Steel girders and stress
As we read earlier, steel girders undergo stresses of both compression and tension (stretching). The Sydney Harbour Bridge is constructed with steel girders as are the frames of many large buildings. Steel girders are similar to solid steel beams but with a chunk of steel removed from the middle on each side giving it its characteristic "I" shape.
Now the "I" shape of the steel girders, at first, appears strange. Surely a solid beam of steel is stronger than the "I" configuration. Surprisingly the "I" shaped girder is nearly as strong as a solid steel beam of identical dimensions. The advantage of the "I" beam configuration is that it is light. If all bridges or frames were constructed of solid steel beams the mass they would have to support would be a severe handicap.
Most of the material in a steel girder is concentrated on the top and bottom parts, called the flanges. The piece of steel connecting the bottom and top flanges is known as the web. A steel girder placed horizontally experiences both tension and compression as it supports a load. The top flange experiences compression forces while the bottom flange experiences tensile forces. The web between the top and bottom flanges experiences little stress.
The stress applied to a particular surface of a structure is given by the expression on the right. If a given force is spread over a large surface area then the stress on the structure is less than if the force is concentrated over a small area.
Consider a horizontal girder supporting a load with the cross-section shown on the right. The red line indicates tension. Notice that the flange on the bottom has a greater surface area than the flange at the top. Tensile forces are spread over a wider area and the stress on the bottom flange is minimised.
As we read earlier, materials have a greater compressive strength than tensile strength. They can cope well with high compressive forces but not with high tensile forces. Increasing the surface area of the bottom flange spreads the force over a greater area and reduces tensile stress. The girder pictured above can withstand greater loads than the one pictured below.
Explain why this girder can withstand less load than the girder pictured above.
The diagram on the right shows the cross-section of 4 horizontal girders.
a) Which girder can support the least load?
b) Which girder can support the greatest load?
c) Girders a) and b) were subjected to a steadily increasing load. Which girder will fail first and why?
Girders c) and b) were subjected to a steadily increasing load. Stephen predicted that girder b) will cope with twice the load placed on girder c). Is he correct? Explain.