Rough Guide to Rebuilding Nepal, Part I
On April 25, at precisely 11:53 am Kathmandu time, two things happened — 1) a massive earthquake struck, razing 2000 years of history to the ground, and 2), a global effort to come up with the best seismic designs for Nepal began.
Organizations that had been focusing on seismic building in Nepal have been inundated with support and requests for collaboration. Teams of architects and engineers descended upon the country, gathering data, conducting analysis, and studying existing building techniques (if they aren’t they should be). International firms based thousands of km away have jumped in. Popup collectives of people trained in the field have come together, and individuals as well. I am running one myself, one which contains a mix of people in the field (builders, architects, and engineers), most based in NYC, but several in Europe and the West Coast.
While professionals are scratching their heads regarding engineering, logistics, local aesthetics, and most of all, cost, I keep getting asked questions by those fortunate enough to not be in the building world (a notoriously difficult business, one which seems to draw unseemly elements of all sorts, myself no exception).
Those questions are:
- ‘What is a good seismic design?’
First, we have to define ‘a good seismic design,’ and then go into the prevalent ones out there. I will cover that in this post, the first in a series of three about rebuilding Nepal.
2. ‘Who should I support in the rebuilding process?’
The real question here is ‘what does a good rebuilding project look like?’ I will answer that in my next post.
3. ‘As a Master Builder with extensive experience in Nepal, what are you doing, other than writing lengthy posts?’
Well, they don’t actually preface the question that way, but they do ask about my personal take on all this, so they are assuming it to some extent.
That will be the subject of my third post. I will go into my highest hopes, and my worst fears. No one will be spared in the process.
Basic Principles of Seismic Engineering
What does it mean to build seismically? Without getting lost in geeky engineering terms (which I love), here it is: a seismically engineered building is meant to withstand seismic loading.
Seismic loading is a subset of dynamic loading, which is a sudden increase of stress placed on a structure, for a short amount of time. For a bridge, this is a car moving across it; for a jet, it is when munitions are launched; for a building, it is uplift from wind, or vibration from an earthquake.
A typical residential building is engineered for two forms of loading:
- static/dead loads (unmoving weight; the building itself)
- small live loads (moving weight; people walking, moving furniture, etc…).
Dynamic loading is a different beast.
Two main factors determine a building’s performance when a dynamic load is placed on it:
- Flexibility. You want a building to flex when a load hits it. If not, it breaks. Buildings are engineered to move as a result of wind and seismic loads, as it is easier to accommodate force than to resist it. Skyscrapers often sway up to 10m at their top; one can feel wood structures move in the wind, and I was just in a 15m steel fire tower from 1919 (on top of a peak in the Catskills) that swayed noticeably in the wind (given that the original steel cables securing it to the ground were removed during a recent restoration, I was concerned).
2. Weight. The heavier the building, the more likely it is to collapse under its own weight (whether in an earthquake or not). What happens here is that the upper floor/floors stress the first floor, which collapses, bringing the rest of the building with it (opposite of the way the WTC collapsed). The important point to note here is that materials that are heavy and strong (such as stone) stress the structure with their own weight. Materials that are light and strong easily hold themselves up. The better the strength to weight ratio, the better the material is for seismic engineering.
Wood and bamboo are both excellent examples of good materials to use for seismic engineering. Being light, they do not stress the structure with their own weight. Being flexible, they move with force, instead of resisting it and shattering from it. Finally, they have high tensile strength, which is a measurement of how much stress a material can take before deforming and breaking. Tensile strength refers to how bound a material is; wood and bamboo have high tensile strength due to the fibers running through them lengthwise, the result of growing upward.
Finally —vibration is what destroys buildings during an earthquake. Vibration can be addressed in two ways:
- seismic isolation: the building is isolated from the ground using a design that absorbs the vibration before it enters the structure
- seismic dissipation: the vibration is absorbed within the building, typically using several different methods
Materials that absorb vibration, such as rubber, are excellent for both applications. After the 1906 San Francisco earthquake, cast iron-framed buildings were retrofitted with rubber gaskets in between each floor, so that vibration would not travel up the building (an example of seismic dissipation). In contemporary sustainable/seismic designs, discarded rubber tires are often used, in a variety of applications. Given that 1.000.000.000 tires are discarded annually, they are an abundant resource.
Existing Sustainable/Seismic Designs Suitable For Nepal
Thousands of people are working on new designs as you read this. Surely, a few of them are neurotic gardeners like myself.
Most likely, what they will come up with will be based on existing sustainable/seismic styles. So what is out there? My rebuilding partners and I have done some research, here is a rundown of the current sustainable/seismic building techniques we are likely to see in the rebuilding process. This is not meant to be a comprehensive list; it is meant to give a broad overview of major design trends.
While not a sustainable building method, reinforced concrete is the most popular building style in areas with road access, so it needs to be addressed. It makes sense for urban areas, where population density calls for buildings higher than 2 stories. For everywhere else, however, sustainably designed structures are a better alternative.
Imported into Nepal from India, ‘reinforced concrete’ refers to any design which calls for concrete poured over an internal steel structure. The steel structure can range from rebar and wire mesh assembled by hand, to massive steel beams moved with cranes.
‘Reinforced masonry’ refers to masonry structures (anything built of stone or brick, with or without mortar in between them) with reinforcement (typically steel) embedded within it. These structures are not very common, as weaving rebar through brick and stone is difficult.
To clarify a general misconception: cement and concrete are two different things. Cement is made by heating limestone in a kiln and grinding it up. It is one part of the mixture that is concrete; the other three parts are gravel, sand, and water.
