Abstract:
An improved method and apparatus for anchoring tensioning members in a concrete structure and the like wherein there is provided a wedge support member having wedge-receiving pockets coaxially aligned with the anchor seats in an anchorage block permitting the anchorage wedges to first be inserted into the pockets not allowing the tensioning members to slip during tensioning. This method is used in post-tensioning of monostrand cable. The apparatus anchors the tensioning members by installing wedges into their respective wedge seats. A magnetic tip is utilized to hold the wedges in place while a slide hammer provides and transmits the energy to forcibly insert the wedges.

Description:
TECHNICAL FIELD 
     The present invention relates in general to post tensioning concrete construction and, in particular to the anchoring of monostrand cables extending through a concrete structure. 
     BACKGROUND INFORMATION 
     Concrete is strong in compression, but relatively weak in tension. In most structures, such as buildings or bridges, forces such as wind, gravity, and earthquakes subject the structure, and therefore, the concrete to compressive and tensile stresses. Steel reinforcement is typically used to resist the tensile forces within a concrete structural member. The steel reinforcement is placed in the form before the concrete is poured within the tension zone. The steel reinforcement is designed to work with the concrete to resist tension forces and to control concrete cracking. For additional efficiency and economy, steel reinforcement, such as prestressing steel, can be stressed before the building forces are applied to the concrete member in the opposite direction of the to-be-applied force. Such stressing counteracts the applied force and allows the use of less steel reinforcing. 
     There are two basic methods of stressing concrete before the concrete is subjected to design forces (i.e., prestressing): post-tensioning and pre-tensioning. Pre-tensioned prestressed concrete is usually fabricated at a plant remote from the final construction site. In that case, the steel tendons are stressed before the concrete is placed. With post-tensioned prestressed concrete, steel tendons are stressed after the concrete has been placed and gained sufficient strength at the construction site. The present invention is primarily concerned with post-tensioned prestressed concrete. 
     With post-tensioned prestressed concrete, after the concrete has hardened, a hydraulic jack is used to pull strands of steel or tendons encased in the concrete. The prestressing steel strands are separated from the concrete by polyethylene sheathing. This sheathing allows the encased steel move with respect to the concrete. Thus, the steel can be stressed without frictional resistance from the concrete which effectively eliminates tensile stresses in the concrete due to the prestressing. The tension force in the steel is then permanently transferred to the concrete as a compressive force through anchoring devices at the end of the concrete member. 
     In post-tensioned concrete, the concrete is simply poured over the unstressed steel strands or tendons. Initially, therefore, both concrete and steel have no stress. As the concrete cures or hardens, it gains its compressive strength. After about 24 hours, it usually has reached about 75 percent of its full design strength. It is at this point when the steel is stressed. The steel is then stressed to high stresses, (e.g., 216,000 psi) but within its elastic limit. While stressed, the steel is then permanently attached to anchors at the ends of the concrete beam. Thus, the steel is in tension (i.e, pulling on the anchors) while the concrete remains in compression because both anchors are pushing on the concrete. The result is a considerable reserve of compressive stress at the bottom of the beam that in turn can be used to counteract the tensile stresses from an applied load. 
     Before the concrete is poured, anchorages are set and attached to the form. Once the concrete has hardened, the anchorages are embedded into the concrete. Typically, the anchorages are not flush with the concrete edge. Pocket formers are used to separate the anchorages from the edge of the concrete. Thus, after the concrete has hardened, these pocket formers leave anchorage cavities in the concrete. Anchorage cavities have tapered wedge-receiving seats and passages through which the tensioning cables extend. FIG. 1 is a cross-section of an end of a concrete member  100  through anchorage  105  and anchorage cavity  102 . Steel strand  104  extends from polyethylene sheathing  106  through the center of anchorage  105  and anchorage cavity  102 . 
     A stressing ram (not shown) is used to pull the tendon  104  to the correct pressure and elongation. Then, anchoring wedges  108   a  and  108   b  are driven between tendon  104  and the surrounding anchorage  105 . The wedges grip tendon  104  as the jack releases it. Thus, the wedges  108   a  and  108   b  keep tendon  104  from slipping back through anchorage  105  into the concrete. Once the all of the tendons have been properly stressed, the ends are cut, the pockets are grouted, and the beam ends are encapsulated to protect the tendons from corrosion. 
     Wedges  108   a  and  108   b  are pieces of tapered metal with teeth that bite into the prestressing tendon during transfer of the prestressing force. The teeth are beveled at the front end to ensure gradual development of the tendon force over the length of the wedge. Two piece wedges are normally used for monostrand tendons. Proper installation of the wedges into the concrete is critical. The wedges must be evenly spaced and exactly fitted into anchorage  105 . If the wedges are unevenly spaced or not properly fitted, the wedges could slip and ruin the entire prestressing operation. 
