Patent Publication Number: US-2004055234-A1

Title: Reinforced concrete column or bridge pier

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001] This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-273001, filed Sept. 19, 2002, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates to a reinforced concrete column or bridge pier.  
       [0004] 2. Description of the Related Art  
       [0005] In a conventional reinforced concrete column or bridge consisting of a long concrete block, a plurality of deformed steel bars are arranged in the longitudinal direction, and hoops arranged with desired intervals around the deformed steel bars along the longitudinal direction of the bars. The concrete directly adheres to the deformed steel bars.  
       [0006] In countries with frequent earthquakes, such as Japan, the problem to be solved for reinforced concrete columns or bridge piers constructed as above is to prevent destruction due to the abrupt and excessive shear stress caused by earthquakes. To this end, according to the current design, the reinforced concrete column or bridge pier has a number of hoops resistant to a shear stress.  
       [0007] However, production of a reinforced concrete column or bridge pier having a number of hoops requires a large number of steel bars, and is very expensive. In addition, construction work to erect the steel bars and casting of concrete are difficult.  
       [0008] Unbonded prestressed concrete (unbonded PC) is known as a concrete. An unbonded PC structure is formed by inserting a PC structural material into a sheath located in concrete, thereby pre-stressing the concrete. The space between the sheath and the PC structural material is left unfilled or filled with oil to prevent corrosion. This structure is frequently used in beams for buildings. It has advantages that the span can be longer and flexing or cracking can be prevented. The reason why unbonded PC is employed is that work can be simplified, since no grouting is performed. Thus, unbonded PC is clearly distinguished from the reinforced concrete (RC) structure.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009] The present inventor found the following. When abrupt and excessive shear stress is exerted on a reinforced concrete column or bridge pier in an earthquake, if a plurality of deformed steel bars are arranged in direct contact with a long concrete block, the shear stress causes cracks due to high adhesion between the concrete and the deformed steel bars, if the number of hoops is small. Further, the shear stress is transmitted through the deformed steel bars in the longitudinal direction (height direction), resulting in cracks in a wide area of the column or bridge pier. As a result, shear failure in diagonal directions occurs.  
       [0010] Based on the above findings, the inventor arrived at the present invention for a reinforced concrete column or bridge pier with high toughness, wherein a sheath covers at least a portion of each deformed steel bar on which shear stress is exerted, so that the deformed steel bars are not in direct contact with the concrete. Surprisingly, even if the number of hoops was small, cracks due to shear stress occurred only in a limited portion and could be prevented from propagating. As a result, shear failure in diagonal directions was effectively avoided. The sheath is formed of a thin steel or plastic plate, which has flexibility. Therefore, even if it directly adheres to the concrete block, the propagation of the shear strength, which occurred in the deformed steel bars, can be prevented from occurring.  
       [0011] The inventor also arrived at the present invention for a reinforced concrete column or bridge pier with high toughness, in which smooth round bars with coated with a lubricant, such as grease, are used instead of the deformed steel bars to lessen adhesion to the concrete. Surprisingly, even if the number of hoops was small, cracks due to shear stress occurred only in a limited portion and could be prevented from propagating. As a result, shear failure in diagonal directions was effectively avoided.  
       [0012] The inventor conducted experiments using concrete specimens in which a deformed steel bar, a deformed steel bar covered with a sheath, a smooth round bar, and a smooth round bar coated with grease are arranged. These bars were extracted from the concrete blocks to examine the relationship between slip and adhesive stress. FIG. 3 shows the results. In FIG. 3, lines A-D represent the slip-adhesive stress characteristic of a deformed steel bar, a deformed steel bar covered with a sheath, a smooth round bar, and a smooth round bar coated with grease, respectively.  
       [0013] As seen from FIG. 3, the adhesive stress of the deformed steel bar covered with a sheath is always zero. Further, the adhesive stress of the smooth round bar coated with grease is smaller than those of both the deformed steel bar and the smooth round bar. Thus, the smooth round bar with grease can reduce the adhesion to concrete.  
       [0014] According to an aspect of the present invention, there is provided a reinforced concrete column or bridge pier comprising a plurality of deformed steel bars arranged in a longitudinal direction, and hoops arranged at desired intervals around the deformed steel bars along the longitudinal direction, wherein a sheath covers at least a portion of each deformed steel bar on which shear stress is exerted.  
       [0015] According to another aspect of the present invention, there is provided a reinforced concrete column or bridge pier comprising a plurality of smooth round steel bars coated with lubricant and arranged in a longitudinal direction, and hoops arranged at desired intervals around the smooth round steel bars along the longitudinal direction.  
       [0016] Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
     
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
     [0017] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.  
