US3891731A

US3891731A - Method of pre-stressing form tie systems

Info

Publication number: US3891731A
Application number: US867508A
Authority: US
Inventors: Chester I Williams
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-10-20
Filing date: 1969-10-20
Publication date: 1975-06-24
Anticipated expiration: 1992-06-24
Also published as: CA918388A; GB1278926A; JPS5311786B1

Abstract

A form anchor embedded in concrete is pre-stressed exclusively with respect to a connecting shebolt and the surrounding concrete to a substantial portion of the working load prior to the securing of the form to the shebolt.

Description

United States Patent 11 1 Williams 1111 3,891,731 June 24, 1975 METHOD OF PRE-STRESSING FORM TIE SYSTEMS [76] Inventor: Chester l. Williams, 347 Greenbriar,

S.E., Grand Rapids, Mich. 49506 22 Filed: 0C1. 20, I969 1211 Appl. No.: 867,508

521 US. Cl. 264/33; 249/21; 264/228; 264/229; 425/65 151 1m. 0.. E04g ll/28 581 Field of Search 25/118 T, 131 D; 249/20-22; 264/33, 228, 229; 425/65 56] References Cited UNITED STATES PATENTS 2,967,343 l/l96l Williams 25/l3l D 3355.529 ll/l967 Easterday 2. 264/228 X OTHER PUBLICATIONS P. W. Abeles, Fully and Partly Prestressed Reinforced Concrete", Journal of the American Concrete Institute, Vol. l6. N0. 3, 1-1945, pp. l8ll87.

Primary Examinerjan H. Silbaugh [57] ABSTRACT A form anchor embedded in concrete is pre-stressed exclusively with respect to a connecting shebolt and the surrounding concrete to a substantial portion of the working load prior to the securing of the form to the shebolt.

4 Claims, 6 Drawing Figures PATENTEUJUN 24 I975 SHEET PATENT-LU Jun 3 m5 Fig. 5

I IIIIIIIIHIIL ymmg 'IIIILL H! IO N METHOD OF PRE-STRESSING FORM TIE SYSTEMS BACKGROUND OF THE INVENTION A vertical or near-vertical face of a concrete structure is developed through repeated use of forms that confine the poured concrete to the plane of the face. It is common practice to secure anchor rods in the newly-poured concrete for use later in tying the forms in position to confine the succeeding pour. The anchors are preferably in the form of undulated rods threaded at the end nearest the formed face, and this threading is engaged by internal threading in the end ofa bolt that transfers the loading to reinforcing beams of the form structure. The anchors are placed in position for engagement with the newly-poured concrete through the use of dummy bolts mounted at the upper portion of the form, and it is the usual practice to use a dummy bolt configuration such that a tapered socket is formed in the concrete exposing the threaded end of the anchor at a position set back somewhat from the plane of the face. The stress bolt has essentially the same configuration as the socket, the purpose of the arrangement being to provide an enlarged bearing area against the concrete for the transmission of the vertical loads representing the weight of the form structure.

If the tie systems are not pre-stressed prior to the pouring of the concrete. it is inevitable that the high loads involved result in stretching the anchor rod through the concrete, and thus permitting the form to back off slightly from the original position bearing solidly against the formed face. When this occurs, some degree of leakage of the newly-poured concrete is inevitable, resulting in a distortion of the formed face. A shifting of the plane of the form from the vertical is also inevitable under these conditions. To correct this problem, it has been common practice to tighten the connections between the shebolt and the form structure to a point representing a substantial portion of the working load of the tie system. The problem is that this procedure necessarily results in backing the shebolt out of the conical recess receiving it to the extent of the elongation and pull-out of the anchor rod. It is obvious that any axial shift of the conical end of the stress shebolt out of a similarly-shaped conical recess will largely eliminate the substantial advantage of a large surface area in solid bearing engagement. The result of the pull-out is to transfer the area bearing into a relatively narrow line contact which so increases the intensity of local stress as to induce spall-out of the concrete, to say nothing of the loss of the positive positioning of the shebolt as a result of looseness in the socket. Placement of forms should be kept as accurate as possible to elimi nate the development of cumulative error as the construction of the concrete mass proceeds in successive pours. In the use of so-called climbing" forms, or in other types of forms in which the securing ties are mechanically actuated, a type of shebolt is frequently used in which a relatively large-diameter flange is positioned directly against the formed face of the concrete mass in order to provide a bearing support for the weight of the form structure. It is obvious that the standard prestressing arrangement that results in backing out the shebolt can easily move this load-transfer flange out to the point where the beam flange representing the lower extremity of the form structure can slip in behind the shebolt flange with a disastrous effect. These problems have largely been responsible for the relatively slow development of automatically-actuated tie systems that would otherwise produce substantial savings in the development of concrete structures.