     It has been found that it is difficult to properly align, install and seat the wedges. The wedges must be installed into the back surface  103  of anchorage cavity  102  which has limited access. Furthermore, both wedges need to be equally spaced and driven into anchorage  105  by an equal amount. Currently, a small hand held device is used to install the wedges in the anchorage cavity. This small hand held device consists of a bent longitudinal bar. The user positions one wedge into the space between anchorage  105  and protruding tendon  104 . Then with the other hand, slides the hand held device along tendon  104  with enough force to partially anchor the wedge into anchorage  105 . The user then repeats the procedure for the other wedge on the opposite side of the strand. The tool only allows for partial installation of one wedge at a time. Thus, the user must alternate seating the wedges on both sides of the cable constantly to ensure that the wedges are evenly aligned as they enter their respective wedge seats. This process is time consuming, inefficient and difficult due to the limited space available in the anchorage cavity. 
     What is needed, therefore, is an efficient and cost-effective device to place the anchorage wedges into the anchorages prior to tensioning the monostrand members. 
     SUMMARY OF THE INVENTION 
     The previously mentioned needs are fulfilled with the present invention. Accordingly, there is provided, in a first form, a hand-operated tool with a u-shaped bar with a handle, sliding hammer, a concentric stop and magnetic tip. The cross section shape of the tool is u-shaped to allow the tool to straddle and slide along the tendon. The slide hammer action in the handle of the tool allows the user to apply force to firmly drive the wedges into the respective anchor-wedge seats. The concentric stop is a means transferring the energy of the hammer&#39;s momentum to the magnetic tool head. The magnetic tip is designed to hold the wedges around the strand until they can be seated into the concrete anchorage. Thus, the installer can simultaneously set two wedges evenly and correctly in the cavity. After the wedges have been partially seated, the magnetic connection is easily broken as the tool is retracted. 
     The present invention allows for two or more wedges to be simultaneously installed, as opposed to a single wedge. Furthermore, both wedges can be aligned simultaneously on the magnetic head of the tool, as opposed to working within the confined space of the anchorage cavity. Finally, the seating of the wedges can occur simultaneously, and because the tool places a nearly equal force on both wedges, there is no need for repeated attempts of alignment and seating. 
     These and other features, and advantages, will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. It is important to note the drawings are not intended to represent the only form of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a cross-section a cavity in a concrete structure and tendon running through the concrete structure and extending beyond the concrete structure; 
     FIG. 2 is an isometric view of the present invention; 
     FIG. 3 is a longitudinal cross sectional view of the present invention straddling a tendon protruding from a concrete structure; 
     FIG. 4 is a cross sectional view of the present invention; 
     FIG. 5 is an isometric view of another embodiment of the hammer member of the present invention; and 
     FIG. 6 a  and  6   b  are isometric views of two embodiments of the tool tip of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The principles of the present invention and their advantages are best understood by referring to the illustrated embodiment depicted in FIG. 2 of the drawings, in which like numbers designate like parts. In the following description, well-known elements are presented without detailed description in order not to obscure the present invention in unnecessary detail. For the most part, details unnecessary to obtain a complete understanding of the present invention have been omitted in as much as such details are within the skills of persons of ordinary skill in the relevant art. 
     FIG. 2 depicts a isometric view of the present invention. The wedge installer tool  200  is constructed from hardened steel or similar material and includes a magnetized tip  202 , shaft  204 , stop  206 , hammer member  208  and handle member  212 . Shaft  204  of wedge installer tool  200  generally extends along longitudinal axis  201 . The cross-section shape of shaft  204  is generally u-shaped cross-section so that it can slide along and straddle over tendon  104  (see FIG.  4 ). The interior diameter of shaft  204 , therefore, must be large enough to fit over tendon  104  with sufficient clearance that shaft  204  can slid along tendon  104  (see FIG.  4 ). Typically, tendons currently in use for post-tensioning are approximately one half inch in diameter. One end of shaft  204  is bent to form transition member  210  and, then bent again to form handle member  212 . Handle member  212  is offset from longitudinal axis  201  and has a sufficient length so that a human hand can grab handle member  212  and use it to slide wedge installer tool  200  along tendon  104  without touching or interfering with tendon  104  (FIG.  1 ). 
