Gernot Minke

Construction manual forearthquake-resistant houses

built of earth

Published byGATE - BASIN (Building Advisory Service and Information Network) atGTZ GmbH (Gesellschaft für Technische Zusammenarbeit)

P.O.Box 5180D-65726 Eschborn

Tel.: ++49-6196-79-3095Fax: ++49-6196-79-7352e-mail: gate-basin@gtz.deHomepage: http://www.gtz.de/basin

December 2001

3

Contents

Acknowledgement 4Introduction 5

1. General aspects of earthquakes 61.1 Location, magnitude, intensity 61.2 Structural aspects 62. Placement of house in the case of 8

slopes3. Shape of plan 94. Typical failures, 11

typical design mistakes5. Structural design aspects 136. Rammed earth walls 146.1 General 146.2 Stabilization through mass 156.3 Stabilization through shape of 16

elements6.4 Internal reinforcement 197. Adobe walls 227.1 General 227.2 Internal reinforcement 247.3 Interlocking blocks 267.4 Concrete skeleton walls with 27

adobe infill8. Wattle and daub 289. Textile wall elements filled with 30

earth10. Critical joints and elements 3410.1 Joints between foundation, 34

plinth and wall10.2 Ring beams 3510.3 Ring beams which act as roof support 3611. Gables 3712. Roofs 3812.1 General 3812.2 Separated roofs 3813. Openings for doors and windows 4114. Domes 4215. Vaults 4616. Plasters and paints 49

Bibliographic references 50

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Acknowledgement

This manual was first published in Spanish withinthe context of the research and developmentproject “Viviendas sismorresistentes en zonasrurales de los Andes”, supported by the Germanorganizations Deutsche Forschungsgemeinschaft,Bonn (DFG), and Deutsche Gesellschaft fürTechnische Zusammenarbeit (GTZ) GmbH,Eschborn (gtz). It also comprises the results offormer research projects of the “Forschungslaborfür Experimentelles Bauen” (Building ResearchLaboratory, University of Kassel, Germany),directed by the author and sponsored by gtz,DAAD and the University of Kassel.The layout and computer drawings were preparedby Friedemann Mahlke, the freehand sketchesby Vera Frey.

Kassel, December 2001Gernot Minke

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Introduction

The solutions proposed in this manual concentrateon low-cost single-story houses, built from earthin rural areas of earthquake-prone zones. Theyare based on research projects carried out at theForschungslabor für Experimentelles Bauen(Building Research Laboratory) of the Universityof Kassel, Germany, on the analysis of earthquakedamage in Latin America, on studying relevantliterature and on the implementation of severaltest structures in Germany and prototype housesin Guatemala, Ecuador and Chile.

Using locally available building materials as wellas the skills of local craftsmen should beconsidered for the design of seismic-resistant(earthquake-proof) houses and it should be provedthat the solutions are accepted by the users.

Earth as a building material has lost its credibilitymainly because of the fact that most modernhouses with earth walls could not withstandearthquakes, and also since earth is consideredas the building material for the poor. In thiscontext it is worth mentioning that a censusconducted by the Salvadorian Government afterthe earthquake in January and February 2001states that adobe houses were not worse affectedthan other houses.

In many areas of the Andes regions building withadobe (unburned, unstabilized handmade soilblocks) is forbidden nowadays. Nevertheless, themajority of the rural population still builds withthis building material, as it cannot afford to buildwith bricks or concrete blocks.

When designing low-cost houses for rural areas itshould be taken into account that structural failuresas a consequence of an earthquake have to beavoided, whereas minor damage like small cracksmust be tolerated if it can be easily restored.

For more information about the different buildingtechniques with earth, the physical and structuralcharacteristics of earth and the possibilities ofimproving them, reference is made to the “EarthConstruction Handbook” by the author,published at WIT Press, Southampton, UK 2000,or to the “Manual de Construcción en Tierra”,publicadora Nordan, Montevideo, Uruguay 2001.

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1. General aspects of earthquakes

1.1 Location, magnitude, intensity

An earthquake is produced either by movementof tectonic plates or by volcanic activity. The areasof the world that are most earthquake-prone areshown in Fig. 1-1. Earthquakes of intensity 8 onthe Richter scale have been recorded in Asia andof up to 8.7 in the Andes . Nearly a hundredearthquakes of intensity higher than 6 and twentyof intensity higher than 7 on the Richter scaleare recorded annually. Several thousand peopleare affected by earthquakes every year.

The magnitude (M) of an earthquake usually ismeasured on the Richter scale, which islogarithmic with an open end. It is a measure ofthe energy produced in the epicenter, the placewhere the earthquake is generated. The Mescaliscale, on the other hand, is divided into 12 gradesand indicates the intensity of the local impact.

The local impacts on a structure depend not onlyby the magnitude of the earthquake, but also onthe depth of and distance from the epicenter, thegeology and topography, the kind of local soil andlast but not least on the duration, frequency andacceleration of the impacts.

1.2 Structural aspects

Structures are mainly affected by the horizontalforces created by the earthquake. The verticalforces are usually less than 50% of the horizontalones.The main danger due to horizontal movementsof the earth is that the walls of buildings mightfall outwards and consequently the roofs collapse.The main aim of building earthquake-resistanthouses, therefore, is to avoid walls being able tofall outwards and to ensure that the roofs are fixedwell to the walls, or even better that they standon a system of posts separated from the wall, sothat the roof system and the walls can swingindependently due to their differing frequency.

With a “medium” earthquake the followingmeasures have to be taken into account:horizontal deformation: h = 0.1 to 0.3 mhorizontal velocity: v = 0.1 to 0.3 m/shorizontal acceleration: a = 0.1 to 0.3 m/s2

= 0.15 to 0.30 gA horizontal acceleration of 0.3 g means that 30%of the dead load of the structural elements actsas horizontal force against the structure(“equivalent force”). Usually simple structuresare calculated by the method of “equivalent

1-1Earthquakezones (Houben,Guillaud 1984)

7

force”, in which the horizontal impact is taken asa static force and not as a dynamic one.However, the higher the ductility, the capacityfor deformation without structural failure, thelower the equivalent force is and the lower thestructural resistance must be.

