StructuresReal World Engineering

Ms. Sicola

Live loads vs dead loads

Live loads  refer to loads that do, or can, change over time. Objects that move around in or on a structure, like people, furniture, and cars, are all examples of live load. 

In addition to live loads, what is known as environmental loads are loads that are created naturally by the environment and include wind, snow, seismic, and lateral soil pressures.

 Dead loads: The weight of the structure itself is called the dead load. Anything permanently attached to the structure is part of its dead load -- including the columns, beams, nuts, and bolts. These are loads that typically do not change over time.

http://www.pbs.org/wgbh/buildingbig/lab/loads.html

Static Loads

Static loads are usually gravity-type loads. These would include the weight of the structure itself or any other load that is stationary.

Dead loads are static forces that are relatively constant for an extended time. An elevator is an example where static loading occurs. When ten people stand in an

elevator waiting for the doors to close, they are exerting a load on it that is static because the people and the elevator are not moving relative to each other. The stresses within the elevator have time to reach equilibrium under such conditions.

Compression vs. Tension

Compression is a force that squeezes a material together. When a material is in compression, it tends to become shorter.

http://www.pbs.org/wgbh/buildingbig/lab/forces.html

 Tension is a force that stretches a material apart. When a material is in tension, it tends to become longer.

The weight of the roadway and all the cars traveling on the bridge pull on the vertical cables in this suspension bridge. The cable are in tension.

The lower columns of the skyscraper are squeezed by the heavy weight above them. This squeezing force in called compression.

Buckling, Deflection, and Neutral Axis

Buckling occurs when an object bends and gives way under pressure or strain.

Example above shows a pressure from both the top and sides causing buckling of this column.

https://www.youtube.com/watch?v=NBcToktU2oI

http://www.pbs.org/wgbh/buildingbig/lab/forces.html

 Pressure is the continuous physical force exerted on or against an object by something in contact with it. Pressure can cause buckling in an object.

• Deflection is the degree to which a structural element is displaced under a load. It may refer to an angle or a distance.

• Example shown below a bam subject to a point load at its mid-span where it will experience deflection.

The neutral axis is an imaginary plane that runs through the center of a structural member and along which no stress occurs.

Columns, Beams, Girders

Column or pillar in architecture and structural engineering is a structural element that transmits, through compression, the weight of the structure above to other structural elements below.

In other words, a column is a compression member.

A Beam is a structural member which spans horizontally between supports and carries loads which act at right angles to the length of the beam. Furthermore, the width and depth of the beam are "small" compared with the span.

Beams are considered secondary beams or, its main job is to first transfer loads on it to Girders or Primary beams which in turn transfer the loads to columns supporting it.

Girders are a type of beam. There are two kinds of beams namely 'Primary beam' and ‘Secondary beam'. Girders comes under the category of primary beam, its main job is to directly transfer loads coming over it to columns upon which it rests.

Cross Bracing

Cross Bracing is a system utilized to reinforce building structures in which diagonal supports intersect.

Dampening

Dampening reduces the amplitude of an oscillation on an object or structure◦ Examples include dampening on shock

absorbers of the car suspension when it moves up and down,

Tacoma Narrows Bridge collapse◦ https://

www.youtube.com/watch?v=xox9BVSu7Ok

Taipei 101 Tower would sway back and forth up to 60cm (2 feet) each way within five secondsThe Taipei 101 engineers included a 730-ton counter giant pendulum to act as a counter weight.

Shock absorbers or dampers are added to the structure to dissipate the seismic shock

Twisting and Shearing

Torsion is a twisting force upon an object or structure

Shear is a force that causes parts of a material to slide past one another in opposite directions

The column above from the Parthenon shows how the column segments have slid past each other

The Tacoma Narrows Bridge collapse is an example of the torsion created from the violent winds/ oscillations that caused it to ultimately fail

Winds, Oscillations, Earthquakes

WIND When wind blows on a structure, it is

called wind load. Wind loads push horizontally on a structure.

Cross-Bracing: Cross-bracing is an excellent way to stiffen a structure experiencing wind load. When the wind blows, the diagonal brace squeezes together and prevents the structure from flopping over.

EARTHQUAKES When the ground beneath a structure

jerks back and forth during an earthquake, the structure is experiencing an earthquake, or seismic load. Earthquake loads push and pull horizontally on a structure. They can also move vertically up and down

https://www.youtube.com/watch?v=fqzTBdFa884

“P” waves (up and down) vs. “S” waves (side to side)

OSCILLATION Oscillation is a motion that

repeats itself in a regular cycle, such as a pendulum.

Wind Force Earthquake ForceVs.

Winds, Oscillations, Earthquakes

EARTHQUAKES When the ground beneath a structure

jerks back and forth during an earthquake, the structure is experiencing an earthquake, or seismic load. Earthquake loads push and pull horizontally on a structure. They can also move vertically up and down

https://www.youtube.com/watch?v=fqzTBdFa884

“P” waves (up and down) vs. “S” waves (side to side)

A P wave, or compressional wave, is a seismic body wave that shakes the ground back and forth in the same direction and the opposite direction as the direction the wave is moving.

An S wave, or shear wave, is a seismic body wave that shakes the ground back and forth perpendicular to the direction the wave is moving. 

PSF vs PSI

PSF or Pounds per Square Foot is a system of measurement used in engineering and architecture to understand the live load and dead load weight imposed on a structure

Pound-force per square foot (psf) is a unit of pressure where a force of one pound-force (lbf ) is applied to an area of one square foot (ft²).

1 psf = 1 lbf / ft²

PSF or Pounds per Square inch is a system of measurement used in engineering and architecture to understand the live load and dead load weight imposed on a structure

Pound-force per square inch (psi) is a unit of pressure where a force of one pound-force (lbf ) is applied to an area of one square inch (in²).

1 psf = 1 lbf / in²

PSI is most commonly used in architecture and engineering