     Tip  202  is at the opposite end of shaft  204  from handle member  212 . Tip  202  is either magnetized or has a sufficient diameter to contain recesses for magnets. FIG. 6 a  an exploded isometric view of one embodiment of tip  202 . Magnets  602   a  and  602   b  are embedded in cylindrical openings  604   a  and  604   b  in the front face of tip  202 . Cylindrical openings  604   a  and  604   b  have longitudinal axises substantially parallel to axis  201 . FIG. 6 b  is an exploded isometric view of another embodiment of tip  202 . Annular magnet  606  is mounted in concentric circular recess  608  on the front face of tip  202 . In both embodiment, the magnets releasably hold anchorage wedges  108   a  and  108   b  to tip  202  (See FIG.  3 ). This support arrangement for anchorage wedges  108   a  and  108   b  permits the anchorage wedges to be inserted into anchorage cavity  102  without the need for additional support. 
     Farther along shaft  204 , is stop  206 . Stop  206  extends radially outwardly from shaft  204 . Stop  206  has a cross-section greater than the cross-section of shaft  204 . Hammer member  208  is slidably mounted on shaft  204  for longitudinal movement along the shaft. The cross-section shape of hammer member  208  is also generally u-shaped which allows for the entire tool to straddle or to be slidably mounted on a steel tendon. This interengagement is more clearly shown is FIG.  4 . FIG. 4 is a transverse cross-section view showing tendon  104  partially surrounded by shaft  204 . Shaft  204  is also partially surrounded by hammer member  208 . Hammer member  208  is a combined weight and handgrip. In one embodiment, hammer member  208  is roughly cylindrical in shape and includes a roughen cross-hatched exterior surface for providing positive, easy gripping by the user of the tool (See FIG.  3 ). FIG. 5 is an isometric view of another embodiment of the hammer member. Hammer member  500  has enlarged ends  502  and  506  and a handle portion  504 . The overall shape of hammer member  500  is similar to a dumbbell. Enlarged ends  502  and  506  provide extra weight to hammer member  506  and handle portion  504  acts as a handle. The cross-section shape of hammer member  500  is similar to hammer member  208 . The cross-section is generally u-shaped and is designed such that hammer member  500  can straddle or partially surround and fit over shaft  204  such that hammer member  500  can be slidably mounted onto shaft  204  (see FIG.  4 ). 
     FIG. 3 shows a longitudinal cross section of tool  200  mounted on tendon  104  which is protruding from anchorage  105 . Wedges  108   a  and  108   b  are shown magnetically attached to tip  202  before they are seated into anchorage  105 . Stop  206  includes pounding surface  205 . Hammer member  208  also includes a pounding surface  207  corresponding to pounding surface  205 . Hammer member  208  and stop  206  operate as a conventional slide hammer to impart a pounding force or longitudinal force in direction  209  on the pounding surface  205 . Direction  209  is generally parallel to the longitudinal axis of shaft  204 . The pounding force is then transferred to shaft  204 , which imparts a similar force to tip  202 , and then to wedge  108   a  and  108   b . 
     OPERATION 
     To begin the installation of wedges  108   a  and  108   b , wedge installer tool  200  is positioned such that it slides over tendon  104  as illustrated in FIG.  3 . Next, the user positions wedges  108   a  and  108   b  on tool tip  202 . The user then slides wedges  108   a  and  108   b  in direction  209  driving the wedges into anchorage  105 . The user slides hammer member  208  towards handle member  212 . The user then slides hammer member  208  along shaft  204  parallel to direction  209  until pounding surface  207  of hammer member  208  makes contact pounding surface  205  of stop  206 . This sliding action and contact causes the momentum of the slide hammer  205  to be transferred as a longitudinal or pounding force to stop member  206  which imparts a force down shaft  204  to wedges  108   a  and  108   b  which forcibly drive the wedges evenly into anchorage  105 . The user may have to repeat the last step of this process (sliding of hammer member  208  along shaft  204  in direction  209 ) until both wedges  108   a  and  108   b  are completely seated around tendon  104 . Once the wedges  108   a  and  108   b  are completely seated, the user pulls on handle member  212  which slides wedge installer tool  200  away from the concrete. This disengages the magnetic force used initially to hold the wedges  108   a  and  108   b . The user then removes the tool simply by sliding it off tendon  104 . 
     The wedge installation tool has substantial advantages over the prior art. Rather than having to hold each wedge individually while the wedges are driven into the cavity, the wedge seating tool&#39;s magnetized head properly hold the wedges in place before they are placed into the cavity. Furthermore, the wedge seating tool saves the operator a significant amount of time because the operator can seat both wedges simultaneously. Additionally, because both wedges are seated by the same surface and impact, both wedges are seated an equal amount. Unequal seating results in excessive tendon slippage and tension failure of the tendons. 
     Although the invention has been described with reference to specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore, contemplated that the claims will cover any such modifications or embodiments that fall within the true scope of the invention.