The quality of an earthquake-resistant structurecan be expressed in the formula

structural quality = resistance x ductilityThis means the lower the resistance of thestructure is, the higher the flexibility must be, andthe higher the flexibility is, the lower theresistance must be (Grohmann, 1998).

The historical rammed earth houses with walls of60 to 100 cm thick had enough resistance towithstand earthquakes and did not need to beflexible. For instance in Mendoza, Argentina,these houses withstood all earthquakes of the lastcenturies, whereas all modern buildings built ofadobe or bricks collapsed. However, thesestructures are not economic nowadays. Economicsolutions have less rigidity, therefore they mustallow deformation during seismic shocks withoutcollapse.

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2. Placement of house in the caseof slopes

In earthquake-prone areas, where the site isinclined, the following rules must be taken intoaccount:

a) The house should not be cut into the slope,as the adjacent wall might collapse due tothe horizontal forces of the earth, seeFig. 2-1

b) The house should not stand on the slope asit might slip down

c) The house should not stand near steep slopesas it might collapse due to falling rocks orearth avalanches, see Figs. 2-3 and 2-4

d) If a slope is given, a platform has to be formedand the house has to be placed at sufficientdistance from the slopes, see Fig. 2-5

e) It is recommended that massive and heavyhouses stand on soft sandy soils, whereas lightflexible structures can stand on rocky soils.

f) Different floor levels should be avoided.

If it is necessary, the rooms should be separated.

2-1 to 2-5 Location of a house on the slope2-5

2-4

2-3

2-2

2-1

dangerous

safe

dangerous

dangerous

dangerous

safe

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3. Shape of plan

The shape of the plan of the house might havean important influence on its stability. Thefollowing rules must be considered:

a) The more compact a plan, the better thestability. This means a square plan is betterthan a rectangular one, and a circle is betterthan a square.

b) L-shaped plans are less stable. The bestsolution in this case is to separate theelements as shown in Fig. 3-2.

At the University of Kassel a simple test methodwas developed within a doctorate thesis in orderto show the influence of the wall shape onresistance to seismic shocks. A weight of 40 kg atthe end of a 5.5 m long pendulum was droppedagainst the models, see Fig. 3-3. The rammedearth house with square plan showed the first largecracks after the second stroke, see Fig. 3-4. Afterthree strokes one part of the wall separated, seeFig. 3-5, and after four strokes the housecollapsed, see Fig. 3-6. The rammed earth housewith circular plan, however, showed the first cracksonly after three strokes, see Fig. 3-7, and onlyafter six strokes did one small part of the wallseparate, see Fig. 3-8 (Yazdani, 1985). SyedSibtain built several houses in Afghanistan utilizingconvex walls with buttresses, which give goodstability similar to that of circular walls, see Fig. 3-9(Sibtain, 1982). But the problem with all wallstructures is that the openings weaken theirstability. Therefore, openings must be carefullydesigned and often require additionalreinforcement.

bad good ideal

dangerous safe

3-3 Simulation of seismic shocks (Minke 2000)

3-1 Ground plans

3-2

10

3-4 to 3-8 Earthquake tests with models of squareand circular shape (Minke 2001)

3-4

3-9 Plan of seismic-resistant houses, Afghanistan(Sibtain 1982)

3-5

3-6

3-7

3-8

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4. Typical failures, typical designmistakes

Typical failures which occurred with simulatedseismic movements on models in the scale of 1:5are to be seen in Figs. 4-1 to 4-3. The mostsignificant are:

- diagonal cracks lead from the edges of windowsto the bottom of the wall,

- the lintel often destabilizes the walls, especiallyif it is not long enough and does not havesufficient bond with the wall,

- if the wall between window and door orbetween opening and corner is not longenough, it might break,

- if the wall has no ring beam at the top it breakseasily when suffering perpendicular loadswhich produce bending.

The houses shown in Fig. 4-4 seem to be welldesigned with the stabilizing buttresses at thecorner. But without a ring beam they do not havesufficient stability against seismic shocks, as Fig.4-5 and 4-6 show.

The 10 main structural mistakes which might leadto a collapse within an earthquake are explainedin Fig. 4-7.

4-1 to 4-3 Typical failures caused by seismic movements(Tolles et al. 2000)

4-4 Earthquake-proof houses, Afghanistan (Sibtain 1982)

4-1

4-2

4-3

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5. Structural design aspects

There are three general principles for designingan earthquake-resistant structure:

1. Walls and roof are well interconnected and sorigid that no deformation occurs in theearthquake.

2. Walls are flexible enough, so that the kineticenergy of the earthquake is absorbed bydeformation. In this case a ring beam, which isable to take bending forces, is necessary andthe joints between wall and ring beam and ringbeam and roof must be strong enough.

3. The walls are designed as mentioned in case2, but the roof is fixed to columns separatedfrom the wall, so that both structural systemscan move independently as they have differentfrequencies.

Case 1 can be a house with very thick rammedearth wall or a reinforced concrete frame structurewith moment-stiff corners at the top and at thebottom, and infills of bricks, cement blocks oradobes.

A variation of a nonflexible structure is a timberframe structure which has less moment-stiffcorners and is therefore stabilized by crossingdiagonals of steel. In this case the danger existsthat the connection of the diagonal or theelements itself may not be strong enough towithstand the concentration of stresses at thecorner and breaks, causing the collapse of the wall,see Fig. 5-1.

4-5 and 4-6 Models of the house in Fig. 4-4 after seismicmovements (Sibtain, 1982)

4-7 Typical design mistakes which might lead to the collapseof the house

1. Ring beam is lacking.2. Lintels do not reach deeply enough into masonry.3. The distance between door and window is too small.4. The distance between openings and wall corner is too

small.5. Plinth is lacking.6. The window is too wide in proportion to its height.7. The wall is too thin in relation to its height.8. The quality of the mortar is too poor, the vertical joints

are not totally filled, thehorizontal joints are too thick (more than 15 mm).

