foundation system of burj al arab

Design Considerations for Portal Frames

Portal frames are constructed mainly using hot-rolled sections, supporting the roofing and side cladding via cold-formed purlins and sheeting rails. They are the most commonly used structural forms for single-storey industrial structures. They may also be composed of tapered stanchions and rafters fabricated from plate elements. Portal frames of lattice members made of angles or tubes are also common, especially in the case of longer spans.

The slopes of rafters in the gable portal frames (Fig.1) vary in the range of 1 in  10 to 1 in 3. Generally, the centre-to-centre distance between frames is of the order 6 to 7.5 m, with eaves height ranging from 6 -15 m.  Normally, larger spacing of frames is used in the case of taller buildings, from the point of economy.  Moment-resisting connections are to be provided at the eaves and crown to resist lateral and gravity loadings.  

Eaves detail of Haunched gable portal frame
Fig: 1 (a) Eaves detail of  Haunched gable portal frame
The stanchion bases may behave as either pinned or fixed, depending upon rotational restraint provided by the foundation and the connection detail between the stanchion and foundations. The foundation restraint depends on the type of foundation and modulus of the sub-grade. Frames with pinned bases are heavier than those having fixity at the bases. However, frames with fixed base may require a more expensive foundation.

For the design of portal frames, plastic methods of analysis are mainly used, which allows the engineer to analyse frames easily and design it economically.
Elevation of Haunched gable portal frame
Fig: 1 (b) Elevation of  Haunched gable portal frame
For the design of portal frames, plastic methods of analysis are mainly used, which allows the engineer to analyse frames easily and design it economically. The basis of the plastic analysis method is the need to determine the load that can be applied to the frame so that the failure of the frame occurs as a mechanism by the formation of a number of plastic hinges within the frame. The various methods of plastic analysis are discussed later.

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