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Learning

Earthquake Design
25 and
Earthquake Tip
Construction

Why are Load Paths Important in Buildings?

What are Load Paths?
Mass is present all through in a building - from Column
Floor
roof parapet to foundation. Earthquake ground Slab
Beam-Column
shaking induces inertia forces in a building where Joints
mass is present. These inertia forces are transferred Floor
downwards through horizontally and vertically
aligned structural elements to foundations, which, in
Wall
turn, transmit these forces to the soil underneath. The Beam Shear
paths along which these inertia forces are transferred Frame Bay
Braced
through building are Load Paths (Figure 1a). Buildings
may have multiple load paths running between locations Z Foundation
of mass and foundations. Load paths are as much a Y
concern for transmitting vertical loads (e.g., self- X Soil
weight, occupancy load, and snow; Figure 1b) as for (a)
horizontal loads (e.g., earthquake and wind; Figure 1c). Inertia Force
in Y-direction
Structural elements in buildings that constitute load
paths include:
(a) Horizontal diaphragm elements laid in horizontal
plane, i.e., roof slabs, floor slabs or trussed roofs
and bracings;
(b) Vertical elements spanning in vertical plane along
height of building, i.e., planar frames (beams and
columns interconnected at different levels), walls
(usually made of RC or masonry), & planar trusses; Not loaded Loaded
(c) Foundations and Soils, i.e., isolated and combined Ground movement
footings, mats, piles, wells, soil layers and rock; and Loaded (b) in Y-direction (c)
(d) Connections between the above elements. Figure 1: Load paths for different load actions –

Importance of Load Paths (a) key structural elements constituting load

Buildings perform best in earthquakes, when paths, (b) vertical load paths, and (c) lateral load

inertia forces generated in them are transmitted to paths
foundation by continuous and direct load paths

without being bent or interrupted. When some structural

elements are discontinued along a direct load path, This edge
loads have to bend and take detours to other load elongates

paths; buildings with discontinuous or indirect load paths
are undesirable, because brittle damage can occur in

structural elements at the interruptions or bends.

Horizontal Diaphragms
No edge shortens
Floor and roof slabs are thin, wide structural or elongates
elements laid in a horizontal plane at different levels.
They transfer inertia forces induced by their own Wall

masses, to vertical elements on which they rest. During This edge
earthquake shaking, horizontal diaphragms act like
shortens
beams in their own horizontal plane and transmit inertia (b)
forces to vertical elements, such as structural walls or

planar frames. Slabs that are long in plan (i.e., flexible in
their own plane), bend and undergo undesirable
stretching along one edge and shortening along the (a)
Figure 2: In-plane deformation in horizontal
other (Figure 2); they perform best when relative
deformations are minimal and in-plane stiffness and diaphragms – (a) absent, if plan aspect ratio is
up to 3, and (b) present, if plan aspect ratio
strength sufficiently large. In general, slabs should be exceeds 5
rectangular with plan length/plan width ratio less than 3.