9. The roof is too heavy.10. The roof is not sufficiently fixed to the wall.

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5-2 Wattle and daub structure, after aheavy earthquake in Guatemala(Minke 2000)

5-1

The systems of case 2 and 3 can be built withoutconcrete and steel and in most regions are muchmore economic. Walls built with the system of“wattle and daub” (in Spanish: “bahareque” or“quincha”) show extreme flexibility. Fig. 5-2shows a house which suffered under a heavyearthquake in Guatemala, but did not collapse.

As the vertical forces created by the earthquakeare less important, we have to decide how thewalls withstand the horizontal forces. There aretwo types of impacts to be considered: thoseforces, which act parallel to the wall and thosewhich act perpendicular to it. (Forces acting atan inclined angle to the wall can be divided intotwo components, one parallel and oneperpendicular to the wall.)The perpendicular forces create a moment whichmight provoke a collapse of the wall if it is notstabilized by intermediate walls, buttresses andring beams. If the walls are very thin and high,they might collapse even though stabilized, dueto the bending forces that create buckling. Theparallel forces are less dangerous. They producethrust within walls which in the case of adobe wallswith poor mortar create the typical diagonalcracks, shown in Fig. 4-1 and 4-2.The most dangerous effects result when the wallsfall outwards and the roofs collapse. Thereforethe safest solution is to place the roof on a separate

structure independent of the walls, see chapter12.When designing earthquake-resistant houses, wemust consider that the horizontal force (“equi-valent force”) to be calculated is proportional tothe mass of the structure and the higher the walls,the higher their displacement.

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6. Rammed earth walls

6.1 General

In the rammed earth technique moist earth ispoured into a formwork in layers 10 to 15 cm thickand compacted by ramming. The formworkconsists of two parallel panels, separated andinterconnected by spacers, see Fig. 6-1. Bycomparison with adobe masonry, rammed earthwalls provide more stability as they are monolithic.

Traditional techniques use formwork with bigwooden spacers, which cause openings and weakparts and often show horizontal shrinkage cracksbetween the layers, as the fresh layer on top ofthe old one shows larger shrinkage.

To avoid both disadvantages a special formworkwas developed at the Building ResearchLaboratory (FEB), University of Kassel, whichis spaced only at the bottom by a very thin steelbar and on the top above the wall, see Fig. 6-4.

Traditional techniques use manual tampers withconical or flat heads, see Fig. 6-1. Conical tampersgive a better bond between the different earthlayers, but need more time. It is preferable to usea tamper with two heads, one with a round surfaceand the other with a square surface, see Fig. 6-2.The square tamper has to be used at the bordersof the formwork. Pneumatic tampers and strongerformwork, as used nowadays for instance inAustralia, can reduce the labor input by the factorof 10. (For further details see: G. Minke: EarthConstruction Handbook, WIT Southampton, UK2000)

6-1 Manual tempers(Minke 2001)

6-2 Temper with two «heads», used in Ecuador(Minke 2001)

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6.2 Stabilization through mass

Rammed earth walls 60 to 100 cm thick, whichare not too high, can withstand horizontal seismicshocks without additional reinforcement. Housesbuilt in this manner more than 150 years ago inMendoza, Argentina withstood all earthquakes,whereas newly constructed houses next to themcollapsed, even when they were built with bricksand a concrete ring beam. As thick rammed earthwalls are too labor-intensive and no longeraffordable nowadays, new structural solutionshave to be used, as set out in the followingchapters

6-3 Formwork for rammed earth (Minke 2000)

6-4 Climbing formwork(Minke 2000)

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6.3 Stabilization through shape of elements

A simple solution for stabilizing rammed earthwalls of lesser thickness is to use elements in theshape of L, T, U, X, Y or Z (Fig. 6-5). Due totheir angles they show better stability againstlateral forces. If the wall is 30 cm thick, the freeends of the elements should not be longer than3/4 and not shorter than 1/3 of their height, seeFig. 6-6. This minimal length is necessary totransfer the loads diagonally to the plinth orfoundation. If the free ends are longer than 3/4of its height, they should be stabilized by anotherangle. If the angle is well fixed on the bottom tothe plinth and on the top to a ring beam, it can belarger or higher. Nevertheless, the height shouldnot be more than 8 times the width, see Fig. 6-7.

The forces perpendicular to the wall aretransferred into the angle which is parallel to thedirection of the force. This means it is transferredversus a moment which creates stress con-centration at the inner corner of the angle.

6-5 Wall elements stabilized by their shape 6-6 Recommended proportions

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6-7 Expedient proportion of wall

Therefore it is advisable to enlarge the sectionat this corner, shown in Figs. 6-8 and 6-9. Fig. 6-12 shows different proposals for plans utilizingangular elements.

To improve lateral stability the joint of twoelements should be formed with tongue andgroove, see Fig. 6-10. However, in order to obtaina more flexible structure, elements with shorterlength and no tongue and groove joint should beused (Fig. 6-11), if the elements are well linkedto a ring beam above and to a plinth below. Thiskind of solution is used in the project describedin chapter 6.4.

6-9 Elements with correct corner details

6-8 Corner solution

dangerous correct

18

6-10 Joint with lateral stability 6-11

6-12 Proposals for simple plans utilizing angular elements

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6.4 Internal reinforcement

One method of stabilizing rammed earth wallsagainst horizontal forces is to use vertical rods ofbamboo or wood inside the wall. These elementsshould be fixed to the foundation below and to aring beam above, see Fig. 6-13. Horizontalreinforcement elements usually weaken thestructure and lead to horizontal cracks, as shearforces cannot be transferred by the rods, sincethe bond between these elements and the earthis very poor. Furthermore in practice it is difficultto ram the earth well underneath these elements,due to their elastic behavior when hit.A new system utilizing bamboo as verticalreinforcement for element-type rammed earthwalls was developed in 1978 at the FEB andsuccessfully implemented together with theUniversity Francisco Marroquin (UFM) andCentre of Appropriate Technology (CEMAT),both from Guatemala. The low-cost housingprototype built is depicted in Figs. 6-14 to 6-19.The wall elements were rammed in a metal T-shape form, 40 cm high, 80 cm large and 14respectively 30 cm wide, see Figs. 6-16 and 6-19.The rib plays an important role in the stabilizationof the element against horizontal forces, as it actslike a buttress. The elements are reinforced by 4vertical bamboo rods of 2 to 3 cm diameter. Thebamboo rods were fixed at the bottom to thehorizontal bamboo ring beam and the stretchedvertical rods of the plinth, see Fig. 6-14.

