LoadingThe structure must be designed to resist the gravitational and lateral
forces, both permanent and transient that will be sustained during construction
and during the expected useful life of the structure (from 60 to 100 years).
Probability will be used to consider the simultaneous occurrence of different
combinations of gravity with either wind or earthquake forces. The limit states
method uses prescribed factors.
For dead loads, the construction sequence should be considered to be the worst
case. It is usual to shore the freshly placed floor upon several previously cast
floors. The construction loads on the supporting floors due to the weight of wet
concrete and its formwork will greatly exceed the loads of normal service
conditions. These loads must be calculated considering the sequence of
construction and the rate of erection. However, the designer rarely knows who
the contractor will be, nor his method of construction.
Strength and Stability
The primary requirement of the ultimate limit state of design procedure is that
the structure has adequate strength to resist and remain stable under the worst
probable loads during its lifetime.
This includes all critical load combinations, augmented moments from
second-order deflections (P-Delta) plus an adequate reserve; study all critical
members whose failure may lead to a progressive collapse of part or the whole
structure. Finally, the whole building must be checked against toppling as a
rigid body about one edge of the base. Moments are taken about that edge with
the resisting moment of the dead weight of the structure to be greater than the
overturning moment by an acceptable factor of safety.
The lateral stiffness is a major consideration in the design of a tall building.
Under the ultimate limit state, the lateral deflections must be limited to
prevent 2nd-order P-Delta effects from gravity loading to be large enough to
precipitate a collapse. In addition, serviceability requires these deflections
not to affect elevator rails, doors, glass partitions, and prevent dynamic
motions to cause discomfort to the occupants and sensitive equipment. This is
one of the major differences of tall buildings with respect to low-rise
The parameter that measures the lateral stiffness is the drift index. It is
defined as the ratio of the maximum deflection at the top of the building to the
total height. In addition, each floor has an index called the inter-story drift
index which checks for localized excessive deformation. There is no national
code requirement for the drift index. Different countries use from 0.001 to
0.005. For example, for an office building this would mean a range of 6 to 20
inches in a 33 story building. Lower values are used for hotels and condominiums
because the noise and discomfort at those levels are unacceptable. For
conventional structures, the preferred range is 0.0015 to 0.0030 (in other
words, from 1/650 to 1/350).
Buildings subjected to both lateral and torsional deflections (plus vortex
shedding and other usual effects) may induce in their human occupants from
discomfort to acute nausea. These are major factors in the final design of the
Creep, Shrinkage and Temperature
In very tall buildings, the cumulative vertical movements due to creep and
shrinkage may cause distress in the structure and induce forces into horizontal
elements especially in the upper regions of the building. During the
construction phase, elastic shortening will occur in the vertical elements of
the lower levels due to the additional loads imposed by the upper floors as they
are completed. Cumulative differential movements will affect the stresses in the
subsequent structure, especially in the building that includes both in-situ and
pre-cast components. Buildings subjected to large temperature variations between
their external faces and the internal core, and that are restrained, will
experience induced stresses in the members connecting both.
One of the most extreme conditions placed upon a building is fire. It is a
primary concern during design. Temperature range and its duration must be
estimated from its probable cause and the materials present in the building that
could provide fuel for its continuation. Also of interest are possible sources
of ventilation, and egress from alterative paths must be considered.
The behavior of the different structural components must be known. For example,
mild steel at 700°C is only 15% of the yield strength at 20°C, and its elastic
modulus drops to only 45% of its original value.
The Effect of the Foundations upon the Building
Minor movements of the foundations are greatly exaggerated by a tall building,
leading to very large inclinations of the tower. This topic is complex, and will
be treated later.