US3123942A

US3123942A - Stinton

Info

Publication number: US3123942A
Authority: US
Publication date: 1964-03-10
Anticipated expiration: 1981-03-10

Description

March 10, 1964 F. STINTON 3,123,942 METHOD OF CONSTRUCTION Filed Oct. 5, 1960 2 Sheets-Sheet 1 INVENTOR. Fred Srinton BY C ll/-65 Mar/[9, t gfn ATTORNEYS March 10, 1964 F. STINTON 3,123,942

METHOD OF CONSTRUCTION Filed on. 5, 1960 2 Sheets-Sheet 2 an: may

INVENTOR. Fred" S'rinton BiY Gui/g l/srr/lv, i ygwi A T TOR N E Y3 United States Patent 3,123,942 METHQD 0F (JUNSTRUCTEON Fred Stinton, Mount Prospect, ISL, assignor to The Preload Company, Inc, Mount Prospect, llL, a corporation of Delaware Filed (Pet. 5, 1960, Ser. No. 60,682 8 Qlairns. (Cl. 50-531) This invention relates to construction methods and in particular to a method for prestressing reinforced concrete tanks and the like.

Prestressed reinforced concrete tanks and the like are often more economical to construct than ordinary reinforced concrete structures of equivalent capacity since prestressing permits a large amount of concrete to be eliminated from the tank wall.

A prestressed tank is one where high tensile strength wire or the like has been wound under a high tension about a reinforced concrete tank core wall so that the concrete is in a state of compression when the tank is empty. When the tank is filled, the load developed on the tank wall by the contents of the tank must first overcome the compressive stresses exerted by the wound wire before tensile stresses are developed in the Wall. Thus, it is possible to substantially reduce the quantity of concrete required by prestressing and, accordingly, reduce the cost of the tank.

In order to take advantage of the lower cost of materials to be realized by the saving in concrete, it is paramount that the method of prestressing be as efficient and inexpensive to practice as possible. Heretofore, the most successful prestressing system for tanks and the like has involved the use of a wire winder which places a wire under tension about a core wall by pulling itself on an annulus that is placed about the tank. Such a method is described in US. Patent No. 2,370,780.

Other prior methods have included the use of a tractor arrangement wherein a tractor pulls a wire under tension and circles a tank so that the wire is placed on the tank and the structure is prestressed.

These prior methods have all required the use of heavy and bulky equipment which is not only expensive to manufacture or purchase but, also, costly to transport to the job site and set up for operation.

While the wire winder apparatus and its method of use are the best which have been provided heretofore, it has not been economical to use this equipment and method for tanks of small diameters and low wall height. The cost of set up and transporting the equipment has cut into the possible savings in concrete so that no cost advantage over an ordinary reinforced concrete tank is gained.

Among the other shortcomings of prior methods have been the necessity of spacing a series of tanks sufiiciently far apart so that adequate working space is provided between tanks; the need of constructing and waiting for the hardening of the tank dome ring to support the heavy wire winder before prestressing of the walls can begin; requirement that outer tank Wall be substantially free of obstacles such as piping connections, copings, etc.; the expense of the high cost of extra excavation with buried or hill side tanks beyond the size of the tank itself to accommodate the wire winder; the necessity of using small diameter wire which requires many revolutions to wind a sufiicient quantity of wire about the tank; the constant danger of the small diameter wire comprising many convolutions breaking, thus whipping the broken wire at a great and dangerous velocity; and others of a similar nature.

The present invention permits strand cable to be used for prestressing the tank in place of the heretofore prevailing wire. With wire winding apparatus the small diameter (MW) wire is drawn through a die and there is a resultant danger of fracture and breaking. Accordingly, the initial allowable stress for such wire is about 140,000 p.s.i. With the present invention larger diameter (%s or /2") strand cable is preferably used. Because there is no drawing of the cable, the initial allowable stress is about 170,000 p.s.i. Thus, fewer convolutions of a strand cable than wire are required on a strength relationship, and still fewer convolutions because of the larger diameter of cable which may be used.

Accordingly, it is an object of the present invention to provide a method of prestressing concrete tanks which is efiicient and yet inexpensive. It is a further object to provide a prestressing method which eliminates the need for heavy and costly wire winding equipment. It is another object of the present invention to provide a construction method Which may be carried on within a minimum work area.

