Types of Superstructures

Most commonly used reinforced concrete structures for multistory buildings are of three types:

• Frames

• Structural wall systems and

• Dual systems.

The selection of structural system for a building is primarily determined by the function and architectural considerations, by the total height of the building and by the seismic exposure.

5.2.1 R e i n f o r c e d C o n c r e t e F r a m e s

Frames for buildings are spatial (3D) systems of beams and columns rigidly intercon-nected in joints.

Figure 5.2 General configuration of a frame structure

The structural configuration and proportions, methods of analysis and design are strongly influenced by the building seismic exposure. According to this criterion, broadly, two types of frames exist:

• Frames of buildings situated in non-seismic or very low seismic areas (“non-seismic frames’’) and

• Frames predominantly designed to resist seismic actions.

The design principles and methods of frames subjected to high intensity seismic actions cover a significant part of design problems of high-rise multistory frames located in non-seismic areas.

G e n e r a l L a y o u t o f F r a m e S y s t e m s

The most used frame structures have the columns disposed according to the nodes of a rectangular regular mesh (Fig. 5.2).

Span and bay are chosen according to architectural and functional requirements.

This means that the cells determined by four columns should fit generally with the requirement of column free room.

Generally, span and bay length are less than 5.00 m for residential buildings. For office buildings or other similar facilities like hospitals, the frames’ span is larger and can be as much as 9.00 m or even longer.

There are also special framed structures for show buildings (cinema, theaters, opera) or similar.

When subjected to lateral forces, the building acts as a vertical cantilever. The can-tilever effect generates important global shear forces and overturning moments. They act the frame system considered as a whole, which has to transfer them to the foun-dation soil. Since the most advantageous cross section shape for a flexural element is the close tubular section, attempts to obtain a “tube effect’’ for framed structures have been made. This has been realized providing close spaced columns (at 1.50–2.50 m) along the building perimeter interconnected through very stiff spandrel beams. The resulting structures are tubular frames (Fig. 5.3a).

The concept of tubular frame has been extended for super-tall buildings providing a supplementary internal framed (or wall) tube obtaining tube-in-tube frame structures (Fig. 5.3b). Multiple tubes have also been used (Fig. 5.3c).

Figure 5.3 Different types of tubular frames

5.2.2 W a l l S y s t e m s

Multi-story frame structures subjected to seismic actions have a limited use for high-rise buildings due to large columns’ sizes required at their bottom zone. Consequently, an important part of the building area at bottom floors is not functional.

Search for alternative structural solutions, based on use of vertical elements with high stiffness and resistance, was a condition for extending the reinforced concrete structures for high- and super-high-rise buildings.

One of the most important solution, which in early 1950s, was the use of structural walls as an alternative solution to the traditional frame structures. The idea was to con-vert partitions into structural walls. Due to their cross section shape and proportions, the walls have a high “natural’’ stiffness and a substantial resistance capacity.

T y p e s o f S t r u c t u r a l W a l l s

An ideal solution for structural walls is that of compact solid units called cantilever walls (Fig. 5.4). Cantilever walls are easy to be executed and lead to structures with unambiguous load paths. The most suitable cross section for cantilever walls is rectan-gular having or not reinforced boundaries. More solid walls provided on perpendicular directions are sometimes crossed forming elements with different section shapes: T, L or tubular.

In many situations, openings in structural walls are required to accommodate doors or windows. We call the resulting component structural wall with openings. When openings are provided in uniform regular pattern over the wall height the unit can be considered to be formed by two or more solid cantilever walls coupled through rigid beams (Fig. 5.5). These are coupled structural walls. Besides functional advantages (in comparison with cantilever walls) the coupled walls, appropriately designed and detailed, showed a stable, controllable energy dissipating capacity during high intensity earthquakes. Sometimes, a staggered pattern of opening is preferred. When openings are suitable arranged, with large space between them, the wall has a good stiffness and appropriate reinforcement can be provided to be effective in preventing shear cracks.

Both cantilever and coupled walls can be performed by cast-in-place or by prefab-ricated concrete. Inter-connected rectangular prefabprefab-ricated units called large panels, realize prefabricated structural walls.

Figure 5.4 Types of cantilever structural walls

Figure 5.5 Structural walls with openings: a), b), Coupled walls c),Walls with staggered openings Structural walls supported by columns at the first floor have been used in the past, in order to obtain architectural freedom (large rooms) at this level. These are walls with weak and soft story. They evidenced very poor seismic behavior and nowadays are abandoned.

5.2.3 D u a l S y s t e m s

Structures made by frames associated with structural walls are called dual systems.

The dual systems combine the advantages of both frame and structural wall systems.

Two types of dual systems are defined by codes:

• Frame-equivalent dual systems are those for which the shear resistance of the frame system at the building base is greater than 50% of the total shear resistance of the whole structural system and

• Wall-equivalent dual systems in which the shear resistance of the walls at the building base is higher than 50% of the total seismic resistance of the whole structural system.

Figure 5.6 Examples of dual systems

5.2.4 A d v a n t a g e s a n d D i s a d v a n t a g e s . O p t i m u m U s a g e

The most significant advantage of frame structures in comparison with structural walls systems is the freedom, from architectural and functional points of view, in space use.

The frame structures allow obtaining large rooms free of structural elements. One says that the “flexibility’’ of the building’s architectural configuration is the main advan-tage of the frame structures. This feature is important and useful for hospitals, office buildings, schools, garages but it is less significant for hotels or residential buildings.

Main disadvantages of framed structures are related to the large size of their com-ponents – columns and beams – necessary to comply with strength and stiffness requirements specific to tall multistory buildings. This disadvantage limits the use of pure framed structures for high-rise buildings.

Having large cross section area and substantial lateral stiffness the structural walls seems to be an excellent solution for multi-story buildings. However, obvious structural requirements oblige to keep the walls continuous over the building height. Thus, their position is fixed throughout all building stories. Accordingly, the architectural layout is “frozen’’ over the building height during its lifetime (minor changes can be made only within cells delimited by structural walls). This is a significant restriction for use of structural walls for high-rise buildings. For this reason, the structural wall systems are suitable for apartment buildings or hotels but not for hospitals, offices or other type of buildings which require large open spaces.

Combining the advantages of both framed and wall structures, the dual systems considerably extend the use of reinforced concrete for multistory buildings. The key of developing optimum solutions with dual systems is the harmonious working together of architect and structural engineer. Seeking for best building configuration which complies with functional, esthetics and structural requirements, finding appropri-ate location for structural walls within the building layout, solving correctly the loads transfer to the soil through well-conceived infrastructure are tasks of crucial importance for designers’ team of high- and super-high rise buildings. Dual systems are in many cases the best solutions for tall buildings with various destinations.