Main and Secondary Beams - Anatomy of Steel Structure.

The plan arrangement of main and secondary beams in industrial steelwork structures normally follows both from the layout of the main items of plant and from the column locations. Thus a sequence of design decisions often  occurs in which main beam locations dictate the column locations and not vice versa. If major plant occurs at more than one level then some compromise on column position and hence beam layout may be needed.

Since large plant items normally impose a line or point loading there are clear advantages in placing main or secondary beams directly below plant support positions.Brackets, plinths or bearings may be fitted directly to steelwork, and for major items of plant this is preferable to allowing the plant to sit on a concrete or steel floor.Where plant or machinery requires a local floor zone around its perimeter for access or servicing, it is common practice to leave out the flooring below the plant for access or because the plant protrudes below the support level.

Deflection requirements between support points should be ascertained.They may well control the beam design since stringent limits, for example relative deflections of 1 in 1000 of support spans, may apply. In addition when piped services are connected to the plant then total deflections of the support structure relative to the beam-to-column intersections may also need to be limited. Relative deflections can best be controlled by the use of deep beams in (lower-grade steel if necessary), and total deflections by placing columns as close as possible to the support positions.

It is preferable to avoid the necessity for load-hearing stiffeners at support points unless the plant dimensions are fixed before steelwork design and detailing take place.Where this is not possible then stiffened zones to prevent secondary bending of top (or bottom if supports are hung) flanges should be provided, even if the design requirements do not require load-bearing stiffeners. This then allows a measure of tolerance for aligning the support positions without causing local overstressing problems.

The stiffness of major plant items should be considered, at least qualitatively, in the steelwork design. Deep-walled tanks, bunkers or silos for example may well be an order of magnitude stiffer than the steel supporting structure.

The loading distribution given by the plant design engineers will automatically assume fully stiff (zero deflection) supports. When the stiffness of the supporting structure is not uniform in relation to the support point locations and loadings, then significant redistribution of loads can take place as the structure deflects.Where the plant support positions and loads and the structural steel layout are symmetrical then engineering judgement can be applied without quantitative  evaluation. In extreme cases, however, a plant–structure interaction analysis may be required to establish the loadings accurately.

When hanger supports are required then pairs of beams or channels are a convenient solution which allows for random hanger positioning in  the longitudinal direction (Fig. 3.7).Many hanger supports have springs bearings to minimize variations in support conditions due to plant temperature changes or to avoid the plant stiffness interactions described above.

Detail at hanger support
Fig. 3.7 Detail at hanger support

The method of installation or removal of major items of plant frequently requires that beams above or, less commonly, below the plant must be designed to cater for hoisting or jacking-up the installed plant sections. Structural designers should query the exact method and route of plant installation to ensure that the temporary hoisting, jacking, rolling or set-down loads are catered for by the steel beam framework.

Experience suggests that these data are not provided as a matter of course, and plant designers commonly believe that the steel structure can support these loads any- where.Where, to ease the problems, plant installation occurs during steel erection then the method of removing plant during the building life-span may be the most significant temporary loading for the beam framework.

When main or secondary beams are specifically designed for infrequent but heavy lifting operations, it is good practice to fit a lifting connection to the beam to give positive location to the lifting position and to allow it to be marked with a safe working load. For design, smaller load safety factors are appropriate in these  circumstances.

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