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I hereby declare that, except where specifically indicated, the work herein is my own original work.

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Improving the Seismic Resistance of Adobe Buildings

by

Stephen Jones (R)

Fourth-year undergraduate project in Group D, 2006/2007

i

Improving the Seismic Resistance of Adobe Buildings: Technical Abstract Stephen Jones (R), Fourth-year undergraduate project in Group D, 2006/2007

Adobe is a naturally-dried earthen brick used for building. Unfired earth construction such as

adobe is used by an estimated 30% of the global population, rising to 50% in developing countries

(Houben and Guillaud, 1994). However, adobe buildings are very vulnerable to earthquakes

because adobe is a low strength, brittle material, and so tends to suffer sudden catastrophic failure.

Further common problems include a lack of: maintenance; involvement from professional

engineers; or structural reinforcement (Tetley and Madabhushi, 2007, and Dowling, 2006).

Unfortunately, there is a strong overlap between the regions of the world where significant use of

earthen building occurs and zones of moderate or greater seismic hazard (De Sensi, 2003).

Previous research has aimed to address this problem by providing methods of reducing the seismic

vulnerability of adobe construction which are suitable for use in the low-income regions most at

risk. However, there has been insufficient large-scale take-up of these potential improvements,

because “most of the proposed systems are too complex and/or too costly to be widely used

without sustained external intervention” (Dowling, 2006).

This project firstly aimed to test scale models of unreinforced adobe walls on a 1-g shaking table

to simulate accurately the behaviour of real adobe buildings under earthquake loading by

modelling realistic collapse mechanisms. The second objective was to test scale models of adobe

walls with structural improvements to enhance seismic resistance, in order to assess the relative

effectiveness of these strengthening techniques. The improvements were designed to be simple

and cheap enough for potential widespread use in developing countries.

Models of single in-plane walls and ‘L’-shaped corner wall joints were tested at 1:5 scale and

exhibited typical real-life collapse mechanisms. A suitable material composition, supporting the

findings of Tetley and Madabhushi (2007), was 60% gravel of size 30mm - 80mm and 40% mortar

by weight. The mortar was made from 60% Kaolin clay at w = 0.45 and 40% building sand by

weight. It was necessary either to leave the completed model to dry for 30 or more days or to pre-

dry the sand in an oven to ensure the model had dried sufficiently before testing, to simulate the

drying process in real-life adobe construction.

Improving the Seismic Resistance of Adobe Buildings: Technical Abstract

ii

In the second part of the project, it was found that external vertical bamboo reinforcement, joined

through the walls of the building and tied externally with horizontal wire, can improve the seismic

resistance of the building by increasing the ductility of the walls and providing confinement for the

adobe blocks. In 1:5 scale tests on ‘L’-shaped corner joints, the capacity to resist severe structural

damage (collapse of the transverse wall) was increased by at least a factor of 2 using the method of

external bamboo reinforcement. The effect is of a similar magnitude to that observed in recent

research on a similar technique by Dowling (2006). Further research would be needed to assess if

this factor could be expected in real life if whole buildings were reinforced in this way.

A further test on a model with external vertical bamboo reinforcement with a smaller spacing

between the bamboo canes showed that seismic resistance due to confinement of the adobe is

dependent on the relative sizes of the adobe blocks and the spacing of the bamboo and wire. In real

life it is likely that there would be a compromise between the amount of confinement, the

materials available, and the number of holes which could be drilled through the walls safely.

External bamboo and wire has the potential to be used to retrofit existing houses provided holes

can safely be drilled in the walls to allow the attachment of bamboo. The technique can also be

used in new constructions. Both bamboo and galvanised wire are materials already used in the

internal reinforcement of adobe buildings and are generally low cost and widely available. Further

research is needed in the field to ensure that the technique has widespread suitability in terms of

public acceptance and ease of use. The author will work with the Salvadorean Foundation for

Reconstruction and Development in El Salvador to perform a structural survey of adobe houses in

order to assess the appropriateness of this method for strengthening existing buildings.

A third structural improvement test found that a combination of internal vertical bamboo

reinforcement and internal horizontal wire can improve the seismic resistance of the building by

increasing the ductility of the walls and increasing the connectivity at corner joints. There may be

differences in the effectiveness of this method depending on whether irregular blocks are used

which can fit around the bamboo (as in this project) or regular blocks which must be cut to fit the

bamboo into the wall and may produce weaknesses due to discontinuity problems in the adobe (as

tested by Dowling, 2006). Internal reinforcement is only suitable for newly-built constructions and

so has less widespread application.

