seismic testing of sustainable composite cane and mortar ......and maintained bahareque houses have...

Seismic Testing of Sustainable Composite Cane and Mortar

Walls for Low-Cost Housing in Developing Countries

Institution of Civil Engineers

Research & Development Enabling Fund 2012

Arup and Imperial College London

Project Report

May 2013

A. Y. Elghazouli A. Lawrence

C. Málaga-Chuquitaype S. Kaminski

K. Coates

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Executive Summary

Wattle-and-daub was a popular traditional housing style in many countries around the world,

and continues to be used in some developing areas. These buildings use local materials such

as timber, cane and bamboo to form a composite wall matrix, which is then plastered in mud

to form shear walls. Such houses are inexpensive, sustainable and relatively seismically-

resistant when well-constructed and free from damage such as decay and insect attack.

However, these constructions require a significant amount of maintenance. Also, wattle-and-

daub is not a popular housing choice for poor communities striving to achieve a better

standard of living, due to their association with low-income groups.

An engineered wattle-and-daub type technology has been developed in Latin America. This

construction type builds upon the traditional design but ensures the frame and wall matrix are

properly treated against insect attack, incorporates adequate details for the frame and base

connections and replaces the mud render with a cement mortar. Such houses have been tested

and built successfully in both Colombia and Costa Rica.

Arup and the NGO – REDES – have developed a local bespoke design for El Salvador,

which uses a timber frame clad with local cane. In order to characterise their seismic

response, an experimental investigation has been carried out at Imperial College London into

the behaviour of seven full-scale wall specimens subjected to cyclic loading.

The results from the tests suggest that cane and cement mortar walls behave compositely

under in-plane loads. The tests also showed that the proposed design is able to provide

reasonable capacity levels depending on the material properties. The behaviour of the wall

panel was observed to be stable and relatively predictable. The main failure mechanisms were

identified which include cracking and delamination, beginning at peak stress locations such

as the lintel beams and propagating throughout the panel. Importantly, the use of chicken

mesh ‘reinforcement’ proved useful in ensuring relatively safe failure modes appropriate for

use in house construction.

The results of these tests were used to propose a number of recommendations for the

construction of similar panels in low-cost houses.

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Table of Contents

1.0 Introduction and Background 4

1.1 Introduction 4

1.2 Background to El Salvador 4

1.3 Existing Houses for Low-Income Communities in El Salvador 5

2.0 New Design Proposal 9

2.1 Overview of Design 9

2.2 Structural Design Method 10

3.0 Previous Research 11

3.1 Costa Rica 11

3.2 Colombia 12

4.0 Full-Scale Testing at Imperial College 14

4.1 General 14

4.2 Testing Methodology 14

4.3 Material Details 14

4.4 Construction of Panels 16

4.5 Experimental Set-up 17

4.6 Test Series 21

5.0 Experimental Results 22

5.1 General 22

5.2 Specimen 1 22

5.3 Specimens 2a and 2b 24

5.4 Specimen 3 28

5.5 Specimen 4 31

5.6 Specimen 5 34

5.7 Specimen 6 36

5.8 Specimen 7 40

5.9 Summary of Main Observations 42

5.10 Numerical Modelling 43

6.0 Conclusions 46

6.1 Concluding Remarks 46

6.2 Recommendation for Construction 46

6.3 Recommendations for Future Work 48

References 49

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1.0 Introduction and Background

1.1 Introduction

This report summarises the experimental investigation into the behaviour of sustainable

composite bamboo and mortar walls under lateral cyclic loading, conducted at the Structures

Laboratories of Imperial College London. The research was funded by the Institution of Civil

Engineers 2012 Research and Development Enabling Fund with matched funding from Arup

and Imperial College London. The work is a joint collaboration between Imperial College

London and Arup.

1.2 Background to El Salvador

El Salvador is the smallest country in Central America, bordering Guatemala, Honduras and

Nicaragua. It is about the size of Wales, but with a population of 7.2 million people has the

highest population density of any country in the Americas. Although El Salvador is

approximately average in the World in the UN Human Development Index and the Human

Poverty Index (106th out of 180 countries in the Human Development Index), this masks its

position as the 32nd highest in terms of inequality, as measured by the Gini coefficient (World

Bank, 2012), and 12th highest in terms of ratio of average income of the richest to the poorest

(UNDP, 2009). This gap has been created by a combination of conflict, corruption and

frequent natural disasters.

Running through El Salvador across the East-West axis is a convergent tectonic boundary

between the Cocos and Caribbean Plate, putting El Salvador at risk not only from volcanoes

but also powerful earthquakes (López et al., 2004). Large earthquakes (> Magnitude 6) tend

to hit the country every 12 years, and the capital every 30. The infamous 2001 earthquakes

for example left almost 1200 dead, 1.5 million people homeless and had an economic cost of

$1.6 billion (Bommer et al, 2002) – many communities are still living in either makeshift

shacks or inadequate housing from this disaster. The El Salvadorean Seismic Code

(Asociación Salvadorena de Ingenieros y Arquitectos, 1997), divides the country into two

seismic zones along a crudely North-West to South-East running axis, with a Peak Ground

Acceleration (PGA) of 0.3g inland to the North and 0.4g South towards the coast associated

with a 475 year return period (10% probability of exceedance in 50 years) (see Figures 1 and

2).

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Figure 1: Seismic hazard maps of Central America,

indicating PGA levels at 10% exceedance in 50

years (USGS 2012)

Figure 2: Seismic zones for El Salvador (Asociación

Salvadorena de Ingenieros y Arquitectos, 1997)

Volcanic eruptions are also a significant hazard, but due to early warning systems and good

evacuation protocols fatalities are usually very low.

El Salvador is also exposed to hurricanes which pass through the Caribbean and hit Central

America. Examples include Hurricane Mitch in 1998 which killed over 9000 people and

Hurricane Stan in 2005 which forced 67,000 into shelters nationwide.

Due to the warm tropical climate, termites and borer beetles are prevalent in El Salvador,

which softwoods, many hardwoods and all bamboo and cane are susceptible to.

The climate in El Salvador is tropical, with pronounced wet and dry seasons of average

rainfall per month over 300mm and under 20mm respectively, and temperatures typically

between 20-30 degrees Celsius (Weather and Climate.com, 2013). Timber, bamboo and cane

are all very susceptible to rot in this heavy rainfall and warm climate.

1.3 Existing Houses for Low-Income Communities

Within El Salvador there are three vernacular housing designs that are most common: wood

frames covered by palm fronds (rancho), unfired clay brick (adobe), and a derivative of

wattle-and-daub known as bahareque (in typical order of socio-economic status, lowest first).

All of these use local materials and are relatively cheap to construct. There are also a number

of common forms of more modern designs: wood frames covered by corrugated metal sheets

(lámina), reinforced brick masonry (mixto) and reinforced blockwork (López et al, 2004). See

Table 1 for a breakdown of the various construction types, and Figures 3 to 11 for views of

typical construction types.

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Construction Type Urban Rural Total

Concrete or mixto (confined masonry) 84.5 46.1 71.2

Bahareque 1.5 8.8 4.0

Adobe 7.3 30.6 15.3

Timber 0.5 3.2 1.4

Lámina (metal sheeting) 5.4 8.5 6.5

Rancho (Straw, palm leaves or other plants) 0.2 1.2 0.5

Refuse 0.2 0.7 0.4

Other materials 0.5 0.8 0.6

Table 1: Wall construction type by % of total occupied houses in El Salvador (Ministerio de Economia &

Direccion General de Estadistica y Census, 2008)

Rancho houses are very simple and typically for those at the lowest economic level. They

have no predefined structure since they are in effect simple shacks, and therefore vulnerable

to earthquakes.