     [0018]FIG. 1 is a schematic view showing a reinforced concrete column or bridge pier according to a first embodiment of the present invention;  
     [0019]FIG. 2 is a perspective view showing a sheath and a deformed steel bar arranged in the reinforced concrete column or bridge pier shown in FIG. 1;  
     [0020]FIG. 3 is a diagram showing the relationship between slip and adhesive stress of a deformed steel bar, a deformed steel bar covered with a sheath, a smooth round bar, and a smooth round bar coated with grease;  
     [0021]FIG. 4A is a schematic view showing a test specimen having a reinforced concrete column of an example 1;  
     [0022]FIG. 4B is a sectional view taken along line A-A in FIG. 4A;  
     [0023]FIG. 5 is a schematic view showing a test machine to inspect failure modes and crack patterns of the test specimens having the reinforced concrete columns of the example 1 and a comparative example 1;  
     [0024]FIG. 6 is a diagram showing a load-displacement hysteresis loop of the reinforced concrete column of the example 1;  
     [0025]FIG. 7 is a diagram showing a load-displacement hysteresis loop of the reinforced concrete column of the comparative example 1;  
     [0026]FIG. 8 is a diagram showing crack patterns of the reinforced concrete column of the example 1;  
     [0027]FIG. 9 is a diagram showing crack patterns of the reinforced concrete column of the comparative example 1;  
     [0028]FIG. 10 is a diagram showing a load-displacement hysteresis loop of the reinforced concrete column of a example 2; and  
     [0029]FIG. 11 is a diagram showing crack patterns of the reinforced concrete column of the example 2. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0030] A reinforced concrete column or bridge pier according to the present invention will be described in detail with reference to the accompanying drawings.  
     First Embodiment  
     [0031]FIG. 1 is a schematic view showing a reinforced concrete column or bridge pier according to the first embodiment of the present invention, and FIG. 2 is a perspective view showing a sheath and a deformed steel bar arranged in the reinforced concrete column or bridge pier shown in FIG. 1.  
     [0032] A reinforced concrete column (or bridge pier)  1  has a long concrete block  2 . As shown in FIG. 2, a deformed steel bar  3  is inserted in a bottomed cylindrical sheath  4 . A plurality of deformed steel bars  3  inserted in the sheaths  4  are arranged in the concrete block  2  along the longitudinal direction (height direction). Since each deformed steel bar  3  is covered by the sheath  4 , it does not directly adhere to the concrete block  2 . In other words, the deformed steel bars are arranged so as not directly adhere to the concrete block  2 . A plurality of hoops  5  are arranged at desired intervals along the longitudinal direction around the sheaths  4 .  
     [0033] The sheath  4  covers at least a portion of the deformed steel bar on which shear stress is exerted. However, it may cover the other portion of the deformed steel bar.  
     [0034] The sheath is preferably made of material that has substantially no resistance to tension and that is not deformed by contact pressure in pouring of concrete. For example, it is made of steel or plastic, such as polypropylene, polyethylene and polyvinyl chloride. It is preferable that the sheath made of the steel is 0.25 to 0.32 mm thick and the sheath made of plastic is 0.5 to 1.0 mm thick.  
     Second Embodiment  
     [0035] In a reinforced concrete column (or bridge pier) according to the second embodiment, a plurality of smooth round bars coated with lubricant are arranged in a concrete block in the longitudinal direction. A plurality of hoops are arranged at desired intervals around the deformed steel bars along the longitudinal direction. The lubricant may be, for example, grease.  
     EXAMPLES  
     [0036] Examples of the present invention will be described with reference to the drawings.  
     Example 1  
     [0037] As shown in FIGS. 4A and 4B, a reinforced concrete hooting  11  and a reinforced concrete column  12  integrally constitutes a test body  13 . The reinforced concrete hooting  11  is 300 mm in width, 1200 mm in length and 500 mm in height. The reinforced concrete column  12  is a square prism of 300 mm in width and length and 1200 mm in height. The reinforced concrete hooting  11  has deformed steel bars  14  having a diameter of 13 mm and arranged in the height direction, and hoops  15  having a diameter of 10 mm and arranged around the deformed steel bars  15 . As shown in FIG. 4B, the reinforced concrete column  12  contains twelve deformed steel bars  16  having a diameter of 16 mm and arranged along the height direction. The deformed steel bars  16  are compliant with Japanese Industrial Standards SD345. The reinforced concrete column  12  also contains eight steel hoops  17  having a diameter of 6 mm and arranged around the twelve deformed steel bars  16  at regular intervals along the longitudinal direction of the deformed steel bars  16 . The steel hoops  17  are compliant with Japanese Industrial Standards SD300. Each deformed steel bar  16  is covered with a 0.25 mm thick steel sheath (not shown) over a length of 800 mm from the bottom.  
     Comparative Example 1  
     [0038] A test specimen of the comparative example 1 has the same reinforced concrete column as that of the example 1 except that the deformed steel bars are not covered with sheathes.  
     [0039] The test specimen was tested by a test machine shown in FIG. 5. More specifically, a load-displacement characteristic of the reinforced concrete column and a failure mode of the test piece were inspected and a crack pattern was observed through the procedures described below.  
     [0040] The test machine has a base member  21 . A flat-shaped base  22  is fixed to the base member  21  with bolts and nuts. A first wall  23  is fixed to a left end portion of the base  22  with bolts and nuts. A displacement transducer  24  is attached to the first wall  23  and extends horizontally rightward. A second wall  25  is fixed to a right end portion of the base member  21  with bolts and nuts. An actuator  26  extending horizontally toward the first wall  23  is attached to the second wall  25  so as to face the displacement transducer  24 . A beam  27  extending horizontally is attached to the second wall  25  above the actuator  26 . A loading member  28  is mounted on the lower surface of the beam  27 .  
