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then become overstrained, causing localised hole elongation and tearing of sheeting. Consequently this may allow water to leak into the building. In addition, differential horizontal deflection at the eaves should be visually acceptable.
(3) The difference in vertical apex deflection between the end frame and its adjacent frame should not be large as this may cause ponding of water.
(4) The difference in horizontal deflection at the eaves and vertical deflection at the apex should be visually acceptable.
9.1. Guidance from standards
The design of hot-rolled steel portal frames is often influenced by serviceability requirements. However, mandatory deflection limits for portal frames are not found in the codes of practice. Guidance from Clause
6.2.1 of SANS 10162-1 (2005) is restricted to the general recommended limits provided in Annex I. This is often qualified by the statement that the deflection of portal
frames has no direct significance on the serviceability of the portal frame itself. The following are some of the reasons why deflection limits of portal frames are not stipulated in the codes of practice:
(1) Since excessive deflections affect the serviceability of the cladding of the frame, water tightness and the visual acceptability of the building in general, it is argued that the implications of excessive frame deflections depend on the type of cladding and other constructional details and is therefore outside the scope of the code.
(2) In order to take into account the whole range of portal frames as well as their many uses, any deflection limits stipulated in a Code of Practice would have to be
stringent. The implication of stipulating stringent requirements is that many portal frames constructed would be unnecessarily heavy and expensive.
(3) Methods for predicting deflections in portal frames do not take into account the beneficial effect of having a partially fixed column base and stressed skin action. In addition deflections would be small if member sizes were selected from an elastic analysis.
(4) Limits of portal frame deflections are outside the scope of the code because in most cases this is the responsibility of a designer, working for a fabricator.
Fabricators ensure that the profile of the frame under load corresponds to the design profile by pre-cambering or presetting the members when detailing. Column members are positioned, tilting inwards and the pitch of the frame larger than the intended angle of the frame.
9.2. Deflection limits suggested in literature
Several guidelines for deflection of portal frames are found in literature; however these are intended for use with hot-rolled steel portal frames, which have a greater
variety of uses than cold-formed steel portal frames. Woolcock and Kitipornchai (1986) published the results about the views on deflection limits of portal frames by experienced steel designers. Deflection limits for industrial buildings emanating from this study are shown in Table 9. Values for the lateral spread at the eaves are given for an industrial building with steel-sheeted walls but without
gantry cranes, suspended ceilings and internal partitions. The limits in the Australian Standard AS 4100 (1990) are derived from Woolcock and Kitipornchai (1986). Other deflection limits were published by The Steel Construction Institute (SCI Advisory Desk) (1991) in the UK. In this document deflection limits refer only to live and wind load cases as it was assumed that dead load effects are compensated for by the initial pre-camber of the portal frame. The limits are defined on the basis of the effect that excessive deflection would have on the cladding of the frame and not on the basis of the type of portal frame, nor its use. Different deflection limits are given for different types of cladding. Deflection limits suggested for frames that use profiled metal cladding are shown in Table 10. Table 11 shows the deflection limits suggested by Lim (2001) for use with cold-formed steel frames. Deflection limits are not classified into absolute or relative as such a distinction is only necessary if bay spacings are not equal. It is also assumed that pre-cambering or pre-setting techniques will have to be used during the fabrication of the frames. Most of the limits proposed by Lim are taken from the recommendations suggested by Woolcock and Kitipornchai (1986) and SCI (1991). A lateral deflection limit for roof cladding of hf/150 has however, been used instead of bf/200 to reflect the new recommendation to be published by the Australian Institute of Steel Construction