Adobe consists of unreinforced unfired mud bricks and a simple timber or bamboo roof. It is

inherently very poor seismically, although with recent research its performance can be

somewhat highly improved. Adobe is poorly regarded by low-income communities as it is

seen as a poor-man’s house, and due to its poor performance in recent earthquakes especially

after the 2001 earthquakes which destroyed over 110,000 adobe houses (32% of all adobe

houses in El Salvador) (Dowling, 2002).

Bahareque (also known as quincha in Peru, cuje in Cuba, pao pique in Brazil and tabiquería)

in other countries historically was more popular in wealthier and/or urban areas, and remains

in many older colonial and rural areas. This form of construction typically consists of a

timber or bamboo frame, clad in a matrix of cane, twigs or timber strips, and finally plastered

in manure or soil, sometimes with straw added for strength (Figures 3 to 7).

Historically, the roof was constructed from palm fronds, but switched to cooler yet heavier

tiles after the Spanish invasion of Central America (López et al, 2004). Properly constructed

and maintained bahareque houses have been shown to possess good structural unity and

flexibility, and therefore have a surprisingly high degree of seismic resistance (López et al.,

2004, and Gutiérrez, 2000). However, there are a number of potential issues with bahareque

that if not adequately addressed can lead to a reduced lifespan and possibly damage or

collapse in an earthquake:

1. Quality of construction: Bahareque is normally built by local skilled workers and short-

cuts are sometimes taken to save time and money.

2. Durability details: Well-constructed bahareque has good features such as a wall

elevated on top of a rock or brick upstand to reduce the risks of dampness, a good

overhang, and treated timber and bamboo (often with pig soap). However, while these

may reduce the risk of decay, damage due to termites and borer beetles is still common.

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3. Roofing: Pre-colonial bahareque used palm fronds for the roofing, while the Spanish

introduced and then enforced clay tiles for the roofing – this change greatly increased

the dead loads on the roof and hence the seismic loads.

4. Maintenance: This is key to reducing damage from moisture, and replacing elements

damaged from insect attack, however in some cases the owners cannot afford

maintenance or are unaware of its importance.

Another important disadvantage of both adobe and bahareque is that they are prone to

harbouring insects, notably “the kissing bug” or chinche. This small biting insect can transmit

Chagas Disease, a potentially life-threatening illness that is estimated to currently affect 10

million people worldwide, mostly in Latin America (WHO, 2010). In addition, bahareque is

not endorsed by the seismic code of El Salvador and therefore does not attract the attention of

potential charities to sponsor housing projects.

Lámina houses are also very basic, and are unpopular because of the excessive heat that

builds up inside due to the metal façade. Mixto is relatively expensive to build due to the

materials required, and quality control during construction is particularly important to ensure

the concrete elements are well constructed and the steel tie reinforcement between the

columns and the panels is properly fitted.

The current most common engineered low-cost house that is being built post-2001 in El

Salvador is reinforced blockwork (see Figure 11), which consists of a simple single-storey

reinforced hollow-blockwork design. The foundations are simple strip footings and the roof is

normally of lightweight cement-fibreboard sheeting or zinc-aluminium sheeting. Tens of

thousands of these houses have been built all around the country, mostly by NGOs but also

some self-built, and they are generally popular with the beneficiaries due to their good

thermal mass, relatively low cost, good seismic performance and perceived social status.

However, these houses are difficult to construct in rural areas due to poor access, display

inherently brittle behaviour in earthquakes, use materials which have a large environmental

impact, and do not put any money back into the local micro-economy which could help to

address the social inequality within the country.

Figure 3: A typical shack, El Salvador (Kaminski,

2013)

Figure 4: Adobe house, El Salvador (Kaminski,

2013)

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Figure 5: Lámina house, El Salvador (Kaminski,

2013) Figure 6: Míxto house, El Salvador (confined

masonry) (Kaminski, 2013)

Figure 7: Traditional Wattle-and-Daub, Latin

America (Casas-Aedo, W & Rivero-Olmos, 2013) Figure 8: Poorly maintained bahareque, El

Salvador (Kaminski, 2013)

Figure 9: Insect and rot damage to bahareque, El

Salvador (Kaminski, 2013) Figure 10: Well-maintained bahareque housing,

Colombia (Gutiérrez, 2004)

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Figure 11: Reinforced blockwork house, El Salvador (Kaminski, 2013)

2.0 New Design Proposal

2.1 Overview of Design

The review presented in the previous section has highlighted the necessity to develop an

alternative low-cost housing design that can address the problems of the existing options,

which still prove to be cost effective. This new design must be seismically-designed and

cheaper or the same cost as the existing reinforced blockwork standard, and should be more

sustainable and put more money back into the local micro-economy. It should also be easy to

construct by the beneficiaries as part of a beneficiary-built program.

In order to achieve this, an engineered vernacular bahareque design was proposed (see

Figures 11 and 12), drawing on existing work in Colombia and Costa Rica. This design uses

the idea of bahareque but replaces the mud with cement render, properly treats all of the

timber and cane and engineers the connections, with the goal of having a seismically-

designed house with a minimum 30 year life-span. The new design has the following

characteristics:

• Single storey 3 or 4 room house.

• A lightly reinforced flat slab as a foundation.

• Two courses of reinforced hollow blockwork topped with a damp-proof membrane to

form an upstand to protect against insects and moisture.

• A treated pine or hardwood 2”x4” frame fixed with nails and light gauge galvanised

steel brackets. Vertical wall studs to be at max 600mm c/c.

• A treated cane (caña brava) wall matrix nailed to the timber (typically 20mm thick).

• Galvanised light gauge chicken mesh also nailed to the timber.

• A structural cement render wall finish to both sides of the cane (resulting in a total

wall thickness of around 60mm).

• A lightweight corrugated cement-fibreboard roof.

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The combination of an upstand, DPM and a large roof overhang is expected to reduce the risk

of rotting of the cane or timber, while the treatment of the timber and cane will reduce the

risk of insect attack.

Figure 12: Prototype of new design constructed in

Bérlin, El Salvador, without render (The El

Salvador Project, 2012)

Figure 13: Completed prototype, Bérlin, El

Salvador (The El Salvador Project, 2012)

2.2 Structural Design Method

With negligible roof self-weight, the loads that the structure must resist will be wind

vertically on the roof (resisted by the timber frame and steel tie-downs to the walls), and wind

and seismic in both the in-plane and out-of-plane directions (seismic generated mostly by the

self-weight of the cement render). Assuming no significant ductility, the initial loads

generated in both the in-plane and out-of-plane direction were found to be governed by the

seismic demand, and an equivalent base shear factor of 1.0g was obtained for a 475 year

return period.

Due to no real composite action being likely in the out-of-plane direction between the timber

studs and the wall matrix, the studs are designed to carry the full out-of-plane load by

themselves, spanning between the double ridge beam and the reinforced upstand.

In the in-plane directions, all walls work as shear walls, transferring the shear down to the

reinforced upstand. It is expected that some shear flow will occur at the corners, resulting in

some benefit from the return walls acting as tension/compression flanges. Because flexibility

of the wall and door layout was deemed vital, the aspect ratios of the walls will vary,

resulting in some of the panels experiencing a net overturning load at a horizontal demand of

1.0g. Accordingly, tie-downs have been introduced in the corners of each wall intersection.

