US 3921353 A
A tendon anchorage assembly is disclosed which includes anchorage securement means formed in the convergently tapered bore of the tendon anchorage for securement of mounting means in the bore to cast the anchorage in place in a concrete member for prestressing of the member. The anchorage securement means is preferably formed as a tapered thread in the convergently tapered anchorage bore with the mounting means being formed as a tubular member having a tapered threaded end which mates with the tapered threads and the anchorage bore. The mounting means allows the assembly to be positioned in fixed spaced relation to a concrete member to be prestressed. A cylindrical threaded surface cut into the convergently tapered bore for receipt of a cylindrical threaded end of an assembly mounting tube is also disclosed.
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Description (OCR text may contain errors)
United States Patent [1 1 Howlett et al.
[ Nov. 25, 1975  Inventors: George H. Howlett; James W.
' Howlett, both of Oakland, Calif.
 Assignee: Howlett Machine Works, Berkeley,
 Filed: Feb. 4, 1972  Appl. No.: 223,584
 US. Cl 52/223; 52/699  E04C 3/10; E04B 1/38 Primary Examiner-James L. Ridgill, Jr. Attorney, Agent, or Firm-Warren, Chickering & Grunewald  ABSTRACT A tendon anchorage assembly is disclosed which includes anchorage securement means formed in the convergently tapered bore of the tendon anchorage for securement of mounting means in the bore to cast the anchorage in place in a concrete member for prestressing of the member. The anchorage securement means is preferably formed as a tapered thread in the convergently tapered anchorage bore with the mounting means being formed as a tubular member having a  Field of Search 52/223 L, 223, 230, 699 tapered threaded end which mates with the tapered threads and the anchorage bore. The mounting means  References Cited allows the assembly to be positioned in fixed spaced UNITED STATES PATENTS relation to a concrete member to be prestressed. A cy- 3,605,361 9/1971 Howlett 52/223 lindrical threaded suTface Cut 9" T Convergemly tapered bore for receipt of a cylindrical threaded end FOREIGN PATENTS OR APPLICATIONS of an assembly mounting tube is also disclosed. 704,094 2/1954 United Kingdom 52/230 2 C 4 Drawing Figures US. Patent Nov. 25, 1975 TENDON ANCHORAGE ASSEMBLY AND BORE MOUNTING APPARATUS THEREFOR BACKGROUND OF THE INVENTION In the construction of various types of concrete members, particularly floor slabs and slab walls, it has been found to be highly advantageous to use a prestressed concrete construction. Typically, a plurality of tendons,'in the form of wires, strands or cables, and tendon anchorages are placed within the concrete forms, and the concrete is then poured with the tendons and anchorages being cast-in-place. The forms are removed and the tendons are tensioned, giving rise to a post-tensioned prestressed concrete member. It has further been found to be desirable in many applications to have the tendon anchorage located in a recess in the end of the concrete member, which recess is subsequently grouted so that the anchorage is completely contained within the finished prestressed concrete member. When this type of construction is used, the tendon anchorage must be located and positioned in fixed, spaced relation to the concrete form board by means of apparatus which, after casting of the concrete member, affords access to the anchorage for post-tensioning of the tendon.
Several anchorages have been previously employed in order to provide an anchorage construction which can be cast-in-place at a position interiorly of the end of the concrete member. One type of approach is common to and illustrated in US. Pat. Nos. 3,293,81 1 and 3,399,434.
Another approach has been to provide a tendon anchorage which is formed with screw threads or a bayonet-type socket in advance of, and preferably concentric with, the convergently tapered, wedge-receiving bore of the anchorage. A mounting member is then screwed into the front of the anchorage and extends therefrom to the form board.
While these prior systems have been found to have numerous advantages, certain problems have been encountered which could further be advantageously eliminated. Thus, some prior art systems are relatively timeconsuming to mount to the form board, while others interfere with the surface against which a tensioning ram or jack must sit during a tensioning of the tendon. Still others afford no protection to the convergently tapered, wedge-receiving bore from the ingress of concrete thereto during casting.