Contents Section 1: Introduction .............................................................................................................. 1

1.1. Background ....................................................................................................................... 1 1.2. Project objectives .............................................................................................................. 2

Section 2: Literature Review..................................................................................................... 3 2.1. Seismic performance of adobe .......................................................................................... 3 2.2. Laboratory testing of model adobe walls and buildings ................................................... 3

2.2.1. Modelling adobe buildings and seismic strengthening methods................................ 4 2.2.2. Testing models using a shaking table to simulate an earthquake............................... 5

2.3. Existing guidelines on adobe construction........................................................................ 6 2.3.1. General layout and construction features................................................................... 6 2.3.2. Adobe block construction methods and materials ..................................................... 7 2.3.3. Wall reinforcement methods and materials................................................................ 7

Section 3: Experimental techniques ......................................................................................... 8 3.1. Shaking table and input motion......................................................................................... 8 3.2. Visual data recording ........................................................................................................ 8 3.3. Instrumentation and data recording................................................................................... 9

Section 4: Modelling adobe walls and seismic strengthening methods ............................... 10 4.1. Key issues and summary of model wall designs chosen................................................. 10 4.2. Size of the model walls ................................................................................................... 12 4.3. Material composition and construction of the model walls ............................................ 13 4.4. Method used to model a continuous transverse wall and boundary conditions .............. 16 4.5. Techniques used to model structural improvements ....................................................... 17

4.5.1. External vertical bamboo tied with external horizontal wire ................................... 17 4.5.2. Internal vertical bamboo tied with internal horizontal wire..................................... 18

Section 5: Behaviour of single walls........................................................................................ 20 5.1. Visual observations ......................................................................................................... 20 5.2. Summary of single wall results ....................................................................................... 24

Section 6: Behaviour of ‘L’-shaped corner wall joints ......................................................... 25 6.1. Visual observations ......................................................................................................... 25 6.2. Instrumentation data recorded......................................................................................... 29

6.2.1. Observations on accelerometer data......................................................................... 32 6.3. Summary of results for ‘L’-shaped corner wall joints .................................................... 33

Section 7: Behaviour of ‘L’-shaped corner wall joints with improvements ....................... 34 7.1. Visual observations ......................................................................................................... 34 7.2. Moisture content data recorded ....................................................................................... 39 7.3. Instrumentation data recorded......................................................................................... 39

7.3.1. Observations on accelerometer data......................................................................... 41 7.4. FFT analysis of acceleration data.................................................................................... 41

7.4.1. Observations on FFT analysis.................................................................................. 43 7.5. Summary of results for ‘L’-shaped corner wall joints with improvements .................... 45

Section 8: Conclusions ............................................................................................................. 46 8.1. Conclusions related to simulating adobe buildings using scale models.......................... 46 8.2. Conclusions related to structural improvements of adobe buildings .............................. 47 8.3. Recommendations for future research............................................................................. 48

Section 9: References ............................................................................................................... 49

1

Section 1: Introduction

1.1. Background

Adobe is a naturally-dried earthen brick used for building, usually with mortar made from the

same material as the bricks. Unfired earth construction such as adobe is used by an estimated

30% of the global population, rising to 50% in developing countries (Houben and Guillaud,

1994). Adobe’s widespread use is due to a number of advantages which include: low cost; ease

of use; wide availability; durability; high thermal capacity; and energy efficiency.

However, adobe buildings are very vulnerable to earthquakes, predominantly because adobe is

a low strength, brittle material, and so tends to suffer sudden catastrophic failure. Further

common problems include a lack of maintenance and an absence of involvement from

professional engineers (Tetley and Madabhushi, 2007), and a lack of structural reinforcement

in the majority of constructions (Dowling, 2006). Unfortunately, there is a strong overlap

between the regions of the world where there significant use of earthen building occurs and

zones of moderate or greater seismic hazard (De Sensi, 2003), as shown in Figure 1.1.

(a) (b)

Figure 1.1(a). Regions where significant use of earthen building occurs (De Sensi, 2003).

Figure 1.1(b). Zones of moderate or greater seismic hazard (De Sensi, 2003).

Considerable previous research has aimed to address this problem by providing methods of

reducing the seismic vulnerability of adobe construction which are suitable for use in the

countries at risk (discussed in Section 2.3). However, there has been insufficient large-scale

take-up of these potential improvements, evident in the high numbers of deaths caused by

recent earthquakes in regions of high adobe use. For example, major earthquakes in India, Iran

and Pakistan in the last six years, regions where adobe housing is commonly used, each killed

between 20,000 and 80,000 people (USGS, 2006).

Section 1: Introduction

2

(a) (b)

Figure 1.2(a). Adobe building failure in Pakistan earthquake, 2005 (www.bbc.co.uk).

Figure 1.2(b.) Collapse of many adobe buildings in Iran earthquake, 2003 (www.ngdir.ir).

Recent research has concluded that the reason for the lack of widespread use of adobe

improvements developed so far is that:

“most of the proposed systems are too complex and/or too costly to be widely

used without sustained external intervention” (Dowling, 2006)

Therefore it is clear that further research is needed to develop suitable methods of improving

the seismic resistance of adobe buildings which are simple and cheap enough for widespread

acceptance and use.

