I found out about Sebastian from his bamboo durability studies on the Cox Bazar Refugee Camp in Bangladesh. Sebastian is a Senior Associate Engineer in Arup, London. He is experienced in international development and engineering projects in developing countries, especially permanent low-cost housing in highly seismic areas, specialising in bamboo. He has many technical publications under his name and is actively involved with the Bamboo International Standards work.

A very busy man indeed and I am appreciative that he agreed to us sharing this work of his. Please read on for a clear and jargon free article on bamboo and earthquakes.

Performance of bamboo buildings in earthquakes

There is much anecdotal evidence for the favourable behaviour of bamboo (and often timber) buildings in earthquakes. People often describe seeing these buildings “sway” with the earthquake without experiencing significant damage, while the nearby modern masonry or concrete buildings crumble. But is this really true? And is this only applicable to traditional bamboo buildings, rather than modern ones?

To understand this properly, we need to introduce a seismic principle: ductility.

**Figure 1: Modern open-plan bamboo-framed restaurant, showing little to no damage after the Ecuador 2016 earthquake (EEFIT, 2016).**

Ductility and brittleness

When one applies a load to a material, if the load is released and the material returns to its original position without losing strength or breaking, that is said to be loading the material within its elastic range.

On the other hand, if the load is released and the material remains deformed, that is said to be loading the material within its plastic range.

Ductility is the ability of a material to be loaded in its plastic range without losing strength or breaking. The process of loading it in its plastic range is a convenient mechanism of absorbing energy and converting it into heat.

The best example of this is a metal paperclip or a metal spoon. One can bend it slightly and it will return to its original position – this is keeping it in its elastic range (Figure 2 (L). No energy has been absorbed at this point. One can then bend it further and it will not return to its original position, however it has retained its original strength – this is now in its plastic range. This has absorbed energy. One can bend it very far backwards and forwards several times, however it still does not lose its strength. One can then feel that it has heated up – this heat is the thermal energy that has been transformed from the kinetic energy from the applied movement.

**Figure 2: (L) Bent paper clip (Yusuke, 2008); (R) Broken bamboo (Swift, 2006)**

Metals like steel are typically very ductile and are able to absorb lots of energy, and are therefore used significantly in earthquake-resistant design. But what about bamboo? Let’s apply the same experiment. One can bend a piece of bamboo quite a lot and it will return to its original position – it’s quite flexible, however this is keeping it in its elastic range only, so no energy has been absorbed. However, if we try and bend it into its plastic range, it snaps suddenly losing much or all of its capacity (Figure 2 (R)). Bamboo is therefore brittle in most failure modes, and unable to absorb energy in an earthquake, and the flexibility of bamboo has no relevance on its ability to absorb energy.

Masonry, concrete and timber are also brittle in most failure modes. This is why in good earthquake-resistant design, these materials are always used in conjunction with steel, in such a way that the steel is the material which absorbs the energy, while the brittle material stays generally in the elastic range. Examples include reinforced concrete and reinforced masonry where the steel is designed to fail in a controlled manner, while the concrete/masonry retains much of its strength. The same applies to timber – at least if it’s been correctly designed: steel connections are able to absorb energy, and the timber itself remains elastic. Bamboo connections can similarly be designed to be ductile, which mean they have the potential to absorb some energy, when properly designed, and if using metal connections (bamboo dowels can’t absorb much energy and are generally considered brittle).

There are also other mechanisms for dissipating energy in connections, for example involving friction when two surfaces slide against each other. Examples include traditional bamboo connection systems tied with rope. However, this energy dissipation is likely to be somewhat limited compared with modern well-designed steel connections.

Observations from real earthquakes

It is true that many traditional bamboo building systems have historically performed relatively well in earthquakes. This was documented after the 1999 Colombia earthquake (Gutiérrez 2000 & 2004), the 2001 El Salvador earthquake (López et al., 2004) and the 2016 Ecuador earthquake (Drunen et al., 2016; Franco et al., 2018). This positive behaviour was observed in particular on a vernacular building system known as bahareque common across Central and South America. This system typically consists of a timber or bamboo frame, clad in a matrix of split bamboo, cane, twigs or timber strips, and finally plastered in manure or soil, sometimes with straw added for strength (Figure 3).

**Figure 3: (L) Details of traditional bahareque system in Latin America; (r) 100-year old traditional bahareque house in Colombia**

Like most vernacular systems, bahareque was developed by local communities by trial and error over hundreds of years, learning what worked in earthquakes and what didn’t. In many highly seismic countries where the time between events is short – such as many of those in Latin America – people remember the previous earthquake and the damage patterns that they saw, and so will tend to naturally develop systems which are more resilient to earthquakes. But why do these systems perform well in earthquakes?

The science

The real reasons behind the favourable seismic performance of traditional bamboo buildings can be summarised as follows (see Figure 4 for an illustration of real earthquake damage to a bamboo house):

1. Bamboo has a good strength-to-weight ratio, therefore traditional bamboo buildings tend to be relatively light (especially compared to masonry and concrete). Hence, they are subjected to lower forces during earthquakes – the force that a structure experiences in an earthquake is directly proportional to its mass.