Accordingly, it is an object of the present invention to provide apparatus for mounting a tendon anchorage assembly to be cast-in-place in a concrete member which allows a rapid mounting of assembly to the form board without interfering with jacking or tensioning of the tendon and without interfering with convergent gripping of the tendon by the tendon gripping wedges.
It is another object of the present invention to provide an apparatus for the mounting of a tendon anchorage assembly to a form board for casting of said anchorage in a concrete member which may be economically incorporated into the anchorage assembly, is durable and able to withstand rough treatment under field conditions, and affords protection to the wedge-receiving bore of the anchorage.
Other features and advantages of the tendon anchorage assembly of the present invention will become apparent upon consideration of the drawing and the following description of the preferred embodiment.
SUMMARY OF THE INVENTION The tendon anchorage assembly of the present invention is comprised, briefly, of a form board, spacing means formed to space the anchorage from the form board, tendon anchorage means having an axially convergently tapered bore to provide wedge-supporting surfaces, anchorage securement means formed in said tapered bore for releasable securement of assembly mounting means thereto, and assembly mounting means passing through the form board and spacing means, and having an inner end formed for releasable securement to said anchorage securement means in said tapered bore and an outer end formed for releasable securement to the form board. The anchorage securement means is preferably formed as a tapered thread formed in the convergently tapered bore and the assembly mounting means formed as a tube having a tapered end with threads formed to mate with the tapered threads in the anchorage bore. Alternatively, a cylindrical recess may be cut into the convergently tapered bore in the anchorage and threaded for receipt of a cylindrical tubular member having a threaded end for mounting of the assembly to the form board.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary, side elevational view, in cross-section, of a tendon anchorage constructed in accordance with the present invention and secured to a concrete form for casting of the anchorage into a concrete member.
FIG. 2 is a fragmentary, side elevational view, in cross-section, of the anchorage illustrated in FIG. 1 after casting of the concrete member, removal of the concrete form and tensioning of the tendon.
FIG. 3 is a fragmentary, side elevational view, in cross-section, of an alternative embodiment of the tendon anchorage assembly of the present invention.
FIG. 4 is a fragmentary, side elevational view, in cross-section, of the tendon anchorage assembly of FIG. 3 after insertion of tendon gripping wedges.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2, the anchorage assembly of the present invention can be seen to be comprised of a bearing plate 11, anchor member 12, assembly mounting means 13, and spacing means 14. The anchorage is secured to and extends from concrete form board 16. In order to secure the anchorage assembly of the present invention to form board 16, the frusto-conical bore 36 of anchorage 12 is formed with anchorage securement means 51, which is preferably a tapered thread formed in the surface of the frusto-conical wedge-receiving bore 36. Assembly mounting means 13 is formed at its inner end 18 with inner securement means, in this case tapered thread 52, formed for and in mating engagement with the tapered thread 51 in bore 36.
In order to allow the assembly to be urged into close abutting relationship with the form board, the outer end 19 of mounting means 13 is formed with outer securement means, such as threads 53 for receipt of nut 17 thereon. Form board 16 is provided with an opening 33 therethrough and spacer 14 is provided with a similar channel or opening 32, both of which are in alignment with frusto-conical bore 36. Thus, mounting tube 13 is mounted in opening 33 in the form board and extends from the exterior side of the form board through opening 33 and through channel 32 in the spacing and block-out element 14 to frusto-conical bore 36. In order to secure the assembly against the form board, tapered threads 52 are threaded into mating threads 51 in the anchorage bore to secure the inner end of the mounting means, and nut 17 is screwed down against the form board until the assembly is held tightly in place, as shown in FIG. 1.
It is further preferable that mounting means 13 be tubular in order that tendon 22 may be positioned through the mounting means while the anchorage is being cast into the concrete member. Wrapped around tendon 22 is a paper covering 23 having a layer of grease on the inner surface thereof so that the tendon may be post-tensioned within covering 23 after casting of the concrete member. Sealing of the junction between wrapped tendon 22 and anchor plate 11 may be accomplished by tape 24, which will resist the flow of concrete into the anchorage and mounting means.