6-15

6-14

6-13

20

6-18

6-16 6-17

After drying vertical gaps of 1 to 2 cm appearedbetween the elements, which were then closedby earth. These vertical joints are predesignedrupture joints which can crack in an earthquakeand allow independent movements of eachelement. So the kinetic energy of the seismicshock will be absorbed by deformation, but the

element being fixed at the top and bottom willnot fall. After the earthquake the open joints caneasily be closed again with earth. Using this ideaan earth wall system was developed, which ismassive and flexible at the same time. Thesecond new fact with this prototype structure wasthat the roof rests on posts standing 50 cm inside

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6-14 to 6-19 Earthquake resistant low-cost housing project,Guatemala (Minke 2001)

6-20 and 6-21 Earthquake resistant prototype building, Alhué, Chile 2001

6-19

vertical reinforcementwith bamboo

the walls, so that the roof and the walls can swingindependently of each other within theearthquake.In 1998 the FEB developed another reinforcedrammed earth wall system which was utilized fora low-cost housing project built in cooperationwith the University Santiago de Chile in 2001 inAlhué, Chile, see Figs. 6-20 and 6-21. Here toothe idea was to separate the roof from the wallsystem and to use U-shape and L-shape elements,which stabilize themselves by their shape.The corners of the angular elements were cutunder 45 degrees in order to increase theirstrength as described in chapter 6.3. To obtainadditional stabilization they were reinforced byvertical rods of colligue (similar to bamboo), 3 to5 cm in diameter. Furthermore, the wall elementswere always separated by light, flexible elements,or doors and windows. The lower parts of thewindows and the parts above the doors were notbuilt with massive elements, but instead of lighttimber elements. The gables were built in light-weight straw-loam stabilized by wooden elements,similar to the wattle and daub system.

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7. Adobe walls

7.1 General

Blocks of earth produced manually by throwingwet earth into a formwork are called adobes, ormud bricks or sometimes sun-dried earth blocks.When moist earth is compacted in a manual orpowered press, the compressed elements soformed are called soil blocks. Blocks producedby an extrusion process in a brick plant, are calledgreen bricks in their unburnt state. Larger blocks

compacted in a formwork by ramming are calledrammed earth blocks.There are many different shapes known all overthe world. Fig. 7-1 shows some samples.

Different manually operated presses are knowntoo. Fig. 7.3 shows one of the first, the widespreadCINVA-Ram, and Fig. 7-4 the CETA-Ram, whichcan produce 3 smaller blocks at the same time.Blocks produced by these presses have the

7-1 Adobe forms (Minke 2000) 7-2 Production of adobes in Ecuador

23

7-4 CETA-Ram, Paraguay 7-3 CINVA-Ram, Colombia

lap

lever

mould

piston

advantage of a more exact shape with sharpcorners than handmade adobes. Their disad-vantages are that they usually need the additionof 4-8% cement to the soil in order to obtainsufficient stability and that the production outputis only half by comparison with adobes.

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7.2 Internal reinforcement

The “Instituto nacional de investigación ynormalización de la vivienda (ININVI), Peru,developed a system of adobe walls which arestabilized by vertical bamboo rods that fit intoholes of 5 cm diameter, formed by grooves at theside of square adobes and halved ones, see Fig.7-5. Corner buttresses and intermediatebuttresses stabilize the wall, see Fig. 7-6. In Fig.7-8 it can be seen that the horizontal elements ofthe roof trusses rest on and are fixed to thebuttresses. It is important to mention that if thelength of a wall is 12 times larger than itsthickness, it should have an intermediate buttress,see Fig. 7-6 and 7-9. Interior walls must also havea buttress when they meet the exterior walls. Fig.7-7 shows a simple design with shorter walls anda separate kitchen built in Salvador (Equipo Maiz,2001).

Horizontal bamboo rods, as shown in Fig. 7-5,normally do not strengthen the structure. Theyweaken the walls as they disturb the transfer ofshear forces. This is due to the fact, that in practicethere is not sufficient bonding between the rodsand the adobe, as they are not always covered by2 cm of mortar and as the quality of the mortar istoo poor to take the shear forces.

The buttresses at the corners can be substituted bycolumns of reinforced concrete, see Fig. 7-10. Inthe case of the solution shown on the left, it isnecessary to place horizontal steel bars at leastevery 50 cm which grip into the joints with theirangles. This detail was forgotten in allpublications which illustrate this solution. Withoutthis bond, the column will separate from the wallwhen strong horizontal shock occurs in theearthquake.

7-6 Ground plan with system ININVI, Perú (Pereira 1995)

7-5 System ININVI, Perú

7-7 Improved ground plan with system ININVI,(Equipo Maiz, El Salvador 1995)

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7-10 Stabilized corners

7-8 Educational centreAcomayo, Perú(Pereira 1995)

7-9 Adobe walls reinforced bybuttresses

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7.3 Interlocking blocks

Walls without mortar can be built with interlockingblocks. The blocks have holes for verticalreinforcement elements from steel rods or bamboocanes fixed by pouring cement sludge into theholes. The blocks are pressed in special moldsand normally stabilized with cement, seeFig. 7-11. If they have enough verticalreinforcement elements, at least at corners andintersections, and if these are well fixed to theplinth and the ring beam, these walls are supposedto be earthquake-resistant due to their flexibility.The system was developed at the Asian Instituteof Technology, Bangkok. Figs. 7-12 and 7-13shows the first demonstration building, built in1984 in Thailand. In this case the holes werefilled with a mixture of cement and sand in theratio 1:3.Fig. 7-14 shows a similar system, developed bythe University of the Andes, Mérida, Venezuela.The blocks have grooves and tongues whichinterlock. Horizontal ring beams of reinforcedconcrete are placed at a height of 1.20 m and ontop of the wall.