Although it can be considered that the walls will all work as shear walls, a proper

understanding of the actual behaviour under in-plane load is required to verify the design and

improve it accordingly. Precedence has been set by the numerous observations by engineers

regarding the good behaviour of bahareque in past earthquakes (López et al, 2004) including

the successful performance of 30 houses in Límon in Costa Rica after a large earthquake of

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Magnitude Mw 7.8 resulting in local MMIs up to IX (Gonzalez, & Gutierrez, 2003). Previous

research has been carried out in Costa Rica and Colombia which will be summarized below.

3.0 Previous Research

3.1 Costa Rica

The Costa Rican National Bamboo Project (Proyecto Nacional de Bambú – PNB) was

established in 1988 with funds from the Netherlands and administrative support from the

United Nations Development Program (Gutiérrez, 2000). This project brought together

architects, engineers and bamboo specialists from across the world to develop and implement

an engineered form of bamboo bahareque for low-cost housing, which combined the local

vernacular form of hollow bahareque with bamboo bahareque technology from Colombia.

The aim of the project was to develop a cheaper and more sustainable form of housing.

In order to justify the design, in 1990 the Materials and Structural Models National

Laboratory of the University of Costa Rica constructed 13 engineered bahareque wall panels

and subjected them to monotonic in-plane load (Mendoza & Villalobos, 1990). The panels

consisted of a timber frame clad in either caña brava or esterilla and finally rendered with

cement mortar. These tests demonstrated that:

• The load capacity of the wall panels is considerably greater than the seismic load

demand from the Costa Rican code.

• Failure occurred by either buckling of the leading stud in compression or a tensile

failure of the rear stud.

• The cement render did not tend to spall, regardless of the use of chicken mesh.

In 2004, additional cyclic tests were conducted on similar specimens (Figure 14). This

research confirmed the original test results and suggested that the walls had some ductility

under cyclic loading. The stiffness degradation at increasing cycles and displacement levels

was documented (Gonzalez, & Gutierrez, 2003) (Figure 15, and Tables 2 and 3).

Further confirmation of the strength of these types of panels was obtained when a number of

newly constructed PNB houses survived a Magnitude Mw 7.8 earthquake in Límon in 1991,

with local MMI intensities (Modified Mercalli Intensity) of up to IX (Gonzalez, & Gutierrez,

2003). Based on these tests, it is considered that these wall panels tend to work compositely,

with the cement render taking most of the load as a diagonal compression strut, the wall

matrix controlling cracking and out-of-plane buckling and the timber studs taking the vertical

tension induced by the diagonal strut.

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Figure 14: Set-up for cyclic in-plane testing of

wall panel in Costa Rica (Gonzalez & Gutierrez,

2003)

Figure 15: Hysteresis load displacement curves for

one of the wall panels tested in Costa Rica (Gonzalez

& Gutierrez, 2003)

Test Fill Fu

[kN]

∆ at Fu

[mm]

Failure

B-02 Bamboo 63 20 Anchors are withdrawn

BW-01 Bamboo with window opening 82 50 Timber beam at base is crushed through washers

C-02 Caña brava 157 60 Bending of foundation beam

C-03 Caña brava 161 60 Bending of foundation beam

Table 2: Costa Rican tests: failure loads, displacement and mechanisms for each test (Gonzalez &

Gutierrez, 2003)

Displacement level

[mm]

Test

B-02 BW-01 C-02 C-03

5 + 8.8 8.9 17.7 14.1

- 8.4 7.1 11.2 10.7

10 + 9.3 6.9 14.2 11.1

- 7.6 5.9 9.3 8.5

15 + 7.3 4.8 11.6 8.6

- 7.5 4.9 8.4 7.8

20 + 5.5 2.5 10.4 7.4

- - 4.3 7.6 6.3

30 + - - 8.3 7.1

- - - 6.8 5.2

40 + - - 6.9 6.9

- - - 5.0 4.2

60 + - - 5.5 -

- - - 4.2 -

Table 3: Costa Rican tests: stiffness values derived from the load-displacement curves for different

displacement levels in kN/mm (Gonzalez & Gutierrez, 2003)

3.2 Colombia

In 1999 a Magnitude Mw 6.4 earthquake struck what is known as the coffee-growing region

of Colombia, notably the cities of Armenia and Pereira, resulting in 300,000 people left

homeless (Tistl & Velásquez, 2002). After this event it was noticed that while the more

modern masonry and reinforced concrete buildings suffered significant damage and often

collapse, the vernacular bahareque style of housing fared significantly better (Trujillo, 2007).

As such, a number of NGOs and international development agencies implemented housing

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reconstruction projects that principally used bamboo for the structure following the

bahareque style, but with engineering input and modern details. This interest spurred the

Colombian Earthquake Engineering Association to conduct research into engineered

bahareque, which included a series of wall panel tests.

Following this, the Construction Manual for Seismically-Resistant Housing using Mortared

Bahareque was published (Prieto, Mogollón & Farbiarz, 2002), to which some of the new

bamboo houses were designed (some of the projects were implemented before this new

research was completed). This work was based around the traditional form of bahareque that

uses esterilla nailed onto a guadua frame, but improved by engineering the joints and

replacing the mud or manure render with cement mortar.

The seismic testing conducted after the coffee-growing region earthquake in 1999 involved

monotonically-increasing racking tests on a variety of panels with different aspect ratios, with

and without in-plane bracing, using principally guadua clad in esterilla, fine steel chicken

mesh and cement render. The results of the tests were generally similar to those obtained in

Costa Rica.

Further testing has also recently been conducted on a full-scale two-storey braced engineered

bamboo house subjected to uni-directional dynamic loading from a shake table (Figures 16

and 17). This testing was successful, and the design was found to comply comfortably with

the Colombian seismic design code (Francisco-Correal, 2012).

Figure 16: Full-scale two storey house after uni-

directional shake table test at the University de los

Andes, Bogota, Colombia (Kaminski, 2013)

Figure 17: Inside view of tested house, Bogota,

Colombia (Kaminski, 2013)

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4.0 Full-Scale Testing

4.1 General

This section presents and discusses a series of tests performed in order to examine the

behaviour of composite cane and mortar walls subjected to lateral cyclic loading. The aims of

this experimental campaign are to characterise the lateral response of these wall panels as

well as to study the influence of factors such as mortar strength, frame layout, stud size and

orientation, mortar thickness, cane spacing and the use of chicken mesh. Additionally,

preliminary estimations of ductility levels and failure mechanisms are reported.

4.2 Testing Methodology

Since the new house design relies on all walls of the house to contribute to its lateral stability,

including those with windows, the test will model a typical individual wall with a central

window (these walls are typically around 3m long due to the timber length available in-

country). Most walls will be connected to return walls, which are expected to contribute to

the flexural behaviour of the in-plane wall, and therefore a 1m return wall section will be

incorporated into the test.

Because the roof cladding and structure is comparatively light, the majority of the seismic

load attracted by the house will be generated from the self-weight of the walls, and therefore

the most appropriate and realistic method of modelling the lateral load would be by applying

a horizontal UDL along the height of the structure. However, since this is difficult to achieve

in practice, and in order to make the testing more comparable to other in-plane cyclic tests, a

horizontal point load will be applied along the length of the wall instead.

Because the self-weight of the test panel that provides a restoring moment is expected to be

less than the overturning moment generated by the lateral load, tie-down straps will be

provided in addition to the existing tie-down straps in the panel design itself. These will

encourage a shear failure to form instead of a flexural or overturning failure

4.3 Material Details

Cane

Two different canes were used: caña brava (which is the same cane to be used for the

houses) for the main structural walls for tests 2-7 and off-the-shelf bamboo for the first pilot

test and some of the less critical return walls. Importantly, due to availability constraints,

smaller and less representative bamboo was used for the pilot test (Specimen 1). Similarly, in

light of the low level of structural engagement observed for the cane on the return walls,

smaller bamboo was employed in the return-walls of latter specimens.

