Design and Materials for Low Cost Housing for Earthquake Resistance

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The World Population And Earthquake Facts

Note: The techniques and materials discussed in this article are for general information only. Seismic building design is a complex engineering undertaking and should only be performed by consulting qualified structural engineers.

A few interesting earthquake facts as related to the world population: While approximately 20% of the world population lives in or near earthquake zones, more than 70% of that percentage (a staggering one billion people) is represented by low income or poverty level socio-economic classes. The statistics show that countries with high population density and lower standards of living also happen to be countries situated in seismically active regions. Indonesia, India, China, Asia, and portions of the South Americas are examples. A large majority of the population in these areas, in turn, live in low income housing constructed of earth, stone, thatch, hollow block, and other types of low cost materials. From popular media reports and footage, it may seem that these structures are always devastated in large magnitude earthquakes. But in reality archeological evidence, such as mortar-less masonry construction at the Machu Picchu site, indicates buildings constructed of the same materials in seismically active areas have withstood earthquakes for centuries. How can this be?

Low Cost Earthquake Resistant Design

In seismic building design, whether a certain type of construction material can withstand an earthquake is a function not only of the material’s resilience and strength, but also how it is incorporated into the structure. A long, straight stone wall, for example, using only friction and geometry to keep stones in place, will almost certainly topple in an earthquake. This is because the mass, or inertia, of the wall tends to keep its motion lagging behind the motion of the earth during a quake. This not only begins to dislodge stones from their initial resting position, but without lateral support can shift the weight of the wall so far out of the stable load line that the top of the wall begins to topple over.

However, simply changing the course of the wall from a straight line to a zigzag line will improve its stability significantly. Changing the straight zigzags to s-curves will improve stability even further. Modifying the geometry of the individual stones to incorporate interlocking grooves, diagonal faces, and/or “L” posts can greatly improve the base to top integrity of the wall. Adding a cap or lintel at the top and post-tensioning elements such as bands or rods to pre-stress the stones in compression will guarantee structural integrity in all but the most severe seismic activity. Building the wall to follow a zigzag path works by providing more lateral support at each change in the wall path. Conforming to one or more s-curves provides almost continuous lateral support along the entire length. Adding a wall cap or lintel helps ensure the integrity of the top course of the wall stones, tying them together without benefit of the weight of the wall.

And so it goes with housing structures. While there is no such thing as totally earthquake proof buildings, earthquake resistant construction is readily achievable. If a house is rectangular in shape, without adequate ties between roof and walls, the walls will most likely become unstable near the top and topple or collapse in the direction of least lateral support. Changing the footprint to a more compact, square geometry with internal cross walls will improve this stability. Changing the footprint to a round or oval geometry will improve the stability even more, as exemplified by the bhongas structures in India. Adding a plate or rim band at the top of the walls helps ensure integrity is maintained at that location. Incorporating a lightweight structural roof with positive connection to the top plates and/or wall rims will also greatly improve the structure’s earthquake resistance, providing almost continuous lateral support at the top of the walls. In addition, if ties to an appropriate foundation are not sufficient, the structure stands a good chance of sliding off the foundation during a quake effectively destroying the structure from the bottom up. So positive connection to the ground, which can be achieved by foundation anchors, trenching, post and rod embedment, and other techniques that minimize horizontal displacement, is also mandatory.

Low Cost Seismic Reinforced Materials

All of the methods mentioned so far can be implemented with little or no additional expense, regardless of the specific construction materials being used. Other low cost, material specific methods can also be incorporated into wood frame, adobe, rammed earth, and masonry construction typically found in seismically active areas. For example, wood frame construction is seismically strengthened using adequate brackets, hold-downs, and fasteners of the correct type. For example, screws would seem to have greater holding power, but in fact are typically more brittle than nails and tend to shear under load. The use of shear walls, brackets and gussets at beams, joints, corners, sill plates, and roof truss connections are well documented, standard reinforcement techniques for this type of construction.

Due to their massive weight and brittle composition, adobe and rammed earth walls are prone to seismic failure by cracking especially at corners and long walls, allowing large pieces of the structure to fall or collapse. This can be mitigated by incorporating reinforcing fibers, bars, rods, or mesh into the walls to reduce cracking and keeping the pieces in place. Fibers can be straw, vines, even synthetic yarns worked into the material which acts as an internal reinforcing matrix. Bars and rods are especially effective and can be in the form of bamboo, reeds, cane, vines, steel bars, or any similar available ductile material, placed at intervals and connected to wall caps, foundations, and each other with rope, vines, or twine. Mesh reinforcement in the form of purpose made screen, chicken wire, etc. is especially effective at corners, serving to tie walls together and distributing horizontal forces during a quake. Buttresses and pilasters can greatly improve lateral resistance to shaking while minimizing the wall mass and cost. Using a wall cap or ring beam with this type of construction is also pivotal in maintaining not only integrity of the walls but also in providing a positive connection point for roof trusses and bar reinforcements. Finally, refraining from designing multistory structures also improves earthquake resistance when building with these materials.

Masonry walls consisting of hollow block, bricks, solid stone, and cast concrete panels are a popular low cost construction material due mainly to local availability. However, these materials are also most likely to be improperly incorporated into buildings subject to seismic stresses. Hollow block construction can be inexpensively reinforced by inserting short rods consisting of steel rebar, bamboo, cane, or similar materials at alternating intervals and then backfilling the rod cavities with mortar or compacted soil. Cast concrete panels and blocks can be similarly reinforced with rods at the time of manufacture, then tying the rods together during construction. All of these materials should incorporate a flat face geometry with interlocking grooves, with L-shaped units used at corners and wall intersections. And again, buttresses, pilasters, and wall caps or ring beams can greatly improve resistance to lateral movement, provide positive connection tie points for roofs and foundations and serve to maintain the position of the individual units. Interestingly, some historical evidence points to the successful use of diagonal bonding, as opposed to horizontal bonding, for masonry blocks and stones to better resist lateral displacement.

About The Author:

John Moehring is a practicing Engineering Technologist in civil, geological, biological, and electrical engineering fields. And one of these days he may actually get it right.