Sway Load Paths - Anatomy of Steel Structure.

For two fundamental reasons sway load paths require particular  consideration in industrial steelwork. First, the plant or equipment itself can produce lateral loading on the structure in addition to wind or seismic load where applicable. Therefore, higher total lateral loadings can exist whose points of application may differ from the usual cladding and floor intersection locations.The type and magnitudes of such additional loads are covered later.

Second, many industrial steelwork structures lack a regular and  complete (in plan) floor construction that provides a convenient and effective horizontal diaphragm.Therefore, the lateral load transfer mechanism must be considered carefully at a very early stage in the design (Fig. 3.5).

Establishment of sway load paths
Fig. 3.5 Establishment of sway load paths

Naturally, there are many different methods of achieving lateral stability.Where floor construction is reasonably complete and regular through the height of the structure, then the design can be based on horizontal diaphragms transferring load to vertical stiff elements at intervals along the structure length or width.Where large openings or penetrations exist in otherwise conventional concrete floors, then it is important to design both the floor itself and the connections between the floor and steelwork for the actual forces acting, rather than simply relying automatically on the provision of an effective diaphragm as would often and justifiably occur in the absence of such openings.

It can be worthwhile deliberately to influence the layout to allow for at least a reasonable width of floor along each external face of building, say of the order of  1 – 10 of the horizontal spacing between braced bays or other vertical stiff elements.

When concrete floors as described above are wholly or sensibly absent, then other types of horizontal girders or diaphragms can be developed. If solid plate or open mesh steel flooring is used, then it is possible, in theory at least, to design such flooring as a horizontal diaphragm, usually by incorporating steel beams as ‘flange’ members of an idealized girder, where the floor steel acts as the web. However, in practice, this is usually inadvisable as the flooring plate fixings are rarely found to be adequate for load transfer and indeed it may be a necessary criterion that some or all of the plates can be removable for operational purposes. A further factor is that any line of beams used as a ‘flange’ must be checked for additional direct  compression or tension loadings; the end connections also have to be designed to transfer these axial forces.

It is therefore normal to provide plan bracing in steelwork in the absence of concrete floor construction. The influence that this will have on plant penetrations, pipework routeing and many other factors must be considered at an early stage in liaison with the plant engineers. Naturally, the design considerations of steel beams serving a dual function as plan bracing are exactly as set out above and must not be neglected. Indeed, it may be preferable to separate totally the lateral load restraint steelwork provided on plan from other steelwork in the horizontal plane.

This will avoid such clashes of purposes and clearly signal to the plant designers the function of the steelwork, as a result preventing its misuse or abuse at a later stage.

Many examples exist where plan bracing members have been removed owing to subsequent plant modifications.A practical suggestion to further minimize the possibility of this happening is to paint such steelwork a completely different colour as well as to separate it completely from any duality of function with respect to plant support or restraint.

Where it proves impossible to provide any type of horizontal plan bracing, then each and every frame can be vertically braced or rigid-framed down to foundations.

It is best not to mix these two systems if possible, since they have markedly differing stiffnesses and will thus deflect differently under loading.

If diaphragms or horizontal girders are used, then the vertical braced bays that receive lateral loading as reactions from them are usually braced in steelwork. In conventional structures tension-only ‘X’ bracing is frequently used, and whereas this may be satisfactory for some straightforward industrial structures such as tank support frames and conveyor support legs, it often proves necessary to design combined tension/compression bracing in ‘N’, ‘K’, ‘M’ or similar layouts depending on the relative geometry of the height to width of the bay and on what obstruction the bracing members cause to plant penetrations. Indeed, it is sound advice initially to provide significantly more bracing than may be considered necessary, for example by bracing in two, three or more bays on one line.When later developments in the plant and equipment layout mean that perhaps one or more panels must be altered or even removed, it is then still possible to provide lateral stability by rechecking or redesigning the bracing, without a major change in bracing location.

One particular aspect of vertical bracing design that requires care is in the evaluation of uplift forces in the tension legs of braced frames.Where plant and equipment provide a significant proportion of the total dead load, then it is important that minimum dead weights of the plant items are used in calculations. For virtually every other design requirement it is likely that rounding-up or contingency additions to loadings will have been made, especially at the early stages of the design.

It is also important to establish whether part of the plant loading is a variable contents weight and, if so, to deduct this when examining stability of braced frames. In some cases, for example in hoppers, silos or tanks, this is obvious; but boilers or turbines which normally operate on steam may have weights expressed in a hydraulic test condition when flooded with water. The structural engineer has to be aware of these plant design features in order to seek out the correct data for design.

0 comentarios:

Post a Comment