1.2. Project objectives

(a) To test scale models of unreinforced adobe walls on a 1-g shaking table to accurately

simulate the behaviour of real adobe buildings under earthquake loading.

(b) To test scale models of adobe walls with structural improvements to enhance seismic

resistance, which are simple and cheap enough for potential widespread use in developing

countries, to assess their relative effectiveness.

(c) To disseminate the results in appropriate useful formats to relevant organisations and

practitioners working in the field.

3

Section 2: Literature Review

2.1. Seismic performance of adobe

Experience from past earthquakes has shown the most common ways in which adobe buildings

fail under seismic loading. These are summarised in Figure 2.1. The main collapse mechanisms

are: overturning of transverse walls; vertical cracking at corner joints; and diagonal shear

cracking of in-plane walls. The diagrams are simplified to assume that earthquake motion is

parallel to one set of walls and perpendicular to the other. In reality this is not usually exactly

true so combinations of the different mechanisms are likely.

(a) (b)

Figure 2.1(a). Typical failure mechanisms of adobe buildings (from Blondet et al, 2003).

Figure 2.1(b). Typical failure mechanisms of walls only (from Tolles and Krawinkler, 1990).

The key criteria considered suitable for measuring the seismic performance of adobe buildings

(cited by Dowling, 2006, and Tolles and Krawinkler, 1990) are: no damage during minor

earthquakes; tolerable damage during moderate earthquakes; and heavy damage but no

collapse during extraordinarily severe earthquakes. The evidence of past earthquakes shows

that large numbers of adobe buildings do not currently meet these criteria.

2.2. Laboratory testing of model adobe walls and buildings

A variety of research projects which have included dynamic adobe model testing has been

undertaken over the past 30 years. The main outcomes most relevant to this project are


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summarised as follows, in two categories: the techniques used to model the adobe buildings

and any improvement methods used, and the experimental procedures used for dynamic testing

of models using a shaking table.

2.2.1. Modelling adobe buildings and seismic strengthening methods

Scale model size: Previous testing on 1:2 and 1:5 scale models has concluded that in general

the behaviour of the smaller models could accurately predict the behaviour of the larger

models, with similar failure modes and similar response to improvement methods at the two

different scales (Tolles et al, 2000). Previous tests on 1:5 scale models in Cambridge

University were considered to model typical real-life collapse mechanisms well (Tetley and

Madabhushi, 2007). However, testing at 1:20 scale did not simulate common real-life collapse

mechanisms consistently (Malton, 2005).

Modelling adobe bricks: Some previous research has used gravel to simulate adobe blocks

and simplify the modelling procedure, instead of making individual bricks as is usual in real

life. This is based on the assumption that failure planes usually pass through the mortar which

surrounds the blocks, rather than through the bricks (Tetley, 2006). However, some other

testing has observed failures through bricks themselves (Dowling, 2006). Also, the bond

strength between the mortar and the brick is affected by the brick material. This is because the

bond is a function of the amount of water absorbed into the brick, which softens the brick to

allow a stronger brick-mortar bond (Tolles and Krawinkler, 1990). A piece of gravel is able to

absorb less water than an adobe brick. The two factors of differing bond strength and possible

failure through blocks would suggest that constructing individual adobe blocks to scale would

provide a more realistic model than the use of gravel, if this was practical at the scale used.

Considering the effect of gravity: Some previous research has neglected the effect of gravity

on scale models, by assuming that gravity-induced stresses on adobe buildings (which are

usually single-storey) are much less than stresses induced by seismic loading (Tolles and

Krawinkler, 1990, Malton, 2005, and Tetley, 2006). However, other projects have noted

differences in performance in models with different gravity loads and concluded that increased

gravity loads increase diagonal cracking and subsequent damage (Tolles et al, 2000).

Considering the effect of roof loads: Some previous research has considered the effect of

varying roof loads by adding different masses to the tops of the model walls (e.g. Flores et al,


5

2001). Other research has assumed that the light roofs typical of adobe buildings have

negligible effect (Dowling, 2006).

Modelling realistic failure patterns: Previous research has emphasised that adobe scale

models should exhibit real-life failure patterns (such as those described in Section 2.1) if they

are to provide useful information on the likely behaviour of real-life buildings (e.g. Dowling,

2006). Two key factors which affect failure mechanisms are the materials used to simulate the

adobe (Tetley, 2006), and the boundary conditions imposed on models which are partial

sections of a wall (usually ‘I’, ‘L’ or ‘U’ shaped) instead of complete houses (Dowling, 2006).