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Caña brava

The cane used for the testing was Gynerium sagittatum, also known as caña brava in Costa

Rica and vara de castilla in El Salvador. This hollow giant reed can grow up to 14m high and

to a diameter of 40mm (Francis, 2009), although more typically it reaches 6-8m high with a

diameter at maturity of 10-30mm. From the outside caña brava looks nearly identical to

smaller diameter bamboos, sharing similar properties. No test data for caña brava is known

to the authors.

The caña brava for the testing was boron treated by dip diffusion and air dried for around 4

weeks before being shipped direct from Costa Rica. The canes were generally 3m long, with

an external diameter of 10-30mm and in good condition. The cane walls are strongest where

the diameter is thicker – where the diameter is thinner, nailing tends to split the cane fibres.

The treatment method is not expected to change the structural properties of the canes, and in

addition is the same as the expected method to be used on site when constructing the houses.

The cane from Costa Rica is not expected to vary structurally significantly from that typically

found in El Salvador.

Bamboo

The bamboo used for the testing was 12-14mm 4ft off-the-shelf bamboo.

Timber

The timber used for the testing was Grade C16 Scandinavian Pine, 2”x4” (planed to

1.5”x3.5”), pressure treated and kiln dried. The timber to be used for the actual design of the

houses is 2”x4” Grade 2 Southern Pine, planed to 1.5”x3.5”, pressure treated and kiln dried.

Because the timber frame is not expected to be working near failure and the strength and

stiffness properties of the two timbers are very similar, any difference is not considered

significant.

Cement Mortar

The cement mortar used for the testing was made with a simple sand and Portland cement

mixture. In order to consider the realities of construction in developing countries, a weak mix

was typically used to provide a lower bound of the strength of the panel. For example, the

typical sand:cement ratio was around 7:1 and the water:cement ratio used was higher than

might be expected for a conventional render – the latter was important since it is known that

both skilled and unskilled labour routinely add in extra water in hot climates in order to

improve workability, and controlling this is very difficult.

Strengths that were considered varied from 2 to 20 N/mm2 from cube testing, differing water-

cement ratios contributing most to the difference in strengths observed.

Chicken Mesh

The chicken mesh used for the testing uses lightweight steel wire with hexagonal 25mm

holes. The chicken mesh to be used for the house design is expected to be the same if not

denser than the mesh used in the testing.

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Steel Ties

The steel ties used for the latter stud-to-sole plate tie-downs were Simpson Strong-Tie Light

Engineered Strap 600x100 straps. These have a tensile Safe Working Load of 4kN. The

straps for the house design are to be USP steel U-straps, gauge 18, with an allowable seismic

tensile capacity of 7.7kN.

Hurricane strapping

The steel ties used for the first stud-to-sole plate tie-downs were Simpson Fixing Band FB20

ties. These ties were 20mm wide and 0.9mm thick.

Nails

The nails used for the testing were as follows:

a) For the structure: round wire nails, 3.0mm dia., 75mm long. b) For the small diameter bamboo: 2.0mm dia., 50mm long.

For the caña brava: round wire nails, 2.0mm dia., 50mm long.

These nails are essentially the same type and size as those to be used on the houses.

4.4 Construction of Panels

The construction method for each panel was as follows:

1. Assemble timber frame.

2. Lay timber frame onto floor and nail cane on.

3. Nail chicken mesh onto frames (on later tests this was done before fixing the cane).

4. Assemble frames together on rig.

5. Apply cement mortar.

6. After 7-20 days, connect rig and test.

The following are various observations in relation to the different components of the panels:

Timber

The nailing did not split the timber at any location.

Cane

Where the cane was thin or the end distance short (<50mm), the cane often split when nailed.

This was purposely not replaced since it is expected that some of the cane will split during

construction on site.

Mesh

While the mesh was tied as tightly as possible, in some locations it was a little loose to the

wall – these were fixed with tying wire where particularly bad, however again this is

expected to be mildly representative of the finished product on site.

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Mortar

The mortar was applied to the wall over between 3-10 days, depending on the manpower

available and the quality of the previous layer. The layers were typically as follows, and

match the procedure to be used on site for the final design:

1. First layer applied to the chicken mesh side of the panel. This layer needed to be

relatively dry in order to stick well to the mesh. Only one side could be applied at once

otherwise the mortar would be forced off the mesh by the other side’s layer.

2. Second layer applied to the other side of the panel. This layer could be wetter than the

first.

3. Third layer applied to both sides of the panel simultaneously. This layer could be wet.

4. Fourth and final layer applied to both sides of the panel simultaneously. This layer could

be wet. Final finish was made as smooth as possible in order not to introduce potential

weaknesses in the wall, and so that cracking could be easily seen during testing.

Application times between layers varied from 1-7 days, depending on manpower available.

Layers were cured by covering with a plastic sheet and normally spraying with water every

day for 3-5 days post-application. Between layers, the wall was roughened by scoring before

drying in order to provide a good key for the next layer. Before applying a new layer, the wall

was cleaned with a brush to remove any loose material and then dampened with water. The

mortar was applied by unskilled labour which is taken to be representative of the auto-

construction levels expected on site.

The application method used is considered generally representative of the type to be used in

El Salvador, with the main difference that in-country the ambient temperature during

application is typically 30-40 degrees Celcius, while the laboratory temperature for the

testing was approximately 20 degrees. This difference will encourage faster setting of the

mortar on site, however rapid evaporation of the moisture within the mortar may affect its

strength, as might the builder’s enthusiasm to add water back to the mix to compensate –

good quality control is essential to reduce the risk of both of these occurring.

4.5 Experimental Set-up

In order to evaluate the lateral response of the wall panels, a total of seven large scale tests

were performed on specimens with different structural and loading configurations. A

purpose-built test-rig was constructed to facilitate a versatile experimental assessment of

panels. The layout of the test-rig is depicted in Figure 18 and Figure 19. All Specimens were

fixed at the base to a steel rig by means of 10 mm steel bolts. The steel rig was constructed

from welded square hollow sections (SHS 100x10) pre-stressed to the strong floor as

depicted in Figure 18.

A hydraulic actuator operating in displacement control was employed to apply lateral forces

at a given height along the specimen. Different heights were considered throughout the

testing program as explained in the following sections. In order to facilitate a uniform load

18

distribution along the length of the panel, a loading beam formed of two parallel U100

Channel sections was employed as depicted in Figure 19. The loading beam was connected to

the Specimen via 10 mm bolts that were allowed to yield. Lateral restraint was provided by a

SHS100 column guiding the loading beam at its extreme as shown in Figure 19. Additional

panel lateral stability was supplied by the 1 metre-long perpendicular walls constructed at

each end of all Specimens.

Nominal vertical loading was applied at the beginning of the test by means of strong stressed

ties attached to the steel base as presented in Figure 20. The level of load was continuously

monitored by means of load cells attached to the top of the Specimen as can also be

appreciated from Figure 20 (right).

Lateral displacements and forces were recorded by the load cell and displacement transducer

incorporated within the actuator. Displacement transducers and string-potentiometers

installed at selected locations were used to monitor in-plane deformations and distortions. All

tests were conducted under displacement-control. The cyclic testing protocol shown in Figure

21 was used, where ∆ is the applied displacement and ∆y is the estimated yield displacement

similar to the EN12512 protocol (CEN, 2005).

