Page 36 - EQTips_Eng

Learning

18 Earthquake Design
Earthquake Tip and

Construction

How do Beams in RC Buildings resist Earthquakes?

Reinforcement and Seismic Damage (b) Shear Failure: A beam may also fail due to shearing
In RC buildings, the vertical and horizontal action. A shear crack is inclined at 45° to the
members (i.e., the columns and beams) are built horizontal; it develops at mid-depth near the
integrally with each other. Thus, under the action of support and grows towards the top and bottom
loads, they act together as a frame transferring forces faces (Figure 2b). Closed loop stirrups are provided
from one to another. This Tip is meant for beams that to avoid such shearing action. Shear damage occurs
are part of a building frame and carry earthquake- when the area of these stirrups is insufficient.
induced forces. Shear failure is brittle, and therefore, shear failure
Beams in RC buildings have two sets of steel must be avoided in the design of RC beams.
reinforcement, namely: (a) long straight bars (called Design Strategy
longitudinal bars) placed along its length, and (b) closed Designing a beam involves the selection of its
loops of small diameter steel bars (called stirrups) material properties (i.e, grades of steel bars and concrete)
placed vertically at regular intervals along its full and shape and size; these are usually selected as a part
length (Figure 1). of an overall design strategy of the whole building.
And, the amount and distribution of steel to be provided
Vertical Stirrup

Smaller diameter steel in the beam must be determined by performing design
bars that are made into Beam calculations as per is:456-2000 and IS13920-1993.
closed loops and are

placed at regular Column Column

intervals along the full
length of the beam
Beam

Bottom face stretches in tension
and vertical cracks develop
(a) Flexure Failure

Inclined crack Column
Lon gitudinal Bar
Larger diameter steel bars that

go through the full length of the

beam

Figure 1: Steel reinforcement in beams - stirrups Beam 45°

prevent longitudinal bars from bending outwards.
(b) Shear Failure
Beams sustain two basic types of failures, namely:

(a) Flexural (or Bending) Failure: As the beam sags under Figure 2: Two types of damage in a beam:
flexure damage is preferred. Longitudinal bars
increased loading, it can fail in two possible ways. resist the tension forces due to bending while
If relatively more steel is present on the tension vertical stirrups resist shear forces.
face, concrete crushes in compression; this is a brittle
failure and is therefore undesirable. If relatively Longitudinal bars are provided to resist flexural
less steel is present on the tension face, the steel cracking on the side of the beam that stretches. Since
yields first (it keeps elongating but does not snap, as both top and bottom faces stretch during strong
steel has ability to stretch large amounts before it earthquake shaking (IITK-BMTPC Earthquake Tip 17),
snaps; see IITK-BMTPC Earthquake Tip 9) and longitudinal steel bars are required on both faces at the
redistribution occurs in the beam until eventually ends and on the bottom face at mid-length (Figure 3).
the concrete crushes in compression; this is a ductile The Indian Ductile Detailing Code IS13920-1993
failure and hence is desirable. Thus, more steel on prescribes that:
tension face is not necessarily desirable! The ductile (a) At least two bars go through the full length of the
failure is characterized with many vertical cracks beam at the top as well as the bottom of the beam.
starting from the stretched beam face, and going (b) At the ends of beams, the amount of steel provided
towards its mid-depth (Figure 2a). at the bottom is at least half that at top.
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