07.DEC.07

by Desmond Pompey

Engineer Buildings Maintenance and Construction – Ministry of Transport and Works

Introduction

People said the buildings in Kingstown, St.Vincent, were swaying from side to side and moved up and down during the earthquake last Thursday, November 29, 2007. Your house or building moving like that during an earthquake is not necessarily a bad thing. Well, for one thing, it provided the necessary warning, and people wisely evacuated the buildings and moved to a clear area.{{more}} The building’s flexibility (deformity and ductility properties) performance could be its seismic/earthquake resistance design ability. Of course, it will not be good for this response to continue for a long period nor for the displacement to be too great. This could result in eventual collapse.

Earthquake Resistance Design Properties

It is desirable that homes and other buildings exhibit properties of ductility, deformity and damageability. The whole point of such earthquake or seismic design is to ensure the health, safety and security of the occupants inside. Ductility and deformity are interrelated concepts signifying the ability of the structure to sustain large deformations (twisting and swaying) without collapse.

Timber and steel buildings are inherently ductile. However, as illustrated in the Building Guidelines, bracing will be required for steel and timber structures. Your building ductility property depends on these materials; they are not brittle so as to break without warning. Ductility can be introduced into concrete buildings on the other using steel reinforcement in the right proportion (i.e. by design). It is important the beam and columns connections be properly reinforced. Pay attention to spacing of stirrups and links – see guideline illustrations.

Deformity is the ability of the structure to be distorted/deform substantial amounts without collapsing. While deformability relies on the inherent ductile materials, deformability requires that the structure be well proportioned and tied together to reduce excessive stress concentration. The guidelines provide information and illustrations regarding the tying of foundations, ring beams and walls or columns from foundation to roof.

Damageability is the ability of the structure to undergo substantial damage without collapse. Good damageability is achieved by providing additional support. This is termed as redundancy. An example is to prop a cantilever.

The St. Vincent and the Grenadines Building Code and Guidelines consider earthquake and hurricane resistance of structures. The guidelines are for structures up to 2500 square feet and two stories. The building codes are for larger structures and a design professional should be consulted. For residential construction, however, the building guidelines earthquake considerations indicate that the desirable resistance properties can be achieved by vertically reinforcing column and external walls from floor to ring beam. Foundation beam, column, wall, stairs, floor and roof slab reinforcement illustration details are provided.

Other considerations



There are simple things a home owner can pay attention to when building a house (buildings with in the scope of the guidelines).



l Building Shape: It is, therefore, important who you choose to design you new house. The designer determines the conceptual design of the building and in so doing largely determines the type and effectiveness of the earthquake resistance systems which can be used. The guidelines indicate that a simple rectangular building with no projections performs well. Rectangular buildings with interconnecting cross walls are inherently stronger.

l Soil and site location: the type of soil and location may have a significant effect on the resistance of your building to earthquake. I do not think, however, that for small residential structures, the impact is significant, provided that the building is not constructed on loose saturated sand, which may liquefy during an earthquake and cause the building to collapse.

l Effect of height seas: Coastal areas may suffer due to high waves and there have even been tsunamis in some areas of the globe.

l Appendages: extreme care must be taken to secure appendages. I also prefer not to have long cantilevered floors or balconies spaces.

l Openings: Attention must be given to the size, number and location with walls. Generally, openings in walls (doors, windows etc.) reduce its ability to resist horizontal/lateral forces due to earthquakes.



Other Design Strategies



Earthquake motion can be from side to side or up and down or a combination. This creates forces in the building which in turn causes most seismic damage.

Simply: Force = Mass x Acceleration



The greater the mass (or the heavier the building) the greater are the forces on the building.

Another important consideration is the fundamental period of vibration of the structure or building. If the ground vibrates at a period equaling the fundamental frequency of the structure/building then resonance occurs and increases the seismic effect.

Structural elements designed into the building to resist earthquakes include:

l Diaphragms (floors and roofs) transfer lateral/horizontal forces.

l Shear walls or strategically located stiffened walls to transfer horizontal forces from floors and roof to the foundation.

l Bracing or braced frames to transfer horizontal forces from floors and roof s to foundations.

l Strong columns weaker beams: The question could be asked, which should fail first during an earthquake, the beam or the columns? Failure does not mean immediate collapse. The structural beam member may crack but adequate steel reinforcement present can hold it in place until eventual collapse, giving occupants time to evacuate. But it stands to reason that if the columns fail there is a greater chance that the building will tumble down more quickly. Therefore, designers must ensure that there are strong columns

l Other strategies include the use of moment resistant frames, energy dissipating devices and base isolation technology.