Joints in Modern And Older RC Structures
Unlike steel structures where the type of connection between the structural members is determined by the engineer-designer, in modern RC structures the beam-column, beam-wall or pier-deck connections are usually considered to be rigid compared to the members that are bridging in the joint. However, the degree of joint rigidity largely depends on the joint detailing, mainly the content and arrangement of joint reinforcement. This short article aims at explaining the main aspects of Eurocode provisions with regard to detailing of RC joints and the importance of joint reinforcement on the local and global behaviour of the structure.
With regards to older RC structures, the use of pinned supports / connections is usually encountered in bridges. For instance, one can refer to the use of Gerber joints close to the location of zero bending moments in bridge beams or the arrangement of pin-type connections between superstructure (bridge deck and beams) and substructure (piers) through use of diagonal reinforcement (typically in X form). Nowadays, such types of supports / connections are rather abandoned due to the following reasons:
(a) They offer a low degree of indeterminacy and consequently limited load redistribution capacity under vertical loads,
(b) They increase the flexibility of the structure which leads to cracking of the concrete zones subjected to tensile stresses,
(c) Shop-fabricated bearings made of various materials like steel, rubber, and lead offer much higher quality control and more clear structural behaviour in terms of transfer of internal forces to the concrete structural members.
It should also be noted that in older RC building the beam-column joints were commonly non-engineered or designed, which means that they can become weak points of the structure.
Here's an interesting publication from 1959 discussing joints in large buildings: https://nrc-publications.canada.ca/eng/view/fulltext/?id=c4f8f86c-e2c0-4c4e-9356-da276b81c2e7
The role of reinforcement in the strength and stiffness of RC joints
Joints between structural members must have the capacity to transfer the combination of the following internal forces:
(a) Axial forces acting on the top column,
(b) Bending moments acting on the horizontal members (e.g. beams) and the vertical members (e.g. columns), and;
(c) Shear forces acting on the horizontal members and vertical members (under wind or earthquake loading).
According to Eurocodes 2 and 8 [1; 2] the horizontal reinforcement that is placed in vertical members (columns or walls) must be extended within the joint with the horizontal members (typically beams).
It must be noted that unlike the members that are connected, the joints between them develop a triaxial state of stress. This means that diagonal tensile failure does not have dramatic consequences for the joint integrity but may influence the behaviour of the joint under diagonal compression.
The horizontal shear stress acting on the concrete core is given by the following relation (EC8 §184.108.40.206):
where As1 and As2 is the top and bottom beam reinforcement, and Vc is the shear force from the column above due to lateral loading, bj is the joint width, hjcis the distance between extreme layers of column reinforcement and γRd is the reinforcing steel overstrength factor (equal to 1.2).
The total area of necessary horizontal hoops Ash to avoid diagonal tensile failure of the joint is given by the following expression (EC8 §220.127.116.11):
where fywd is the design yield stress of the hoop reinforcement, fctd is the design tensile strengh of concrete and hjwis the distance between top and bottom beam reinforcing bars.
If this inequality is not satisfied, diagonal tensile failure is anticipated in the joint. It should be noted that if the joint is unreinforced (Ash = 0) diagonal tensile failure occurs when τj ≥ √ fctd ∙ ( fctd + νd ∙ fcd). This failure mode will not have dramatic consequences if beams are connected to two or more faces of the joint.
The diagonal compression stress acting on the joint must be limited by the compressive strength of concrete in presence of transverse tensile strains as given by the following formula (EC8 §18.104.22.168):
where νd is the normalized axial force acting on the top of the joint, fcd is the design compressive strength and the coefficient η represents the reduction of the uniaxial concrete compressive strength due to transverse tensile strains and is equal to 0.6 ∙(1-fck / 250). If inequality (2) is not satisfied (diagonal tensile failure occurs) the value of η is set to unity.
If inequality (3) is not satisfied, diagonal compressive failure will occur in the joint. This failure mode can have significant impact on the integrity of the structure (Figure 1).
With regards to rebar anchorage, it is suggested that the longitudinal reinforcing bars of the vertical members should pass through the joint to be anchored preferably close to next storey mid-height. Moreover, the anchorage of the horizontal beam reinforcement within the joint core increases the joint behaviour efficiency significantly compared to anchorage outside the joint core.
Premature failure of RC joints due to poor detailing can also result in a significant increase in the flexibility of the entire structure under vertical and/or lateral loads which can affect the response of the structural members.
Figure 1: Catastrophic earthquake-induced beam-column joint failure (2003, Boumerdes, Algeria)
 Eurocode 2: Design of concrete structures - Part 1-1: General rules and rules for buildings
 Eurocode 8: Design of structures for earthquake resistance - Part 1: General rules, seismic actions and rules for buildings