2. Traditional bamboo connection systems, such as nails, traditional carpentry connections and even rope, can usually resist relatively large deformations without failing, and some of them can absorb limited amounts of energy through friction and local damage.

3. Traditional wattle-and-daub type systems that have a mud plaster can also absorb some energy through local damage (Figure 4).

4. Unlike many traditional unreinforced masonry buildings, traditional bamboo buildings generally have a degree of tying, which helps to distribute the forces and ensure building components move together.

5. Since traditional bamboo buildings tend to be relatively light, their collapse is less of a life safety risk to the occupants. In addition, repairs may be easier.

There is a sixth reason which is often quoted, which is that bamboo buildings are flexible enough to sway in earthquakes. It's true – seismic forces do tend to decrease in a building as it becomes more flexible, although in modern construction the main benefit is for higher rise buildings (of say 4 or more storeys). For traditional bamboo or timber lower rise buildings, there may be some limited force reduction due to flexibility, however this comes at the expense of large movements as the building is twisted and distorted by the ground’s shaking. Typical roped connections simply can’t withstand this kind of distortion. Modern brittle finishes, facades and glazing in bamboo houses will also suffer without a stiff frame to limit seismic movements. Therefore, unfortunately modern low-rise bamboo buildings cannot typically benefit from this effect. Formal vernacular bamboo housing systems, such as bahareque, are also considered too stiff to benefit from this effect.

Therefore, when people observe modern bamboo buildings “swaying”, what they are normally really seeing is just the normal movement of the earthquake, potentially combined with local deformation, friction and damage at connections, which can absorb some energy and is a real positive effect.

Are bamboo buildings automatically earthquake-resistant?

No, and far from it. Like any material, the principles outlined here are dependent on good design, construction and maintenance. Observations from the El Salvador 2001 earthquake for example identified that older traditional bamboo buildings showed considerably more damage than younger ones, due to increased insect and fungal attack. Figure 5 clearly illustrates how the areas that showed most damage in traditional bamboo bahareque houses were those most exposed to rain.

Recommendations for modern bamboo structures

For modern bamboo structures, the above principles still apply. Combining them with good practice earthquake engineering principles gives us the following recommendations:

1. Keep the structure light – this reduces the loads that the structure has to resist.

2. Tie the structure together well – this helps to distribute the forces.

3. Use a bracing system, moment-frame or shear walls to resist the earthquake loads, incorporating connections that are able to absorb the forces and hence the earthquake energy without failing.

4. Ensure the structure is designed to be durable – most failures of bamboo structures have occurred because of rot or insect attack.

Possible bamboo structural systems to resist and absorb earthquakes

We recommend considering the “Composite Bamboo Shear Wall” system as a very effective way of forming walls that resist earthquake loads. This will be described more in a future post, but for now, more information can be found in Kaminski et al. (2016). A video of a full-scale shake-table test under a very large simulated earthquake of one of these bamboo houses can be found here: https://www.youtube.com/watch?v=Oqyyky1vUJ8.

Further reading

Drunen N, Cangas A, Rojas S, Kaminski S (2016) Post-earthquake report on bamboo structures and recommendations for reconstruction with bamboo on the Ecuadorian coast. INBAR.

EEFIT 2016 (2016) Muisne Ecuador Field Investigation Team.

Franco G, Stone H, Bayes A, Chen Chang S, Hughes F, Jirouskova N, Kaminski S, Lopez J, van Drunen N, Molina Hutt C, Querembas M (2018) The Mw7.8 Muisne Ecuador Earthquake of 16 April 2016 – A Field Report by EEFIT.

Gutiérrez J (2000) Technical Report 19: Structural adequacy of traditional bamboo housing in Latin America. Beijing: INBAR.

Gutiérrez J (2004) Notes on the seismic adequacy of vernacular buildings. In Proceedings of 13th World Conference on Earthquake Engineering, Vancouver, Canada.

Kaminski S, Lawrence A, Trujillo D (2016) INBAR Technical Report No. 38: Design Guide for Engineered Bahareque Housing. INBAR, Beijing.

López M, Bommer J, Méndez P (2004) The seismic performance of bahareque dwellings in El Salvador. In 13th World Conference on Earthquake Engineering, Vancouver, Canada.

Swift, B. (2006) Broken bamboo. Licensed under CC BY 2.0. Available from: https://www.flickr.com/photos/metalcowboy/427791396. [Accessed January 20201].

Yusuke, D. (2008) Bent spoon. Licensed under CC BY 2.0. Available from https://www.flickr.com/photos/yd/2625186734[Accessed February 2021].

Note: This article has replaced an earlier post with revisions and updates from Sebastian.

’As surely as the sun rises in the morning, bamboo will surely rise ’ - Ewe Jin Low.