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Stress and Strain
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There are two types of forces in structural engineering: tension and compression.

TENSION: Pulling force
Imagine the force felt in your arms while you hang from a bar.
A structural element subjected to tension is elongated.
 
COMPRESSION: Squeezing force
Imagine the force felt in your arms while you stand on your hands.
A structural element subjected to compression is shortened.

Stress is defined as force per unit area that the force acts upon.  Thus,

Stresses are either tensile or compressive. Structural materials are chosen by their ability to resist tensile or compressive forces, depending upon the application.  Most materials are better at resisting one or the other.  For instance, concrete is strong in compression and relatively weak in tension. Steel is equally strong in both tension and compression.
An example of a tensile structural element would be a cable.   An example of a compressive structural element would be a column.

Strain is defined as the change in length of a stressed structural element divided by the original length of the unstressed element. Thus,

where
Compression: comp_schematic.gif (1739 bytes)
Tension: tension_schematic.gif (1846 bytes)

A material’s tensile strength is determined in the laboratory by pulling on a specimen until it breaks. While the test is conducted, both the stress and strain are recorded. The maximum stress that the specimen can withstand is called the ultimate strength of that particular material. From a design stand-point, we are mainly interested in the stress where the material stops behaving elastically.

A material behaves elastically when it returns to its original shape when an applied load is no longer applied. This point is found by plotting stress versus strain during the test and determining the stress at which the plot becomes non-linear. This stress is called the yield stress, sy.

The slope of the stress-strain curve in the elastic region is defined as the elastic modulus, E. Structures should be designed so that any applied load would not cause the stress in the structure to be greater than sy.

Beams are structural elements that are subjected to bending forces. When bending occurs, the beam is subjected to tension and compression simultaneously.

Imagine a sponge beam. Say we draw a grid on the side of the beam, so that the sponge is divided into two rows of rectangles of equal length, Lo, and height, h/2.

When a force is applied to the beam, the rectangles deform.

The tops of the upper row of rectangles are shortened, and the bottoms of the lower row of rectangles are elongated. Thus, we see that the top of the beam is in compression and the bottom of the beam is in tension.

Notice that the middle of the beam is in neither tension or compression. This is called the neutral axis. The bending stress at the neutral axis is zero.

The key to designing a beam is to locate the point of maximum stress. For a simply-supported beam under a uniform load, the maximum stress occurs at the center point. The maximum compressive stress at the top of the beam, scmax, and the maximum tensile stress at the bottom of the beam, stmax, are given by the following equations:

where h is the height of the beam, b is the width of the beam, and Mmax is the maximum moment at the midspan of the beam.

 

    
 
Last modified:  5 Oct 2000 Count: 11,702