ANCHORING METHOD AND APPARATUS OF END BEARING SUPPORT TYPE IN EXTERNAL PRESTRESS REINFORCEMENT Technical Field The present invention relates to an anchoring method and apparatus of an end bearing support type in an external prestress reinforcement for repair and reinforcement of a concrete structure which has been severely damaged, for example, a crack generated because a concrete structure such as a bridge or a pier bends or sags, and more particularly, to an anchoring method and apparatus of an end bearing support type in an external prestress reinforcement in which a rib plate is provided on an extension line of an end plate and welded to the end plate so that the concrete contacting the end plate located at the end portion of a girder can bear the tension of an external steel wire. Thus, an increase in the welding length between the rib plate and the end plate improves the load bearing capacity. Also, the strength of the welding portion at the end portion which is a weak portion in the conventional end bearing support type anchoring method is improved.

Background Art For all frame structures, the load bearing capacity decreases and the life span becomes shorter as time passes. Also, as the number of large vehicles and heavily loaded trucks increase with industrial developments, damage to bridges becomes severe. Currently, since most bridges are grade II bridges capable of bearing a total weight of 32 tons, destruction of bridges is accelerated due to passing of vehicles weighing more than the load bearing capacity of the bridge, which may result in collapse of the bridge.

To prevent the above, there is a strong desire to develop an effective and economical repair/reinforcing method for increasing the load bearing capacity of existing bridges so that the existing bridges become grade I bridges and bear a total weight of 43 tons. As a method for solving the above problem, there is a deck plate reconstruction method, a steel plate reinforcement method, and an external steel wire reinforcement method.

In the deck plate reconstruction method, a high strength concrete is used after the deck plate of an existing bridge is completely removed.

However, the deck plate reconstruction method is not widely used due to huge construction costs and manpower and a long-term construction period according to reconstruction of the deck plate. Further, traffic must be completely blocked during the construction period.

The steel plate reinforcement method has been mainly used for most reinforcement construction of bridges and piers. However, the use of this method has decreased recently due to matters of securing quality materials used and construction and problems in maintenance and management. Also, although use of the reinforcement method using a high strength and high elastic material such as a carbon fiber has been gradually increased, actual application thereof to bridges is still rare.

Contrary to the above methods, the external steel wire method which is a reinforcement method adopted to existing structures using external steel wires is widely used for reinforcements of various structures due to its advantages in adaptability to structures and the efficiency of reinforcement.

The method using external steel wires is generally divided into four types according to installation of an anchoring device and has been widely used for actual reinforcement of structures. However, the anchoring methods have problems. That is, the methods are unstable, require a considerably high compression force, or design of the anchoring device is not possible due to a complicated structure.

The types of the conventional anchoring devices are briefly described as follows. In reinforcement of existing bridges using an external prestress reinforcement method, various types of anchoring devices are used as a method for transferring tension of an external steel wire to the structure.

The above anchoring devices can be typically divided into four types, i. e., a shear support type, an end bearing support type, a friction support type, and a hybrid support type. These anchoring methods exhibit a wide difference in load transfer concept according to the structure and installation position of the anchoring device. The features and advantages and disadvantages of the

anchoring methods when being carried out in actual construction are as follows.

In the shear support type, as shown in FIG. 1, a tension P of a PS <BR> <BR> <BR> <BR> (prestressed) wire 17 is supported by a shear force Van of an anchor bolt 13.

As the tension P increases, the number of anchoring bolt 13 required increases.

In construction, an anchoring plate 11 is manufactured and a punching work for making holes in an original PS concrete girder 18 is performed.

Anchoring bolts 13 are inserted into the holes to fix the anchoring plate 11.

A gap between the anchoring plate 11 and the concrete is filled with epoxy and anchoring devices 14 and 15 are installed and a steel wire 17 is tensioned.

The load is transferred as follows. When the steel wire 17 in a wire pipe 16 is tensioned, tension P is applied to a cross anchoring plate 12 and a rib plate 10 via the anchoring devices 14 and 15. The tension is transferred to the anchor bolt 13 installed at the concrete along the anchoring plate 11 and supported by a shear force V, of the anchoring bolt 13.

This method has advantages in that purchase of materials are easy and that construction is simple. However, the method has the following disadvantages.