Since the assembly mounting apparatus of the present invention is effected by securement of the mounting tube 13 to frusto-conical bore 36, anchor plate 11 must be affixed in some manner to anchor member 12, such as by means of welds 21. Thus, securement of the mounting means to anchor member 12 similarly holds the bearing plate 11 in place. Alternatively, bearing plate 11 and anchor member 12 may be integrally formed or fastened together by other techniques, and if welding is used, only a few tack welds need be employed.
Prior tendon anchorage assembly mounting techniques have sought either to reach through the wedgereceiving bore or to secure the mounting member to the front or outwardly facing side of the anchor member in advance of the tendon receiving bore. These approaches have been taken, in large part, because of the hesitancy to tamper with the wedge-receiving bore for fear that the anchorage will fail. It has been found, however, that axial loading forces on the tendons are so high that the presence of a tapered thread 51 in bore 36 does not interfere with the convergent axial movement of wedges 31, which are positioned in the bore after removal of the form, as shown in FIG. 2. Thus, the smooth outer frusto-conical surface of wedges 31 merely rides on the peaks of threads 51 as the wedge moves axially toward the small diameter end of the anchorage bore 36 during a gripping of tendon 22. Since wedges 31 are preferably surface-hardened, the axial tension forces may cause the wedges to deform the peaks of threads 51 slightly, but this does not interfere with the gripping engagement of the tendons by the wedges.
One important advantage that accrues by reason of the use of a tapered thread as the anchorage securement means is that only a few turns, on the order of about four or less, of mounting tube 13 will result in a complete disengagement of the inner threaded end of the mounting tube from the tapered bore. In fully secured position, all of the threads 52 on end 18 of the mounting tube engage threads 51 in the anchor member to provide a secure connection. On disengagement, however, a few turns will completely disengage all of the threads allowing removal of the mounting tube. In order to allow rotation of the mounting tube 13, a second nut or non-circular surface (not shown) can be affixed to or formed in end 19 of the mounting tube.
'Another advantage of the tendon anchorage assembly mounting apparatus of the present invention is that opening 25 in bearing plate 11 and opening 37 at the small diameter end of bore 36 may be formed to have a dimension only slightly larger than tendon 22. In systems in which the mounting tube passes through the anchor plate, the small diameter end 37 must accommodate not only the tendon, but the full diameter of the mounting means. When a tapered thread is used, opening 37 can be much smaller with the result that wedges 31 can similarly be smaller in their thickness between the serrated teeth 39, which engage the tendon 22, and the outer surface 30, which engages anchorage 12. The reduction of the size and volume of wedges 31 and accordingly, the wedge containing barrel or housing 12, results in a significant costs savings in the overall anchorage by reason of a decrease in the cost of materials, the cost of fabrication, and the cost of heat treating. This is particularly important in large slabs which include many tendon anchorages.
Still further, securement of the anchorage mounting apparatus in the wedge-receiving bore of the anchorage has the advantage of leaving the front surface 27 unimpeded so that a ram or jacking head may be inserted in pocket 28 left by spacer 14 for tensioning of the tendon. Thus, the ram or jacking head may bear directly on anchor member 12 during tensioning of the tendon, rather than having to bear upon a mounting element affixed to front 27 of anchor member 12 or to span the anchor member and bear directly on concrete member 38.
In order to cast the anchorage assembly of the present invention in place in concrete member 38, anchor plate 11 and wedge-containing housing 12 are preferably slipped over tendon 22, with the tendon passing through opening 25 and out bore 36. Paper tape 23 may be removed back to the position of the anchor plate and tape 24 used to effect a seal between the tendon and the anchor plate. The spacer means 14 is then slipped over the tendon, and the tendon passed out through opening 33 and form board 16. Mounting tube 13 is inserted through the form board and spacer means and screwed into the threaded tapered bore 36. Nut 17 may then be threaded on and adjusted until the anchorage is pulled up firmly against the form board so that spacer 14 blocks out the entry of concrete between the spacer element and the anchorage into the frusto-conical bore 36. As so mounted in place, concrete may be poured and allowed to set up. Once the concrete has become solid, nut 17 is loosened and mounting means 13 rotated a few turns to release the same from the anchorage. The mounting means is withdrawn from the form board and spacer 14, and the form board and spacer stripped from the concrete member, leaving the anchorage cast-in-place. Wedges 33 are inserted in bore 36 and a ram used to tension the tendon 22, after which the tendon is cut off so that it will not extend beyond pocket 28. Finally, although not shown, grout may be placed in pocket 28 to seal the anchorage inside the end of the concrete member.
