Sei sulla pagina 1di 12


1.1 Definition of Structure

A building structure is an assemblage of elements used to channel and
direct loads present in a building to the foundation.
To Structure means to build - to make use of solid materials (steel,
concrete, timber) in such a way as to assemble an interconnected whole.
Primarily, structures must resist against the pull of gravity and to
withstand the pressures of wind and the inertial forces resulting from
Statics is the analysis of forces on an individual structural element or an
entire structural assembly that is in a state of balance or equilibrium.
Strength of materials constitutes the basis for structural design.
Strength of materials relates the external forces applied to the structural
assembly or element with the internal forces (stresses) within the

1.2 Structural Design

Structural design involves a process of balancing between applied
forces and the materials that resist the forces.
A building must never collapse under the action of design loads
nor from large forces due to wind or earthquake.
A general procedure of designing a structural system (called
structural planning) consists of the following phases:
Seattle Public Library Architect, Rem Koolhaas

conceiving of the basic structural form.

devising the gravity and lateral force resisting system.
roughly proportioning the component parts.
developing a foundation scheme.
determining the structural materials to be used
detailed proportioning of the component parts
devising a construction methodology
Chinas Olympic stadium under construction

1.4 Loads on Structures

All structural systems are subjected to a variety of load conditions.
Forces on various parts of the structure induce stresses and deformations on the
elements within the framework.
Structural systems essentially exist to resist static and dynamic forces.
Static forces are gravity type forces, applied gradually to the structure and over
a relatively long period of time.
Dynamic loads are due to inertial forces caused by sources like earthquakes and
Dead loads are static, fixed loads that include the building structure weight,
exterior and interior cladding, flooring, and fixed equipment that generally
remains on the structure throughout its lifetime.
Live loads are transient and moving loads that include occupancy loads,
furnishings, and storage.
Live loads are extremely variable by nature and normally change during a
structures lifetime as occupancy changes.

Loads on Structures - Dead and Live Loads

Dead loads can be computed quite accurately using
standard material weight tables of common building
Building codes specify minimum uniform live loads for
the design of roof and floor systems based on a history of
many buildings and types of occupancy conditions.
They incorporate safety provisions for overload
protection, allowance for construction loads, and
serviceability considerations (such as vibration and
deflection behavior).
The occupancy loads specified in building codes are
generally conservative enough to account for the
increased stresses caused by the vibration of the
structure (people dancing, bouncing down a flight of
A large portion of a building structure exists for loads

Selected Building Material Weights

Loads on Structures - DL & LL (contd)

Building designers have always strived to reduce the
ratio of dead to live load.
New methods of design, new and lighter materials,
and old materials used in new ways have contributed
to the dead/live load reduction.
As spans increase, so do the bending effects caused
by dead and live loads; therefore, more material must
be introduced into the beam in order to resist the
increased bending effects.
This added material weight itself adds further dead
load and pronounced bending effects as spans
The dead/live load ratio has considerable influence
on the choice of structure and especially on the
choice of beam types.

Selected Live Load Requirements

Loads on Structures - Snow (contd)

Snow loads represent a special type of live load because of
the variability involved.
Generally, snow loads are determined from a zone map
reporting 50-year recurrence intervals of an extreme snow
Design loads can vary from 10 psf on a horizontal surface to
400 psf in some specific mountainous regions.
The accumulation depth of the snow depends on the slope of
the roof.
Steeper slopes have smaller accumulations.
Special provisions must also be made for potential
accumulation of snow at roof valleys, parapets, and other

Failure from snow load

Loads on Structures - Wind (contd)

Wind loading on buildings is dynamic in nature.
When buildings and structures become obstacles in the path of wind
flow, the winds kinetic energy is converted into potential energy of
pressure on various parts of the building.
Wind pressures, directions, and duration are constantly changing.
The fluctuating pressure caused by a constantly blowing wind is
approximated by a mean pressure that acts on the windward and
leeward sides of the structure.
Direct wind pressures, also referred to as stagnation pressure, depend
on several variables: wind velocity, height of the wind above ground
(wind velocities are lower near the ground), and the nature of the
buildings surroundings.
The wind creates a negative pressure, or suction, on both the leeward
side of the building (the side opposite the windward side), and the side
walls parallel to the wind direction.
Uplift pressure occurs on horizontal or sloping roof surfaces.

Building damaged by a tornado Oklahoma

FEMA/ Win Henderson

Loads on Structures - Wind (contd)

The corners, edges and eave overhangs of a building are subjected to
complicated forces as the wind passes these obstructions, causing higher
localized suction forces than generally encountered on the building as a
Wind is a fluid and acts like other fluids, where a rough surface causes
friction and slows the wind velocity near the ground and increases with
Wind speeds are measured at a standard height of 10 meters (33 feet)
above the ground and adjustments are made when calculating wind
pressures at higher elevations.
Other buildings, trees, and topography affect how the wind will strike the
Buildings in vast , open areas are subject to larger wind forces than those
in sheltered areas or where the building is surrounded by other buildings.
The size, shape, and surface texture of the building also impacts the
design wind forces.

Loads on Structures - Earthquake (contd)

Earthquake loads (seismic) are inertial forces develop in the structure due
to its weight, configuration, building type, and geographic location.
Inertial forces are the product of mass and acceleration (Newtons second
law: F= m x a).
Heavy, massive buildings will result in larger inertial forces, hence, there
is a distinct advantage in using a lighter weight construction when
seismic considerations are a key part of the design strategy.
Earthquake, like wind, produces a dynamic force on a building.
Lateral forces developed in the structure are a function of the buildings
mass, configuration, building type, height, and geographic location.
In actual earthquakes, there are continuous ground motions which cause
the building structure to vibrate.
All objects, including buildings, have a natural or fundamental period of

2010 Chile Earthquake Damage

U.S. Geological Survey, Department of the Interior/USGS

U.S. Geological Survey photo by Walter D. Mooney Ph.D.

Loads on Structures - Earthquake (contd)

When an earthquake ground motion begins a building vibrating, the building
begins to displace (sway) back and forth at its natural period of vibration.
Shorter, lower buildings have very short periods of vibration (less than 1
Tall high rises can have periods of vibration that last several seconds. The
ground also vibrates at its own natural period of vibration.
Many of the soils in the United States have periods of vibration in the range
of 0.4 seconds to 1.5 seconds.
Instability of the soil due to liquifaction - Niigata, Japan

Short periods are more characteristic of hard soils(rock), while soft ground
(some clays) may have periods of up to 2 seconds.
Many common buildings can have periods within the range of the supporting
soils, making it possible for the ground motion to transmit at the same natural
frequency as that of the building.
This may create a condition of resonance (where the vibrations increase
dramatically), in which the inertial forces might become extremely large.

Photo Credit: National Geophysical Data Center

Courtesy of the U.S. Geological Survey

1.5 Basic Structural Requirements

Stability - a configurational property that preserves the geometry of a
structure through its elements strategically arranged and interacting
together to resist loads.
Equilibrium - a state of rest and the balancing of forces
Strength - ability of the structural material or element from breaking
Stiffness - ability of the structure and its elements to resist large
Continuity - a direct, uninterrupted path for loads through the building
structurefrom the roof level down to the foundation
Wind damage in North Carolina

Redundancy - the concept of providing multiple load paths in a structural

framework so that one system acts as a backup to another in the event of
localized structural failure
The requirements of economy, functionality, and aesthetics are usually not
covered in a structures course

FEMA/ Dave Gatley