A wood-framed building with soft, weak or open-front walls is a multi-story wood structure where the first story is substantially weaker and more flexible than the stories above. This is typically due to lack of walls or frames at the first floor. These buildings were commonly designed prior to the 1980's and contain large open areas for tuck-under parking or commercial space, such as restaurants or convenience stores. 

Buildings with a soft-story condition are particularly vulnerable to severe damage and collapse during an earthquake. Without proper strengthening, the soft-story floor may suffer structural damage or complete failure during or after an earthquake. 

View Existing Wood-Frame Buildings with Soft, Weak, or Open-Front Walls Presentation

Non-ductile concrete buildings are structures that contain brittle concrete elements (columns, beams, walls, and connections) that tend to perform poorly during earthquakes due to the limited amount of provided reinforcing steel. In a concrete building designed per today’s code standards, the reinforcing steel is designed to act as a strong cage, holding the concrete intact inside of the elements during an earthquake and allowing it to continue to support the building weight.

 In a non-ductile concrete building, typically designed using older pre-1980’s building codes, the elements have too little steel reinforcement to form a confining cage. As a result, the concrete element cracks and crushes, losing its ability to continue to support load as the concrete pieces spall or fall out of the cage. The building consequently may partially or fully collapse because of a large earthquake.

During the 1994 Northridge Earthquake in the Los Angeles area, many steel moment-resisting frame structures experienced unexpected brittle damage to their beam-to-column connections. Steel moment frames resist lateral earthquake loads primarily through bending of the beams and columns so the joint between these two elements becomes critical in their performance. Experts discovered that these buildings suffered from a wide range of flaws resulting from shortcomings in established design and construction procedures, including poor basic connection geometry, poor connection welding materials, and inadequate quality control, among other factors. Some column fractures were even observed in several buildings at the beam-to-column connections.

Many of these buildings have not been retrofitted and may be susceptible to similar severe structural damage or even building collapse in a major earthquake. Since the Northridge Earthquake, building code revisions have changed the way these buildings are designed, detailed, and the way quality control during construction is performed to ensure the steel moment frames perform as intended.

View Pre-Northridge Steel Moment Frames and Non-Ductile Concrete Presentation

Rigid-wall-flexible diaphragm buildings are buildings that consist of concrete/masonry wall with wood panelized roof system. The connection between the wall and the roof is generally weak and during a major seismic event the probability that the roof to wall interconnection will separate is high and could also result in structural failure. Buildings built prior to the 1994 Uniform Building Code are considered at-risk structures because of the lower construction standards at that time.

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