Introduction

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 earthquakes.

• 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 element(s) themselves.

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:

• 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

Seattle Public Library – Architect, Rem Koolhaas

China’s 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 explosions.

• 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 structure’s 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 materials.

• 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 stairs).

• A large portion of a building structure exists for loads that may never occur or will be present at much lower magnitudes.

Selected Building Material Weights

Loads on Structures - DL & LL (cont’d)

• 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 increase.

• 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 (cont’d)

• 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 depth.

• 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 uneven roof configuration.

Failure from snow load

Loads on Structures - Wind (cont’d)

• Wind loading on buildings is dynamic in nature.

• When buildings and structures become obstacles in the path of wind flow, the wind’s 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 building’s 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 (cont’d)

• 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 whole.

• 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 height.

• 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 building.

• 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 (cont’d)• 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 (Newton’s 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 building’s 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 vibration.

2010 Chile Earthquake DamageU.S. Geological Survey, Department of the Interior/USGSU.S. Geological Survey photo by Walter D. Mooney Ph.D.

Loads on Structures - Earthquake (cont’d)

• 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 second).

• 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.

• 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.

Instability of the soil due to liquifaction - Niigata, Japan

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 deformation

• Continuity - a direct, uninterrupted path for loads through the building structure—from the roof level down to the foundation

• 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

Wind damage in North CarolinaFEMA/ Dave Gatley

END OF LECTURE 1