Slide Homes built with reinforced concrete walls have a record of surviving earthquakes intact and remain structurally sound. ICFs Remain Strong,
Even in Earthquakes

Earthquake Resistant

ICF Structures Are Stable, Even in an Earthquake

Why Buildings Survive

First, it’s important to identify what makes a building strong enough to withstand an earthquake. The three most important properties for earthquake resistance are stiffness, strength, and ductility. 

Optimal design conditions include shear walls that extend the entire height and located on all four sides of a building. The reason being, long walls are stronger than short walls, and solid walls are better than ones with a lot of opening for windows and doors. These elements are designed to survive severe sideways (in-plane) forces, called racking and shear, without being damaged or bent far out of position. Also, shear walls also must be well anchored to the foundation structure to work effectively. Properly installed steel reinforcing bars extend across the joint between the walls and the foundation. This provides a secure anchorage to the foundation.

Scientists study damage from earthquakes to determine what types of buildings best withstand seismic forces. The modern earthquake-resistant design relies on research studies conducted by the University of California Berkeley, Stanford University, the National Institute of Standards and Technology, and the NAHB Research Center. These studies of earthquake damage consistently show that well-anchored shear walls are the key to earthquake resistance in low-rise buildings.

Why Buildings Fail

When a low-rise building fails during an earthquake, it’s typically because it didn’t meet the necessary stiffness, strength, and ductility to resist the forces of an earthquake, had walls that were not well anchored to a solid foundation, or both.

Research studies identified which building types sustained the most damage and how the damage was caused. These were the results:

  • Multi-story buildings with a ground floor consisting only of columns:
    • Most of these buildings were 3 to 4 stories tall with a parking garage or a living area with many large windows on the ground level. While the columns may have been strong enough to hold up the structure, they did not provide an adequate amount of racking resistance during a seismic event. As a result, when the earthquake shook the building side-to-side, the upper stories sometimes tipped over to one side. Whether built of wood, steel, or concrete—they all suffered damage.
  • Wood-frame houses with weak connections between the walls and foundation:
    • Wood-framed buildings are inherently ductile (flexible), which is an attribute during an earthquake. However, the shaking sent some of these houses sliding to one side. Frequently, the shear walls were strong enough, but the connection to the foundation was a weak point that gave way.
  • Un-reinforced masonry or concrete buildings:
    • Masonry or concrete walls not reinforced with steel bars were not ductile enough to be effective shear walls. And if there is no steel connecting them to their foundation, the joint between walls and foundation can be a weak point.

Why Reinforced Concrete is Safe

Reinforced concrete walls are a composite system: Concrete resists compression forces, and reinforcing steel resists tensile forces produced by an earthquake.

The concrete is cast around the steel bars which locks it in place. The exceptional ductility of the steel to resist tensile forces, coupled with the rock-like ability of concrete to resist compression. This exceptional durability results in an excellent combination of the three most important earthquake resistance properties: stiffness, strength, and ductility. In fact, a study at Construction Technology Laboratories revealed that even a lightly reinforced concrete shear wall has over six times the racking load resistance as framed wall construction.

In the end, it’s no wonder that modern reinforced concrete buildings were found to be earthquake resistant with rarely any significant damage.

(Source: Portland Cement Association’s Concrete Homes Technology Brief 10.)

Concrete Homes Stand Up to Earthquakes

In an ICF building, the combination of concrete and steel provide the three most important properties for earthquake resistance: stiffness, strength, and ductility.

In conclusion, ICF buildings can be among the safest and most durable types of structures in an earthquake. As a result, homes built with reinforced concrete walls have a record of surviving earthquakes intact and remain structurally sound.

Click here for a detailed comparison of ICF walls and wood frame walls.

Earthquake Resistant July 9, 2013