First, when the punching work for installation of the anchor bolt is performed, the steel wire 17 of the existing concrete structure or the anchoring device may be damaged so that load bearing capacity of the original structure is deteriorated. Also, destruction of an end portion of the PS concrete girder where stress is concentrated due to the holes formed therein is possible and, when the strength of the concrete structure is weak, the anchor bolt may be pulled out. Thus, since the shear load bearing capacity of the anchor bolt is determined by the strength of the concrete, quality management or design becomes difficult. Further, since no method for estimating the strength of the concrete of the existing structure is available, the reliability with respect to the measurement of the strength becomes low.

Second, there is a limit to available tension according to a limitation on

installation of the anchor bolt due to the first disadvantage. Since most of tension is supported by the anchor bolt in the shear support type, when the existing girder is damaged due to installation of the anchor bolt or the quality of concrete cannot be accurately estimated, the installation of the anchor bolt should be restricted. Thus, there is a limitation on introduction of tension.

Third, since a large number of anchor bolts are required for securing tension, the size of the anchoring device increases accordingly. The anchor bolt can obtain the required shear force when the anchor bolts are installed over a predetermined distance. When the interval between the bolts installed is narrower than the predetermined distance, the shear force of the bolt must be recalculated using a corresponding reduction coefficient. Thus, as the tension increases, since a large number of anchor bolts are needed and the corresponding interval between the anchor bolts must be secured, the size of the anchoring plate increases accordingly. Thus, this method cannot be used when the area of the girder is small.

Fourth, although the load bearing capacity of the bolt is proportional to the diameter of the bolt, since the load bearing capacity per bolt is about 2-3 tons or less, a sufficient number of the bolts with appropriate intervals must be used to secure a load bearing capacity necessary for external prestressing.

This is due to the same reason applied for the above-mentioned increase of size of the anchoring plate.

In the friction support type, as shown in FIG. 2, a large compression force is applied to an anchoring plate 21 using bolts which fix PS steel rods 29 and tension P of a PS steel wire 27 is supported by fixed shear friction force between the anchoring plate 21 and the original structure due to the above compression force. In general, the steel rod 29 tensions between the anchoring device and a PS concrete girder with an average tension of about 40 tons.

According to the order of construction, the anchoring plate 21 is manufactured and holes are made in the PS concrete girder. The steel rods 29 are inserted into the holes and the anchoring plate 21 is installed. Empty space between the anchoring plate 21 and the concrete is filled with epoxy

forming a seal. The anchoring device 25 is installed and tensioned.

In the transfer of a load, when the steel wire 27 in a steel wire pipe 26 is tensioned, a tension P is directly applied to the anchoring plate fixing the anchoring device since there is no cross anchoring plate or rib plate as in other methods. The tension P is supported by a friction force generated by the steel rod.

This method is characteristic in that construction by this method is possible when the interval between a girder and the next girder is small so that construction by the end bearing support type is not possible. Also, the present method is aesthetically leasing and small.

However, the disadvantages of the above method are as follows.

First, the cost of the PS steel rod is high and special technology is required for installing the PS steel rods. For the anchoring device to bear the tension, compression force of tens of tons corresponding to the tension should be applied to the anchoring plate and accordingly the steel rod and the bolts and nuts are necessarily manufactured in a special manner. Also, to apply a tension of about 40 tons, a fastening work or a special equipment is needed.

Further, since measurement by torque and compression force by fastening do not increase in a predetermined proportion when the nut is fastened, an accurate calculation of compression force with respect to tension is not possible.

Second, since the final anchoring position of a tension member does not extend to a pier, a review of a stress distribution and stability between the anchoring device and a supporting point is needed. This is because, in a state of entirely receiving compression force, tensile stress is generated or stress distortion phenomenon can be generated around a hole for a tensile rod.

In the end bearing support type, an end plate 39 and an anchoring plate 31 are installed in a"c"shape at an end portion of PS concrete so that a concrete surface contacting an end plate 39 supports the tension of an external steel wire 37. As shown in FIG. 3, unlike the afore-mentioned shear support type, the size of the anchoring plate 31 and the number and length of

anchor bolts decrease. This is to receive the tension at the end portion of PS concrete.

According to the order of construction, the anchoring plate 31 is manufactured and holes are made in a PS concrete girder. Anchor bolts 33 are inserted in the holes and the anchoring plate 31 is installed. Here, since the anchor bolts are only for fixing the anchoring plate 31 at a corresponding position during construction, there is no need to make holes deep unlike the shear support type. Next, the end plate 39 is welded to the anchoring plate 31 and the space between the anchoring plate 31 and the end plate is filled with epoxy forming a seal. Finally, anchoring devices 34 and 35 are installed and the steel wire 37 in a steel wire pipe 36 is tensioned.

