate association with said body in elongated condition to compress said body therewithin, providing an element reinforced with a substantial proportion of stressed fibers.

6. A reinforced element as claimed in claim 5 wherein .said body is of wood.

7. A reinforced element as claimed in Claim 5 wherein said body is of aluminum.

8. A reinforced element as claimed in claim 5 wherein said body is of plastic.

9. A reinforced ladder including rigid, non-yielding solid side members having reinforcing strand means composed of a multiplicity of synthetic organic plastic fibers having a high coeficient of contraction of at least l0 percent relatively to the coefiicient of contraction of said body extending continuously in a closed curve encircling said side members in a longitudinal direction and maintaining said side members therewithin in compressed condition, said reinforcing strand means being permanently maintained at a relatively fixed length and in intimate association with said side members in elongated condition to compress said body therewithin, providing a ladder reinforced with a substantial proportion of stressed fibers.

10. A reinforced element as claimed in claim 9 wherein said side members are of Wood.

11. A reinforced element as claimed in claim 9 wherein said side members are of aluminum.

12. A reinforced element including a rigid, non-yielding aluminum tube having longitudinally extending central reinforcing strand means composed of a multiplicity of synthetic organic plastic fibers having a high coefficient of contraction of at least 10 percent relatively to the coeflicient of contraction of said body extending continuously throughout the length of said tube internally thereof and maintaining said surrounding tube in compressed condition, said reinforcing strand means being permanently maintained at a relatively fixed length in elongated condition to compress said tube, providing an element reinforced with a substantial proportion of stressed bers.

13. A stressed structural element having a relatively rigid, incompressible body and tensioning means comprising a multiplicity of synthetic organic plastic fibers having a recoverable stretch of at least about 10 percent, and a tensile strength of at least about 10,000 p.s.i., said tensioning means being maintained at a relatively fixed length in intimate association with said body at -at least about 5 to 10 percent elongation and under tension of at least 10 percent of its ultimate breaking strength to compress said body.

14. A stressed structural member as claimed in claim 13 wherein said tensioning means is free of supporting terminations.

15. A stressed structural member as claimed in claim 14 wherein said tensioning means is curved.

16. A stressed structural element as claimed in claim 13 wherein said fibers are of nylon, and said tensioning means has a recoverable stretch of at least about percent and a tensile strength of at least about 20,000 p.s.i.

17. A stressed structural element as claimed in claim 13 wherein said fibers are of polyester and said tensioning means has a recoverable stretch of at least about 20 percent and a tensile strength of at least about 20,000 p.s.i.

18. A stressed structural element as claimed in claim 13 wherein said bers are of polypropylene and said tensioning means has a recoverable stretch of at least about 20 percent and a tensile strength of at least about 15,000 p.s.i.

UNITED References Cited STATES PATENTS Muntz 52-223 Burton.

Seckel.

Evans et al.

Bussard et al.

10 2,336,529 5/1953 Morris 52-3o9 2,850,890 9/1958 Rubenstein 52-309 2,359,936 11/1953 Wamken 13 36 X FOREIGN PATENTS 5 452,126 11/1934 Greatritain.

FRANK L. ABBOTT, Primary Examiner J. L. RIDGILL, JR., Assistant Examiner U.S. C1. X.R.

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