Referring now to FIGS. 3 and 4, an alternative embodiment of the tendon anchorage assembly of the present invention is illustrated. Bearing plate 61, having opening 65 for receipt of tendon 72, is secured by tack welds 71 to anchor member 62 in a manner similar to the embodiment shown in FIGS. 1 and 2. In this embodiment, however, frusto-conical wedge-receiving bore 76 is not formed with tapered anchorage securement threads. Instead, the anchorage securement means is formed as a threaded cylindrical surface 77 between the large and small diameter ends of bore 76. As so formed, anchorage securement means 77 is used with a mounting tube 63, shown in phantom, having an end 64, which is cylindrical and formed with threads mating with threads 77. The anchorage is held in place against a spacer (not shown) and a form board (not shown) in a manner analogous to the assembly of FIGS. 1 and 2. Once mounting tube 63 is removed and the form board and spacer stripped away from the concrete member, wedges 66 may be placed in bore 76, and they will span cylindrical threaded recess 77 so that the wedges will converge and move axially without being interfered with by the cylindrical threaded mounting .portion 77. The cylindrical threaded surface 77 is preferably further formed with a relieved annular groove 78 which allows the threading tap to run out so that the entire cylindrical surface 77 may be threaded and the chips formed during the threading operation allowed to freely pass out of the threads.
The use of a cylindrical threaded section in the tapered frusto-conical bore has the advantage of allowing somewhat more conventional machine operations in forming the mounting apparatus. It should be noted, however, that one further advantage of a tapered thread, as shown in FIGS. 1 and 2, is that damage to the thread, which may easily occur under the rough field conditions which anchorages are typically subjected to, will not be serious since tapered threads tend to correct themselves when screwed together whereas a cylindrical thread may more easily become cross-threaded and permanently damaged.
It has been found that the cylindrical recess 77 will not materially affect the convergent movement of the wedges as long as the recess does not eliminate too substantial a portion of the wedge-engaging surface of frusto-conical bore 76. Thus, if threaded recess 77 is on the order of about A to A; of the length of bore 76, wedges 66 will have sufficient strength to span the recess and impose a gripping force on tendon 72 which is sufficient to withstand the axial load on the tendon. Additionally, since some leakage of concrete into bore 76 through opening 65 may occur, it is preferable that securement means 77 be located adjacent the small diameter end of bore 76, causing leaking concrete to pass down the center of tube 63. Similarly, the diameter of tubular mounting means 13 is preferably selected so that threads 52 engage the small end of bore 36 and the inner diameter of tube 13 is aligned with opening 25.
1. In a tendon anchorage assembly to be cast-in-place in a concrete member, said assembly including anchorage means having a wall defining an axially convergently tapered bore providing a supporting surface for mounting of tendon gripping wedges therein, spacing means formed with an opening therein and formed to space said anchorage means from a form board to which said anchorage means is to be mounted, and assembly mounting means extending through said spacing means to said anchorage means, the improvement comprising:
a. anchorage securement means in the form of a cylindrical surface formed in said tapered bore intermediate the ends of said bore, said surface being formed with cylindrical thread means for receipt and releasable securement of said assembly mounting means thereto, said surface being further dimensioned for spanning thereof by said tendon gripping wedges and for convergent axial movement of said tendon gripping wedges thereover upon removal of said tendon mounting means from said tapered bore; and
b. said assembly mounting means includes a tubular pipe having a threaded cylindrical inner end formed and dimensioned for and in releasable mating engagement with said cylindrical thread means formed in said surface, said assembly mounting means being further formed proximate an outer end of said pipe for releasable securement to said form board.
2. The tendon anchorage assembly as defined in claim 1 wherein,
the inner end of said cylindrical surface in said tapered bore is formed with a relieved section of greater diameter than said cylindrical surface to facilitate threading of said cylindrical surface.