19

Figure 18: Test-rig layout

Loading studs (10 mm diameter) U100 Loading beam

Base fixing studs

Floor bolt

3.050

0.635

Column

Bracing - lateral restraint

0.260 0.440

0.100

SHS 100x100x10

SHS 100x100x10Steel plate t= 10mm

0.260 2.750 0.440

1.500 1.000

0.220

Harnesses for vertical

load application

Specimen

Floor bolt

Fixing studs @ 0.12

25t Reaction frame

25t Actuator

Column - lateral restraint

Hinge

Specimen

Hinge

Strong Floor

Actuator

0.610 3.050 0.610

Variable

RIG ASSEMBLY PLAN VIEW

RIG ASSEMBLY ELEVATION

A B C

20

Figure 19: General view of test set-up

Figure 20: Details of the vertical ties and load cell

Figure 21: Loading protocol

-8

-6

-4

-2

0

2

4

6

8

0 4 8 12 16

Cycle number

.∆

/∆y

Specimen

Loading beam

Actuator

Column

(lateral restraint)

Vertical ties

21

4.6 Test Series

A total of seven large scale panels were tested. A summary of the test series is given in Table

4. After two test variations where the load application point was placed at the top of the panel

(in Specimen 1) and at the middle of the panel (in Specimen 2), the location of the actuator

and loading beam was kept constant at a height of 1.70 m for Specimens 3 to 7. Also, the

number of vertical ties was increased from 2 in Specimen 1 to 4 in Specimens 2 to 7 in order

to prevent an undesirable global overturning failure mechanism. Similarly, in order to avoid

shearing of the stud fixings (which can be determined easily through hand-calculations

therefore does not require testing), the number of 10mm dia. steel studs at the base of the

main panel was increased from 6 in Specimens 1 and 2 to 16 in Specimens 3 to 7.

Specimen 1 Specimen 2 Specimen 3 Specimen 4 Specimen 5 Specimen 6 Specimen 7

Panel No window

With 1m x 1m window

With 1m x 1m window

With 1m x 1m window

With 1m x 1m window

With 1m x 1m window

Independent walls

Loading Top of panel (18 studs)

Mid-height (18 studs)

Top of window (18 studs)

At top of window (14 studs), slotted

holes


holes


holes

2/3 up wall (7 studs per panel)

Vertical ties 2 (on main panel)

4 (on return walls)

4 (on return walls)

4 (on return walls)

4 (on return walls)

4 (on return walls)

2 per wall (on return walls)

Base shear fixing from sole plate to foundation

6 bolts 6 bolts 16 bolts 16 bolts 16 bolts 16 bolts 8 bolts per wall

Fixing from studs to sole plate, in addition to nails

Nailed 150mm stub, no straps

L-straps (return wall and window) and U-straps (to

all)

L-straps (return wall)





Cane in main panel Small-diameter bamboo

Caña brava Caña brava Caña brava Caña brava Caña brava Caña brava

Cane spacing (mm) 10-20 5-10 10-20 10-20 10-20 10-20 10-15?

Spacing between vertical studs (mm) 500 500 1000 1000 1000 1000 1000

Vertical stud strong axis orientation Out-of-plane Out-of-plane In-plane In-plane In-plane In-plane In-plane

Chicken mesh Outer face Outer face Outer face Inner face Inner face None Inner face

Mortar strength [Mpa] (number of samples)

Not obtained 11 (2) 5 (2) 5 (12) 12 (12) 3 (8) 3 (6)

Table 4: Summary of test series

22

5.0 Experimental Results

5.1 General

Table 5 summarizes the main results from the experimental programme. The initial stiffness

represents the tangent stiffness at small displacement levels whereas the maximum measured

force (Fmax) and its corresponding displacement (∆u) are also reported. Table 5 also includes

an estimate of the ultimate load (Fu) corresponding to the stage at which a significant

deterioration in strength was observed. Similarly, estimates of ductility were calculated as a

function of the ultimate displacement (∆u) and the displacement at which full plasticity was

achieved (∆u). The results for individual tests are presented and discussed in the following

sub-sections.

Maximum

Force, Fmax

Displacement

at Fmax

Displacement

at ultimate, ∆u

Ultimate

load, Fu

Estimated

ductility

Initial

stiffness

Test [kN] [mm] [mm] [% of Fmax] ∆u/∆p [kN/mm]

Specimen 1 53.2 10 NA - - 20

Specimen 2 77.4 35 40 79.8 2.7 20

Specimen 3 45.8 25 45 88.4 4.5 20

Specimen 4 42.1 25 45 89 4.5 18

Specimen 5 50.3 55 NA - - 14

Specimen 6 37.2 25 30 84.4 3 14

Specimen 7 27.7 35 - - - 7.6

Table 5: Summary of test results

5.2 Specimen 1

Description

Figure 22 presents a view of Specimen 1 which consisted of a single 3.00 m by 2.10 m panel

with two perpendicular one-metre wide walls at both ends. 18 equally spaced bolts (10 mm

diameter) were used to transmit the load from the loading beam at the top of the panel to the

panel itself. The specimen was fixed at its base to the steel rig by means of 6 10 mm steel

bolts on the main panel and 2 additional bolts on each perpendicular wall. All vertical timber

studs were nailed at their base as depicted in Figure 23. Importantly, vertical restraint was

provided by means of 2 tie-downs at the extremes of the main panel as can be appreciated

from Figure 22. Applying the load at the top of the panel is in line with typical in-plane tests,

however will increase the overturning load on the panel.

Results

The force-displacement curve at the actuator level obtained for Specimen 1 is presented in

Figure 24 whereas the corresponding vertical forces in the ties are shown in Figure 25. It can

be appreciated from both figures that the Specimen behaves nearly elastically until a force

plateau is developed starting at a lateral displacement of around ±15 mm due to overturning

failure of the panel. The base shear capacity of Specimen 1 was around 50 kN and the peak

vertical load reached 12 kN at maximum displacements. This overturning failure prevented

23

the development of the full shear lateral capacity of the panel. Figure 26 presents the typical

failure of the base of the panel at peak deformation (approximately 55 mm of lateral top

displacement).

Figure 22: General view of Specimen 1 before test

Figure 23: Detail of timber connections at the base in Specimen 1

Figure 24: Force-displacement hysteresis for Specimen 1

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

Displacement [mm]

Forc

e [k

N]

24

Figure 25: Vertical forces in Specimen 1

Figure 26: Overturning failure in Specimen 1

5.3 Specimens 2a and 2b

Description

Figure 27 presents a view of Specimen 2 before testing. Specimen 2 included a 1.00 m by

1.00 m window within the main panel. It can also be seen from Figure 27 that the loading

beam in Specimen 2 runs through the middle of the panel while the vertical ties were moved

to the perpendicular walls and doubled up (2 at each side). Additional L-straps and u-straps

were employed to reinforce the timber connections at the end of the panels either side of the

window as presented in Figure 28. The combination of these two changes will halve the

overturning load for the same lateral load, and at the same time increase the capacity of the

panel to resist overturning, with the aim of forcing a shear failure to develop. In addition, the

repositioning of the tie-down straps was to avoid the risk of the pre-tensioning loads in the

straps affecting the results in the main panel (by enhancing the shear capacity). Lastly, caña

brava was used for all wall panels instead of small-diameter bamboo.

0

2

4

6

8

10

12

14

16

18

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

Actuator Displacement [mm]

Ver

tica

l F

orc

e [k

N]

1

2

1 Actua tor

2

25

Results of Specimen 2a

Figure 29 presents the force-displacement relationship for Specimen 2 at the actuator’s height

for the first stage of testing denoted herein as Specimen 2a. Shear cracking started to develop

within the bottom part of the specimen at lateral displacements of 2-7 mm and continued

throughout the test. Mortar cracking was accompanied by deviations from linearity of the

force-displacement curves as presented in Figure 29. Shear failure of the steel studs

connecting the panel to the rig was observed during the first 25-mm cycle at a corresponding

lateral load of around 70 kN. Due to this failure, the specimen was unable to carry further

loads and the test was stopped. Figure 30 presents the state of Specimen 2a at the end of this

phase of testing.