In the transfer of load, when the steel wire 37 is tensioned, tension P is applied to a cross anchoring plate 32 and the force is transferred to a rib plate 30 and the anchoring plate 31, as in the shear support type. However, the subsequent processes are different from the shear support type. That is, the force applied to the anchoring plate 31 is transferred to the end plate 39 via the anchoring plate 31 and the force is finally supported by a concrete girder f, contacting the end plate 39. In this method, as the tension arrives at the end plate 39 via the anchoring plate 31 as can be seen from the above load transfer process, as shown in FIG. 4, considerable stress is applied to the corner between the anchoring plate 31 and the end plate 39 so that a welding portion can be detached. That is, when tension is applied to the anchoring plate 31, the end plate 39 transforms and a tensile crack is generated on an inside surface of the welding portion of the anchoring plate 31 and the end plate 39 and the welding portion is damaged.

This method has the advantage of using materials that can be purchased easily and simple construction and uniform distribution of the effect of external steel wire to the entire effective span.

However, the above method has the following disadvantages.

First, there is high probability that the welding portion of the corner of the end portion of the PS concrete girder can be destroyed. The end portion of the PS concrete girder is a portion where existing steel wires are

concentrated so that considerable stress is concentrated. Thus, more reinforcing bars are packed in the end portion of the PS concrete girder.

However, when a considerable compression force is applied to the end portion, a partial destruction is likely to occur. Also, there is a recess block for camouflaging the protrusion of a tension installation device at the end portion of the PS concrete girder, which is filled with common cement mortar after tensioning work is completed. However, since there are many cases in which the filled state is poor, when a considerable compression force by external steel wires is applied, cracks may be generated. To solve the problem of crack generation, the empty space between the concrete structure, the end plate and the anchoring plate should be filled with a material such as epoxy.

Second, there may be a limit in carrying out construction. That is, since an application of the method is available when there is an interval over at least 3 cm at the end portion of the PS concrete girder, when the PS concrete girders are disposed in sequence or the intervals between the girders are narrow, it is difficult to secure a sufficient thickness of the end plate.

Third, clearance of a bridge is limited. The interval between one PS concrete girder and the next PS concrete girder is designed to absorb expansion of the bridge generated due to a rise in temperature. However, when an iron structure for anchoring is inserted in that interval, the space for absorbing the expansion of the PS concrete girder is consequently reduced.

Thus, the above method can be applied only for cases in which the interval between the girders is 3 cm or greater.

Finally, the hybrid support type mixing the shear support type and the end bearing support type will be described. The hybrid support type is devised since compensation for the above disadvantages is not easy and is a mere method expecting a mutually complementary action by appropriately mixing the above methods, rather than suggesting a logical solution.

According to the order of construction, an anchoring plate 51 is manufactured and holes for installing anchor bolts 53 are made in a PS concrete girder as shown in FIG. 5. A steel rod 53'to be installed midway of the girder can be installed when there exists a hole already in the existing

girder, and since it is nearly impossible to make a new hole, there are not many cases of installing the steel rod 53. After installation of the anchor bolts 53, the anchoring plate 51 and an end plate 59 are fixed and welded. Finally, anchoring devices 54 and 55 are installed and a steel wire 57 in a steel wire pipe 56 is tensioned.

The load transfer process of this method is similar to those in the afore- said shear support type and the end bearing support type since this method is a mixture of the above two methods. That is, load is transferred to the end plate 59 and the anchoring plate 51 via a rib plate 50 and an anchoring device 52, but the only differing portion is that both the anchor bolts 53 and the concrete receive tension P finally in the load transfer process.

The hybrid support type utilizing various anchoring methods can increase load bearing capacity on the whole. However, the method has the following disadvantages.

First, various support types do not operate concurrently. When the anchoring plate 51 advances after the end plate 59 functioning as an end bearing is sufficiently deformed, a shearing force acts on the anchor bolts 53 or the steel rod 53'. Since the size of a bolt hole made in the anchoring plate 51 is larger than the size of the bolt itself and since a shearing force does not act on the bolt until deformation within 1-2 mm or less is generated, a concurrent occurrence of the effects of shear support and end bearing hardly occurs. Thus, the hybrid support type does not have the load bearing capacity corresponding to the sum of the respective support forces, but the load bearing capacity of each anchoring device occurs in sequence. Therefore, when the anchoring force of this hybrid support type method is calculated, a simple addition of the load bearing capacity of each method may result in excess calculation of the load bearing capacity and damage the anchoring apparatuses. Also, since it is difficult to calculate the load distribution rate of each anchoring method, design of the anchoring device is nearly impossible.

