In view of the important role of the force transfer performance of reinforcement joints in concrete structures, the code for design of concrete structures (2015 Edition) (GB 50010-2010) specifically lists in Chapter 8 “structural provisions” and makes corresponding provisions in part 8.4 “connection of reinforcement”. The basic principles of reinforcement connection are put forward in article 8.4.1 at the beginning of this section and the following corresponding articles, the main contents of which are as follows.

 

 

(1) Connector location. Since the force transmission performance of the reinforcement joint is inferior to that of the overall reinforcement in any case, the code for design of concrete structures (2015 Edition) clearly stipulates that “the connection joint of the stressed reinforcement in the concrete structure should be set at the place with less stress”. In general, the most appropriate position is near the reverse bending point. Since the moment is the main internal force in the member, the moment at the reverse bending point is zero. In the region near the reverse bending point, the bending moment is small. It is a traditional custom in China to arrange the connection joints of reinforcement at the column base and beam end. Because this is the area with the largest bending moment of beams and columns during the earthquake, this is a very inappropriate practice. The earthquake damage investigation has repeatedly shown that the stirrup densification area at the column end and beam end is also the place most vulnerable to damage during the earthquake. If considering the convenience of construction, it is not too difficult to move the reinforcement joint to the place outside the stirrup densification area at the column end and beam end to realize the connection.

 

In this regard, the designers and constructors should be reminded to pay special attention: it is best not to set the connection joint of reinforcement, including mechanical connection joint, at the stirrup densification area at the column end and beam end.

 

 

(2) Number of joints. Because the reinforcement connection joint weakens the force transfer performance, it is not desirable to have too many connection joints on the same stressed reinforcement. The specification stipulates that “less joints should be set on the same stressed reinforcement”. Generally, more than two reinforcement connection joints should not be set in the longitudinal range of the same span of the beam and the same floor height of the column, so as to avoid adverse impact on the structural performance of the members.

 

(3) Percentage of joint area. Similarly, because of the adverse effect of reinforcement connection joints on force transfer performance, the area percentage of reinforcement connection joints should also be controlled in the same connection section in the transverse range of members. Code for design of concrete structures (2015 Edition) specifies the scope of different connection sections and the limit of the area percentage of reinforcement connection joints within the same connection section. Generally, the area percentage of welded and mechanically connected joints is limited to 50%, that is, all reinforcement shall be connected in two batches. There are more detailed provisions for binding and overlapping connection. In the project, only the connection joints of prefabricated members of fabricated structure can be treated as individual cases and exceptions when more strict structural measures are taken, because they can only be connected by all reinforcement (the percentage of joint area is 100%).

 

 

(4) Avoidance principle. Special attention should be paid in the design: the connecting joints of stressed reinforcement should not be set at the key parts of stress. The code for design of concrete structures (2015 Edition) specially emphasizes that “in important components and key force transmission parts of the structure, the longitudinal stressed reinforcement should not be provided with connecting joints”. This is because if there is a problem in the force transmission of reinforcement in the important harm of stress, it will cause serious consequences. Especially the column end and beam end of seismic frame structure, which is the key area where plastic hinge is most likely to occur under earthquake. Setting the connecting joint of reinforcement will change the “resistance” there, which is greatly detrimental to the seismic performance of the structure.

 

 

It should also be pointed out that “resistance” is not only “strength”, but also “ductility”; The strength of the joint is too high to be completely beneficial. If the connection joint at the beam end is “super strong” (such as upsetting straight thread joint), not to mention the impact on ductility, it is likely to cause the transfer of plastic hinge, thus affecting the ductility design principles of “strong column and weak beam” and “strong shear and weak bending” of seismic structures. If the plastic hinge is transferred to the column end, it may cause more serious consequences – structural collapse. Therefore, in the seismic design of structures, the “super strength ratio” of seismic reinforcement is strictly limited in the form of mandatory provisions. There are the same requirements for the connection joints of reinforcement. Therefore, one-sided pursuit of “high strength” or “ultra-high strength” of connecting joints is not only useless, but even harmful.