     [0041] First, the test specimen  13  was fixed to the base  22  of the test machine with bolts and nuts. The displacement transducer  24  attached to the first wall  23  was brought into contact with the left side surface of the reinforced concrete column  12  of the test body  13  via a spring  29 . The actuator  26  attached to the second wall  25  was fixed to the right side surface of the reinforced concrete column  12  so as to face the displacement transducer  24  with the column  12  interposed therebetween. The loading member  28  attached to the beam  27  was brought into contact with the upper end of the reinforced concrete column  12 .  
     [0042] In this manner, the test specimen  13  was placed in the test machine as shown in FIG. 5. The loading member  28  applied a normal load to the reinforced concrete column  12  under the conditions indicated in Table 1. In this state, the actuator  26  was reciprocated in the horizontal direction, thereby applying a horizontal load to the reinforced concrete column  12 . At this time, yield displacement (δy) and ultimate displacement (δu) of the example 1 and the comparative example 1 were measured with the displacement transducer  24 . Further, under these displacement conditions, the failure modes of the reinforced concrete columns of the example 1 and the comparative example 1 were inspected. The results of measurement and inspection are indicated in Table 1. Table 1 also shows yield loads (Py), maximum loads (Pmax) and δu/δy.  
                           TABLE 1                                       Comparative           Example 1   Example 1                                                            Load (kN)   Py   119.4   117.02               Pmax   120.17   118.21           Displacement   δy   13.7   9.66           (mm)   δu   46.8   15.74           δu/δy       3.42   1.63           Failure mode       flexure   shear                      
 
     [0043]FIGS. 6 and 7 show load-displacement hysteresis loops of the reinforced concrete columns of the example 1 and the comparative example 1, respectively.  
     [0044] The reinforced concrete columns of the example 1 and the comparative example 1 were reciprocated three times with displacement of 5.2 mm, 10.4 mm and 15.6 mm, and the exteriors thereof were observed. FIG. 8 shows a crack pattern of the reinforced concrete column of the example 1, and FIG. 9 shows a crack pattern of the reinforced concrete column of the comparative example 1.  
     [0045] As seen from Table 1 and FIGS. 6 and 7, both the yield displacement (δy) and the ultimate displacement (δu) of the reinforced concrete column of the example 1 are greater than those of the comparative example 1. In addition, the value of δu/δy of the example 1 is considerably greater than that of the comparative example 1. Moreover, the failure mode of the reinforced concrete column of the example 1 is flexure, while that of the comparative example is shear.  
     [0046] Further, as seen from FIGS. 8 and 9, in the example 1 (FIG. 8), cracks appear only in a bottom portion of the column when the displacement is 5.2 mm, 10.4 mm and 15.6 mm. In the comparative example 1 (FIG. 9), cracking occurs in diagonal directions from the bottom of the column when the displacement is 10.4 mm, and diagonal shear cracks extend all over the column when the displacement is 15.6 mm.  
     [0047] From the above results, it is understood that the reinforced concrete column of the example 1 has high toughness; that is, even if abrupt and strong shear stress caused by earthquakes is exerted on the column, shear failure in diagonal directions does not occur.  
     Example 2  
     [0048] A test specimen of the example 2 has the same reinforced concrete column as that of the example 1 except that the deformed steel bar covered with the sheath is replaced by a smooth round bar made of steel, compliant with Japanese Industrial Standards SR295, having a diameter of 16 mm and coated with grease.  
     [0049] The test specimen was subjected to a loading test of the reinforced concrete column by the test machine shown in FIG. 5 through the same procedures as in the case of the example 1. Also, as in the example 1, the failure mode was flexure.  
     [0050]FIG. 10 shows a load-displacement hysteresis loop of the reinforced concrete column as a result of the test. Like the example 1, as seen from FIG. 10, both the yield displacement (δy: about 12 mm) and the ultimate displacement (δu: about 58 mm) of the reinforced concrete column of the example 2 are greater than those of the comparative example 1. In addition, like the example 1, the value of δu/δy of the example 2 is considerably greater than that of the comparative example 1.  
     [0051] As in the example 1 and the comparative example 1, the reinforced concrete column of the example 2 was reciprocated three times with displacement of 5.2 mm, 10.4 mm and 15.6 mm, and the exterior thereof was observed. FIG. 11 shows the results.  
     [0052] As seen from FIG. 11, in the example 2, a little amount of cracks appear only in a bottom portion of the column and slightly above the bottom portion, when the displacement is 5.2 mm, 10.4 mm and 15.6 mm. When the displacement is 10.4 mm, cracking that occurs in diagonal directions from the bottom of the column in the comparative example 1, as shown in FIG. 9, is prevented from occurring in the example 2.  
     [0053] From the above results, it is understood that the reinforced concrete column of the example 2 has high toughness; that is, even if abrupt and strong shear stress caused by earthquakes is exerted on the column, shear failure in diagonal directions does not occur.  
     [0054] Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.