Lateral loads affecting Tall Buildings - Tall Buildings: Structural Systems and Aerodynamic Form - page 9

laTeral loads aFFecTing Tall
Buildings
From the structural design point of view, tall (high-rise) buildings, because of their
extraordinary height, show a greater sensitivity to wind and earthquake induced
lateral loads than low-rise buildings. Estimating those lateral loads which play an
important role in the design of tall buildings is more difficult than estimating vertical
loads.
Earthquake loads increase according to the building weight, and wind loads
increase according to the building height. For this reason, wind loads, while they
are generally an unimportant issue in the design of structural systems for low- and
mid-rise buildings, play a decisive role in that of tall buildings, and can even be a
cause of large lateral drift (sway) that is more critical than that from earthquake loads.
Consequently, the occupancy comfort takes prominence in the design of structural
systems in tall buildings, and it is necessary to limit the building sway. In tall buildings,
which can be described as vertical cantilever beams, the maximum lateral top drift
caused by lateral loads is expected to be approximately 1/500 of the building height
(structural height), according to Bennett (1995) and Taranath (1998), and in limits
ranging from 1.5/1000 to 3/1000 according to Smith and Coull (1991). In this context,
the drift index is defined as the ratio of the maximum lateral top displacement of the
building to the building height (Δ/H); and the inter-storey drift index as the ratio of the
lateral displacement of the floor relative to the floor below, to the floor-to-floor height
(Δ/h). Generally in wind design of tall buildings, 1/400-500 is commonly preferred as
both the drift index and the inter-storey drift index.
2.1 wind loads
At first wind loads were ignored because the weight of the construction materials
and structural systems used in the first skyscrapers made vertical loads more critical
than lateral loads, but over time wind loads became important, as the strength to
weight ratio of construction materials and the ratio of floor area to structural weight in