Page 40 - EQTips_Eng

Learning

20 Earthquake Design
Earthquake Tip and

Construction

How do Beam-Column Joints in RC Buildings resist Earthquakes?

Why Beam-Column Joints are Special Further, under the action of the above pull-push
In RC buildings, portions of columns that are forces at top and bottom ends, joints undergo
common to beams at their intersections are called beam- geometric distortion; one diagonal length of the joint
column joints (Figure 1). Since their constituent elongates and the other compresses (Figure 2b). If the
materials have limited strengths, the joints have limited column cross-sectional size is insufficient, the concrete
force carrying capacity. When forces larger than these in the joint develops diagonal cracks.
are applied during earthquakes, joints are severely Reinforcing the Beam-Column Joint
damaged. Repairing damaged joints is difficult, and so Diagonal cracking & crushing of concrete in joint
damage must be avoided. Thus, beam-column joints region should be prevented to ensure good earthquake
must be designed to resist earthquake effects. performance of RC frame buildings. Using large column
sizes is the most effective way of achieving this. In
Beam-Column Joint addition, closely spaced closed-loop steel ties are required
Overlap volume
common to beams around column bars (Figure 3) to hold together
and columns concrete in joint region and to resist shear forces.
Intermediate column bars also are effective in confining
the joint concrete and resisting horizontal shear forces.

Closed ties

10 times
diameter of tie
Beam
Figure 1: Beam-Column Joints are critical parts
of a building – they need to be designed. 135º

Earthquake Behaviour of Joints Column Intermediate
Under earthquake shaking, the beams adjoining a Column Bars
joint are subjected to moments in the same (clockwise Figure 3: Closed loop steel ties in beam-column
or counter-clockwise) direction (Figure 1). Under these joints – such ties with 135° hooks resist the ill
moments, the top bars in the beam-column joint are effects of distortion of joints.

pulled in one direction and the bottom ones in the
opposite direction (Figure 2a). These forces are Providing closed-loop ties in the joint requires
balanced by bond stress developed between concrete some extra effort. Indian Standard IS:13920-1993
and steel in the joint region. If the column is not wide recommends continuing the transverse loops around
enough or if the strength of concrete in the joint is low, the column bars through the joint region. In practice,
there is insufficient grip of concrete on the steel bars. this is achieved by preparing the cage of the
In such circumstances, the bar slips inside the joint reinforcement (both longitudinal bars and stirrups) of all
region, and beams loose their capacity to carry load. beams at a floor level to be prepared on top of the
beam formwork of that level and lowered into the cage
(Figures 4a and 4b). However, this may not always be
Compression possible particularly when the beams are long and the
Gripping of entire reinforcement cage becomes heavy.
bar inside Anchoring Beam Bars
joint region Tension The gripping of beam bars in the joint region is
improved first by using columns of reasonably large
cross-sectional size. As explained in Earthquake Tip 19,
the Indian Standard IS:13920-1993 requires building
(a) Loss of grip on beam bars (b) Distortion of joint:
in joint region: causes diagonal columns in seismic zones III, IV and V to be at least
Large column width and good cracking and crushing 300mm wide in each direction of the cross-section
concrete help in holding the of concrete when they support beams that are longer than 5m or
beam bars
when these columns are taller than 4m between floors
Figure 2: Pull-push forces on joints cause two
(or beams). The American Concrete Institute
problems – these result in irreparable damage in recommends a column width of at least 20 times the
joints under strong seismic shaking.
diameter of largest longitudinal bar used in adjoining beam.
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