Reinforcement materials and techniques: Research should consider both horizontal and

vertical reinforcement. Reinforcement can be either internal or external to the walls, but only

external reinforcement is suitable for reinforcing existing houses. A wide variety of ductile

materials have been investigated for internal reinforcement but less research has focused on

external methods. These include geogrids or polymer mesh attached with polypropylene ties

through the wall (Blondet et al, 2005), bamboo tied with wire and string (Dowling, 2006) and

wire mesh nailed with metal bottle caps (e.g. Zegarra et al, 1999, and Tetley, 2006).

2.2.2. Testing models using a shaking table to simulate an earthquake

Sequence of earthquakes which are input to models: A consistent sequence of earthquakes

simulated for each model tested in a project is not always used (e.g. Yamin, 2005, and Tetley

and Madabhushi, 2007) but is desirable if possible (Dowling, 2006) to provide a more

consistent basis for comparing the results of different models.

Frequency of earthquakes which are input to models: Differing approaches have been taken

to selecting the earthquake excitation frequencies used in model testing. Dowling (2006)

proposes that for dynamic similitude, the frequency ratio used in each model test should be

identical, where the frequency ratio is defined as the “ratio of dominant input excitation

frequencies to structural frequencies (first natural frequency of each specimen)”. This requires

the input spectra to be uniquely time-scaled for each model test to change the dominant input

frequency, since each model will have a different first natural frequency. This method then

allows each model to be tested at near-resonant conditions by matching the dominant

frequencies of the input earthquake with the natural frequency of the model. This ensures that

each model experiences the most damaging conditions possible. However, this approach has


6

not been followed in other research. Instead, usually an unscaled input earthquake series has

been used and each model has been tested with the same set of earthquakes. As discussed in

Section 3.1, this approach can still provide useful information about structural response at non-

resonant frequencies, provided that it is established during testing that the input frequencies are

not near to the natural frequency of the model.

Data recording: Much of the previous research has produced qualitative data only. However it

is recommended that greater quantitative data would be useful to examine more detailed

aspects of model behaviour and to provide information for the development and validation of

possible finite element models of adobe structures under earthquake loading (Dowling, 2006).

2.3. Existing guidelines on adobe construction

A variety of guidelines exists on adobe construction for improved seismic resistance. The

scope of the guidelines tends to cover the following key areas for construction: general layout

and construction features; adobe block construction methods and materials; and wall

reinforcement methods and materials.

In the existing guidelines there is little discussion of the varying cost and complexity of the

different methods proposed, nor issues such as local availability of materials or techniques for

retrofitting improvements onto existing buildings. These themes have all been proposed for

further investigation (Tetley, 2006, and Dowling, 2006).

The form of horizontal and vertical wall reinforcement is the main aspect within the scope of

this project. However, issues which are important in the other areas of construction are also

described here. This is because any wall reinforcement methods developed must take into

account the other strengthening techniques that may be used in the construction of adobe

buildings, to ensure compatibility.

2.3.1. General layout and construction features

The guidelines typically include information on site selection, building plan layout, dimensions

of walls and openings, construction of foundations and roofs, the use of buttresses or pilasters,

and the use of a ‘ring beam’ (a continuous beam, usually made of timber or concrete, which

runs around the tops of the walls and ties the walls together). Despite details on particular


7

dimensions and specifications differing between the guidelines, there seems to be a broad

consensus on the factors that are most important. However, since the construction of complete

model houses was beyond the scope of this project, detailed consideration of house layout and

features was not required.

2.3.2. Adobe block construction methods and materials

Dowling (2006) observes that there is no overall agreement in the literature on the best

proportions of different soil types that should be mixed together to produce adobe blocks.

However, a simple test is available to ensure sufficient strength in whatever soil mixture has

been produced. This involves making dried balls of soil which cannot easily be crushed

between finger and thumb (IAEE, 2004). There are also tests suggested to assess the strength

of finished adobe blocks after four weeks’ drying by supporting in bending the weight of a man

(IAEE, 2004) or by checking for specified maximum crack lengths, widths and depths

(Middleton, 1987). These tests avoid the need to rely on following prescriptive mix

proportions, which may be difficult to satisfy due to limited local availability of soil types.

Overall the guidelines recommend that block size and strength are much less important than the

mortar properties because the mortar must dry in-situ between the blocks and so is more

susceptible to weaknesses from micro-cracking than the blocks, which dry separately (IAEE,

2004). This supports the argument of Tetley (2006) that adobe buildings can be modelled using

gravel to mimic adobe blocks instead of making individual blocks themselves.

2.3.3. Wall reinforcement methods and materials

A key aspect in all guidelines is the use of ductile horizontal and vertical reinforcement.