Figure 27: General view of Specimen 2 before test

Figure 28: Detail of connections at the base on Specimen 2

26

Figure 29: Force-displacement hysteresis for Specimen 2a

Figure 30: Damage state of Specimen 2a after test. Outside (left), inside (right). The coloured lines show

the progressive cracking at each level of displacement

Results of Specimen 2b

Specimen 2a was demounted and 10 additional 10mm dia. bolts were installed to connect the

bottom of the panel to the steel base. Testing of the specimen was resumed after the

installation of the additional studs starting with cycles at ± 15 mm. The load-displacement

hysteresis obtained for this second stage of testing (referred herein as Specimen 2b) is

depicted in Figure 31. It is clear from this figure that Specimen 2b exhibited a stable cyclic

response with large levels of pinching. This pinching effect occurs due to the accumulation of

mortar cracking and residual plastic deformations within the timber frame connections. The

peak shear forces recorded are in the order of 75 and 80 kN at peak displacements of +35 and

-35 mm at mid-height, respectively. At this level of displacements, significant spalling of the

cement render of the inner face of the panel was also observed together with important

cracking in the outer face. This accumulation of damage marked a diminution in the

specimen capacity during the last cycles of loading as depicted in Figure 31. Overall views of

the Specimen damage at the end of the test are shown in Figure 32 and Figure 33 for the outer

and inner faces, respectively.

-85-75-65-55-45-35-25-15

-55

1525354555657585

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

Displacement [mm]

Fo

rce

[kN

]

27

It is important to note that the drift distribution along the height of Specimen 2b was not

uniform. This can be further appreciated with reference to Figure 34 which presents the

absolute difference between the displacements measured at the top of the panel and at mid-

height. It is evident from Figure 34 that for mid-height displacements of 20 mm or higher, the

difference between top and mid panel deformations remains effectively constant, indicating a

low level of participation of the top part of the specimen above the loading apparatus. This

partial engagement is also reflected in the damage patters presented in Figure 32 and Figure

33.

Figure 31: Force-displacement hysteresis for Specimen 2b

Figure 32: Damage state of Specimen 2b (outer face) at the end of test

-85-75-65-55-45-35-25-15-55

1525354555657585

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

Displacement [mm]

Forc

e [k

N]

28

Figure 33: Damage state of Specimen 2b (inner face) at the end of test

Figure 34: Difference between top and mid-height displacements in Specimen 2b

5.4 Specimen 3

Description

Figure 35 presents a general view of Specimen 3 before testing. In order to favour a more

uniform distribution of deformations along the height of the panel and more closely model

the real position of the net lateral load on the house, the loading system in Specimen 3 was

placed at 1.70 m from the strong floor (coinciding with the lintel of the window as can be

seen in Figure 35). Other changes included the reduction in the number of vertical timber

studs from the 3 at each side of the window employed in previous specimens (Figure 27) to

only 2 at either side of the window used in Specimen 3 (Figure 35). Also, the orientation of

the timber studs was varied, with the main axis of the timber studs aligned to the main panel

plane in Specimen 3. Similarly, all L-straps at the base were removed and only 3 L-straps

were left in the return walls at the extremes. These three changes were aimed to more closely

mirror the final house design.

0

2

4

6

8

10

12

14

16

18

20

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

Displacement mid height [mm]

Ab

solu

te d

isp

lace

men

t d

iffe

ren

ce

top

to

mid

hei

gh

t [m

m]

29

Results

The cyclic response of Specimen 3 is depicted in Figure 36 while Figure 37 presents the

corresponding vertical loads. It can be seen from Figure 36 that Specimen 3 exhibited a stable

cyclic response with appreciable levels of pinching. The peak lateral forces were about 45 kN

and 51 kN for +25 and -27 mm of lateral displacement at the actuator height, respectively.

Observable deviation from linearity happened at around ±3 mm of lateral displacement

whereas significant non-linear behaviour was evident at around ±10 mm. Peak vertical forces

of around 5 kN were observed at maximum displacement demands as depicted in Figure 37.

Overall views of the damage in Specimen 3 at the end of the test are shown in Figure 38 and

Figure 39 for the outer and inner faces, respectively. Significant spalling of the mortar in the

inner face of the panel was observed starting at around +30 mm and continuing throughout

the test leaving large sections of bamboo exposed as shown in Figure 39.

Figure 35: General view of Specimen 3


-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

Displacement [mm]

Forc

e [k

N]

30


Figure 38: Damage state of Specimen 3 (outer face) at the end of test

Figure 39: Damage state of Specimen 3 (inner face) at the end of test

0

2

4

6

8

10

12

14

16

18

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

Actuator displacement [mm]

Ver

tica

l fo

rce

[kN

]

1

3

1 Actua tor 2

3 4

31

5.5 Specimen 4

Description

Figure 40 presents a general view of Specimen 4 before testing. It is important to note that the

chicken mesh reinforcement was moved to the inside (between the timber frame and the

bamboo) in Specimen 4 in order to test the effect of the chicken mesh on potentially

hazardous spalling. Additionally, the loading beam was not connected to the panel over the

section of the window in Specimen 4 whilst the steel beam holes were slotted as shown in

Figure 41. This was done in order to reduce artificial restraints imposed by the loading

apparatus which could act to inadvertently stiffen and strengthen the lintel portion of the

panel.

Results

The cyclic force-displacement relationship obtained for Specimen 4 is depicted in Figure 42

whereas Figure 43 presents the corresponding loads measured at the vertical ties. It can be

seen from Figure 42 that Specimen 4 exhibited a stable cyclic response with maximum lateral

forces reaching ±42 kN. As with previous specimens, cracking initiated around the corners of

the window at around ±7 mm of lateral displacement, whereas crushing of mortar in the

compression zones at the base of the panel was evident at ± 25 mm. It is also shown in Figure

43 that peak vertical forces of 5 kN were reached at maximum displacement levels.


Figure 45 for the outer and inner faces, respectively. Figure 46 shows close views of

damaged locations after testing. It is important to note that the provision of chicken mesh on

the inner face of the panel was effective in preventing spalling of the mortar even after

significant cracking had occurred as can be seen from Figure 46.


32

Figure 41: Detail of the slotted holes in loading beam of Specimen 4



-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

Displacement [mm]

Fo

rce

[kN

]

0

2

4

6

8

10

12

14

16

18

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70


Ver

tica

l fo

rce

[kN

]

1

3

1 Actua tor 2

3 4

33



Figure 46: Details of damage at the end of the test in Specimen 4

34

5.6 Specimen 5

Description

Figure 47 presents a general view of Specimen 5 before testing. All geometric and structural

characteristics of the previous test (Specimen 4) were conserved in Specimen 5 except for the

mortar strength. The mortar strength was increased from 5.2 MPa in Specimen 4 to 11.5 MPa

in Specimen 5 by halving the sand:cement ratio, reducing the water:cement ratio, and

increasing the quality control and curing time.

