Saturday, March 19, 2011

Materials Science in Seismic and Earthquake Engineering

Toronto's CN Tower was one of the first skyscrapers built with "mass dampers" for increased structural stability and earthquake resistance (Source).
A topic currently on the minds of many people, I thought in this post I'd take a look at material science in seismic and earthquake engineering.  In general, successful engineering for earthquakes, tsunamis, and other natural disasters requires intimate knowledge of the landscape, the likely magnitude and type of natural disaster, as well as the likely failure mechanism(s) of the building or structure.  In this regard, Japan has become one of the world leaders in earthquake engineering research, and possesses a full-scale "shaking table" at the Hyogo Earthquake Engineering Research Center [1].  The table is used to test and investigate failure modes of full-scale buildings and structures (see here for testing videos).  In order to engineer a building or structure so that it may withstand an earthquake and associated effects - for a given landscape, magnitude, and failure mechanism(s) - a number of solutions are being developed which combine architecture, civil engineering, and materials science.  (For a more general outline of these solutions, see Wikipedia.)

Springs and frequency "dampers" act to reduce the potentially catastrophic resonant coupling between external forces (e.g. from an earthquake) and the building or structure.  Because springs and dampers undergo cyclic loading, the materials used to make them must possess i) consistent cyclic mechanical properties such as high fatigue strength and low hysteresis, as well as ii) environmental stability, e.g. resistance to corrosion and embrittlement.  Friction pendulum bearings (FPBs) and metallic roller bearings operate cyclically like springs and dampers, but with the added complexity of contact dynamics.  In particular, the use of friction, which generates heat, further reduces the energy transferred to the building or structure.  In addition to high fatigue strength, low hysteresis (in some cases), and environmental stability, the materials designed for use in these bearings must also possess high wear resistance, high thermal stability, and resistance to spall formation. 

Other approaches make use of more fundamental and advanced material science concepts.  For example, Lead Rubber Bearings (LRBs) (developed in the 1970's [2]) are short blocks that consist of a core made of Lead surrounded by a rubber skin.  By utilizing strict lateral (shearing) deformation of these blocks, the building or structure above is effectively isolated from the ground below (see test video here from UCSD).  Lead is compliant (has low Young's modulus) and is very ductile, which translates to a large amount of energy that can be absorbed in the bearing during its deformation.  Lead also possesses a great deal of density (or inertia) to assist with mechanical damping.  Like the Lead component, the rubber is also compliant; but its higher elastic limit (strength) maintains the bearing shape and keeps the cyclic properties intact (i.e. low hysteresis).  The combination of these material properties in LRBs has lead to some significant improvements in seismic energy attenuation [2].

In contrast to LRBs, hysteretic dampers make specific use of materials' non-linear mechanical properties.  For example, viscous fluid dampers for structures are similar to automotive shock absorbers - they consist of a piston filled with a fluid that possesses non-linear viscosity (e.g. oil or honey, but not water).  Viscoelastic dampers behave similarly but consist of largely solid materials with visco-elastic (i.e. non-linear and hysteretic) mechanical properties.  These (or metallic dampers or yielding dampers) transform the non-linear deformation energy into an alternate form, such as heat, which can be dissipated.  A subset of metallic dampers are "superelastic" dampers made of shape-memory alloys.  Shape-memory alloys demonstrate complicated and temperature-sensitive mechanical behaviour - a thermal treatment will return a deformed piece to its shape before deformation - which has been utilized to further dissipate energy as part of a seismic damper, e.g. [3-5].

There is considerable and growing research and development in the area of materials with novel properties for seismic control and earthquake engineering - see reviews [3-5].  A great deal of this work has been done and is underway in Japan, among other earthquake-prone countries.  Engineering solutions will continue to address the diverse and unique landscapes, types and magnitudes of disaster, and failure mechanisms.  In general, however, and as I've written in the case of Haiti, the most appropriate engineering solutions must also consider geographical location, available resources, long-term outlook, and, of course, cost.

[1] Hyogo Earthquake Engineering Research Center.  Facility website:
[2] Robinson Seismic Ltd.  Company website:
[3] S. Valliappan and K. Qi.  Review of seismic vibration control using 'smart materials'.  Structural Engineering and Mechanics, Vol. 11 (2001), pp. 617-636.
[4] P. Towashiraporn et al.  Passive control methods for seismic response modification.  Progress in Structural Engineering and Materials, Vol. 4 (2002), pp. 74-86. 
[5] Y.M. Parulekar and G.R. Reddy.  Passive response control systems for seismic response reduction: a state-of-the-art review.  International Journal of Structural Stability and Dynamics, Vol. 9 (2009), pp. 151-177.

No comments:

Post a Comment