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3.12 SPECIAL LOADINGS
The designer should be alert to the possible load and deflection implications in buildings from various items not all of which are within his control. A few are discussed below.
Fig. 3.5
3.12.1 Sliding doors
A sliding door can run on a bottom track, in which case the designer has to ensure that the deflection of any component above the door will not prevent the door from opening or closing. Often the extent of vertical adjustment or tolerance built into door runners is unrealistically small.
A sliding door can be top hung, in which case one can either provide a separate support beam or beams (Fig. 3.6) so that deflection of the main support beams will not cause the door to jam, or provide short beams between the main beams (Fig. 3.7), so that if it is found that too little adjustment has been provided in the door mechanism, adjustment to the secondary beams can be carried out without major structural alterations. It is not advisable to hang a door directly under a main beam (also see section 12.6.5).
3.12.2 Water tanks
Tanks are usually quoted as having a nominal capacity together with an actual dimensional size say 300 litres for a tank 0.60 m ¥ 0.45 m ¥0.50 m deep. These actual dimensions give the ‘overflowing’ capacity (in this example it is 450 litres).
By taking the weight in design calculations as the ‘overflowing’ capacity, an Loading 61
Fig. 3.6
Fig. 3.7
allowance is automatically allowed for the self-weight of the tank (a load rarely provided to the designer). If the overflow from the tank fails and the tank fills to its maximum then it is reasonable to assume that this ‘flood’ overloading is only of temporary duration and that a medium duration of load factor is appropriate.
Normally the critical design condition for the supporting structure occurs with the tank full (a long-term loading condition). If the tank is placed to give unbalanced loading on a component (see section 3.10), the act of emptying it could lead to reversal of stress.
3.12.3 Roofs under high towers or masts
Roofs under high towers or tall masts have been penetrated by icicles which form then break off. This potential problem should be discussed in the design of such roofs and possibly with regard to the location of the building itself.
3.12.4 Accidental loadings and disproportionate collapse
3.12.4.1 General
The various Building Regulations operating in the UK call for special procedures to be employed to guard against building failures disproportionate to the cause of the failure where buildings are more than four storeys in height. It would be un- reasonable to expect a normal structure to survive an atomic blast but a localized internal explosion or a vehicular impact would come within this consideration.
Until the early 1990s timber-framed buildings were limited to three storeys in height by fire requirements. Changes in the UK Building Regulations now allow timber constructions of five storeys in Scotland. In England, Wales and Northern Ireland the practical limit is eight storeys. This topic is therefore an essential design consideration for the taller buildings.
The designer’s tasks have also been made more onerous with the introduction of The Construction (Design and Management) Regulations 1994which requires consideration of the safety of personnel not only during construction but while routine maintenance is being carried out.
3.12.4.2 Disproportionate collapse
The Building Regulations for the UK offer three ways of satisfying this statutory requirement:
(1) by providing adequate horizontal and vertical ties through the structure;
these forces are given in the relevant design code of practice;
(2) with horizontal ties being effective, but not vertical ties, to remove the ver- tical supports one at a time to check by calculation that the remaining structure can span over the removed member;
(3) where neither horizontal nor vertical tying can be achieved, each support member should be removed one at a time and on its removal the area at risk of collapse should not exceed 15% of the area of the storey or 70 m2, whichever is the less.
Options (1) and (2) are applicable to framed buildings, i.e. one relying on the strength and stiffness of beams and columns. It is unlikely that a timber building of five storeys or more will be a framed structure as the racking resistance required for normal loading conditions will generate a panellized, platform frame type of construction. Vertical, storey height diaphragm walls with continuous edge floor members providing the flanges of a vertical beam have been shown to perform exceptionally well in disproportionate collapse tests giving compliance with option (3).
3.12.4.3 Safety during construction and maintenance
The kind of problem that has to be considered is best illustrated by an example of a problem that occurred when CDM was not even a twinkle in the eye of the Health and Safety Executive (indeed the HSE did not exist). A very large (5000 m2) single-storey, open plan, flat-roofed building was queried by the Factory Inspectorate with regard to stability if at any time during construction or subse- quently one of the internal, free-standing columns was removed. The Inspectorate had in mind, particularly, a dumper truck colliding with a column during the construction stage.
A timber-framed building has little ‘intentional’ continuity. For example, there are considerable continuity effects with insitu concrete that may be enhanced by the introduction of reinforcing bars and the bolted joints in steel construction afford horizontal and vertical ties. Precast concrete, on the other hand, can have the same problems as timber and this was the probable reason why the Inspectorate took such an interest in the timber building described above.
A timber-framed building, on the other hand, has considerable ‘coincidental’
continuity through overlapping timbers and sheet materials as well as the nailed, screwed and bolted joints. These elements can be developed as often none will nor- mally have been considered as contributing to the overall stability. The solution to the particular problem described was to arrange for the sheet of plywood in the roof construction immediately over a beam to ‘straddle’ the beam and for this sheet to be site fixed. This meant that the ends of the roof joists bearing on the beam were exposed and could be spliced by working from above. The plywood decking was then completed which further added to the continuity of the roof construction.
If a column was removed it could be shown that the remaining roof would hang as a catenary from the adjoining beams. The duration of load in this condition would be short term and provided the dumper driver did not panic and remove other columns then the building would survive this accidental loading.
By careful consideration of the layout of the component parts of a timber build- ing it is possible to insert continuity ties between joists and beams and also to make sure that sheathing can be arranged to bridge discontinuities in the support- ing structure.
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