Principles to Consider When Designing Seismically

Natural catastrophes can't be avoided but the way our buildings react to them can be controlled. Countries with a seismic nature such as New Zealand, Chile, and Japan are at the vanguard of seismic design, but that doesn't mean other countries with fewer incidences should stay behind. Preparation against earthquakes will never be a waste of resources but rather a worthwhile investment.

Structural movements can happen for a variety of reasons; the most apparent and destructive being a tectonic movement––strong winds, heavy traffic on the streets near a building, occupation stress, construction work and demolitions in the vicinities can cause movement stress in a structure. While that kind of motion is not as catastrophic, with time, the structure weakens. These small seismic stressors can be avoided by designing seismically.

Following is advice to design seismically and enhance a building's resistance to any movement-induced damage. 

Structural Configurations

Placing the project’s materials in a way that promotes structural stability is a key element of seismic design. 

Placing diaphragms, usually made of plywood or OSB panels, on structural and non-structural timber walls can provide a building with room to move without causing important structural damage. Diaphragms bend and work as a supportive net for wall studs and insulation, providing more stability than a timber brace. Diaphragms also act as infinite braces, one next to the other. Roofs and floors can benefit from the placement of diaphragms as well.

Masonry walls benefit from confined configurations, a mix of reinforced concrete and masonry. In the event of a seismic occurrence, the reinforced pillars and beams of the wall will compress and keep the masonry in place. If the masonry collapses, the building will remain standing thanks to the reinforced concrete bones surrounding them, keeping the integrity of the building mostly intact. Reinforced brickwork, or hollow bricks reinforced with steel bars, gives the design an extra layer of structural stability, allowing it to resist compression and tension stress. 

Reinforced concrete behaves well in seismic events but its performance can be further improved by pre-stressing its internal steel bars.

Materials

Any material can be used for seismic design, provided it’s done properly. Materials must be tested for fatigue and compression; usually, this is done by the providers, but in some cases, these tests must be conducted privately in labs.

Wood, a cheap and renewable source, is the hardest material to calculate, however, it can be one of the best-behaved materials when it comes to seismic resistance, thanks to its individual strand internal configuration. Avoiding wood knots in beams and trusses can improve the overall stability of a structure. Favoring OSB or plywood elements, such as l-joists, over natural wood allows a more predictable structural behavior, with the added stability composite wood provides. Thanks to its layered strands, OSB is ideal for use as structural diaphragms and other structural elements.

Testing and Calculations

To understand how a certain material or structure will behave under seismic conditions, a series of calculations must be made. These conditions can be simulated in software programs where the material’s physical properties and the design’s structural configuration are tested under different conditions. 

Shape Configurations

A building’s shape can directly influence its stability in the face of a seismic event. Unstable shapes should be either avoided or thoroughly calculated. Sometimes damage can be unavoidable, but buildings can be designed to collapse in strategical points and avoid important structural damage.

Continuous bearing walls, uninterrupted from bottom to top, are crucial when it comes to designing highrise buildings. This feature is all the more important when including basements in the designs, and should be considered especially in basements including subterranean parking lots.

Soil Testing

Not all soils are created equal, thus it’s important to take the material's properties into consideration at the moment of building. The properties of the soil will define the kind of foundations to be used. The wrong foundation/soil combination could seriously damage a building’s seismic resistance, causing fissures and in the worst case: structural collapse. 

External Solutions

Sometimes a sound configuration and the right materials are simply not enough. Buildings such as skyscrapers need an extra layer of protection. Tuned mass dampers, like the one used in the Taipei 101 building, can be used to prevent damage either from the wind or seismic movements. These massive pendulums are encased into the skyscraper––taking up to a thousand square feet of space––and connected to the structure’s bones. These dampers cancel a seismic movement by applying force in the opposite direction. 

Smaller buildings can be benefited by the addition of lead-rubber bearings which work by separating the structure from its base, allowing it to move separately and decreasing foundational structural damage. Roller Bearings and friction pendulum bearings act similarly, allowing the building to slide back and forth safely during a seismic movement. 

Hysteretic dampers are another good seismic solution. These externally placed braces absorb seismic movements, stopping the structure from shaking. 

Have you tried any of these methods or have others you recommend? Comment below.