In RC buildings, the parts of the pillars that are common to the beams at their intersections are called beam-pillar joints (Figure 1). Since their constituent materials have limited strength, joints have limited force bearing capacity. When forces greater than these are applied during an earthquake, the joints are badly damaged. Damaged joints are difficult to repair, and therefore damage should be avoided. Thus, beam-pillar joints must be designed to resist the effects of earthquakes.
earthquake behavior of joints
Under earthquake shaking, beams adjacent to a joint are subject to moments in the same (clockwise or counterclockwise) direction (Figure 1). Under these moments, the top bars in the beam-pillar joint are pulled in one direction and the bottom ones in the opposite direction (Figure 2A). These forces are balanced by the bond stress developed between the concrete and steel at the joint area. If the column is not wide enough or if the strength of the concrete at the joint is low, there is insufficient grip of the concrete on the steel bars. Under such conditions, the bar slips inside the joint area, and the beams lose their load bearing capacity.
In addition, under the action of the above pull-push forces at the top and bottom ends, the joints undergo geometric deformation; One diagonal length of the joint is elongated and the other is constricted (Figure 2b). If the column cross-sectional size is insufficient, the concrete at the joint develops diagonal cracks.
reinforcing beam-column joint
Diagonal cracking and crushing of concrete in the joint area should be prevented to ensure good earthquake resistance of RC frame buildings. The most efficient way to achieve this is to use larger column sizes. In addition, closely spaced closed-loop steel ties are required around the column bars (Figure 3) to hold the concrete together in the joint area and resist shear forces. Intermediate column bars are also effective in limiting combined concrete and resisting horizontal shear forces.
Some extra effort is required to provide closed-loop ties at the joint. Indian Standard IS:13920-1993 recommends continuation of the transverse loops around the pillar bars through the joint area. In practice, this is achieved by preparing a cage of reinforcement (both longitudinal bars and stirrups) of all beams to be laid on top of that level beam formwork and lowered into the cage (Figures 4a and 4b). . However, this may not always be possible, especially when the beams are long and the entire reinforcement cage becomes heavy.
Anchoring Beam Bars
The gripping of the beam bar in the joint area is first improved by using pillars of significantly larger cross-sectional size. Indian Standard IS:13920-1993 requires the seismic zones III, IV and V to be at least 300 mm wide each. The direction of cross-section when they support beams that are longer than 5 m or when these columns are between floors (or beams) longer than 4 m. American Concrete Institute at least . Recommends a column width of 20 times the diameter of the largest longitudinal bar used in the adjoining beam,
In external joints where beams end on pillars (Figure 5), longitudinal beam bars need to be anchored to the column to ensure proper grip of the bar at the joint. The anchor length for a bar of Fe415 grade (characteristic tensile strength of 415MPa) is approx. 50 times its diameter, This length is measured from the face of the column to the end of the bar attached to the column. In columns of smaller width and when the beam bars are of larger diameter (Figure 5A), a portion of the beam top bar is embedded in the column which is inserted up to the soffit of the beam, and a portion of it is overhanged.
Such an overhanging beam top bar is difficult to position when casting the column to the beam’s soffit. In addition, the vertical distance beyond the 90º bend in the beam bars is not very effective in providing anchorage. On the other hand, if the column width is large, the beam bars may not extend below the beam soffit (Figure 5b). Thus, it is better to have columns with sufficient width. This kind of approach is used in many codes [e.g., ACI318, 2005], In internal joints, the beam bar (both top and bottom) has to pass through the joint without any cuts in the joint area. In addition, these bars must be placed within the column bars and must be without bending (Figure 6).
Article written by:
Indian Institute of Technology Kanpur
building materials and technology promotion
Council, New Delhi, India
BMTPC Earthquake Tips