Non-Ductile Concrete Buildings
 
OVERVIEW

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 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 theelements during an earthquake and allowing it to continue to support load. 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 it's ability to continue to support load. The building consequently may partially or fully collapse as a result of a large earthquake. This potential for damage puts the occupant's safety at a tremendous risk, in addition to reducing or eliminating its ability to function. This type of non-ductile concrete construction has caused the loss of lives during earthquakes including the Mexico City earthquake (1985), Northridge earthquake (1994), and Christ Church earthquake (2011). Buildings designed and constructed prior to the 1976 building code are usually non-ductile, even if they are categorized as "reinforced concrete" buildings on building permits.

 

 

Properly Reinforced Columns During an Earthquake

 

            1. A Reinforced Concrete Column with adequate steel reinforcement has
                many reinforcing bars which form a strong confining cage around the
                concrete core
of the column.

            2. Cracks may form on the column during shaking from an earthquake.

            3. After extensive shaking, many cracks have formed and some of the
                outer concrete may spall off, but the concrete core of the column
                remains strong
and intact due to the reinforcing steel that forms a
                tight cage around it.

            4. Although the column looks extensively damaged after the
                earthquake shaking has stopped, the column is still
                supporting the weight of the building above it.

 

 

Non-Ductile Column During an Earthquake

 

            1. A Non-Ductile Concrete Column has insufficient Steel Reinforcement
                to form a strong confining cage around the concrete core of the
                column.

            2. The concrete outside and inside of the column begins to crack and 
                crumble during an earthquake.

            3. Without the strong confining cage of steel to hold the concrete core of
                the column in pace, the column can no longer support the
                wright of the building above it, and cumbles.

            4. The building may partially or fully collapse.

 

 

Non-ductile concrete buildings must be retrofitted to improve the building's lateral force-resisting system and prevent extreme structural damage or collapse in the event of an earthquake. This can be accomplished by adding sturdy shear walls to the interior of the building, adding moment resisting frames like an exoskeleton, or by other structural improvements that correct the deficiencies of the building.

 

 

Image Source: Link