If stacked without mortar these walls do not showany high resistance to horizontal forces as theinterlocking effect is only given by a height ofsome millimeters.Therefore the author developed an improvedinterlocking system, with blocks showing tonguesand grooves 40 mm high, in horizontal as well asin vertical joints, see Fig. 7-15. If these blocksare displaced or lifted up by seismic shocks, theyalways fall back into their normal position. Theholes act as gripholes for easy0 handling, but canalso be used to install vertical reinforcementelements. But these additional reinforcementelements are not necessary if the wall corners areformed by concrete columns, which interlock withthe block as shown in Fig. 7-15, and thesecolumns are interconnected by a ring beam.

7-11 Interlocking blocks (Weinhuber 1995)

7-15 Improved interlocking system of FEB, Kassel, 2001

7-12 and 7-13 Prototype house, Thailand 1984 (Weinhuber 1995)

27

7-14 Building system invented by Universidad de los Andes, Mérida, Venezuela (Pereira 1995)

7-16

7.4 Concrete skeleton walls with adobe infill

Normal masonry walls are not very stable againstseismic shocks. Therefore, nowadays themasonry walls are often framed by concrete whichforms a skeleton structure with adobe (or brick)infill, see Fig. 7-2 and 7-3. The vertical concretecolumns should have 4 steel bars of at least 14mm diameter. It is important that the concreteand the adobes interlock, as shown in Fig. 7-16.For low-cost housing projects this solution isnormally too expensive. Moreover it shows hardlyany flexibility.

cyclopean concretefoundation withcyclopean concrete

reed

reinforcement with reed

interlocking blocks

front door made oftongued andgrooved wood

timber window,fixed glazing

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8. Wattle and daub

The wattle and daub wall system, which is calledbahareque or quincha in Latin America, consistsof vertical and horizontal elements made of timberor bamboo, forming a double layer grid which isfilled with earth. Single-layer systems also exist,see Fig. 8-1. The vertical elements are usuallytree trunks, the horizontal ones bamboo, reed ortwigs. This is the most flexible system, as it isbasically a timber grid structure with flexible jointsand earth infill.The disadvantage of this system is that it needs alot of maintenance, as it cracks easily due to thethin cover of the wood elements and the swellingand shrinking of the wood. In practice there areoften cracks and holes, where erosion starts andwhere insects can live; for instance in LatinAmerica those that create the “decease mal dechagaz”. Fig. 8-3 shows a system, developed byCEPED, Camari, Brazil, with prefabricated wallelements, to be filled locally with earth. Thedesign in Fig. 8-4 by the architects Kühn, Pobleteand Trebilcock show an interesting combinationof rammed earth columns and wattle and daubintersections. This design was developed duringa workshop on earthquake-resistant house design,sponsored by DFG and held by the author in 1998at Santiago de Chile.

A very poor, unstable solution is shown in Fig. 8-5, sometimes built in Latin America afterearthquakes as a quick solution to create shelter.The timber frame walls are filled with adobes,mounted upright and held by barbed wire on bothsides. As the distance between the vertical postsis too large and the wires are not tightenedenough, the adobes easily fall down.

8-1 Traditional wattle and daub system, Venezuela(Minke 2001)

8-2 Wattle and daub systems(after Vorhauer 1979)

29

8-3 Prefabricated system CEPED, Brasil

8-4

8-5 Dangerous solution, Chile

bedroom

bedroom

bathroom

kitchen

living/dining room

movable windows

30

9. Textile wall elements filled withearth

At the Building Research Laboratory (FEB),University of Kassel, different solutions usingtextile earth-filled elements for walls were tested.Fig. 9-1 shows a prototype building, built in 1978in Kassel. The wall consists of hoses made of jutefabric filled with earth and pumice aggregate,stacked without mortar but fixed with strips ofcut bamboo driven through the layers. Theelements were laid in a U-shape, the top of thewall was fixed to a ring beam, see Fig. 9-2. Inorder to avoid rotting of the fabric, the wall wascovered with 4 layers of thin lime paint, see Fig.9-3. The roof structure, built of tree trunks, restson posts separated from the wall.

Within a research project concerning seismic-proof low-cost housing the FEB together with theUniversity Francisco Marroquin and CEMAT,Guatemala, built a prototype house of 55 m2 inGuatemala in 1978, see Fig. 9-7. In this casecotton hoses of 10 cm diameter were sewed, filledwith earth and pumice (Fig. 9-4), dipped into limemilk (Fig. 9-5) and stacked between thin bamboosticks (Fig. 9-6).

9-1 Low-cost housing prototype, University of Kassel, Germany 1978

9-2 and 9-3 Wall with filled textile hoses

31

9-4 to 9-6 Filling and stacking of textile elements

9-7 Earthquake resistant low-cost housing prototype, Guatemala 1978

32

9-8 Earthquake resistant low-cost housing prototype, Guatemala 1978

33

9-9 to 9-11 Textile wall elements filled with earth

Four vertical posts every 2.15 m and thinbamboo rods of 2 to 4 cm diameter every 45 cmstabilized the wall. The surfaces were paintedwith a paint made of 1 bag of lime, 4 kg kitchensalt, 2 kg alum and 30 liters of water. In theprototype structure, shown in Fig. 9-1, anothernew textile system was tested, which is to beseen at the right side of the house. It consists ofa prefabricated U-shaped wall of jute fabric,kept by wooden sticks pushed into the earth.The container (“bag”) formed in this way wasthen filled with pumice and earth, see Fig. 9-9.The model of this system is shown in Fig. 9-10and 9-11.

