Expert: Jeanette A - 9/28/2008

Question
after an earthquake,what is the best way in general for specifying measures for structural strengthening

Answer
Hello Liaqat,

The main innovative methods of seismic retrofitting may be grouped into the following categories


• Stiffness Reduction

This is a period elongation and consequent reduction of the seismic action in affect this reduces the seismic strength demand. It may be achieved by the principle of springs in a series where the equivalent stiffness of the two springs in series is smaller than the single springs. Significantly low stiffness could result in large displacements which might conflict with the function of the structure and hence cause damage to non-structural components. This method is not effective in long period structures or structures on soft soils.
• Ductility Increase
This may be achieved locally by means of confining reinforced concrete flexural and compressed structural members. This method although traditional, may now be applied with the use of new materials such as Fibre Reinforced Polymers (FRP).


• Damage Controlled Structures

Damage Controlled Structures (DCS) has been an important development in engineering over the last 10yrs. This method involves two parallel structures; a primary structure and an auxiliary structure. The primary structure behaves elastically under the most severe design earthquake whilst the auxiliary structure responds to the seismic action. The auxiliary structure establishes a stiffness increment and a large energy dissipation capacity. With these two components, the only damage incurred applies to only the auxiliary structure where the damaged elements may be replaced after the event of an earthquake. This makes the structure remain operative even under the most severe design earthquake. The primary and auxiliary structures are in parallel therefore the total deformation is the same. The primary structure can deform elastically until the inter-story drift is 1/100. Plastic deformations happen in devices which have been designed not to affect the primary structure. This method may be applied to new and old structures.


• Composite Materials

Masonry has always been unfavourable in regards to earthquake prone areas. However, composite materials have made this material attractive. Crack opening plus degradation in strength and stiffness is the major limitation for the use of masonry in seismic regions. Application of composite laminae to masonry panels gives strong traction resistance which limits crack extension and width. Therefore, strength and stiffness degradation is reduced, if not completely removed.


• Active Control

This method involves the use of servo-actuated devices with apply opposite forces to the seismic action. It requires sensors to read the motion of the structure followed by hardware & software to evaluate the forces to be applied to minimise the structural response. For the equipment to continuously function there must be an energy source available at all times. This method is still undergoing research and no actual application exists as of yet.


• Any suitable combination of the above methods


Hope this answers your question.


Warm Regards,
Jeanette