How do Beams in RC Buildings Resist Earthquake ?
Reinforcement and seismic damage
In RC buildings, the vertical and horizontal members (i.e, the beams and columns) are built integrally with each other. Thus, under the action of loads, they act together as a frame transferring forces from one to another. This tip is meant for beams that are part of a building frame and carry earthquake induced forces.
Beams in RC buildings have two sets of steel reinforcement, namely: (a) long straight bars (called longitudinal bars) placed along its length, and (b) closed loops of small diameter steel bars (called stirrups) placed vertically at regular intervals along its full length (figure 1).
Beams sustain two basic types of failures, namely:
(a) Flexure (or Bending) Failure: As the beam sags under increased loading, it can fail in two possible ways. If relatively more steel is present on the tension face, concrete crushed in compression, this is a brittle failure and is therefore undesirable. If relatively less steel is present on the tension face, the steel yields first (it keeps elongating but does not snap, as steel has ability to stretch large amounts before it snaps and redistribution occurs in the beam until eventually the concrete crushes in compression; this is a ductile failure and hence is desirable. This, more steel on tension face is not necessarily desirable! The ductile failure is characterized with many vertical cracks starting from the stretched beam face, and going towards its mid-depth (Figure 2a).
(b) Shear Failure: A beam may also fail due to shearing action. A shear crack is inclined at 45 degree to the horizontal; it develops at mid-depth near the support and grows towards the top and bottom faces (Figure 2b). Closed loop stirrups are provided to avoid such shearing action. Shear damage occurs when the area of these stirrups is insufficient. Shear failure is brittle, and therefore, shear failure must be avoided in the design of RC beams.
Designing a beam involves the selection of its material properties (i.e, grades of steel bars and concrete) and shape and size; these strategy of the whole building. And, the amount and distribution of steel to be provided in the beam mist be determined by performing design calculations.
Longitudinal bars are provided to resist flexural cracking on the side of the beam that stretches. Since both top and bottom faces stretch during strong earthquake shaking, longitudinal steel bars are required on both faces at the ends and on the bottom face at mid-length (Figure 3).
The American standard, ACI 318 – Chapter 21 prescribes that:
At both ends of the beam, hoops shall be provided over lengths not less than 2h measured from the face of the supporting member toward midspan.
The first hoop shall be located not more than 50 mm from the face of the supporting member. Spacing of hoops shall not exceed the smallest of (a), (b), (c), and (d):
(b) Eight times the diameter of the smallest longitudinal bar enclosed;
(c) 24 times the diameter of the hoop bar;
(d) 300 mm