In the accompanying drawings and in the description thereof a particular embodiment of the present invention is shown and described. However, it is to be understood that this embodiment is not intended to be the limitation of this invention but is for the purpose of illustration so that those skilled in the art may practice hereinafter.

In the drawings:

FIGURE 1 is a perspective view of a reinforced concrete tank which is being prestressed;

FIGURE 2 is a schematic plan view of a tank core wall before prestressing is begun;

FIGURE 3 is a view similar to FIGURE 2 with one end of the tensioning cable anchored in place;

FIGURE 4 is a view similar to FIGURE 2 with a first length of cable in position about the tank prior to tension- FIGURE 5 is a view similar to FIGURE 2 with the first length of cable tensioned to the desired elongation; and

FIGURE 6 is a view similar to FIGURE 5 with a second length of cable tensioned to the desired elongation completing a convolution about the tank.

As shown in the drawings and in FIGURES 26 in particular, the present invention provides a method of tensioning a cable about the core wall of a tank by elongating successive continuous lengths of cable placed about the tank in convolutions. In carrying out the present invention it has been found that once the length of cable has been placed under an initial tension and elongation of the cable has begun, for best results, the elongating is continued uninterrupted until the desired total elongation and the accompanying developed tensile stress in the length of cable is achieved. This continuous and uninterrupted elongating gives a substantially uniform elongation to the entire length of cable with a corresponding uniform developed stress throughout the length.

When elongating a cable about a tank or the like, static friction between the cable and the tank surface must first be overcome before elongation can begin. With static friction overcome, the force exerted on the cable need only be that required for the desired total elongation plus that needed to overcome the kinetic friction developed between the elongating cable and the tank surface. If the elongation of the cable is interrupted for any reason after it has begun and then begun again, static friction again becomes effective. The static friction developed between a partially and materially elongated cable length and the wall surface is greater than that in the case of a tensioned cable Without any substantial elongation. This is caused by the radial forces developed in the partially elongated cable. Since the friction between two given bodies is directly proportional to the pressure, the developed radial forces increase the static friction which must be overcome.

The cable used in the presently described example is of the strand type. By use of this type of cable a corrugated cable surface is presented to the concrete core wall so that less than a full cable surface contacts the wall.

In overcoming this static friction it is believed that the static friction between the length of cable and the wall nearest the source of applied elongating force is first overcome. This results in the length of cable being non-uniformly elongated and a corresponding non-uniform development of stress in the tension length of cable. Accordingly, best results are obtained when the length of cable is elongated in a continuing and uninterrupted action. Further, if elongation is interrupted at an intermediate stage, positive uniform elongation can best be obtained by releasing the load on the length of cable and beginning anew the elongating of the cable.

An example of the present invention may best be explained by reference to the drawings. In FIGURE 1 a prestressed tank is wound with a series of convolutions of strand cable 12 which are applied in accordance with the present invention. The tank 10 is constructed of reinforced concrete by any well known method. After the tank wall 14 and the dome ring 16 have been prepared prestressing of the tank may be commenced.

As shown in FIGURE 2 in particular, the wall 14- is provided with a series of spaced vertical grooves 13 on its outer surface. These grooves 18 are spaced at any selected distance, 30 inches having been found to be satisfactory. In addition, two oppositely positioned vertical channels 29 and 21 are also recessed in the outer surface of the tank wall 14.

Before beginning prestressing operations, a supply reel 22 of strand cable is positioned conveniently within the tank wall 14 as shown in FIGURE 3, or if desired, it may be set outside the wall and fed into the center of the tank through an opening (not shown) in the wall. One end of the cable 12 is withdrawn from the reel and alfixed to the first recessed channel 2% by an anchor 24. The anchorage point may be near the top of the wall 14 or near the bottom according to the particular field conditions. In normal operation the anchorage point is near the bottom.

With the end of cable 12 anchored to the channel 20, a first length 25 of cable is payed out from the reel 22. The extent of this first length 25 is suificient to reach to the second channel 21 when the cable 12 is placed about the wall 14 to form a half convolution as shown in FIGURE 4. At channel 21 a jacking and clamping device 26 is removably afiixed to the channel 21.

The device 26 may be of any suitable type (such as a screw jack, a hydraulic ram jack, or any similar arrangement) which permits the cable 12 to be gripped and gradually tensioned and elongated.

In a preferred embodiment the device 26 is comprised of a hydraulic ram jack With a split cone gripper for engaging the cable 12. The device is in an extended position at the start of tensioning operations. By this is meant that the rams are fully extended and may be retracted their full extent.