Reinforcement materials include bamboo, reeds, cane, vines, rope, timber, chicken wire,

barbed wire, or steel bars (Blondet et al, 2003). All reinforcement should be attached to the

other reinforcement and structural elements of the building (the foundations, roof, and ring

beam, if present) to provide a ‘stable matrix’ which restrains bending and therefore minimises

cracking and collapse of the walls. Horizontal reinforcement also assists in the transmission of

forces from out-of-plane transverse walls to in-plane shear walls. Vertical reinforcement also

helps minimise out-of-plane bending and in-plane shear. Most methods proposed so far use

internal reinforcement, which is a method only feasible on newly-built adobe houses.

8

Section 3: Experimental techniques

3.1. Shaking table and input motion

All earthquake simulations were undertaken on a uni-axial shaking table at the Schofield

Centre, University of Cambridge. For each earthquake simulated, one frequency (set via a

control box) and one maximum peak displacement level (set by manually changing the crank

radius of the table) were chosen. Unlike some shaking tables, the table used in this project is

not capable of modelling complex input spectra. The specification of the shaking table is

shown in Table 3.1.

Table 3.1. Specification of 1-g shaking table at the Schofield Centre.

Size of table 900mm × 450mm Nominal maximum displacement ±11mm Nominal maximum acceleration ±1.02g Nominal maximum frequency 4.8Hz

After the initial tests to establish realistic collapse mechanisms, the procedure used for all

experiments for comparing structural improvements was first to subject the model to a small

amplitude simulated earthquake at a particular frequency to establish that the test was not at

resonant frequency. (If the frequency was near the natural frequency of the model then there

would be a disproportionate response observed even at small amplitudes). The model was then

subjected to a series of increasing amplitude earthquakes at the same frequency until collapse.

This approach ensured that none of the tests was performed at near-resonant frequencies and so

allowed comparisons of the response of the different models in different tests. This was a more

suitable approach for the facilities available than changing the input frequency to ensure all

tests are at near-resonant frequencies, as proposed by Dowling (2006) and discussed in Section

2.2.2.

3.2. Visual data recording

All tests were recorded as videos using a standard Canon digital camera. The camera was also

used to take photos to show the stages of collapse of a model. A high speed Phantom video

camera (with a capacity of 1000 frames/second) was used to record video of the tests at 100

frames/second, to allow detailed observation of collapse mechanisms.

Section 3: Experimental Techniques

9

3.3. Instrumentation and data recording

A series of accelerometers and an LVDT (Linear Variable Differential Transformer)

displacement transducer were used to record the acceleration and displacement of particular

locations on the shaking table and models. These approximate locations are shown in Figure

3.1 (model design is discussed in Section 4). Each accelerometer location on the model varied

up to 25mm between different tests because accelerometers had to be fixed to a smooth, flat

section of adobe on the model wall (despite the use of wooden moulds to assist construction,

discussed in Section 4.3, the whole surface of the walls was not completely smooth).

Figure 3.1. Locations of accelerometers and LVDT on model walls and shaking table.

The accelerometer and LVDT data was recorded via junction boxes (which converted the

LVDT and accelerometer signals to volts) onto a computer using DasyLab and then

manipulated after testing using Microsoft Excel and MathWorks MatLab. The accelerometers

(DJ Birchall A/32 piezo-electric charge output model) were calibrated using a Brüel and Kjaer

Accelerometer Calibrator Type 4291.

However, the model of accelerometer used has limited accuracy at the frequencies (5Hz or

below) used in this project. As discussed in Section 6.2, there was a consistent difference found

between the recorded acceleration data from the accelerometers and the calculated acceleration

determined from the displacement and frequency measurements. This difference is likely to be

due to the inaccuracy of the accelerometers at the frequencies used. However, since the

difference was consistent it was concluded that the results obtained were still valid for

comparisons between different wall positions, earthquakes and tests.

Ac5

Ac7

Ac6 Ac3

Ac2

Ac1

Ac4

SHAKING TABLE

MODEL WALL

LVDT

Direction of earthquake motion

10

Section 4: Modelling adobe walls and seismic strengthening methods

4.1. Key issues and summary of model wall designs chosen

The main considerations in designing the model walls to be tested were the need to ensure:

• Failure of the model walls within the capacity of the shaking table so that the collapse of all

models could be observed and comparisons made of the improvements tested.

• Failure of the model walls by realistic collapse mechanisms to accurately simulate real-life

behaviour of adobe buildings under earthquake loading.

To address these needs, two types of model adobe walls were tested during this project:

• Single unreinforced model walls were tested in-plane with the motion of the shaking table

to check that the in-plane walls (and therefore also the more vulnerable out-of-plane walls)

of unreinforced corner joint models would fail within the capacity of the shaking table.

• ‘L’-shaped corner wall joints were tested (with and without structural improvements) to

observe behaviour of the corner and the out-of-plane wall section, typically the two most

vulnerable parts of adobe buildings. This allowed confirmation that typical real-life collapse

mechanisms were modelled, as discussed in Section 2.1.