Results

The cyclic force-displacement relationship obtained for Specimen 5 is presented in Figure 48

whereas Figure 49 presents the corresponding vertical loads measured at the ties. As

expected, the increment in the mortar strength has a direct influence on the maximum base

shear capacity of the panel which attained ±51 kN of maximum lateral load (20% more when

compared with the 42 kN of Specimen 4). However, despite the increment in the lateral

resistance of the panel, no significant change in the failure mechanism was observed. As with

previous specimens, cracking initiated around the corners of the window at ±7 mm of lateral

displacement in Specimen 5, whereas crushing of mortar in the compression zones at the base

of the panel was evident at ± 25 mm. With respect to the applied vertical forces, Figure 49

shows that peak vertical forces of nearly 7kN were attained at maximum displacement.


Figure 51 for the outer and inner faces, respectively. Figure 52 shows close views of the

damage observed on the lintel and window sections of the panel after testing. It is important

to note that the failure of the timber stud to the right of the window occurred during the ± 55

mm cycle and was caused by unavoidable stress concentrations brought about by the loading

system rather than a natural lateral (seismic) response of the panel – the effect of

concentrating the lateral load onto the horizontal timber beam, acted to load the studs

flexurally, at which point they failed. The wall also failed in shear along a line at the top of

the window, however this was exacerbated by the stress concentrations at the bolt holes

leading to crack propagation.


35




-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

Displacement [mm]

Forc

e [k

N]

0

2

4

6

8

10

12

14

16

18

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70


Ver

tica

l fo

rce

[kN

]

1

3

1 Actua tor 2

3 4

36


Figure 52: Details of damage at the end of the test in Specimen 5. Lintel (left) and window timber stud

(right)

5.7 Specimen 6

Description

Figure 53 presents a general view of Specimen 6 before testing. Specimen 6 has the same

geometric characteristics of Specimens 4 and 5 but does not incorporate chicken mesh

reinforcement. This was done in order to investigate the response of these panels if the

chicken mesh reinforcement fully corrodes within the mortar.

Results

Figure 54 shows the cyclic force-displacement hysteresis obtained for Specimen 6. The initial

quasi-elastic stages are similar to those observed for Specimens 4 and 5 up until ±10 mm of

lateral displacement. From that point onwards, a relatively constant strength response was

developed with base shear values in the order of 33-37 kN. Delamination of both faces of the

panel started during the 10 mm cycles and continued until significant spalling of mortar on

both faces at the end of the test. The test finished after a 25% deterioration in the panel

capacity was evident during the first cycle at ±40 mm.

37

Figure 55 shows the evolution of vertical forces as measured in the ties against actuator

displacements. Maximum forces of nearly 5 kN and 7 kN were observed for ties Number 1

and 3, respectively. It is important to note the initial difference of 1kN between the two sides

of the panel which was unavoidable due to the tightening procedures.

Figure 56 presents a general view of the damaged state of Specimen 6 at the end of the test

for the outer face of the panel whereas Figure 57 presents the corresponding damage state for

the inner face. After this test was finalized, it was decided to remove all residual rendering

within the main panel and resume testing. This was done in order to evaluate the contribution

of the outstanding timber-bamboo frame to the overall lateral response of the panel. Figure 58

shows the state of the specimen after removal of rendering. The same loading protocol

defined in Figure 21 was employed for the subsequent test of the timber-bamboo structure.

Figure 59 presents the force-displacement relationship of the timber-bamboo frame of

Specimen 6 after removal of the rendering. It can be seen from Figure 59 that the contribution

of the timber frame to the full panel lateral response amounts to less than 15 %. A peak shear

force of 5kN was observed for maximum displacement demand levels of 70 mm.


38




-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

Displacement [mm]

Forc

e [k

N]

0

2

4

6

8

10

12

14

16

18

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70


Ver

tica

l fo

rce

[kN

]

1

3

1 Actua tor 2

3 4

39


Figure 58: State of Specimen 6 after removal of rendering

Figure 59: Force-displacement hysteresis for Specimen 6 without mortar on the main panel

-6

-4

-2

0

2

4

6

-100 -50 0 50 100

Displacement [mm]

Forc

e [k

N]

40

5.8 Specimen 7

Description

Figure 60 presents a general view of Specimen 7 before testing. It can be seen from this

figure that Specimen 7 is composed of two separate panels acting in tandem. The aim of this

Specimen is to determine the behaviour of a shorter panel, which is analogous to a typical

panel after the two coupling beams have broken down. Importantly, Specimen 7 incorporates

pinned connections between the horizontal and vertical studs at the location of the loading

beam that was incorporated in order to limit the transition of moments at these locations and

reduce the effect of the stiffness of the timber frame itself. Figure 61 presents a detail of the

pinned connection employed.

Results

The hysteretic response of Specimen 7 in terms of force-displacement relationship is

presented in Figure 62. A smooth transition between elastic and inelastic behaviour can be

appreciated from Figure 62 with a lateral strength capacity in the order of 25 kN. Crushing of

the mortar in the compression zones (bottom corners) was evident during the first 25mm

cycle whereas significant delamination of the rendering in the outer face was observed during

the third cycle at 25 mm amplitude. Shear cracking of the inner face started to develop at 40

mm of lateral displacement demand whereas extensive shear damage was observed during the

55 mm cycles. Figure 63 shows the evolution of vertical forces as measured in the ties against

actuator displacements. The vertical forces remained uniform throughout the test at values of

less than 4 kN.

Figure 64 presents a general view of the damaged state of Specimen 7 at the end of the test

for the outer face of the panel whereas Figure 65 presents the corresponding damage state for

the inner face.


41

Figure 61: Detail of the connection between horizontal and vertical timber studs at the loading beam of

Specimen 7



-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

Displacement [mm]

Forc

e [k

N]

0

2

4

6

8

10

12

14

16

18

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70


Ver

tica

l fo

rce

[kN

]

1

2

1 Actua tor 2

3 4

42



5.9 Summary of Main Observations

A total of seven large scale specimens of composite cane and mortar panels were tested under

prescribed cyclic loading. From this set of tests, the following main observations can be

drawn:

• All specimens (except Specimen 1) observed a stable hysteretic behaviour with

smooth transition between elastic and non-linear response.

• All specimens presented significant levels of pinching that can be attributed to the

accumulation of plastic deformation in the connections and damage in the mortar.

• The number of L-straps and u-straps employed in Specimens 3 to 6 seem to be

sufficient to ensure adequate panel fixing at the base for the range of geometries and

loads studied.

43

• Chicken mesh reinforcement proved to be effective in preventing spalling of mortar

on the face to which it is attached. This effect was evident even after the panels had

accumulated significant cracking and damage.

• Chicken mesh reinforcement proved to be effective at retaining significant enough

portions of mortar on the wall for the mortar to still contribute to the strength and

stiffness of the panel.

• The contribution of the timber-bamboo frame without mortar in Specimen 6 reached 5

kN at maximum displacement demands, equivalent to less than 15% of the total

lateral resistance of the composite panel.

• As expected, the strength of the mortar stands in direct relationship with the lateral

capacity of the panel. A two-fold increment in the mortar strength (in Specimens 4

and 5) caused a 20% increase in the overall panel resistance.

• The cane, mortar, frame and chicken mesh all initially work compositely.

• The bond between the cane and the mortar is sufficient to provide some composite

action.

• Where the previous mortar layer was not well prepared by scoring, debonding often

occurred early. Where the previous layer was well prepared, debonding rarely

occurred.

• The beam above and below the window acts as a coupling beam.

• Failure of the panel typically occurs due to tension within the mortar, beginning in the

coupling beam and spreading to the side walls. Crushing of the mortar may also

occur.

• The method of application of the mortar and the spacing of the canes greatly affects

the strength of the wall panel, therefore quality control of the method of construction

is important.

5.10 Numerical Modelling

A number of preliminary numerical simulations were carried out in order to inform the

testing process and rig-design as well as to provide continuous feedback during the

experimental campaign. To this end, continuum Finite Element (FE) models were developed

in ABAQUS (2012) (Kontovazainiti, 2012). This section presents preliminary results from

such simulations.

