Low-damage design
Following the Canterbury earthquakes in 2010 and 2011, many commercial building owners were dismayed by the extent of damage to their properties, with even relatively modern buildings suffering extensive damage. The fundamental problem was the design philosophy in use when these buildings were constructed.
A U-shaped flexural plate between two rocking timber shear walls. (Stephen Hogg)
Around the middle of the 20th century, engineers began to realise that it was not practical to resist earthquake forces by designing buildings expecting only an elastic response. Instead, buildings were designed to provide a ductile response so they yielded in a controlled manner. From around the 1970s, buildings began to be engineered with ductile failure mechanisms in place. In the majority of commercial buildings in Christchurch, these mechanisms worked well. However, it rendered many of the city’s commercial buildings unusable and unrepairable after the earthquake had passed.
For any building, when seismic forces exceed the building’s design limits, damage will occur. However, the type and extent of the damage depends on the characteristics of the building. A building designed to have an elastic response may collapse when the forces exerted on it are stronger than its design limits. On the other hand, a building designed with a ductile or inelastic response may be badly damaged but can continue to withstand seismic forces that exceed its design limits without failing completely.
Low-damage design is a new approach to building earthquake-resilient structures. It not only focuses on preserving life in a major event but also on preserving the primary structure of the building, leaving it usable or easily repairable following an earthquake. Building owners, occupant businesses and insurers see obvious advantages in low-damage design.
Elasticity is the ability of a material to deform under load and return to its original shape and size when the load is removed. A building with this ability is said to exhibit an elastic response.
Damage limitation strategies
The three basic strategies for limiting damage to a building during a major seismic event are to:
- resist damage by increasing strength and stiffness
- avoid damage by isolating the structure from the ground
- reduce damage by dissipating earthquake energy externally to the structural frame.
The first option is the simplest and oldest method and has been used for thousands of years. The building is greatly over-engineered, making it as stiff and as strong as possible. The design limit may not be reached even by the strongest expected earthquake, and the building remains elastic and undamaged.
This is the approach used by many ancient monuments and is one of the reasons many still stand today. In certain situations, over-strengthening may be suitable in a modern setting as well, such as schools or important industrial buildings, but for most commercial buildings, it would be prohibitively expensive.
The second option is to isolate the building so that damaging seismic forces are not transferred into the structure. Even though the earthquake may be stronger than a design-level event, the full seismic forces do not reach the components of the structure, and the building’s limit is not exceeded. One way this can be achieved is through base isolation , which places bearings between the foundation and the bottom of the structure. During a seismic event, the isolation bearings effectively separate the superstructure from the foundation, limiting its displacement and thereby avoiding damage to the superstructure.
Isolation techniques are often combined with the third option, which uses special design techniques and damping devices incorporated at particular locations within the structure to increase the amount of seismic energy the building can dissipate. In this case, the dissipation further decreases the amount of seismic energy that is exerted directly on the structural elements, and damage is limited or only occurs at easily replaced components.
Residential timber construction
NZS 3604:2011 Timber-framed buildings, which is referenced by Building Code Acceptable Solution B1/AS1, is used to design most homes and other low-rise timber-framed buildings in New Zealand.
In contrast, typical engineered structures use fewer, larger and stronger members, each carrying much larger loads. However, the philosophy of light timber framing is the use of many small section timber elements of low structural capacity. This distributes the applied loads in conjunction with non-structural elements, such as linings and claddings. This approach builds redundancy into the structure in two ways:
- Failure of one member allows redistribution of loads through alternative load paths.
- Practical framing arrangements provide more structure than is strictly necessary (for example, walls that are ignored for bracing purposes).
This redundancy goes a long way to providing resilient buildings that are able to cope well with seismic loads.