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minimum so as to increase the stiffness of the columns. An increase in the stiffness of the column reduces the rotation and horizontal movements of the eaves and the
vertical movement of the apex. Keeping the eaves height to a minimum also reduces the construction and side cladding material costs. However, the eaves height should
not be too low, as this reduces the clear headroom. A reduction in the headroom results in industrial buildings failing to accommodate large machinery or provide free air circulation in residential houses. It has been found in practice, that small industrial buildings rarely receive containers and stack or stock material to heights in excess of 3 m (De Clercq, 1991). An eaves height of 3 m was
therefore adopted for all the portal frames. In South Africa the slopes of roofs for portal frames are generally in excess of 5ο, although roof slopes as low as
1ο are becoming increasingly popular with the advent of new cladding systems such as standing seam roofs. The pitch of a roof has significant implications not only on the cost of the structure, but also on the performance and appearance of the structure. Factors influenced by the pitch of the roof include the following:
• Cost of the portal frame. Under a defined vertical load, the elastic bending moment at the eaves joint and the elastic deflection at the apex are at their maximum values when the roof is flat. The vertical loading case is more critical than the wind loading case in Southern Africa. As the pitch increases, the eaves bending moment and the apex deflection of the frame decrease (axial force in the rafter increases).The design of a portal frame is influenced to a large extent by the maximum moment and deflection considerations in the structure rather than axial and shear forces. A reduction in the eaves bending moment and deflection of the frame means that a lighter profile may be used for the portal frame, thus cutting down on the cost of the frame.
• Cost of the cladding material. For a low pitch roof, the slope length and therefore cost of the cladding material increases with increase in the pitch of the roof. Thus, a roof with a low pitch of roof results in a lower cost of the cladding material.
• Construction costs-The ease of construction of the structure is an important practical consideration. Apart from the increase in the cost of the scaffolding,
the erection of frames becomes more difficult as the height of ridge increases. The fixing of roof cladding by a crew with hand held equipment becomes difficult on high roof pitches.
• Aesthetics-From an aesthetic point of view, a flat roofed portal structure is not very pleasing. The most common type of roof is the dual-pitched roof. Based on the analysis given above, a steeper pitch of the roof of 10ο is adopted for all the portal frames, since roofs with steeper slopes have smaller bending moments and deflections, and are less susceptible to leaks. The cladding material used for the proposed structural system constitutes a significant part of the total building
costs. It is consequently essential that the most economical profile be employed for the proposed structures. An assumption was made during the investigation to have
four rows of purlins in each half of the portal frame. This means that each structure would have a total of three spacings between purlins in each half (Fig. 3). According
to the Southern African Steel Construction Handbook (2005), the corresponding roof cladding that might be selected, would be 0.8 CQ Corrugated iron for the 10 m
span frame and 0.45 Ekono Flekspan for the 11 and 12 m span frames. The type of cladding is different for the frames as it is based on different end and internal spans of the sheeting. A 0.8 CQ Corrugated iron has a maximum end and internal spans of 1.65 m, and a 0.45 Ekono Flekspan sheeting has a maximum end span of 1.90 m
and an internal span of 2.00 m.