Seismic vibrations that occur during an earthquake cause buildings to sway, which may dislodge them from their foundations and cause significant structural damage and even collapse, which in turn can cause a lot of damage to human life.
Structural damage depends on five factors:
The strength of the earthquake waves that reach the surface
The duration of the tremors
Proximity to the epicentre
Geologic foundation
Structural design and construction quality
Today, exactly 25 years after the devastating 1989 Loma Prieta earthquake, 6.9 on the Richter scale, that shook Northern California, Stanford University unveiled the design of a house (built and tested) that can withstand three times the intensity of that quake.
The house that Stanford built has two major modifications to stave off earthquake damage. First, it is not affixed to a foundation, but rests on twelve steel-and-plastic sliders, each about 4.5 inches in diameter. Under those sliders are either plates or bowl-shaped dishes made of galvanised steel. These units are called seismic isolators. Seismic isolators will make the building ‘slide’ when the ground is moving.
However, the isolators are designed such that the house will settle back into its original position once the tremors cease.
Second, it has a ‘unibody’ design, a term predominantly used in the automobile industry to imply that every element of design and construction contributes to its strength. Instead of simply screwing drywall to the wooden framing, as in typical construction, glue is used to affix extra-thick, 5/8-inch drywall more securely. On the outside, strong mesh and additional screws attach the white stucco tightly.
These elements make the house stiffer and stronger, leading to a significantly better seismic performance.
Testing the Design
The three-bedroom, 36 x 22-foot house was constructed in seven weeks atop a shake-table to test the design, one of the largest of its kind. First the isolators were tested.
The table was shaken at three times the intensity of the Loma Prieta, and the house stayed intact , though it slid from left to right. Next, they bolted the ‘unibody’ to the shake table, which outperformed the experimental expectations. Other than a few minor cracks in the stucco, everything remained intact. Even the test furniture in the house did not move.
Ready to Roll it Out
The modifications come at a very do-able and a reasonable price. Though existing homes can be retrofitted, this technology will be easy to incorporate in new construction. For a mere $10,000-$15000 and just four extra days of labour, contractors can add seismic isolators and a unibody system to a new 1500-2000 sq ft house.
This one-time cost is nothing compared to an average of $675 earthquake insurance that the home owners have to shell out every year. Contractors and home owners can start incorporating these modifications in the homes under construction immediately.
Very soon, these will be included in the building codes as well.