US3041702A

US3041702A - Method of making a prestressed reinforced concrete structure

Info

Publication number: US3041702A
Application number: US690295A
Authority: US
Inventors: Alvin R Schwab
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1957-10-15
Filing date: 1957-10-15
Publication date: 1962-07-03
Anticipated expiration: 1979-07-03

Description

METHOD OF MAKING A PRESTRESSED REINFORCED CONCRETE STRUCTURE Filed Oct. 15, 1957 A. R. SCHWAB July 3, 1962 2 Sheets-Sheet 1 lNl/ENTGR AL VIN E. .SCHWAB Attorney United States Patent 3,041,702 METHOD OF MAKING A PRESTRESSED REINFORCED CONCRETE STRUCTURE Alvin R. Schwab, Glen Rock, N.J., assignor to United States Steel Corporation, a corporation of New Jersey Filed Oct. 15, 1957, Ser. No. 690,295

6 Claims. (Cl. 25-154) This invention relates to a method of making a prestressed reinforced concrete structure, particularly beams or other structures in which the reinforcing steel, for best etficie'ncy, should be bent. There are two methods of prestressing concrete, namely, pretensioning and post-tensioning. In pretensioning high strength wire, wire strand or rod is stretched between two fixed points or abutments. While the steel is maintained in its elongated condition, concrete is poured around it and after the concrete has attained sufficient strength, the ends of the steel are released. The stress in the steel is then transferred to the concrete by means of the bond between the two materials. In post-tensioning, the concrete is poured before the steel is stretched with bond between the two materials being prevented. Ordinarily, the stress in the steel is transferred to the concrete by means other than bond. The result produced is essentially the same as that produced by the pretensioning method. The pretensioning method has many advantages over post-tensioning, but pretensioning has a disadvantage that, in the method ordinarily used, the steel is most conveniently stretched in a straight line from abutment to abutment. For maximum economy, the prestressing force at the center of the span of a beam should be applied as closely as possible to the bottom of the beam, While to eliminate tension in the top fibers it must ordinarily be placed somewhat closer to the centroid at the ends of the beam. However, if the steel is to lie in a straight line, it is impossible to place the steel in the position described. There have been various attempts to overcome this difficulty in pretensioning concrete. The shape of the member has been varied toward the ends (as, for example, by haunching or deepening the beam) and the amount of pretensioned force has been decreased by destroying or preventing bond between some portion of the steel and the concrete near the beam end, but these expedients only partially solve the problem. Methods have also been devised to raise the steel at the ends of the beam or depress it near the center of the beam, but this is difficult on long prestressing beds or with heavy beams requiring a large amount of bent-up steel.

One difficulty in bending up the steel is that large vertical forces occur at the point where the steel is bent upwardly. For example, when stressing a group of ten diameter steel strands to 14,000 lbs. per strand and bending up through an angle of lo degrees, there will be a vertical component of 25,000 lbs. so that there is a problem in holding down the prestressing strand. Either massive weights or prestressing beds with great beam strength may be required.

It is therefore an object of my invention to provide a method of making a prestressed reinforcing concrete structure by pretensioning in which the reinforcing steel is bent up without applying any appreciable vertical forces to the prestressing bed.

This and other objects will be more apparent after referring to the following specification and attached drawings. in which:

FIGURE 1 is a schematic sectional elevation of a prestressing bed showing intermediate steps of my method;

FIGURE 2 is a view, similar to FIGURE 1, showing a second embodiment of my invention;

FIGURE 3 is an enlarged view of a detail of FIGURES 1 and 2;

3,041,702 Patented July 3, 1962 FIGURE 4 is a fragmentary view, showing a modification of the detail of FIGURE 3;

FIGURE 5 is a transverse sectional view of the detail of FIGURE 4;

FIGURE 6 is a transverse sectional view of a further embodiment of my invention;

FIGURE 7 is a schematic elevation showing still another embodiment of my invention;

FIGURE 8 is a view of a detail of my invention;

FIGURE 9 is a view, similar to FIGURE 7, showing a modification thereof; 6

FIGURE 10 is a view, similar to FIGURE 7, showing a modification thereof;

FIGURE 11 is a view, similar to FIGURE 7, showing a modification thereof;

FIGURE 12 is a view of a detail of a part of FIGURES 10 and 11; and

FIGURE 13 is a view of another detail of a part of FIGURES 10 and 11.

