What Is a Short Column?
A short column is a structural member whose relatively short length virtually ensures it will fail in compression if it is evenly loaded along its axis. Columns can be classified as short, intermediate, or long, depending on their lengths, their material properties, and the lengths of other columns in the same structure. A short column differs from a medium or long column, each of which can fail by bending when evenly loaded along the axis.
No single specific length or range of lengths defines a short column. Rather, whether a column is considered to be short depends on a variety of factors. A short column may be made of various materials including structural steel, aluminum alloys, reinforced concrete, or even wood. Depending on which of these materials they are made from, short columns may be different lengths due to the different strengths of these materials. Structural columns that are short in length relative to the other columns on a particular story of a building are also considered short columns.
Columns most typically fail in one of two modes. They can be crushed, meaning that they failed in compression, or they can bend, which is a phenomenon known as buckling. Which mode they fail in depends on a variety of factors including the geometry of the column and the material properties of the column. Distribution of the compressive forces across the cross-sectional area of column is also important. If compression is unevenly applied across the column, with more force on one edge than another, the chances of buckling are increased.
Relatively short columns are most likely to fail in compression. Stress on a column due to loading must be less than the strength of the column in order for the column to support the load. Buildings are specifically designed to ensure that this is the case; otherwise, the building would collapse.
For safety, columns are designed to be stronger than necessary under expected conditions. This is known as a safety margin. A safety margin helps ensure that the amount of stress each column is able to withstand exceeds the stress it experiences in actual use. In rare circumstances, such as a natural disaster, however, the stress on a column can unexpectedly exceed its strength, causing failure of the column. This poses an important concern for short column design.
In comparison to other columns, short columns are relatively stiffer, resulting in greater susceptibility to damage during an earthquake or other extreme events. Special attention must therefore be given to their design to ensure sound construction. For this reason, short column design is particularly important in less-developed nations in areas prone to earthquakes, such as Honduras and India.
Due to their relative stiffness compared to other structural members, short columns experience increased seismic demand, subjecting them to more force in an earthquake. This phenomenon is known as the short column effect. Short columns that fail in this manner are characterized by cracks in the shape of an X visible on the outside of the column. Designers generally avoid the use of such columns in buildings whenever possible to avoid the short column effect. Where their use cannot be avoided, designers will typically increase the robustness of the structural design to compensate for the increased risk.
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