Skip navigationBuilding big, building strong.

1. Scraping the sky
2. New York decides on Trade Center
3. Skyline or bottom line
Structurally speaking

Photo of Chrysler building by Library of Congress.



Stresses and strains
When they design buildings, mechanical engineers worry about three types of stress: compression, tension and torsion.

Compression squeezes stuff together; the ability to resist compression is called compressive strength. Gravity puts the biggest compressive load on buildings. Steel has a higher compressive strength than the same amount of concrete. But because concrete is much cheaper, it's often used under compression.

Tension pulls stuff apart. The tensile strength of plain concrete is much lower than steel, so when concrete is placed under tension, it is reinforced with steel. (Composite materials are being tested to replace steel as reinforcement.) The downward load of a floor puts a tensile load on beams, which explains why steel is placed at the bottom of a concrete beam.

Torsion is twisting. Building components suffer torsion when wind or an earthquake move one side more than the other. Resistance to torsion is greatly affected by shape: a steel pipe or tube has much greater torsional strength than the same amount of steel in a band shape.

Drawing of the building, showing  forces of compression, tension and torsion.
Background: The American Surety Building (1894) was the first in New York to use a complete steel frame to support interior and exterior masonry. Courtesy The Skyscraper Museum.

This bibliography is solid.


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