(a) (b)

Figure 4.1(a). Single unreinforced in-plane model wall [Test 1].

Figure 4.1(b). ‘L’-shaped corner wall joint [Test 3].

Tetley (2006) also performed tests on corner joints, and proposed that this model could

simulate the real-life situation where there is an opening (i.e. a door or window) in the


11

transverse wall of the building which could permit rotation of the transverse wall if cracking

occurs near the opening, as shown in Figure 4.2.

Figure 4.2. Situation represented by corner joint model (from Tetley, 2006).

However, in this project it was decided to model a more general situation of a continuous

transverse wall without existing cracking near openings. To simulate the effect of a continuous

transverse wall, a support must be provided to the free end of the transverse wall in the joint

model to prevent rotation of the whole joint. This support is shown schematically in Figure 4.3

and the actual construction of the support is described in Section 4.4.

Figure 4.3. Support provided to prevent rotation and simulate continuous transverse wall.

After determining the types of model walls to test, the design issues remaining were then:

• The size of the model walls.

• The material composition and construction of the model walls.

• The method used to model a continuous transverse wall and boundary conditions.

• The techniques used to model structural improvements.

Direction of earthquake motion

Transverse wall

In-plane wall

Support required by transverse wall to simulate continuous wall


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4.2. Size of the model walls

It has been shown by dimensional analysis (by Tolles and Krawinkler, 1990, p44, and Tetley,

2006, p16-17) that for creating a scale model wall to have dynamic similitude with a real-life

full-size prototype wall, the following must be fulfilled:

rr

r lE

=ρ

(4.1)

where E = Young’s modulus, ρ = material density, l = length and the subscript r denotes the

ratio of units which defines the model [m] to real-life prototype [p] relationship for one of the

relevant physical properties (i.e. Er = Em / Ep).

Based on Tetley (2006), equation (4.1) implies that for dynamic similitude, the model must

have higher density or lower stiffness than the real-life prototype. Higher density can be

achieved by the technique of ‘artificial mass simulation’, which involves adding extra masses

to the model, in a near-uniform distribution. The research of both Tolles and Krawinkler and

Tetley concluded that this method is not feasible for small-scale models. Using materials of a

significantly lower stiffness was also considered too difficult.

However, if gravitational effects are neglected then the dimensional analysis simplifies so that

material properties can be equal in the model and the prototype (i.e. density ratio ρr = 1 and

stiffness ratio Er = 1). As discussed in Section 2.2.1, the assumption of negligible effect due to

gravity has been considered valid by previous researchers because gravity-induced stresses on

adobe buildings (which are predominantly single-storey) are much less than stresses induced

by seismic loading. This simplified analysis leads to the relationship ar = 1 / lr . Therefore the

length ratio (lr) is the important ratio which determines the acceleration required in the model

test compared to the prototype.

Tetley used this relationship ar = 1 / lr to conclude that the model should be built as large as

possible to ensure that model collapse can be achieved within the acceleration capacity of the

shaking table. Previous experiments on the shaking table at the Schofield Centre by Malton

(2005) and Tetley (2006) assumed a typical real-life prototype single wall size of 2.8m × 2.2m

× 0.5m. Loaiza et al (2002) noted typical real-life wall thicknesses of 0.3m - 0.8m, and IAEE

(2004) recommends a maximum wall height to thickness ratio of 8:1 and a maximum wall

length to thickness ratio of 10:1, so this assumed prototype size matches these real-life

observations and suggested criteria.


13

Based on these dimensions and the size of the shaking table available, the maximum single

wall model size possible was 1:5 scale, i.e. 0.56m × 0.44m × 0.10m. This was chosen as the

most suitable single wall model size based on the arguments of the other researchers outlined

above and to allow direct comparison of model tests with the work of Tetley (2006), who used

models with these dimensions. The corner joint model wall was chosen to have the same 1:5

scale and the same dimensions as the corner models used by Tetley, shown in Figure 4.4.

Figure 4.4. Dimensions of corner joint wall model (from Tetley, 2006).

4.3. Material composition and construction of the model walls

As discussed in Section 2.2.1, model adobe walls can be made either by manufacturing

individual adobe blocks (e.g. Dowling, 2006) or using gravel to simulate adobe blocks (e.g.

Malton, 2005, and Tetley, 2006). Walls are then made by joining the chosen ‘bricks’ with

mortar. For simplicity of construction, it was decided in this project to use gravel as the

‘bricks’ in the walls because constructing individual blocks at 1:5 scale would have been too

time-consuming. The possible limitations of this approach in modelling adobe walls are

discussed in Section 2.2.1. For consistency and simplicity, it was decided to use wooden

moulds, shown in Figure 4.5, to assist the construction of the walls, as used by Tetley (2006).

The gravel ‘bricks’ were laid layer by layer to imitate real-life construction, for all models.