With the jacking device 26 in place the cable 12 is inserted therein and an initial tension applied to the first cable length 25 to snug it up against the Wall 14 so as to remove the slack and hold it in position. After the cable is snugged up, vertical rods 28, such as steel reinforcing rods or the like, are inserted into the grooves 18 so that a minimum bearing surface is provided for the cable length 25. If desired, the rods 23 may be rotatably mounted for further ease in elongating the cable. In most cases solid rods, not rotatably mounted, are satisfactory.

A satisfactory rod size for use with a inch diameter cable is inch when the grooves are spaced inches on center. The rods 28 provide a minimum friction contact area for the cable, thus providing a substantial friction free extent along the cable length 25 during the elongation operation.

With rods 28 in place the next step is to elongate the cable length 25. To accomplish this the ram of the jacking device is retracted in a continuous and uninterrupted motion until the desired total elongation of the first cable length 25 is obtained. During the operation of elongating the length 25 a continuous elongation must be maintained so that only kinetic friction will be encountered in the operation. As discussed hereinbefore, a stoppage in the elongating operation, say at the halfway point, and a restarting and continuation from this point will not accomplish a substantially uniform elongation over the entire extent of length 25.

After the desired elongation of length 25 has been accomplished, a second length 39 of the cable 12 is laid out and positioned about the tank wall 14. Cable length 34) extends from the jacking device 26 at channel 29 to a second jacking device 32 which is temporarily afiixed at channel 29.

In order to insure that length 25 has been properly tensioned a double check is available. First the total elongation will be one indicator, and the jack pressure developed will be the second. By having this double indication system proper elongation is assured.

In developing the desired tension in the length of cable the strand cable will be elongated a definite amount. This amount of elongation is readily determined by well known methods and it is a direct measure of the tensile stress developed in the length of strand cable. Accordingly, when the predetermined total elongation in the strand cable is obtained, the developed stress will also be obtained.

To develop the desired tension and accompanying total elongation of the strand cable, a load is applied to the cable by the jacking device 26. The load applied by the device 26 is indicated by the pressure developed in the device 26. Thus, the load on the cable is accurately shown by the developed pressure in the device 26.

As shown in FIGURE 6, the elongating procedure with the second cable length 34 is similar to that used with the first cable length with the exception that the second cable length is anchored to the first jacking device 26 since the second length it) is continuous from the first length 25.

When the desired total elongation is obtained in the second length, the forces exerted on each side of the gripper portion of the first jacking device 26 are opposite and substantially equal. Therefore, the holding effect of the gripper is substantially eliminated and, also, the developed pressure in the first jacking device substantially zero. In actual operation a small residual developed load may remain in the first jack 26, in which case a small increase in the pressure developed in the second jacking device 32 will overcome this.

With the elongation and developed stress in the first and

second cable lengths

25 and 30 substantially equal the first jacking device can be disengaged. The procedure is then repeated for further cable lengths until the required number of convolutions are obtained about the tank.

The rods 28 may be removed from the wall of the tank as required and reused, or, if desired, they may be left in the tank wall.

The several convolutions of cable may be anchored off at any desired intermediate point or points. When the desired number of convolutions have been obtained a suitable anchor is used to permanently affix the last cable length to the tank.

Each length of cable is of substantial extent. In wrapping a tank of diameter, each length of cable may be one half the circumference of the tank.

The various convolutions about the tank are aesthetically pleasing. This is caused by the limited number of cables which are required compared to the large number of wires used in wire wound tanks.

In addition to the pleasing effect of the cable, it also permits quicker and easier repairs to the cable and the tank to be made. Since the cable may be anchored off after a few convolutions a severed cable may be replaced with a few windings using the exceedingly portable equipment required for the present invention. With wire wound tanks frequent anchoring is expensive and impractical due to the large number of convolutions required. Also, the expense of moving in and setting up a wire winder is great. The substantially widely spaced convolutions of cable accomplished by the present invention also permit alterations to tank walls, for change of piping, etc., to be made more readily and conveniently since a large amount of wall space is not covered with the tensioning cable.

