|Supplement: Wind Forces on Emergency Storage Structures (NRI)|
This investigation of the nature of weather in tropical climates shows that at the extreme, 3-second gust speeds of up to 90 m/s (201 mph) are possible. These can be in excess of the 3-second gust speed for the United Kingdom (BSI CP3, 1972). Commonly in the Carribean, Mali, Mauritius, Niger, parts of India, Taiwan, Bermuda and the Philippines, 3-second gust speeds exceed 60 m/s (see Table 1). If these speeds are underestimated this has serious consequences for design because the square of the wind speed is employed to calculate wind load, which will therefore be much reduced. In this context it has also been shown how wind loads on rigid structures may be derived from local 3-second gust speeds. There is no straightforward way of calculating wind loads on plastic-clad steel frame structures, but professional advice is obtainable.
Because emergency storage structures are used for food relief in developing countries where no supervision is easily available, it is important that they are accompanied by clear instructions in diagram form to overcome language problems.
Robertson (1988) has suggested that failure of plastic-clad structures is commonly caused by inadequate foundations and this is ODNRI experience too. Mayo (1988) suggests that failures often relate to:
· use of the incorrect design wind speed
· inapplicability of Code CP3 - for example, because the building shape is not included in those covered
· unknown cladding properties at the design stage. In addition, Robertson suggests that some manufacturers of film plastic-clad structures may use incorrect design procedures. Because of failures caused by wind it is considered that this may also be true of emergency stores.