Types of Structural Systems

Buildings have to support their own weight (gravity loads) and also any horizontal (lateral) loads which might be imposed on them, such as wind or seismic loading.

In Japan, buildings are very widely categorised into three main types based on their seismic resisting capabilities and performance.

In principle, these categories can also be applied to other countries. However, they are always not appropriate in countries like the UK where there is almost zero risk of a large magnitude earthquake, and where the design of the structure may only be governed by gravity loads or wind loads.

Structural Systems

The three main structural systems are presented below:

Of these, Seismic-Resistant Structures are the ones most suspectible to damage to the main building frame, but usually of typical construction including braces and shear walls.

By adding Damping devices, such as oil dampers, the energy dissipation capacity of the building is significantly improved, and damage is largely limited to the damping elements. These are known as Vibration-Controlled Structures.

The final and most effective solution but challenging solution is to adopt a seismic-isolation system, typically at the base of the building (Base-Isolation). By doing so, the building natural periods become longer and more separated from the seismic ground motions. Therefore the building sees far smaller Dynamic Loads.

Seismic-Resistant Structure

In **seismic-resistance structure**s, the building relies only on the strength of the main structural members (beams, columns, shear walls, bracing etc.) to resist lateral loads. Therefore these elements must be designed to withstand large seismic forces.

In Japan, one of the most commonly applied systems is the steel special moment-resisting frame, which can follow large deformations and has significant ductility.

Vibration-Controlled Structure

Seismic-resisting structures usually have very small inherent structural damping which means that they cannot effectively dissipate energy. Vibration-Controlled Structures are usually just seismic-resistant structures with added vibration control elements.

By increasing the damping (energy dissipation capability) of the building, it is possible to significantly reducing the seismic demand on the main structural members and therefore size them to be much smaller.

※ What is Damping? Explained in 1-minute

Seismically-Isolated Structures

By adding an isolation system, the structure above can move more freely since it is allowed to "slide" on the isolation layer. The buildings natural periods (its natural movements) become much longer and shift away from the periods or frequencies where the seismic energy is strongest. 

The isolation layer will also usually have vibration control members to further reduce the extent of seismic forces experience by the building. It is also common to introduce steel dampers to improve the damping and provide improved restoring force capabilities to the building.

Because the seismic loads on the above building are much smaller, they can be sized much smaller than their conventional seismic-resisting counterparts, and may provide significant savings in the steel structure as well as improved performance. 

Other

As mentioned before, these categories are based on a seismic design paradigm. The other more common way to categorise buildings internationally (including in Japan) is based on their structural framing system. For example, a building structure might be based on a moment-frame construction, braced-frame construction, or a combination of both.

※ Different Types of Structural Framing Systems Explained

These classifications are not exclusive from the categories above. Rather, the three main types of structural systems mentioned above should be considered as a very broad categorisation for buildings in seismic areas.