Steel Design Practical Design Notes.docx

STEEL DESIGN PRACTICAL DESIGN NOTES COLUMNS: Columns in multi-storey steel frames are generally H sections, predominantly carrying axial load. When the stability of the structure is provided by cores, or discreet vertical bracing, the beams generally designed as simply supported. The generally accepted design model is that nominally pinned connections produce nominal moments in the column, calculated by assuming that the beam reaction is 100 mm from the face of the column. If the reactions on opposite side of the column are equal, there is no net moment. Columns on the perimeter of the structure will have n applied moment, due to the connections being on one side only. Typical internal column sizes are given in the table below:

Although small column sections may be preferred for architectural reasons, the practical issues of connections to the floor beams should be considered. It can be difficult and costly to provide connection into the minor axis of a very small column sections.

Deep sections with relatively narrow flanges are preferred for roof beams, , where they primarily resist bending. Columns, which primarily resist compression, are usually thicker, shallower sections with wider flanges. The span/depth ratio for the roof beams is typically 30 to 40 for spans up to 20 m.

The span will generally be modest, up to approximately 20m. The roof beam may be pre-cambered. Bracing will be required in roof and all elevations, to provide in-plane and longitudinal stability.

A Portal frame is a continuous frame with moment resisting connections to provide stability in plane. A Portal frame may be single bay or multi bay. The members are generally plain rolled sections, with the resistance of the rafter enhanced locally with a haunch. In many cases, the frame will have pinned bases. Stability in the longitudinal direction is provided by a combination of bracing in the roof, across one or both end bays, and vertical bracing in the elevations. If vertical bracing cannot be provided at the elevations (Due to industrial doors, for example) stability is often provided by a rigid frame within the elevation.

The truss buildings have roof bracing and vertical bracing in each elevation to provide stability in both orthogonal directions. The trusses may take a variety of forms, with shallow or steep external roof slopes. A truss building may also be designed as rigid in-plane although it is more common to provide bracing to stabilize the frame.