Referring more particularly to the drawings, the reference numeral 2 indicates a prestressing bed having abutments 4 at the ends thereof. An eyebolt 6 passes through each of the abutments 4 and has

nuts

8 and 10 thereon, one on each side of the abutment 4. Two

tension members

12 and 14 have their ends fastened to the eyebolts 6. The

tension members

12 and 14 may be of any of the types commonly used in making prestressed concrete. In most instances the

members

12 and 14 will be of greater length than the distance between eyebolts 6. Two or more struts or compression members 16 are inserted between the

tension members

12 and 14 intermediate their ends and hold the tension members in spaced apart relationship. The necessary tensioning force is then applied to the

members

12 and 14 in any suitable conventional manner. As shown, this may be done by loosening one or both of the nuts 10 and tightening one or both of the nuts 8. Another common method is to utilize a jack to move one or both of the eyebolts 6 outwardly. Concrete is then poured into form 18 around the tension member 12. After the concrete sets the required amount the

tension members

12 and 14 are released from the eyebolts 6 and that portion of the tension member 12 extending beyond the form may be cut on. The member 14, which acts as a dummy member, may be reused. After suitable curing the beam B is ready for use. In making the beam disclosed in FIGURE 2 the procedure is the same as that described above except that the dummy tension member 14 is only partially outside the beam B1 with a recess 20 being provided at each end of beam B1 for receiving the member 14. If desired recess 20 may be omitted and tension member 14 covered in any suitable manner to prevent bonding of the member 14 to the concrete. In the embodiments of FIGURES 1 and 2 a third tension member 22 may be provided adjacent the bottom of the prestressing bed 2. This member 22 is horizontal and is fastened to the abutments 4 and tension applied thereto in the same manner as to the

members

12 and 14. The strut or compression member 16 may assume different forms. As shown in FIGURE 3 a saddle 24 rests on the tension member 12 and the strut in the form of a bar 26 extends upwardly through a tube 28 to a second saddle 24 which receives the tension member 14. The tube 28 may be made of metal, cardboard, plastic or the like and merely functions to prevent the concrete from bonding with the member 26 After the prestress has been transferred to the concrete the

members

14 and 26 are removed and the hole in the tube 28 filled with concrete. In some cases it is desirable to use a tie wire 30 from the

strut

16 or 26 to each of the

members

12 and 14, thus preventing the strut from moving longitudinally of the prestressing bed 2.

In the embodiment shown in FIGURES 4 and S the members 12 are bent around a tube 32 which passes tr-ansversely from wall to wall of the form 18. A pin 34 passes through the tube 32 and projects beyond the tube on both sides of the form. The strut in the form of two bars 36, one bearing on each end of the pin 34, extends upwardly outside the form 18 to a second pin 34 around which the members 14 pass. A tie wire 37 extending from tube 32 around member 12 may be provided. After the concrete is poured and the prestress transferred to the concrete the

members

34 and 36 are removed and the tube 32 filled with concrete.

In FIGURE 6 side forms 38 for the beam are utilized as the strut or compression member. A tube 40 extending between the sides of form 38 holds down the prestressed strands 12. A pin 42 passes through the tube 40 and extends outwardly at each end of form 38. A second pin 44 rests on top of the form 38 and holds up the dummy strands 14. Flange stiffeners 46 may be fastened to the form 38 to stiffen the same adjacent the

pins

42 and 44. After prestress has been transferred to the concrete the

pins

42 and 44 are removed along with the dummy strands -14 and reused and the hole in the tube 40 filled with concrete.

FIGURE 7 shows an embodiment of my invention in which a plurality of prestressed concrete structures may be formed at one time. In this embodiment a prestressing bed 48 has a plurality of forms 50 therein spaced apart end to end. A tension member 52 extends between and is fastened to end abutments 54 in the same manner as in FIGURE 1 by means of eyebolts. A second tension member 56 is arranged above the member 52 and is also fastened to abutments 54 by means of eyebolts arranged above the first set of eyebolts. Compression members 58', which may be of the same types described above, are arranged to separate the

tension members

52 and 56 within the forms 50. A tension collar 60 holds the

members

52 and 56 together between each pair of forms 50. The tension collar may assume Various forms but is shown in FIGURE 8 as made of two bent bars held together with bolts. The prestressing is applied in the same manner as described above and, after the concrete has set the desired amount, dummy tension member 56 is removed along with the struts 58 and tension collars 60. Tension member 52 is separated between the forms 50 so as to form a plurality of beams B.

In the embodiment shown in FIGURE 9 three

prestressing tension members

62, 64 and 66 are fastened to abutments 54, one above the other in the same manner as that described above. A dummy tension member 68 is fastened to the abutments 54 above the member 66. Forms 50 are provided in the same manner as described above. A strut 70 is provided between the

members

62 and 64 adjacent each end of each form 50 and a strut 72 is provided between

members

64 and 66 at each end of each form 50 but toward the center of the form from struts 70. A strut 74 is provided between

members

66 and 68 inwardly from each of the struts 72. A tension ring 76 is provided between each pair of forms 50 to hold the

members

62 and 68 together. This method of making the beams requires less dummy steel because the amount of external vertical force necessary to keep the steel in a deflected position is reduced. The upward force from the member 62 at strut 70 and the downward force from the member 64 at the strut 70 balance each other and in a similar manner the upward force from the member 64 and the downward force from the member 66 at the strut 72 balance each other. The downward force in the member 62 and the upward force in the member 66 at the strut 74 are carried by the member 68.