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10. Critical joints and elements

10.1 Joints between foundation, plinth andwall

For a wall 30 to 40 cm of thickness the foundationshould usually be 20 cm wider and 40 cm or morehigh, see Fig. 10-1, depending on the rigidity ofthe soil. With a 50 cm thick rammed earth wallthe plinth and foundation can be of the samewidth. The plinth is usually built of rubble randomstone or bricks, but can also be of concrete withlarge stone aggregates. As it shelters the wallagainst splashing rain water, the height should beat least 30 cm. The joints between foundationand plinth as well as between plinth and wall haveto have a good bond in order to be able to transfershear forces. They should be situated every 30to 50 cm. The easiest solution is to integrate avertical wooden rod and to create a rough plinthsurface , see Fig. 10-2. In the case of adobe wallsthe mortar must have a very good adhesion and ahigh bending strength. Horizontal damp-proofcourses will interrupt the necessary bond.A proposal by the author, not yet tested, is a“floating” foundation created by a channel ofround pebbles which reduce the kinetic energyof the horizontal shocks, see Fig. 10-3.

10-1 Foundation of external walls

10-2

10-3 Floating foundation

≥3

0cm

rammed earth

≥3

0cm

foundation

rammed earth

plinth

35

10.2 Ring beams

Walls always have to be kept on top by a closedring beam, which must be able to take bendingloads when there are lateral forces against the wall.In order to prevent the walls from buckling and

10-4 Fixing of ring beam 10-5 Fixing of ring beam

10-6

10-7

10-8

falling, the connection between wall and ringbeam must be very strong. The ring beams canalso act as a support for the roof structure. Fig.10-4 shows one way of fixing a wooden ring beamto a rammed earth wall. A better solution is shownin Fig. 10-5 and 12-6, where a vertical interiorreinforcement element of wood or bamboo isfixed to the foundation at the bottom and to adouble ring beam at the top.

With adobe walls without vertical reinforcementelements it is not so easy to obtain a good bondbetween the masonry work and the ring beam. Inthe case of a reinforced concrete ring beam it isnecessary to leave the last layer of adobes withopen vertical joints so that the concrete will gointo the gaps. In the case of adobe walls, if thering beam is made from timber, as seen in Fig.10-6, these elements must be covered by 2 cm ofmortar with good adhesion values.

As corners of ring beams have to be able totransfer moments under seismic forces, they mustbe stiff. Figs. 10-6 to 10-8 and 10-12 showsolutions for stiffening the corners for timber ringbeams, while Figs. 10-9 to 10-11 show solutionsfor reinforced concrete ring beams.

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10.3 Ring beams which act as roof support

If the ring beams act as support for the roofstructure, they have to be positioned centrallyover the wall (Fig. 10-13). In the case of adobewalls the upper layer of adobe may break underseismic movement, therefore it is recommendedthat a top layer of burnt bricks be built for betterstress distribution, see Fig. 10-14. In order totransfer the load uniformly from the roof beamsto the wall, wedges of wood or concrete shouldbe used. Also additional fixing is advisable, seeFig. 10-15.

10-9 10-10

10-11

10-12

10-6 to 10-12Solutions ofstiffening of corners

dangerous

dangerous safe

10-13

10-14

10-15

safe

adobe burntbrick

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11. Gables

If gables are part of the wall, they are very weakagainst perpendicular forces. The best solutionfor avoiding this problem is to build a roof withfour inclined planes (pyramidal shape), with whichno gables appear. The second best solution is tobuild a light gable which is fixed only to the roof,as Fig. 11-1 shows. The third best solution is tobuild a gable wall and to stabilize it with a buttress,see Fig. 11-2. If a concrete skeleton structure isused, which is the most expensive solution, thegable also has to be stabilized by reinforcedconcrete elements, as shown in Fig. 11-3.

11-1 Gable fixed roof

11-2 Stabilization by buttresses

11-3 Stabilization by reinforced concrete structure

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12. Roofs

12.1 General

The roof should be built as light as possible. Roofswith tiles or stone plates are not recommended,as they are heavy and in case of an earthquakethe tiles or plates might fall into the house.For earthquake-resistant houses a pyramidal roofwith 4 inclined planes, which rest on a horizontalring beam, is the best solution. A simple roof ofthis kind is shown in Figs. 12-8 and 12-9. Themost used solution is a roof with one ridge andtwo inclined surfaces, but in this case the beamson which the roof rests, must form a ring and crossthe gable, which needs extra stabilization, seechapter 11, or must be fixed to the roof insteadbeing a part of the wall, see Fig. 11-1.For smaller houses a roof with a single inclinedplane is more economical, but in this case thebeams on which the roof rests need to beinterconnected, forming an inclined ring beam.

12.2 Separated roofs

As the frequency of the movements of roofs andwalls differs during seismic activities, due to theirdifferent moment and weight, the safest solutionis to separate the roof from the wall and have itresting on columns which are positioned insideor outside the wall. Then the roof and wallsystems can move independently of each other.Figs. 12-1 to 12-4 show different proposals of theauthor, utilizing this idea. It is necessary to fixthe columns to the ground at the bottom and tothe roof structure at the top in such a way thatthese connections are partially moment-stiff, butstill allow some ductility. At the top of woodencolumns short diagonals give best solution, seeFigs. 12-1 to 12-9.

12-1

12-2

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12-312-7 to 12- 9 Proposals for plans with separated roof structure

12-4

12-6 Reinforced rammed earth wall systemAlhué, Chile 2001

12-5 Reinforced rammed earth wall systemGuatemala, 1978 (Minke 2001)

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12-7 to 12- 9 Earthquake resistant low-cost housing projectPujili, Ecuador 1989

Figs. 12-5 and 12-6 show solutions of projectswhich were described in chapter 6.4. In the firstcase the columns are positioned inside, in thesecond case outside the walls.Figs. 12-7 to 12-9 show the construction of anearthquake-resistant low-cost housing projectbuilt in 1989 at Pujili, Ecuador (design: GernotMinke and FUNHABIT, Quito). In this projectthe walls are built of two U-shaped rammed earthelements 40 cm thick, separated by a door or awindow. The roof rests on four wooden columns,which stand outside the walls at the corners ofthe square. Though the columns reach into thefoundation and are fixed to the ring beam bydiagonals, the roof system shows sufficientductility within an earthquake.The roof was built of eucalyptus trunks, coveredby caña brava (a kind of reed) and then plasteredwith a mixture of clayey soil with pumice, animaldung, sisal fibers and waste car oil. After drying itwas painted with white paint..

