|Wind forces on Emergency Storage Structures (supplement) (NRI)|
For film plastic-clad greenhouses which are not dissimilar to emergency stores the Ministry of Agriculture Fisheries and Food (MAFF, 1983) recommend that cladding is anchored at ground level either by gripping with a continuous structural member fixed to the main hoops, or by being buried in the trench not less than 300 mm deep by 300 mm wide, firmly backfilled and rammed with earth. Such structures should be supplied with an erection manual giving:
· erection instructions in diagramatic form;
· a maintenance procedure;
· details of constraints in use.
Correctly applied storm rigging for tents ensures that wind forces are distributed evenly; mountaineering tents are low, steeply pitched and present no vertical faces to the wind. Although such measures may not be applicable to relief stores, efforts can be made to site these structures away from areas which experience strong winds, such as hill tops and valley bottoms, and behind any available cover or wind break.
Rigid rectangular structures should have a roof pitch of well over 10° and if possible greater than 15°. The optimum is 30°-40°. Rigid structures likely to be subjected to strong winds should have hip-angled rather than gable ends.
Once-in-50-year basic gust speeds for selected countries and territories
Sources: Met Office (1987) and Eaton (1981)
Notes: To obtain the design wind speed the basic gust speed must be multiplied by constants, S1, S2 and 53, see Appendix 2.
This figure will be revised on account of Hurricane Gilbert (Lawson 1988)
Large roof overhangs should be avoided, or vents included in these to relieve wind pressure. If eaves ventilators are employed the structure should be strengthened with a ring-beam at eaves' level. Similarly, every part of the structure should be tied together roof to walls, walls to walls, walls to floor, floor to foundations. The latter should have reinforcing bars which anchor the construction. All masonry construction should also be reinforced and horizontal reinforcement used round corners, between intersecting walls and between columns, infill walls and doors. Timber columns should be notched to resist uplift forces and cast into the concrete foundations in situ.
Timber roofs should be connected to masonry walls with a fastening strap or reinforcing bar that is firmly embedded in the concrete or masonry. If timber walls are used it should be ensured that nails are driven in so they act in shear rather than in tension. Purlins should be tied to rafters with strap connectors. When nailing corrugated roof sheeting, the top of the corrugations should be nailed through and a washer at least 20 mm (3/4 inch) in diameter used. Every corrugation of roof edges and every other corrugation elsewhere (see Figure 2) should be nailed.
These measures should reduce and combat wind loads; to be confident that a structure can resist cyclical loads cyclical testing is necessary (see Appendix 3). Normally, full-scale testing is sufficient.