In these models, the timber frame elements were represented by means of three dimensional

B31 beam elements. The mortar infill was modelled using quadrilateral four-node three

dimensional S4R shell elements. Material non-linearity in the mortar was taken into account

by means of the Concrete Damage Plasticity Model (ABAQUS, 2012) whereas equivalent

rebars were introduced to simulate the cane reinforcement. Contact phenomena between pairs

of interacting surfaces were modelled in accordance with the test set-up including non-linear

44

links to simulate the steel connections at the base. The material properties of the mortar

correspond to the values reported in Table 4 whereas nominal values were assumed for the

timber, cane and steel components. The models were subjected to increasing lateral forces

distributed along the line of loading specified during the experiments.

Figure 66: General view of the Finite Element model, Specimen 2 (Kontovazainiti, 2012).

Figure 67: Deformed shape of the Finite Element model of Specimen 2 (Kontovazainiti, 2012).

Figure 66 presents a general view of a typical FE model constructed to simulate the response

of Specimen 2 whereas presents the deformation pattern during a full plastic stage. It can be

seen with reference to and the corresponding experimental results depicted in Figure 32 and

Figure 33 that the model is able to faithfully represent the deformation mechanisms and

damage distribution observed during the experiment.

45

A typical comparison between the experimental force–displacement envelope and the

corresponding FE predictions is presented in Figure 68. It is evident from the plot in Figure

68 that the FE model provides a good prediction of the experimental stiffness. The strength of

the specimen is less well predicted due to variations on the mortar material properties as well

as the influence of the concentration of displacements in the vertical ties that were not

accounted for in the FE model.

Figure 68: Comparison of numerical force-displacement relationship and experimental envelope,

Specimen 1 (Kontovazainiti, 2012).

The numerical simulations carried out to date, have highlighted the two main contributions to

the system stiffness, namely the cement plastered walls and the connectivity between the

timber frame and the cement wall panel. In addition, it has been possible to conclude that the

proposed panel design can act compositely. These models also informed the design of the rig

and the dimensioning of the Specimens. Further studies are currently being carried out into

the refinement of the proposed FE models with a view to extend the experimental database

and explore the sensitivity of the response to additional design parameters.

46

6 Conclusion

6.1 Concluding Remarks

The results of the full-scale testing suggest that wall panels of small diameter cane/bamboo

and cement render work compositely to resist in-plane forces. However, the bond between

cane and the cement mortar is considerably worse than between reinforcement and mortar. In

addition, the cold joint created by rendering from either side separately contributes to create a

weak point from which delamination starts.

The simple low-cost panel design that has been developed is difficult to alter to incorporate

changes that would reduce the effects mentioned above. However, significant modifications

may not be required as the test results indicated that the capacity and ductility of the panel in

its current form will be adequate for typical design scenarios.

The tests indicate a range of ultimate strengths of a typical 3m wall panel with a window

from 37kN to 77kN, which are all greater than the design load of approximately 20kN

(equivalent to a design earthquake of 0.4g, conservatively assuming no reduction for

ductility). At ultimate loads, following failure of the coupling beam, the final failure

mechanism of the wall was found to involve cracking and delamination of the mortar, starting

from the base of the wall where the loads and deformations are greatest, and then propagating

upwards with increasing levels of deformation. The amount of spalling can be controlled by

the incorporation of a chicken mesh in construction.

6.2 Recommendations for Construction

Based on the results of this testing, the following are a list of recommendations for the

construction of future low-cost mortar and cane houses. Some of these recommendations will

apply to traditional vernacular bahareque houses as well.

- A minimum vertical stud spacing of ~500mm – more than this leads to deformation of

the canes during mortaring, which weakens the resulting bond.

- Horizontal infill timbers between studs at the connection to the sole plate is

recommended, as this provides a direct surface on which the studs can bear against –

nailing alone may not work well.

- Fixing the return wall to the in-plane wall with nails is crucial for allowing shear flow

and hence mobilising the return walls as flanges.

- Vertical tie-down straps from the end studs to the sole plate (and therefore to the

foundation) prevent pull-out, and are recommended, especially where there is no

significant return wall. Leaving these straps out may be adequate for the design

earthquake, however may result in pull-out in larger events.

- Vertical tie-down straps are typically not required either side of the windows for the

design earthquake. However, if the coupling beams experience damage, vertical tie-

47

down straps at the windows will considerably increase the resulting capacity of the

walls in-plane.

- A base fixing detail of bolts sized in shear for the design earthquake is sufficient.

- A minimum wall thickness of 60mm – this provides protection to the cane and is

adequate structurally.

- Canes should be 10-30mm in diameter.

- Canes should be well dried before fixing – although this makes them more susceptible

to cracking, otherwise they will dry and shrink away from the mortar. An alternative

is to fix the canes onto the frame and then leave them for a couple of weeks to air dry.

- Clear gaps between canes should be 10-15mm – greater than this leads to difficulty in

mortaring the first layer, and less than this leads to a weak structural bond.

- Canes must be continuous between studs. The canes are stiffest when they are

continuous over multiple studs, therefore long cane lengths are preferable.

- Canes must be nailed to each and every stud – a small nail diameter (e.g. 2mm) is

preferable to reduce the risk of splitting the cane.

- Chicken mesh is strongly recommended to be applied on the inside face of the

external walls, and on both faces of all internal walls, to prevent spalling (which helps

both structurally but importantly also for safety).

- Chicken mesh should be pulled tight and nailed to all studs and sole plates. Excess

mesh should be cut away to ease mortaring. Where the mesh is not tight up against the

cane, it should be tied onto the cane.

- Mortar should be protected from direct sunlight when curing, and should be regularly

sprayed with water for the first 7 days.

- Each mortar layer must be well prepared for the next layer by e.g. scoring – this is

essential.

- The surface of the previous layer should be cleaned to remove loose material, and

moistened, prior to application of the new layer.

- Each mortar layer must be well worked into the prepared previous layer.

- Mortar should be applied in layers <10mm thick – greater than this leads to a weaker

bond as the self-weight of the mortar pulls itself away from the wall.

- Well-graded sand in the mortar provides a much better finish and is easier to work

with.

- A sand-cement ratio of 6:1 or less is recommended – higher cement content will lead

to a higher strength.

- A high water content in the mortar will lead to a mix that is considerably easier to

apply and work into the gaps. Although this will lead to a weaker mix, the strength of

48

the panel is still likely to be adequate for the design earthquake, and ease of mortar

application leads to a better bonded panel.

6.3 Recommendations for Future Work

The findings presented in the previous sections have highlighted the potential advantages of

employing cane mortar composite panels to build low-cost earthquake-proof houses in

developing countries. The experimental results provide direct assessment of the number of

geometric and loading parameters, and represent valuable validation data for the purpose of

future numerical and design models. The studies carried out have suggested the need for

further research in the following areas:

- Additional in-plane cyclic tests, aimed at evaluating the influence of a number of

design modifications and optimizations ought to be conducted. Such modifications

should include the utilization of chicken mesh on both sides of panel, primer or bond

material placed over the cane to improve the bond to the cement render, different

retrofitting alternatives, etc.

- Also, the effects of different loading protocols on the panel structural response

parameters should be studied. This may include testing protocols with increasing

versus decreasing amplitudes and different number of cycles.

- Similarly, the characterization of the out-of-plane seismic response of composite cane

mortar panels is required.

- Full-scale shake table testing of a cane/bamboo and cement render house would be

desirable in order to test its global behaviour under dynamic loading.

49

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