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13. Openings for doors andwindows

Openings within the walls destabilize the wallsystem. In an earthquake diagonal cracks oftenoccur, starting at the window edges, see Figs. 4-1and 4-2. Lintels have to penetrate into the wallfor at least 40 cm in order to achieve a good bond,see Fig. 13-1. However, in this case the part abovethe lintel may be weak and come off in anearthquake, and therefore the best solution is toalso use the lintel as a ring beam on which theroof structure rests. It is also recommended thatthe part below the window be built as a lightflexible structure, for instance from woodenpanels or wattle and daub. The following ruleshave to be taken into account, see Figs. 13-5 and13-6:

a) The length of the windows should not be morethan 1.20 m and not more than 1/3 of the lengthof the wall.

b) The length of walls between openings mustbe at least 1/3 of their height and not less than1 m.

c) Doors must be opened towards the outside.Opposite the entrance door there should be alarge window or another door, which acts asemergency exit, see Fig. 13-6.

13-6 Recommendable positions of openings

13-1 dangerous acceptable

13-2 better

13-3 best

13-4 Stabilized openings

13-5 Recomendable dimensions of openings

13-1 to 13-3

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14-1 Resultant forces and their components(Minke 2000)

14-2 Allowable eccentricity 14-3 Ring beam stabilized by buttresses

14-4 Stabilization of dome entrance

14. Domes

The problem with the structural design of domesis the stress transfer to the foundation. Theinclined thrust force can be divided into ahorizontal and a vertical component, see Fig. 14-1. The steeper the thrust (resultant), the smallerthe horizontal component.The support of a dome must be a circularhorizontal ring of reinforced cement concrete,steel or possibly also timber, and it must be ableto take the horizontal forces of the dome. Thejoint between dome and plinth or foundation mustbe inclined in order to resist the horizontal seismicmovement, see Figs. 14-2 and 14-3. Because ofthe heavy weight of a dome, high ring beams andwalls need to be stabilized by buttresses and the

43

14-7Dome coordinatesfor 7 differentproportions(Minke 2000)

joint of ring beam and wall must be able to transferlarge horizontal forces, see Fig. 14-3.

If the dome rests directly on a low plinth, thestructure is much more stable in an earthquake,see Fig. 14-2. In this case the foundation has toact as horizontal ring beam, and is usually built ofreinforced cement concrete. It is important tocheck that the resultant force of the dome stayswithin the center third of the width of the plinthmeasured above the ring beam, i.e. theeccentricity must not be more than 1/6 of thebase.

If the dome starts above a plinth, it must be takeninto account that openings like doors andwindows destabilize the dome structure.Therefore the tops of door and windows must bedesigned as vaults which penetrate the dome andare able to transfer the stresses from the dome totheir sides, see Fig. 14-4.

An earthquake-resistant dome must have a certainsection, which guarantees that all forces aretransferred vertically to the foundation withoutcreating tensile or compressive ring forces. Theresultant forces must always be within the center

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14-5 A semicircle is dangerous for dome section (Minke 2001)

14-6 Ideal section curve in relation to other well known curves(Minke 2001)

14-9 14-8

parable

ideal curve

semicircle

catenory

of the dome wall, the eccentricity must be lessthan 1/6.

In a dome with a semicircular section theresultants of the forces act inside the center line,thus creating tensile ring forces which can easilylead to collapse, see Fig. 14-5. In Fig. 14-6 theideal section line, which does not create ringforces, is shown in contrast to other usual curves.This curve was derived by a computer program.However, it can be found for 7 differentproportions of height to radius, when using the

coordinates listed in Fig. 14-7, where r is theradius and h the height of the dome - alwaysmeasured to the center of the wall, ααααα is the angleof inclination at the bottom, A the area and Vthe volume.In order to construct such a structurally optimizeddome without formwork, at the FEB a rotationalguide was developed which is fixed to a verticalmast. At the end of the rotating arm an angle isfixed against which the mason lays the adobe orsoil block. So each block can be placed in positionexactly. Figs. 14-8 to 14-11 show the applicationof this construction technique for a dome of 8.80m free span and 5.50 m height, built in La Paz,Bolivia, in 2000. The adobes for this dome weremade by hand in a special mould with roundededges,in order to provide good sound distributionwithin the dome. The acoustic behavior of thedome was further refined by deepening thevertical joints in order to achieve some soundabsorption and by a slight cantilevering position,which avoids the sound focusing effect towardsthe center of the dome.

45

14-8 to 14-11Building of an adobe domeLa Paz, Bolivia 2000

14-10

46

15. Vaults

In an earthquake vaults are less stable thandomes, as described in chapter 14. It is advisableto have a square plan. If a rectangular plan isdesired, buttresses or tensile elements connectingthe beams are required, see Fig. 15-1. To createmore stability, a vault should start directly abovea low plinth instead of above a wall, seeFig. 15-2.The vertical section of a vault should have theshape of an inverted catenary if it only has totransfer its own load. Then it only transfers forcesin compression, see Fig. 15-3.

An important rule for the design of plinth andfoundation is that the resultant force at the bottomof the vault should go through the inner third ofthe surface of the foundation. This means thatthe eccentricity should be less than 1/6, see Fig.15-2. The foundation must have a reinforcedconcrete beam, which can also withstand theadditional horizontal forces created by anearthquake.Fig. 15-4 shows a section of a building which wasbuilt in an earthquake-prone area in Bolivia. Itsplinth has structurally dangerous proportions, asthe resultant force from the vault creates abending moment in the plinth and does not staywithin the inner third of the wall, as necessary.

The facades of vaults should be stabilized likethe gables described in chapter 11. However, thebest solution is to build them light and flexiblewith “wattle and daub”, mats covered with earthplaster, or with timber planks.