From the foregoing it will be realized that the present invention accomplishes the several aforesaid objects as well as others which are apparent from the description of the invention herein:

I claim:

1. A method of prestressing large substantially cylindrical walled concrete structures such as tanks and the like comprising the steps of anchoring one end of a high tensile strength flexible tension member, extending a first length of substantial extent of the tension member along the wall, continuously and uninterruptedly elastically elongating said first length to a desired total elastic elongation whereby a uniform elastic elongation and predetermined stress is developed in said first length, maintaining the elongation of said first length and continuously and uninterruptedly elastically elongating along the wall a series of additional lengths of said tension member, maintaining the elastic elongation and uniform stress in each successive length before continuously and uninterruptedly elastically elongating the next successive length, whereby the uniform elongation and the developed stress in said additional lengths are substantially equal to that of said first length, anchoring the end of the last of said additional lengths whereby a series of convolutions of said tension member are formed about the wall.

2. A method of prestressing large substantially cylindrical walled concrete tanks as defined in claim 1 wherein the flexible tension member is a strand cable.

3. A method of prestressing large substantially cylindrical walled concrete structures comprising the steps of anchoring one end of a high tensile strength strand cable, extending a first length of substantial extent of the strand cable along the wall, continuously and uninterruptedly elastically elongating said first length to a desired total elastic elongation and uniform stress after static friction between the wall and the cable is overcome, maintaining the desired total elongation and uniform stress in said first length, extending additional lengths of the strand cable each of substantial extent about the wall, continuously and uninterruptedly elastically elongating each successive length to a desired total elastic elongation to develop a uniform stress therein substantially equal to the uniform stress developed in the first length, maintaining the elastic elongation and uniform stress in each successive length before continuously and uninterruptedly elastically elongating the next successive length, anchoring the last length after elongation whereby a series of uniformly stressed convolutions of strand cable are permanently maintained about the wall and an inward compressive force is exerted thereon by the convolutions of cable.

4. A method as defined in claim 3 wherein jacking devices are temporarily aifixed to the wall for elastically elongating each length of cable.

5. A method as defined in claim 3 wherein first a jacking device is temporarily afiixed to the wall for elastically elongating each length of cable and said device is maintained in holding relationship with the first length of strand cable each intermediate length until the total elastic elongation and uniform stress is developed in the next succeeding length by a second jacking device afiixed to the wall whereby the developed load in the first jacking device is substantially eliminated.

6. A method of prestressing large substantially cylindrical walled concrete tanks and the like with a series of convolutions of high tensile strength flexible strand cable wrapped about the wall comprising the steps of anchoring one end ofthe strand cable, extending a first length of substantial extent of the cable about a portion of the wall less than the circumference of the wall, applying a pulling force to said first length by means of a first jacking device temporarily a fiixed to the wall, the pulling force being continuously and uninterruptedly applied to the first length of cable to obtain a desired total elastic elongation of said first length and to develop a uniform stress therein, maintaining the elastic elongation of the first length, extending additional lengths of substantial extent of the strand cable about the wall, continuously and uninterruptedly elastically elongating each successive length of cable by a jacking device to a desired total elastic elongation to develop a uniform stress therein substantially equal to the uniform stress developed in the first length, maintaining the elastic elongation and uniform stress in each successive length before continuously and uninterruptedly elastically elongating the next successive length, the jacking device remaining in holding relation with each intermediate length until the desired uniform stress is developed in the next successive length, anchoring the last length while maintaining the uniform stress developed.

7. A method as defined in claim 6 wherein the cable has a corrugated outer surface and vertical supports are placed along the wall to reduce the area of frictional contact of the cable.

'8. A method of prestressing large concrete structures such as tanks and the like comprising the steps of anchoring one end of a high tensile strength flexible tension member, extending from said anchored end a first length of substantial extent of the tension member, continuously and uninterruptedly elastically elongating said first length to a desired total elastic elongation whereby a predetermined uniform stress is developed in said first length by said elastic elongation, maintaining the elastic elongation of said first length and continuously and uninterruptedly elastically elongating successive lengths of said tension member, maintaining the elastic elongation and uniform stress in each successive length before continuously and uninterruptedly elongating the next successive length so that the elastic elongation and uniform stress developed in the successive lengths is substantially equal to the elastic elongation and uniform stress developed in the first length whereby a series of lengths of said tension member are uniformly prestressed and form convolutions of said tension member about said tank, and anchoring the final length of tension member after total elastic elongation of said final length.

References Cited in the file of this patent UNITED STATES PATENTS 2,191,025 Mitchell Feb. 20, 1940 2,321,465 Crorn June 8, 1943 2,579,183 Freyssinet Dec. 18, 1951