FIGURES and 1-1 show a method of making pointed piles or other non-prismatic forms with symmetrical placed prestressed steel. In this embodiment, forms 78 are placed in prestressing bed 48 and tension members 80 are arranged in a circle and attached to abutments 54 by eyebolts 6 in the same manner as described above. A compression ring 82 is provided in each of the forms to change the direction of travel of the members 80. The compression ring 82 has a plurality of notches 84 around its periphery for receiving the members 80. As shown, four forms 78 are provided and a tension ring 86 is provided between each end form and the form adjacent thereto. The tension ring 86 has an opening 88 therethrough with notches 90 therearound for receiving the members 80. When assembled tension is applied to the members 80 in the same manner as described above, concrete is poured and after the prestressing has been transferred to the concrete, strands 80 are cut oflf at the end of the formed concrete.

In FIGURE 11 a plurality of forms 92 are arranged in the prestressing bed 48 and a plurality of tension members 94 are attached to the abutments 54 by eyebolts 6 in the same manner as in FIGURE 1. A compression ring 96, similar to ring 82, is provided between each end form 92 and each abutment and also between the middle forms. A tension ring 98, similar to tension ring 86, receives tthe strands 94 between each end form 92 and the form adjacent thereto. The tension is applied to the members 94 and the piles P formed in the same manner as described when discussing FIGURE 10.

While several embodiments of my invention have been shown and described it will be apparent that other adaptations and modifications may be made without departing from the scope of the following claims.

I claim:

1. The method of making a prestressed reinforced concrete structure comprising providing a prestressing bed having an abutment at each end, positioning two tension members in said bed with the ends supported at said abutments, inserting a compression member between said tension members intermediate their ends to bend them out of alignment, applying the necessary pretensioning force to said tension members, then pouring concrete around only one of said tension members to form -a concrete structure, and after the concrete has at least partially set removing the other of said tension members.

2. The method of making a prestressed reinforced concrete structure comprising providing a prestressing bed having an abutment at each end, positioning two tension members in said bed with their ends adjacent each other at said abutments, inserting a compression member between said tension members intermediate their ends to bend them out of alignment, applying the necessary pretensioning force to said tension members, then pouring concrete around only one of said tension members to form said concrete structure, and after the concrete has at least partially set removing the other of said tension members.

3. The method of making a prestressed reinforced concrete structure according to claim 2 including the steps of positioning a third tension member in said bed with its ends spaced from the ends of the first and second tension members, and applying the necessary pretensioning force to said third tension member, said concrete being poured around said third tension member before removing the first tension member.

4. The method of making a prestressed reinforced concrete structure comprising providing a prestressing bed having an abutment ateach end, positioning two tension members in said bed with their ends adjacent each other at said abutments, inserting two spaced apart compression members between said tension members intermediate their ends, to bend them out of alignment, applying the necessary pretensioning force to said tension members, then pouring concrete around only one of said tension members to form said concrete structure, and after the concrete has at least partially set removing the other of said tension members.

5. The method of making a plurality of prestressed reinforced concrete structures comprising providing a prestressing bed having an abutment at each end, positioning) two tension members in said bed with their ends supported at said abutments, placing a plurality of forms in said bed, inserting a compression member between said tension members intermediate their ends to hold them apart and bend them out of alignment, placing a tension ring around said tension members to hold them together intermediate the compression member and an abutment, applying the necessary pretensioning force to said tension members, then pouring concrete around only one of said tension members to form said concrete structure, and after the concrete has at least partially set removing the other of said tension members.

6. The method of making a plurality of prestressed reinforced concrete structures comprising providing a prestressing bed having an abutment at each end, positioning a plurality of tension members in said bed with their ends supported at said abutments, placing a plurality of forms in said bed in spaced relationship, inserting a compression member between said tension members intermediate their ends within said forms to hold them apart and bend them out of alignment with at least one tension member being above said form, placing a tension ring around said tension members to hold them together between said forms, applying the necessary pretensioning force to said tension members, then pouring concrete into said forms around the tension member therein, and after the concrete has at least partially set removing the tension member above said form.

References Cited in the file of this patent UNITED STATES PATENTS McCarthy July 7, 1891 McCarthy Sept. 29, 1891 Burkholder et al. June 17, 1905 Owens et a1. May 7, 1907 Wilson Mar. 19, 1918 Schorer Dec. 1, 1942 Schorer Aug. 31, 1943 Barger et al. June 13, 1950 Blaton May 12, 1953 Lipski Dec. 6, 1955 FOREIGN PATENTS Canada Aug. 12, 1952 Germany Sept. 15, 1952 Great Britain Apr. 1, 1889 Great Britain June 10, 1920 Great Britain Apr. 21, 1954 Great Britain Oct. 2, 1957 Italy July 25, 1950 OTHER REFERENCES Equpiment (Magazine), 1957, pp. 138-141 and p.