overturning loops below or at the soil surface shouldbe considered to anchor trees adequately. In general,the cost of supporting trees is high and, for economy,they are normally placed over column and/or principalsupport positions.

The features of landscaped areas will normally includeprovision for drainage, irrigation and access formaintenance. Owing to the difficulties of access toinspect membranes and the need to both irrigate anddrain landscaped areas, such areas will normallyrequire an enhanced waterproofing membrane and aroot barrier or positive measures to prevent structuraldamage by root penetration. The membrane must beadequately protected to prevent damage from

horticultural maintenance. The soil must be properlydrained, without being too steep, and falls of 1 :100to 1:40 would normally be used in conjunction with apositive drainage system to ensure the soil does notbecome saturated (see Figure 5.2). Loading mustconsider the effects of blockages to the drainagesystem and the increased loads resulting from storminundation.

The roof loading must allow for the maximum weightof vehicles that will be used for maintenance, such astractors and grass cutters, which can imposeconcentrated wheel and patch loading. Provision ofedge barriers, means of access, and means ofavoiding steep slopes requiring frequent maintenancemust be properly dealt with so that landscaped areascan be safely maintained.

Water features are often considered to be a desirablecomponent of a landscaping scheme. They alsoimpose loading significantly higher than normal carpark loading and, as with landscaping in general,must be carefully considered for each project.

The restrictions on loading, the requirements tocontrol growth, and the maintenance regime toensure loading does not exceed the design levelmust be clearly defined and agreed with the projectclient as part of the brief. This information is anessential part of the operational manual and healthand safety file.

5.2.10 Response of structure to vibration

Human perception of vibration in buildings variesbetween individuals. It also varies depending on theactivity being undertaken by those individuals.Stationary people are more aware of vibrations thanthose who are moving. Those people engaged indetailed operations, e.g. in hospital theatres or onprinted circuit boards, are particularly sensitive tomovement.

Empty car park structures lack the damping normallyprovided in buildings by partitions and finishings.However, the dynamics of most car park structuresare generally found to be satisfactory when thedesign gives natural frequencies above 5Hz, evenwhen compartments within the structure adjoiningthe car parking zones are in use as offices or forsensitive activities. Where car parks form part of amixed use development, measures to reducevibration of the interface or shared floors should be

Figure 5.1 Parc Hector Malot underground car park, Paris,showing a selection of landscaping features

Drainagematerial

Drain laidto fall

Structural slablaid to falls

Waterproofing layer/impermeable layer

Drainagelayer

Root barrier(permeable)

0.4m–1.2mtop soil/planting

Fall

Figure 5.2 Minimal roof planting details

42 The Institution of Structural Engineers Design recommendations for multi-storey and underground car parks – fourth edition

5.2 Car park design and construction