Fig. 15-5 shows a design of the author for anearthquake-resistant low-cost-housing project inthe region of Gujarat, India.

A proposal for stabilizing adobe vaults by bambooarches also guaranteeing a certain ductility wasrealized within a test structure built in 2001 atthe University of Kassel, see Figs. 15-6 to 15-9.This was built with special U-shaped adobeswhich rest on an arch, built of three layers of splitbamboo. The bamboo sections were kept in

15-1 Ring beams stabilized by tensors or buttresses

15-3 Inverted catenary as ideal section for vaults (Minke 2000)

15-2 Permitted eccentricity

47

15-5 Proposal for an earthqauke resistant vault structure forIndia

15-4 Badly designed plinth with eccentric thrust line

resultant

Kitchen Bath

water for 3 days in order to be able to bend them.Then they were bent over sticks, which werepushed into the ground in a catenary line, see Fig.15-7. In order to keep the arch in form, the threebamboo sections were wrapped together withgalvanized steel wire every 50 cm. The arch wasput into a vertical position and fixed to steel barswhich stick out of the plinth. This connectionmust be able to take tensile forces within anearthquake. Above the adobe vault a membraneof PVC-coated polyester fabric is fixed andtightened to the plinth. This has two functions:firstly it gives shelter against rain and wind, andsecondly it pretensions the arch and thereforeincreases its stability against movements createdby the earthquake.These movements may deform the vault to acertain extent, so that the adobe joints may open,but the vault will not collapse as it is held by thetensile prestressed membrane at the top and thecompressive prestressed bamboo archunderneath. Thus the stability of this structuredepends mainly on its ductility. However, it mustbe taken into account that if the pretension ofthe membrane is high, the optimal section of thevault is more like an ellipse.

bracing cable

cementmortar

bamboo gridplasteredwith earth

stabilized mud mortar orstraw fixed on bitumen

48

15-6 Production of special adobes

15-7 Construction of the arc with split bamboo

15-8 and 15-9E a r t h q u a k eresistant vaultreinforced withbambooFEB, Kassel, 2001

49

16. Plasters and paints

Adobe walls have to be plastered by mortars madeof earth or lime, or by earth stabilized withcement, lime or bitumen. A pure cement plastershould never be used, as it is too brittle and tendsto crack under thermal loads (through expansionand retraction) and under mechanical impacts.If water penetrates through these cracks, the earthunderneath will expand creating more cracks, oreven burst off.

The church at Ranchos de Taos, New Mexico,see Fig. 16-1, which was built in 1815 with adobewalls, was plastered with cement plaster during arestoration in 1967. Eleven years later the plasterhad to be taken off, as rain water had penetratedthrough the many cracks and caused thedestruction of many parts of the surface.

16-1Church San Franciscode Asís, Ranchos deTaos, EEUU

If an earth mortar is used for plastering, it isrecommended that the surface be madewaterproof by applying a paint of lime or lime-casein. Rammed earth walls do not needplastering. It is better to smoothe the surface witha trowel while it is still humid and then add twoor three layers of thin lime or lime-casein paint.The first layer must have a high water content,so that it penetrates 2 or 3 mm deep into the wall.

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Bibliographic references

Equipo Maiz (ed.): La casa de adobesismorresistente, El Salvador 2001

Grohmann, M.: Introducción al diseñosismorresistente, in: Laboratorio de ConstrucciónExperimental, Universidad de Kassel (ed.):Viviendas sismoresistentes (Report, notpublished),Kassel, German, 1998

Houben, H.; Guillaud, H.: Earth ConstructionPrimer, Brussels, Belgium 1984

ININVI (Instituto Nacional de Investigacióny Normalización de la vivienda de Perú):Construcciones en adobe, Lima, Peru

Minke, G.: Earth Construction Handbook,WIT Press, Southampton, UK 2000

Pereira, G.H.: Habiterra (Catalogue ofexposicion), Bogota, Colombia, 1995

Sibtain, S.N.: To build a village, Parramata,N.S.W. Australia, 1982

Tolles, E.L.; Kimbro, E.E. et al.: Seismicstabilization of historic adobe structures,Getty Conservation Institute, Los Angeles,USA, 2000

Vorhauer, K.: Low Cost / Self Help Housing(Gate Modul 6/6, Eschborn, Germany, 1979)

Weinhuber, K.: Building with InterlockingBlocks,in: basin at gate/gtz (ed.) Wall Building, TechnicalBrief,Eschborn, Germany, 1995

Yazdani, H.: Erhöhung der Lebensdauer vonLehmbauten in erdbebengefährdeten GebietenAfghanistans (Doctoral thesis, not published),University of Kassel, Germany, 1985

Useful contacts

Building Advisory Service and InformationNetwork (BASIN), Germanyhttp://www.gtz.de/basin

Departamento Ingenieria, PonteficiaUniversidad Catolica de Peru (PUC), Peruhttp://www.pucp.edu.pe

Seismological Regional Centre for SouthAmerica (CERESIS), Peruhttp://sipan.inictel.gob.pe/ceresis

Surfing the Internet for Earthquake Datahttp://www.geophys.washington.edu/seismosurfing.html

Forschungslabor für Experimentelles BauenFEB (Building Research Institute), Germanyhttp://www.uni-kassel.de/fb12/fachgebiete/feb/

The figures without sources quoted are fromthe author.

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Gernot Minke is architect andprofessor at Kassel University,Germany, where he heads theForschungslabor fürExperimentelles Bauen (FEB)(Building ResearchLaboratory). Since 1974 morethan 30 research anddevelopment projects havebeen realized in the field ofecological building, low-cost-housing and especially buildingwith earth.In his private architecturaloffice he has designed manyprivate and public buildings, allof them having earth as apredominant building material.His buildings stand not only inEurope, but also in Central andSouth America and India.He is the author of severalbooks, more than 200 articles,has been invited to more than30 international conferencesand was visiting professor inMexico, Guatemala, Paraguayand Venezuela.

About the author