In general, unreinforced masonry and concrete designs (the vast majority of the buildings in rural Nepal) do not stand up to seismic loading . However, there are examples of unreinforced masonry that have performed well in earthquakes — the Incas built large structures with huge stones, cut in a way to allow them to move slightly during an earthquake. Also, the Swiss have developed their own dry-laid (no mortar), unreinforced big-stone design. These should be looked at for roadless areas without bamboo or wood supplies.
Steel reinforcement plays a critical role, performing two functions:
- it takes up to 50% of the weight of the building, relieving the concrete of that load
- it ties the building together, preventing it from coming apart while vibrating during an earthquake.
Concrete alone is extremely brittle, and will quickly come apart under a seismic load (pictures of Port au Prince after the Haiti earthquake reveal this; there are piles of concrete rubble, with no rebar in it).
Many of the more recent masonry buildings built in Kathmandu have reinforcement in certain areas — in structural columns, the corners, and floors — but the walls are all infilled with brick, which have no steel running through them. During an earthquake, the columns and floors might hold together, but the brick walls come crashing down.
Further, being an old lake bed, Kathmandu has very soft soil, which makes it possible for entire buildings to fall over, even if they hold together. Soil liquefaction is the culprit here. Accordingly, buildings in Kathmandu should have deep foundations and be buttressed (short walls perpendicular to the foundation to keep it from moving) to prevent them from toppling over.
However, the water table in the Valley is high (often only one meter below the ground), so people do not excavate deep enough for their foundations (doing so requires pumping water out, an undertaking few seem interested in). Otherwise, I have never seen buttressing of any building in Nepal.
If cement and steel are brought into areas without road access, using cargo helicopters, mules, or porters, their price drastically increases. Affordability is an important part of any sustainable/seismic design; if it costs too much to build, it will not get built.
However, in roadless areas, small amounts of cement are typically brought in by mule, and mixed with locally sourced sand and gravel to produce concrete, which is used for floors, bathrooms, and other specialized applications.
Bamboo is a fantastic building material, and extensive seismic/sustainable work has been done with it. Among many other applications, bamboo has recently been used to reinforce concrete, and has stood up well in seismic trial testing. Bamboo has also successfully been combined with mud and adobe construction to create a seismic structural wall system; Abari has been doing excellent work along these lines in Nepal. Further, on its own, bamboo, like wood, can be used to frame strong, flexible buildings.
However, in Nepal, bamboo does not grow large enough to be used as a building material above 1700m. It grows until about 2700m, but anywhere above 1700m, it can only be used as material for weaving, as it only gets 2–3 cm thick. Even below 1700m, it is not commonly found; when one does come across it at lower elevations, it is often too small to build with. Unlike other tropical countries, there are not that many bamboo groves in Nepal (which might also explain the lack of giant pandas).
While it is a good building material when available, many of the areas that need to be rebuilt do not have a bamboo supply.
Wood is often used to create seismically engineered structures. The Japanese have been doing it for centuries, and have developed a number of joinery techniques specifically designed to allow movement (but not disconnection) during earthquakes.
However, in many of the roadless areas which suffered extensive damage, wood is not available; as in most of the settled parts of Nepal under 2500m, the forests have been cleared long ago for farming and grazing land. Like cement, steel, and glass, in a roadless area it is possible to use small amounts of wood in the structure (for windows, doors, furniture, etc…), but it cannot be relied upon as the principal structural material. It is scarce, and is often too expensive where it is available (most forests in Nepal are community-managed, and if someone wants to fell trees for lumber, they must gain approval, and pay for the trees at market rates). When it is available, given the terrain, it is rarely moved more than one day’s walk from where the trees are felled.
There are parts of the country with an extensive wood supply. They are generally above 2000–2500m, where population density drops. With proper joinery and engineering, wood is an ideal material for those that have access to cheap lumber.
The principal structural materials in an Earthship are used tires, rammed earth, and reinforced concrete. Seismically speaking, the tires are key to their design, as they are at the bottom, seismically isloating the rest of the structure. Pioneers in the radically sustainable design movement, Earthships also produce their own power, food, drinking water, and process their own waste. They are awesome.
Michael Reynolds and his firm Biotecture have been building Earthships for decades, often in countries right after a natural disaster. Their structures range from small buildings for a family of four to elaborate compounds for hundreds of people. They tweak existing designs for local conditions; in the Phillipines it was for typhoons, in Haiti, it was for earthquakes. They are currently working on a design tailored for Nepal.
While an excellent choice for areas with road access, I do not see mule trains loaded with tires heading into the mountains (2–4 per mule). Mule transport is not cheap (charged per kilo, about 10–15 rupees per day/per kg; a mule typically carries about 50 kg), which kills Earthship affordability in a roadless setting. Further, they require substantial quantities of cement and steel.
For anywhere with road access, however, Earthships are an excellent option. They are affordable, their building style is easily learned, and their principal structural materials are garbage and dirt.
Earthbag buildings utilize woven polypropylene rice sacks (30–50kg) filled with dirt as their principal structural material. They are affordable, and the technique is simple. The dirt moves in an earthquake, and absorbs vibration. They can also be reinforced with rebar or bamboo.
Several earthbag buildings have already been built in the areas affected by the earthquake, and at least one is still standing.
For areas with no road access, and a limited wood/bamboo supply, earthbags are a good option, as they do not rely on expensive materials imported from the lowlands.
Keep white-knuckling your chair — in my next post, I will go into what a good rebuilding project looks like.