How do we design steel portal frames to BS 5950? What are the design considerations?

Question

Steel portal frames consist of rafters and columns. What is the design principle? For what do we design these different members? Is it for tension? For shear? For uplift? Or anything else?

Answer 1

BS 5950 is the British Standard for the design, fabrication and erection of structural steelwork. BS 5950 replaced BS 449, which used a permissible stress approach, and uses limit state design methods. It is written for use in the UK but can be used worldwide. It is expected that the code will eventually be superseded by BS EN 1993 when Eurocodes are more widely adopted.

Limit state design refers to a design methodology used in structural engineering. Most modern buildings are designed in accordance with a code which is based on limit state theory. For example, in the UK, Steel structures are designed in accordance with BS 5950, and reinforced concrete structures to BS 8110, both of which are codes based on limit state theory. Canada, Australia and New Zealand also utilise limit state theory.

A limit state is a set of performance criteria (e.g. vibration levels, deflection, strength), stability (buckling, twisting, collapse) that must be met when the structure is subject to loads. Limit state design requires the structure to satisfy two principal criteria: the ultimate limit state (ULS) and the serviceability limit state (SLS).

To satisfy the ultimate limit state criteria, all bending, shear and tensile or compressive stresses which would be obtained under factored loading, must be below the limit stress (maximum allowable stress dependent on the loading condition - i.e. tensile yielding or fracture, bending, buckling stress, etc., or combinations thereof dictated by the structural system - i.e. edge beams with bending and torsion at the same time) in the structural element being analysed (e.g. a beam or a column or other load bearing element, such as walls). Material and load safety factors are used to ensure that these loads can be carried safely. These factors are customarily determined using statistics and a pre-selected probability of failure. Variability in the quality of construction, consistency of the construction material are accounted for in the factors (One or lower) applied to the resistances of the material, as such, Wood and masonry typically has smaller factors than concrete, which has smaller factors than steel. The factors applied to resistance also account for the degree of scientific confidence in the derivation of the values - i.e. smaller values are used when there isn't much research on the specific type of failure mode). Factors associated with loads are typically greater than one and are normally not dependent on the type of material. More deterministic loads (like dead loads, the weight of the structure and permanent attachments like walls, floor treatments, ceiling finishes) are given lower factors (for example 1.4) than highly variable loads like earthquake, wind, or live (occupancy) loads (1.6). Impact loads are typically given higher factors still (say 2.0). While this technique is not philosophically superior to permissible or allowable stress design, it does have the potential to produce a more consistently designed structure as each element is intended to have the same probability of failure. Permissible stress design simply limits the stress by a value, usually determined by the function of the piece in question (beam, tie rod, column, hanger, etc.) where the probability of failure is not assured to be consistent.

To satisfy the serviceability limit state criteria, the structural element must not deflect by more than certain limits laid down in the building codes. In addition to deflection, there may be other serviceability criteria that apply; for example, crack widths in concrete must be kept below specified dimensions. A structure where the serviceability requirements are not met, e.g. the beams deflect by more than the SLS limit, will not necessarily fail structurally. The purpose of SLS requirements is to ensure that people in the structure are not unnerved by large deflections of the floor, vibration caused by walking, sickened by excessive swaying of the building during high winds, or by a bridge swaying from side to side and to keep beam deflections low enough to ensure that brittle finishes on the ceiling above do not crack, affecting the appearance and longevity of the structure. Many of these limits depend on the finish materials (sheetrock, acoustical tile) selected by the architect, as such, the limits in the building codes on deflections are generally descriptive and leave the choice to the engineer of record (this may not be as true outside the U.S.)

Limit state design has replaced the older concept of permissible stress design in most forms of civil engineering. Notable exceptions are geotechnical engineering and transportation engineering.

It is also true that Limit State(s) design is more thoroughly adopted outside the United States. Inside the U.S. there has been significant resistance to this technique, so much so that the American Institute of Steel Construction (AISC) is now issuing a combined manual of steel construction (the 2005 manual) that contains both methods of design side by side (ASD - Allowable Stress Design, last updated in 1989), and LRFD - load and resistance factor design). Concrete design is most commonly performed using Ultimate stress design / Limit state(s) design.

If there are specific limits, you'd need an actual (current) copy of BS 5950 - in this case, I expect it's the 2003 version.

Answer 2

Refer

Answer 3

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Answer 4

Bs 5950