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Learning

Earthquake Design
26 and
Earthquake Tip
Construction

What Harms Load Paths in Buildings?

Buildings with Moment Resisting Frames column in the lower storey of a building (Figure 2a);
Smooth transfer of inertia forces in a Moment such columns are called floating columns. When a
Resisting Frame (MRF) building is critically dependant column is pushed out of the vertical line in a lower
on the geometry of the frame grid. Some desirable storey, the forces carried by the upper portion of the
features of a frame grid include: column have to bend at the setback location to
(a) Several distinct planar, regular MRFs placed parallel to continue towards the foundation (Figure 2b); such
each other, in each of the two perpendicular plan columns are called setback columns. Presence of a
directions of the building; setback column also leads to poor building
(b) Columns running run through full height and beams performance in an earthquake; brittle damage is
through full width of the building; expected in beam-column joints and beams adjoining
(c) Uniform spacing between parallel planar MRFs in each the setback location.
plan direction; and Floating Column Setback Column
(d) Beams within each planar frame slender enough to
deform in flexure: Concrete beams of very short
span may damage in shear, which is undesirable.
Poor Frame Grid -
For smooth load transfer in an MRF, it is
necessarily for beams and columns to intersect and to
form a well-defined grid. Of the two MRF buildings
shown in plan in Figure 1, the first one has regular
frames in both plan directions (Figure 1a), while the
second has irregular beam and column layout
consisting of a small MRF in the X-direction and (a) (b)
limited frame action in the Y-direction (Figure 1b); this
Figure 2: Discontinuing columns in MRFs
is NOT an acceptable earthquake-resistant solution. detrimental to earthquake safety of buildings
Large detours in load paths result in stress – (a) floating column: the column is discontinued
concentration in the frame and in poor performance. at a lower level, and (b) set-back column: the
This can happen, if frame lines are discontinuous (i.e., column is moved out of plumb
beam lines jog out-of-plane), and if beams frame into
each other instead of into columns.
Buildings with Structural Walls

Structural walls (SWs; also called Shear Walls) have

Uniform large lateral stiffness and lateral strength in the length
Planar Frames direction and provide very good load paths. Buildings
Y in both X- and
Y-directions with SWs have performed well during past earthquakes.

Some desirable features of buildings with SWs include:
(a) Continuous SWs running through full building
X
(a) height generally offer direct load paths for inertia

forces collected from diaphragms at different floor

levels to be carried down to the foundation;
Unequal (b) Uniformly distributed SWs in both plan directions;
Planar Frames
One in Y-direction and

Planar Frame
in X-direction (c) Sufficient wall density, i.e., total cross-section area

of structural walls in plan as a percentage of plan
(b) area of building.

Figure 1: Structural framing in RC buildings in Situations arise when departure occurs from good
plan – (a) good, and (b) poor. earthquake behaviour. These include:
(a) Large and/or irregular openings
Discontinuity in Vertical Elements: SWs with smaller and uniform openings behave
Floating Columns and Setback Columns - better (Figure 3a). In SWs with large and random
Discontinuing a load carrying member along its openings, there are multiple load paths and each of
length or height is harmful to earthquake performance those has long detours. As a result, the load paths
of the building. It is not desirable to discontinue a become long and convoluted instead of being short