Columns - Anatomy of Steel Structure.

Column location in industrial buildings must be decided on practical considerations.

Although regular grid layouts are desirable in normal structures, it is sometimes impossible to avoid an irregular layout which reflects the major plant and equipment location, the irregular floor or walkway layouts and an envelope with irregular wall and roof profiles to suit. Obviously some degree of regularity is of considerable benefit in standardizing as many secondary members  as possible; a common method of achieving this is to lay out the columns on a line-grid basis with a uniform spacing between lines. This compromise will allow standard lengths for beam or similar components in one direction while giving the facility to vary spans and provide direct plant support at least in the other direction.The line-grids should be set out perpendicular to the longer direction of the structure if possible.

When vertical loadings are high and the capacity of rolled sections is exceeded, several types of built-up columns are available.Where bending capacity is also of importance, large plate I-sections are appropriate, for example in frameworks where rigid frame action is required in one direction.However, if high vertical loads dominate then fabricated box columns are often employed (Fig. 3.8). Design of box columns is principally constrained by practical fabrication and erection considerations. Internal access during fabrication is usually necessary for fitting of internal stiffeners, and similarly internal access may be needed during erection for making splice connections between column lengths, or for beam-to-column connections.

 Typical details of box columns
Fig. 3.8 Typical details of box columns

Preferred minimum dimensions are of the order of 1m with absolute minimum dimensions of about 900mm. Whenever possible, column plates should be sized  to avoid the necessity of longitudinal and transverse stiffeners to control plate  buckling. The simpler fabrication that results from the use of thick plates without stiffeners should lead to overall economies, and the increased weight of the member is not a serious penalty to pay in columns; the same argument does not apply to long-span box girders where increases in self-weight may well be of overriding importance. Under most conditions of internal exposure no paint protection to the box interior is necessary. If erection access is needed then simple fixed ladders should be provided.Transverse diaphragms are necessary at intervals (say 3–4 times the minimum column dimension) to assist in maintaining a straight, untwisted profile, and also at splices and at major beam-to-column intersections even if, as is usually the case, rigid connections are not being used.Diaphragms should be welded to all four box sides and be provided with manhole cut-outs if  internal access is needed.

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