To create a suitable mortar, a mixture of clay and sand was used. The initial composition by

weight used was 60% gravel, 40% mortar, where the mortar was 30% sand and 70% Kaolin

clay with a water content of w = 0.45. These proportions had been initially used by Tetley

(2006) with the aim of producing a mortar that enabled wall failure within the capacity of the

Schofield Centre shaking table. Tetley’s further experiments then concluded that a mortar of


14

40% sand and 60% Kaolin clay was preferable to ensure failure. In this project it was decided

to perform tests with both 30% sand and 40% sand contents to confirm the better option.

(a) (b)

Figure 4.5(a). Construction of single wall model in mould [Test 1, unreinforced].

Figure 4.5(b). Construction of corner joint model in mould [Test 6, with internal bamboo].

Prior to testing on the shaking table, shear tests of mortars with different drying times were

performed, summarised in Table 4.1. This highlighted the importance of the length of drying

time of the mortar, and the extra shear strength due to interlocking of the gravel blocks.

Table 4.1. Summary of the results of shear tests on different mortars.

Material tested

Time allowed for drying

Maximum shear strength recorded (kPa)

Mortar of 30% sand, 70% Kaolin None (unset) 4

Mortar of 30% sand, 70% Kaolin 3 days 10

Mixture of gravel + mortar of 30% sand, 70% Kaolin 3 days 22

After each shaking table test, the wall composition and construction method was reviewed to

check the requirements of failure within capacity of the shaking table and by realistic failure

mechanisms. Table 4.2 shows the composition and construction methods used for each test.

Table 4.3 explains the developments in construction and materials as the project progressed,

based on the following materials for the model adobe walls:

• Powdered E Type Kaolin clay, mixed to water content w = 0.45.

• Hostun sand (Tests 1-2), Rugby building sand (Tests 3-7).

• Gravel of length generally 30mm - 80mm.


15

Table 4.2. Summary of construction method and materials used.

Test Wall type

Structural improvements

Mortar sand

content

Sand type

Sand pre-drying

Model drying time before

testing

1 Single in-plane None 30% Hostun

sand Not pre-dried 4 days

2 Single in-plane None 40% Hostun


3 Corner joint None 40% Building


4 Corner joint None 40% Building


5 Corner joint

External bamboo (wide spacing) 40% Building

sand Pre-dried* 4 days

6 Corner joint Internal bamboo 40% Building


7 Corner joint

External bamboo (narrow spacing) 40% Building


* All pre-drying was in an oven for 24 hours at 70oC

Table 4.3. Explanation of development in construction method and materials used.

Test Development in construction materials and method

Issues arising from construction materials and method

1 Used original composition of 60% gravel, 40% mortar, where mortar was 30% sand and 70% Kaolin clay with a water content w = 0.45.

Building sand unavailable so Hostun sand used. Collapse acceleration too near shaking table capacity so needed to weaken mortar in subsequent tests.

2 Increased sand content to 40% from Test 1 to weaken mortar and decrease collapse acceleration.

Building sand unavailable so Hostun sand used. Collapse acceleration still too near shaking table capacity so still needed to weaken mortar.

3 Used 40% coarse building sand instead of 40% fine Hostun sand to increase both mortar drainage speed and amount of drying and therefore reduce both mortar ductility and collapse acceleration.

Collapse acceleration of transverse wall was high compared to equivalent test of Tetley (2006). Observation of mortar showed that there was still dampness and suction after the test (explained in Section 5.1), indicating that further drying was still needed.

4 Allowed model to dry for 37 days to avoid suction remaining.

Collapse acceleration of transverse wall very similar to test of Tetley with pre-dried sand.

5 6 7

Used pre-dried sand to avoid suction remaining in mortar, then allowed model to dry for 4 days.

Measurement of moisture content of mortar samples after drying was equivalent to the model with long drying period in Test 4 so pre-drying was confirmed as valid alternative (discussed further in Section 7.2).


16

4.4. Method used to model a continuous transverse wall and boundary conditions

As discussed in Section 4.1, a support was needed for the free end of the transverse wall in the

corner joint model to prevent rotation of the model, in order to simulate the effect of a

continuous transverse wall. This support was provided by a wooden attachment bolted to the

base of the wooden ‘holder’ for the wall which was fixed to the shaking table.

Before testing, the support was designed to touch the transverse wall but not exert any force on

the wall. For simplicity of construction, the support was only on one side of the wall, so it only

restrained movement in one direction. A full continuous wall would provide some restraint in

both directions but the support in this project was considered a reasonable approximation for

the simple model used, and more realistic than providing no support at all.