SUMMARY OF THE INVENTION This invention provides for a system of pre-stressing that eliminates the axial movement of the stressed shebolt with respect to the concrete. Pre-stressing is developed prior to the application of securing forces to the form. The procedure for the connection of the tie systern involves a preliminary pre-stressing operation in which the shebolt is forcibly rotated in its threaded engagement with the anchor to the point that the pre stress loading is developed by a pulling action resisted by the pressure of the tapered end of the shebolt in the concrete socket. The anchor is pulled until it develops its predetermined loading. which may be approxi mately determined by the amount of torque applied to the shebolt (allowance being made for the frictional resistance against the rotation in the concrete socket). When this degree of pre-stress has been established in the anchor, it is obvious that no outward movement of the shebolt can take place unless the pre-stress loading is exceeded by the loading subsequently applied to the shebolt in securing it to the form structure. The degree of pre-stress can be kept sufficiently close to the ultimate working load to eliminate any substantial axial displacement as the pouring of the concrete proceeds. It is also obvious that the pre-stressed connection of the shebolt and the anchor will result in a solid placement of the shebolt as a firm support for the weight of the form structure as the final securing operation takes place. Best results are obtained through the use of the gradient anchor configuration shown in my US. Pat. No. 3,160,988, as that particular anchor configuration results in the generation of retention forces distributed throughout the length of the anchor, rather than permitting them to become centered in the first undulation near the formed face of the concrete. An anchor rod with a large-amplitude undulation near the formed face will tend to develop such concentrated stresses in the concrete at this point that the result will be to break out the concrete in the very area where it is desired to maintain the solidity necessary to support the shebolt. Where concrete has had a considerable length of time to set, this is a minor problem, but usual practice is to time successive pours of concrete in as rapid a sequence as possible. This inevitably results in the necessity of securing the forms to relatively un-cured or green" concrete that has developed only a fraction of its ultimate stress-carrying capability. Fly ash or other additives that delay the set also delay the strength of the concrete, weakening it substantially (actually reducing the strength of two day concrete from 400 psi to 200 psi).

DESCRIPTION OF THE DRAWINGS The features of the invention will be analyzed in detail through a discussion of the several figures in the drawings, in which:

FIG. 1 is a sectional elevation showing a typical steel form structure modified to the extent necessary to accommodate it to the present invention.

FIG. 2 is a fragmentary sectional elevation on an enlarged scale over FIG. 1 showing a preliminary stage in the attachment of the tie system.

FIG. 3 is a fragmentary sectional elevation similar to FIG. 2, and showing the completed attachment of the tie system.

FIG. 4 is a sectional elevation on an enlarged scale over FIGS. 2 and 3, showing the interengagement of an anchor rod with a dummy shebolt.

FIG. 5 illustrates the subsequent interengagement of the anchor with the stress-carrying shebolt in the prestressed condition.

FIG. 6 illustrates a different form structure and tie arrangement that can utilize the present invention.

DESCRIPTION OF THE PREFERRED PROCEDURE Referring to FIG. 1, the mass of concrete 10 has been developed through successive pours, the last two of which are indicated at 11 and 12. The formed face 13 is developed by the panel 14 of the form structure 15, which includes (in addition to the panel 14) the horizontal beams 16-19, spaced vertical beams 20, and the cantilever beams 21. The latter are normally installed in pairs, with the space between them being traversed by the tie systems. The cantilever beams 21 may be pivotally connected to the beams 20 by the pins 22, with the upper extremity of the form structure being adjustable with respect to the cantilever beam 21 by a jack system (not shown). The abutment beam 23 at the lower extremity of the cantilever beam 21 acts in conjunction with the tie systems to maintain the vertical orientation of the cantilever beam 21, to maintain the erect position of the form structure.

When the form structure is in position preparatory to the pouring of concrete on top of the surface 24, an chor rods 25 are supported in position to be surrounded by concrete through the use of dummy shebolts 26 slipped through appropriate holes in the form structure, and positioned axially by the pins 27 traversing a flange of the angle 28 mounted on the cantilever beams 21. The conical inner extremity 29 of the dummy shebolt 26 forms a socket extending inwardly from the formed face 13 for receiving the similarlyshaped end 30 of the stress-carrying shebolt 31. The anchor rod 32 shown in FIG. 1 was initially supported and placed in position in the same manner as the anchor 25.

