STATIC VERSUS DYNAMIC
Many older houses have had very little, or no seismic design consideration. Most newer houses are designed using Linear Static Procedures (LSP). This is not a true picture of the structural behavior because the house does not respond in a linear manner during an earthquake and the earthquake is not a static event.
Some existing housing is being retrofitted, but there is no controversy amongst professional engineers, professional home inspectors and those contractors knowledgeable in the proper methods for retrofitting a house that much of the retrofitting is being done wrong and is of little or no benefit. This is partly due to the fact that building inspectors, who are normally placed in a position to safeguard our health and safety, have not been provided with that responsibility for the adequacy of earthquake repairs.
TECHNICAL ISSUES AND CONTROVERSIES
MISTAKES ARE COMMON
According to the International Existing Building Code, 2009, the most common structural seismic deficiencies are the following:
1. Lack of a continuous Load Path.
2. Soft or weak story structures.
3. Brittle or deteriorated materials.
4. Deformation incompatibility.
These weak areas were expanded in the same code in 2013 to include:
1. No viable foundation system.
2. Pier and Post foundation.
3. Noncontinuous foundations.
4. Unreinforced masonry.
5. Unbolted foundations.
6. Unbraced cripple walls.
Failure to understand these issues is the reason why so many mistakes are made during retrofitting.
REDUNDANCY AND COLLAPSE
Buildings collapse due to instability. Instability is caused by the lack of redundancy. Houses, due to their design, have very little redundancy. A significant number of seriously damaged houses or collapsed structures can result in what is referred to as the Ninth Ward problem. In New Orleans after this area was badly damaged, no one wanted to live there or to rebuild.
The response of structures to an earthquake follow the rules of a technology called vibration theory. The response of most houses is a fairly simple and quite easy to determine pattern called a fixed free cantilever. This is similar to a flag poles response to someone pushing and pulling on it near the base. Every house has its own natural frequency in each of the two horizontal directions, a weak direction and a strong direction. Problems occur when the weak direction natural frequency is matched up with the strong motion frequency of the earthquake. This is called amplification.
THE DANGER OF RESONANCE
The house structure responds to the earth quake wave motions of the ground surfaces by flexing back and forth for the duration of the earthquake like the cantilevered beam shown above. Analysis of the model determines the natural frequencies of the structure. These results are compared to the frequencies of the Strong Motion Acceleration Frequencies of the earthquake. From this data, it is possible to evaluate the likelihood of frequency matching, which would cause an increase in the response of the house. This is called Resonance and ths infomation is used to determine whether the house has the capacity to withstand the design earthquake event designated for that particular site.
EARTHQUAKE INTENSITY & DURATION
Currently, the earthquake intensity is based on the Magnitude of the Earthquake (Mw). This system has replaced the older Richter Scale system because it more readily reflects the amount of energy released. The expected intensity Mw is based on the location of the property and its position amongst the known active faults in the area. The USGS has done extensive statistical analysis of past earthquake activity and now provides maps and data on the expected intensity of future earthquakes in all areas of the United States. The Duration of the earthquake depends on the length of the fault rupture. The longer the fault rupture the longer the duration, and the larger the number of Strong Motion Acceleration Cycles the house will see. If there are critical local flaws in the seismic design, the house continues to sustain damage during the earthquake. And, in the end if the house does not collapse, it might sustain enough damage that would not allow continued occupancy. This is what is referred to as getting "Red Tagged". This designation is made by local code enforcement agencies and their technical engineering support groups. The goal for any retrofitting should be to attempt to maintain a Shelter in Place status following the earthquake.
CAPACITY VERSUS DEMAND
Just as economics is governed by the law of supply and demand, a house is judged by the Law of Capacity and Demand. Does the house have the inherent strength and capacity to withstand the demands placed on it during an earthquake. The inherent strength in a wood house is quite good due to the flexibility and mechanical properties of the materials. Their ability to absorb energy, during the flexural movements caused by an earthquake is quite remarkable. The ability to absorb the damaging movements of the ground is based on the number and proper design of what are termed Lateral Force Resisting Elements (LFRE's), which protect the house. A properly designed or retrofitted home will have an excess of LFRE's in order to meet the demands placed on it during an earthquake.
Model of House
Fixed Free Cantilever
Capacity vs Demand
Soft or Weak Story
Garage Under Living Area
OF A HOUSE
During an earthquake, a house resting on the ground surface, behaves like a fixed-free beam that is supported and fixed at its base on the ground and is free to shift back and forth at its upper elevations. It is a motion similar to that shown below with the fixed plate being the ground.
Simplified Model of a House
5.0 2 seconds
5.5 6 seconds
6.0 12 seconds
6.5 18 seconds
7.0 24 seconds
7.5 30 seconds
8.0 34 seconds
8.5 37 seconds