Gravity Load Paths - Anatomy of Steel Structure.

Vertical loadings on industrial steelwork can be extraordinarily heavy; some individual pieces of plant have a mass of 10 000 t or more. Furthermore, by their very nature these loadings generally act as discrete point loads or line loads rather than as uniformly distributed loadings. Load values are often ill-defined at the steelwork design stage and frequently additional vertical loadings are introduced at new locations late in the design process.

For these reasons, the gravity load paths must be established at an early stage to provide a simple, logical and well-defined system. The facility should exist to cater for a new load location within the general area of the equipment without the need to alter all existing main structural element locations. Typically this means that it is best to provide a layered system of beams or trusses with known primary span directions and spacings, and then with secondary (and sometimes in complex layouts, tertiary as well) beams, which actually provide vertical support to the plant.

While simplicity and a uniform layout of structure are always attractive to a structural designer, the non-uniform loadings and layout of plant mean that supporting columns may have to be positioned in other than a completely regular grid to provide the most direct and effective load path to the foundations. It is certainly preferable to compromise on a layout that gives short spans and a direct, simple route of gravity load to columns, than to proceed with designing on a regular grid of columns only to end up with a large range of member sizes and even types of beams, girders or trusses (Fig. 3.4).

Support steelwork for an unsymmetrical plant item
Fig. 3.4 Support steelwork for an unsymmetrical plant item

Similarly, although it is clearly preferable for columns to run consistently down to foundation level, interference at low levels by further plant or equipment is quite common, which may make it preferable to transfer vertical loading to an offset column rather than use much larger spans at all higher levels in the structure. A conscious effort to be familiar with all aspects of the industrial process, and close liaison with the plant designers so that they are aware of the importance of an early and inviolate scheme for column locations, are both necessary to overcome this problem.

Different plant designers may be handling the equipment layout at differing levels in the structure, so it is important to ensure that loading information from all parties is received before the crucial decision on column spacing is made.

If a widely variable layout of potential loading attachment points exists, then  consideration must be given to hanging or top-supporting certain types of plant.

A typical example of this is a vehicle assembly line structure, where the overhead assembly line system of conveyors can sensibly be supported on a deep truss roof with a regular column system. If this is contrasted with the multitude of columns and foundations that would be needed to support such plant from below and the prospect of having to reposition these supports during the design stage as more accurate information on the plant becomes available, then it can be an appropriate, if unorthodox, method of establishing a gravity load path.

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