Referring to FIG. 4, the threaded bore 33 in the inner end of the dummy shebolt 26 is provided with a stop plug 34 limiting the penetration of the threaded end 35 of the anchor rod 25. The counterbore 36 at the entrance of the threaded bore 33 is optional, but is desirable as means for facilitating the threaded engagement between the anchor and the dummy shebolt. Prior to the engagement of the anchor with the dummy shebolt, the seal ring 37 is slipped into place to close off the opening at the entrance of the counterbore 36 to prevent the inflow of concrete.

When the stress-carrying shebolt 31 is attached to the anchor 32, and prior to the application of securing forces to the form structure, the bolt 31 is rotated down against the threaded end of the anchor 32 with a sufficient intensity of torque to pull the anchor 32 sufficiently to develop the desired degree of pre-stress. The resulting axial movement of the anchor 32 with respect to the concrete results in a movement of the unthreaded portion of the anchor rod 32 into the counter bore 38 in the bolt 31. The bolt 31 is not provided with the stop plug, thus permitting the end of the anchor rod to move axially into the threaded bore 39 to whatever extent is necessary to develop the desired degree of pre-stress.

The successive stages of completely coupling the tie systems are shown in FIGS. 2 and 3, together with the preferred mechanism for accomplishing these procedures. The stress-carrying shebolt 31 has a square section 40 slidably received in a similarly-shaped opening in the drive hub of the conventional reversible gear motor 41 secured to the flange 42 of the beam 21. Torque delivered by the gear motor will generate the pre-stress condition illustrated in FIG. 5, accompanied by a slipping of the square section 40 through the drive opening of the gear motor 41. This operation is performed while the form structure is suspended either from a conventional overhead crane. or possibly by a lifting mechanism of a climbing form system. In either case. the complete form structure will be brought close to or lightly against the formed face 13, and the presence of the light spring 43 acting against the nut 44 will permit the bolt 31 to back off resiliently and apply a gentle force facilitating threaded interengagement with the anchor rod as the gear motor 41 begins its operation. As the tightening proceeds, the shebolt changes from the FIG. 2 position to the FIG. 3 position, when full pre-stress may be considered to have been developed. To eliminate any interference with the prestressing, the nut 45 is preferably backed off to provide substantial clearance over the bridge plate 46 spanning between the beams 21 for transferring the forces ultimately securing the form structure in position. After the degree of pre-stress has been developed by the gear motor 41, the similar gear motor 47 induces rotation of the shaft 48 on which the axially elongated gear 49 is supported. Operation of the gear motor 47 will shift the nut 45 down into bearing position, as shown in FIG. 3. The extended axial length of the gear 49 will permit the axial movement of the nut 45 from the clearance position required by the pre-stressing operation down into the bearing condition shown in FIG. 3.

In order to provide a solid and positive vertical placement of the form structure, it is preferable to either extend the conical surface 29 to a sufficient degree to provide bearing for the flanges 50, or a cylindrical portion 51 may be provided as an intermediate surface between the conical portion and the square cross-section 40. The latter arrangement has the advantage of providing a more distributed bearing for the weight of the form structure. In either case, the solid pre-stressing of the anchor rod with respect to the surrounding concrete permits the shebolt 31 to provide a firm positioning function. Preferably, there will be a slight degree of lateral lost motion between the square cross-section 40 and the drive opening of the gear motor 41 in order to permit some degree of shifting as the form structure is brought into position preparatory to engaging the anchor rod.

The pre-stressing operation outlined above can be performed manually with the aid of a torque wrench, if desired. The gear nut 45 can also be replaced by a standard nut set by a wrench. An arrangement such as this is shown associated with a conventional wooden form structure in FIG. 6. The form structure generally indicated at 52 includes the plywood panel 53, the horizontal studs 54-59, and the vertical walers 60. The abutment block 61 stabilizes the lower extremity of the walers to provide a cantilever wooden form structure of generally standard design. The vertical walers are secured to the horizontal studs by the use of

L bolts

62 and 63.