Second, since a hole used for movement of the PS concrete girder is most disposed at the position where the steel rod cannot be inserted, i. e., at the lower portion or upper portion of the girder, installation of the steel rod is

not possible in most cases.

Third, due to the installation of the anchor bolts, the original structure can be damaged as in the shear support type. That is, since the depth of the bolts inserted should be elongated to expect a shearing force by the anchor bolts, damage to the original structure, particular internal steel wires or the anchoring apparatus is unavoidable.

As described above, in the conventional anchoring technologies using external prestressed steel wires, stress is concentrated or cracks are generated at the corner of the end portion of the girder. Also, the installation of the anchor bolts is not easy and major members of a bridge can be severely damaged during the installation thereof. Further, since the calculation of the load distribution rate of each portion is difficult, design of the anchoring device becomes very difficult.

Disclosure of the Invention To solve the above problem, it is an objective of the present invention to provide an improved anchoring method and apparatus of an end bearing support type in an external prestress reinforcement, in which a rib plate is provided at an end plate and an anchoring plate to alleviate concentration of stress generated at the corner of the end portion of an anchoring apparatus when a steel wire is tensioned so that damage to the end portion of the anchoring apparatus is prevented.

It is another objective of the present invention to provide an improved anchoring method and apparatus of an end bearing support type in an external prestress reinforcement by which an anchoring device can be installed with ease and in a small space, and damage to an existing bridge can be minimized during construction.

Accordingly, to achieve the above objectives, there is provided an anchoring method and apparatus of an end bearing support type in an external prestress reinforcement in which an anchoring plate is attached to a PS concrete girder by an anchor bolt, an end plate is welded to the anchoring plate at an end portion of the PS concrete girder, a cross anchoring plate is

installed at the anchoring plate, and an external steel wire is tensioned through an anchoring device supported on the cross anchoring plate, characterized in that at least one or more rectangular rib plates are provided on an extension line of the end plate and the rib plate and the end plate are welded together.

Thus, by additionally welding the rib plate to the end portion which is a weak portion in the conventional end bearing support type, load bearing capacity can be increased.

Brief Description of the Drainas FIG. 1 is plan view for explaining an anchoring method of the conventional shear support type in an external prestress reinforcement ; FIG. 2 is a front view for explaining an anchoring method of the conventional friction support type in an external prestress reinforcement ; FIG. 3 is a plan view for explaining an anchoring method of the conventional end bearing support type in an external prestress reinforcement; FIG. 4 is a view showing a portion where the end plate and the anchoring plate of FIG. 3 contact each other ; FIG. 5 is a plan view for explaining an anchoring method of the conventional hybrid support type adopting both the shear support type and the end bearing support type in an external prestress reinforcement ; FIG. 6 is a plan view for explaining an improved anchoring method of an end bearing support type in an external prestress reinforcement according to the present invention ; FIG. 7 is a front view of FIG. 6; and FIG. 8 is a perspective view showing in detail a welding portion according to the anchoring method of the end bearing support type of FIG. 6.

Best mode for carrying out the Invention Referring to FIGS. 6 and 7, in an improved anchoring method according to an end bearing support type in an external prestress reinforcement according to the present invention, a rib plate 60 is provided on an extended

line of an end plate 69 so that tension P of an external steel wire 67 in a steel wire pipe 66 is received by concrete contacting the end plate 69 disposed at an end portion of a PS concrete girder 68. Thus, load bearing capacity increases as a welding length between the rib plate 60 and the end plate 69 increases. Also, the strength of a welding portion of the end portion which is a weak portion in the conventional end bearing support type increases.

Further, deformation in shape due to the weight of the welding portion of the end portion improves. Here, as shown in FIG. 8, at least one or more rib plates 60 can be installed to be perpendicular to an anchoring plate 61.

The anchoring apparatus according to an end bearing support type in an external prestress reinforcement according to the present invention is designed such that the end plate 69 and the anchoring plate 61 are installed to have a""shape at the end portion of the PS concrete girder 68 so that a concrete surface contacting the end plate 69 supports a tension P of the external steel wire 67. As shown in FIG. 6, unlike the conventional end bearing support type, the rib plate 60 is welded to the anchoring plate 61 and the end plate 69 so that the length of the welding portion receiving load can be increased remarkably.

