In general the higher a structure, the more expensive handling of concrete becomes. For this reason, extra attention must be paid to achieving maximum efficiency when there is a need to move concrete above first story levels. Correct choice of equipment becomes especially important. The two essentials to aim for: quick, easy erection and dismantling; and good delivery rates. High lift loaders can be used with special attachments to elevate concrete, but range is limited at the most to three stories. Maximum lift with currently available models is 40 feet; 20 to 32 feet is more usual. With such long lifts a rigid, and above all stable, frame and chassis are essential. Models with tilting upper decks are preferable because these permit safe and level concrete handling on uneven ground. Simple pulley arrangements, with makeshift scaffolding built on site, cannot be justified except for handling very small quantities of concrete. When a job is to be supplied by hoist, a properly engineered elevator tower should be used. A tower system usually consists of a central hopper at ground level, the tower itself, an elevator bucket and a mobile hopper at the level at which the concrete is to be placed. The concrete can then be transferred to hand or power buggies for distribution at the placement level. For most structural work, crane and bucket transport is a highly efficient means of handling concrete. The mix can be delivered directly from supply point of forms in a single operation without intermediate handling. The choice of crane is largely dependent of the desired lifting capacity, and on the required reach and degree of mobility. An efficient design is a cylinder with a center discharge gate of ample size. Self climbing tower cranes have revolutionized the placing of concrete for very high structures. The two basic designs of the tower crane are the counter balanced horizontal T boom type and the slewing type with luffing jib. Both types can operate from either a static foundation or a track mounted carriage. The luffing cranes, however, are mainly used outside the building.