# Factors to be considered when designing a tall building

The 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.

## Stiffness

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 buildings.

## Drift Limitations

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).

## Human Comfort

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 building.

## 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.

## Fire

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.