According to the order of construction, the anchoring plate 61 is manufactured and holes are made in the PS concrete girder 68. Short anchor bolts 63 are installed and the anchoring plate 61 is installed. The end plate 69 is positioned at the end portion of the girder 68 and the end plate 69 and the anchoring plate 61 are welded. Here, the rib plate 60 welded to the anchoring plate 61 is welded to the end plate 61, thus completing the assembly. Then, spaces between the anchoring plate 61 and the concrete, and the end plate 69 and the concrete are filled with epoxy forming a seal.

Here, injection of epoxy is very important because, if there is a tiny gap when tension is active, deformation of the concrete is generated in the space and thus a crack can be generated. Finally, anchoring devices 64 and 65 and a cross anchoring plate 62 are installed and the steel wire 67 in the steel wire pipe 66 is tensioned, thus completing the construction.

In the present invention, it is a great advantage that, since the anchor

bolt 63 only holds the anchoring plate 61 until the anchoring plate 61 is fixed by epoxy, a short bolt of 10 cm or less in length can be used to prevent damage to the steel wire 67. The reason why a steel rod is not used in the present invention is that, when the steel rod is installed at an end portion of an anchoring apparatus as in the conventional hybrid support type, the installation thereof is possible only when a hole for carrying a PS concrete girder is located midway in the girder. However, since the carrying hole is actually located at the lower portion of the girder in most cases and sometimes no carrying hole is made, the installation of the steel rod is not possible.

In a load transfer process of the present method, when the steel wire 67 is tensioned, a tension P is applied to the cross anchoring plate 62 as in the conventional shear support type and the force is sheared to the rib plate 60, the anchoring plate 61, and the end plate 60. That is, the force applied to the anchoring plate 61 reaches the end plate 69 via the anchoring plate 61 and the rib plate 60 and the force is finally supported by the concrete girder f, contacting the end plate 69. In the present invention in which the tension is transferred to the end plate 69 via the anchoring plate 61 as can be seen in the load transfer process, the length of the welding portion becomes long as shown in FIG. 8, and thus the load bearing capacity increases so that tensile stress generated at the corner between the end plate 69 and the anchoring plate 61 is reduced. Since reduction of tensile stress can reduce the possibility of generation of the tensile stress, destruction of the anchoring apparatus due to development of cracks can be prevented.

As shown in FIG. 8, unlike the conventional end bearing support type where the shape of the rib plate is triangular so that the welding portion contacting the end plate is linear, since the shape of the rib plate 60 according <BR> <BR> <BR> <BR> to the present invention is rectangular and the end plate 69 is longer, the length of welding at this portion increases and the load bearing capacity of the anchoring apparatus increases accordingly. The stress concentrated at the corner of a portion where the anchoring plate and the end plate meet in the conventional end bearing support type is received on shares by the welding portion between the end plate 69 and the anchoring plate 61 as the end plate

69 and the rib plate 60 extend in the present invention. Thus, the tensile stress at the corner portion is reduced.

As described above, in the anchoring method and apparatus of an end bearing support type in an external prestress reinforcement according to the present invention, since a short anchor bolt can be used, damage to the steel wire or the anchoring apparatus of the original structure is avoidable.

Designing is possible since an anchoring force is exerted by securing the length of welding at the end portion. Also, the anchoring device can be made small and lightweight, which is an advantage of the conventional end bearing support type anchoring method, and damage to the anchoring apparatus due to the stress concentrated at the corner between the anchoring plate and the end plate, which is a disadvantage in the conventional end bearing support type anchoring method. Further, since the shape of the welding portion is not a plane, when damage is generated at the corner portion between the end plate and the anchoring plate, deformation occurs to a great degree while the crack develops to the welding portion of the rib plate so that observation of the appearance thereof for maintenance is easy. Also, since the increased anchoring force means that a large tension is possible, tension can be increased by a re-tensioning process, if necessary. According to the present invention, purchase of material and construction is simple and the effect of external steel wire can be uniformly applied to the entire effective span.

Industrial Applicability The anchoring method and apparatus of an end bearing support type in external prestress reinforcement according to the present invention adopts an improved end bearing support type and used for reinforcement of a bridge or a pier. Particularly, installation is easy, damage to existing bridge and pier is minimized, and a reinforcement work is possible in a small place.