Stability Systems in Highrise Buildings

The last post was an introduction to the design of high-rise buildings, the main focus of that post was on the design considerations and how their design differs from low-medium rise buildings. It was discovered that the influence of lateral forces is critical and very crucial to the design of a highrise building. This post thereby highlights some of the possible structural systems that can be adopted in resisting these forces.

The structural options that are available can be sub-divided into four main categories:

Moment frames

Shear wall systems
Tube systems
Outrigger Braced-Systems

The selection of the most suitable structural system for a tall building is determined by factors such as, but not limited to, geographical location, building height, plan dimensions and planned use, as well as desired visual appearance and architectural specifications. Not all of these parameters are within the reach of structural engineers.

Highrise buildings will almost always depend on the lift and stair center for a large proportion of their lateral stability and lateral load-bearing capability. Structural engineers ought to pay special attention to the location, size and design of the core. Centrally placed cores are favoured, although they are not an absolute prerequisite. Positioning the core too far away from the centroid of the building might generate significant torsional stresses which might necessitate the use of other lateral stability systems.

Moment Frames

This is a fairly simple structural system, in which beams and columns are rigidly attached in two orthogonal directions to form moment-resistant frames, able to withstand lateral and gravity loads.

Each frame resists a proportion of the lateral load, determined by its relative stiffness compared with the frames total stiffness. In order to increase the structural height, the size of the frame components is directly increased to satisfy lateral drift and deflection limits

This structural system applies to buildings of up to about 75 m in height but is most economical for buildings of less than 50 m.

Shear Wall Systems

This system consists of shear walls designed to resist lateral forces in two orthogonal directions. Figure 2 shows a typical arrangement, with shear walls, arranged close to the middle of the structure to house lifts, fire-escape stairs and other building maintenance, thereby providing a rigid structural base to withstand horizontal loads in two directions.

It is also referred to as a ‘core system,’ the core is built to operate as a single vertical cantilever with enough lateral, torsional and bending rigidity to withstand the worst combinations of operation and ultimate conditions. A variation on this system involves the dispersal of additional shear walls uniformly across the building’s plan area. If this style is implemented, it is desirable to achieve a level of symmetry across the plan in the wall dimensions and positions to minimize twist of the structure.

The very high lateral stiffness of the walls compared with the remaining vertical elements (columns) ensures that lateral loads are fully resisted by the main shear walls for this typology of framing. The columns are then only designed to withstand gravity loads, simplifying the design process and the construction of the slabs.

Generally, this system is adequate for buildings up to 120 m tall – Although having shear walls larger and longer within the limits of the floor plan will reach considerably greater heights.

Tube Systems

This system allows the full width of the building footprint to be used to withstand the lateral loads on the building. It provides a very stiff structure but requires the structural elements to be arranged in some manner. Typically, columns are placed in relatively close centres of 2-4 m, connected by beams to create rigid frames around the perimeter. The resulting form is a closed tube that acts as a hollow vertical cantilever.

The framed tube system is suitable for buildings with heights up to approximately 150-170 m

Tube design variations include tube in the tube; packed tubes; and braced tubes, where diagonal braces are mounted on the outer faces. Such variants usually provide increased flexural rigidity and allow increased spacing of the perimeter columns. Buildings up to nearly 300 m in height can be realized by implementing these structural systems. The John Hancock Centre in Chicago as shown in figure 4 is one of the famous examples of braced tubes.

Figure 4: John Hancock Center in Chicago (Photo credit by E. Kvelland)

Outrigger-Braced Systems

The outrigger system is used when it is desirable to use the maximum width of the building footprint and to mobilize the perimeter columns as a fundamental part of the structural structure to achieve the greatest flexural rigidity.

This can also be achieved by adding horizontal outrigger elements (often trusses) of one or two floors deep, linking the core to the outside columns at regular intervals of height up the building. At the same level as the outriggers, sturdy exterior walls or trusses – also referred to as ‘belt trusses’ – can be up to two floors tall, linking the perimeter columns with the outriggers and distributing vertical loads.

This system is used to build structures with a height of up to 350 m, or super-tall buildings. Nevertheless, the braced system principle of using outriggers can be extended to much shorter constructions.

As shown in figure 6 the shard in the U.K is the tallest building in London and the sixth tallest building in the whole of Europe. It is a perfect example of a building that uses the outrigger system to resist lateral forces.

For most tall buildings, system combinations of the systems described in the preceding sections may be applicable. Generally, engineers should allow a reasonable amount of time to determine the most appropriate structural system for each case.