The placement of the anchors 64 for the new pour is handled in exactly the same manner as that illustrated in FIG. 1. The dummy shebolts 65 are supported by the form structure in the position shown at 66. The stress shebolt 67 differs from standard in the provision of the square or hex section 68. lf the circumscribed diameter of the section 68 is larger than that of the threaded portion 69, the shebolt 67 can be tightened on the anchor 66 through the use of a socket wrench slipped in between the waler pairs 60. If the circumscribed diameter of the section 48 is less than that of the threaded section 69, it is preferable that the pre-stressing operation of the shebolt 67 on the anchor 66 be accomplished prior to placement of the form structure 52 on the shebolt, so that there will be clearance for the application of a wrench to the portion 68 from the side. As an alternative, the section 68 can extend outwardly beyond the plane of the walers 60, accompanied by the use of a bearing plate 70 which is sufficiently deep (in the direction of the axis of the bolt 67) to assure that the wing nut 71 will have sufficient axial threading to secure the form structure. Such an arrangement would permit the use of somewhat smaller diameter rod stock in the manufacture of the shebolt 67, so that the flats forming the peripheral surface of the portion 68 could be milled or broached. If the circumscribed diameter of this portion were to be larger than that of the rest of the bolt, the bolt would probably be manufactured from square or hexagonal rod stock, with the cylindrical portions machined down from that configuration. It is obvious that this will produce quite a bit of extra machining, and considerable waste of material. The extent of the axial projection of the section 68 outward beyond the walers 60 should be sufficient for the convenient engagement of a wrench without interference from the presence of the walers. Whichever of these various alternatives is selected will normally be determined by a balance of the various cost factors involved, including labor on the job.

In the arrangement shown in FIG. 6, and also that in FIG. I, disengagement of the form after the new pour of concrete has set involves the release of the form structure from the concrete mass. In the FIG. 6 arrangement, this is done by the removal of the nut 71, the bearing plate 70, and the pin 72. After this has been accomplished, the form structure can be moved horizontally outward to a position free of the stress shebolt and the dummy shebolt. In the FIG. 1 arrangement. the shebolt remains associated with the form, and the disengagement is accompanied by back-rotation ofthe reversible gear motor 41, possibly preceded by removing the load on the nut 45 by back rotation of the gear motor 47. In a fully automated form structure. the dummy shebolt 26 would also be provided with a power-actuated arrangement utilizing a reversible gear motor functioning in the manner of the motor 41. A spring arrangement similar to the spring 43 would be desirable to assure the proper axial placement of the dummy shebolt through the gear motor. A fully automated arrangement of this type can make possible a very efficient operation. as it may be controlled from remote positions with a high degree of accuracy, the torque applied to the shebolts being easily determinable by notation of the power being consumed by the gear motors as determined by standard metering arrangements.

I claim:

1. method of securing a form structure to an existing mass of concrete, said method including attaching in threaded engagement an anchor rod to a dummy bolt mounted in a form panel and projecting inwardly there from, pouring concrete around said anchor rod and the inner extremity of said dummy bolt device, removing said dummy bolt, attaching a stress bolt to said anchor in threaded engagement, and securing a form panel for the succeeding pour by securing the same to said stress bolt, wherein the improvement comprises:

generating predetermined pre-stress in said anchor rod at least to the extent of the working load to be applied thereto by tightening the threaded engagement of said stress bolt therewith prior to the securing of said form panel to said stress bolt, said stress bolt remaining in axially substantially fixed position with respect to said concrete mass during said generation of pre-stress.

2. A method as defined in claim I, wherein said stress bolt is placed in greater axial threaded interengagement with said anchor rod than is said dummy bolt.

3. A method as defined in claim 1, wherein said dummy bolt and stress bolt have similar inner end configuration.

4. A method as defined in claim I, wherein form structure is rested on said stress bolt exclusively adjacent the face of the said concrete mass.

Claims

1. A method of securing a form structure to an existing mass of concrete, said method including attaching in threaded engagement an anchor rod to a dummy bolt mounted in a form panel and projecting inwardly therefrom, pouring concrete around said anchor rod and the inner extremity of said dummy bolt device, removing said dummy bolt, attaching a stress bolt to said anchor in threaded engagement, and securing a form panel for the succeeding pour by securing the same to said stress bolt, wherein the improvement comprises: generating predetermined pre-stress in said anchor rod at least to the extent of the working load to be applied thereto by tightening the threaded engagement of said stress bolt therewith prior to the securing of said form panel to said stress bolt, said stress bolt remaining in axially substantially fixed position with respect to said concrete mass during said generation of pre-stress.

2. A method as defined in claim 1, wherein said stress bolt is placed in greater axial threaded interengagement with said anchor rod than is said dummy bolt.

4. A method as defined in claim 1, wherein form structure is rested on said stress bolt exclusively adjacent the face of the said concrete mass.