Dowling (2006) proposed that restraint is also needed for the in-plane wall to simulate the

effect of this being a continuous wall and joining the rest of the building in real life. Dowling’s

experiments used an applied downward force at the free end of the in-plane wall to simulate

this restraint, with the aim of preventing overturning of the in-plane wall. However, Zegarra et

al (1999) did not use any restraint on the in-plane walls in model tests. Dowling argued that the

results of Zegarra’s experiments (including shear cracking of the in-plane walls) were less

consistent with real-life failure patterns. However, as discussed in Section 2.1, other research

has observed diagonal cracking of in-plane walls as a typical real-life failure mechanism. It

was decided for this project not to provide further restraint to the in-plane walls.

Figure 4.6. L-shaped joint with wooden support to simulate continuous transverse wall.


17

4.5. Techniques used to model structural improvements

The structural improvement techniques tested were chosen to build on previous laboratory

testing and research in the field, with the aim of investigating methods that are appropriate (in

terms of cost and complexity) for the regions that require them. The methods proposed in

previous research in Cambridge University by Tetley (2006) were not considered suitable for

further investigation because they were either impractical in terms of cost (steel reinforcement

or cement), practicality of construction (using a mesh made from strips of plastic bags, which

would require prohibitive amounts of labour time in real life) or had limited scope for further

investigation using the facilities available (wire mesh).

4.5.1. External vertical bamboo tied with external horizontal wire

Previous research (especially by Dowling, 2006) has indicated that bamboo and wire offers

significant extra seismic resistance, but this method has not previously been tested in

Cambridge University. These tests were chosen to provide a useful comparison with tests

performed by other researchers, by offering further insight into both the reinforcement and the

testing process. Two tests were performed using external bamboo: with spacing of bamboo

larger than average gravel size, 100mm [Test 5], and with spacing of bamboo smaller than

average gravel size, 50mm [Test 7]. The two experiments were designed to allow observations

of the effect of the bamboo and wire in adding ductility to the structure, and also the effect of

the bamboo and wire matrix acting as a mesh to provide confinement for the gravel and mortar.

The technique of external bamboo and wire reinforcement also has potential as a retrofitting

measure for existing adobe buildings if holes can be drilled through the walls of the structure.

The method involves putting wire (or string) horizontally through the wall at various points and

then using this to attach vertical bamboo canes either side of the wall. The canes are then

linked by horizontal wires parallel to the wall, which can be looped round adjacent canes and

tied and tightened with pliers. The technique is likely to be much more difficult for buildings

made of stones (which are more difficult to drill through than adobe). However, although this

project used stones to model the adobe, the wires through the wall could be placed during

construction, within the mortar layers (as shown in Figure 4.7). The bamboo was not attached

to the wooden base (to match as near as possible the technique used by Dowling, 2006), but

could have been attached to a ring beam or other structural features if these were present.

Dimensions of bamboo and wire in Tests 5-7 are listed at the end of Section 4.5.


18

(a) (b)

Figure 4.7(a). Construction with wires placed in mortar layers [Test 5 with external bamboo].

Figure 4.7(b). Construction after mould removed, with wires ready to attach bamboo [Test 5].

(a) (b)

Figure 4.8(a). Wall joint with some vertical bamboo attached [Test 5 with external bamboo].

Figure 4.8(b). Wall joint with all vertical bamboo attached, before wooden support attached

[Test 5 with external bamboo].

4.5.2. Internal vertical bamboo tied with internal horizontal wire

Internal reinforcement such as bamboo provides extra ductility for the structure, but does not

have the possible advantages of containment or retrofitting that external reinforcement has.

Internal bamboo or other materials are already widely recommended, as discussed in Section

2.3.3. However, research by Dowling (2006) indicated that internal bamboo could in fact cause

extra weakness due to discontinuity in the adobe, and proposed further investigation.

Wire spacing = 180mmBamboo spacing =

100mm in Test 5, 50mm in Test 7

Position of wire represented by black lines


19

This discontinuity in real-life adobe buildings occurs because some adobe blocks will have to

be cut to shape to accommodate the reinforcement (usually, semi-circles would have to be cut

from the ends of adjacent adobe blocks to provide a circular gap for the bamboo). This also

makes construction more difficult and so makes the technique less suitable for communities in

poorer regions which have less access to training in building techniques. However in this

project it was possible to temporarily hold the bamboo canes in place using the mould, build

the wall up around the canes, and then attach horizontal internal wire within the mortar layers.

(a) (b)

Figure 4.9(a). Wall joint with internal bamboo held temporarily in mould during construction.

Figure 4.9(b). Completed wall joint with tops of internal bamboo and joining wires visible.

Summary of materials used for structural improvements:

• Bamboo of diameter in the range 5mm - 12mm, sourced from HomeBase and B&Q in

Cambridge. Dowling (2006) used bamboo of 16mm - 22mm diameter in tests of 1:2 scale

wall units, indicating that 5mm - 12mm is of the correct order of magnitude for testing 1:5

scale models. The diameter used is equivalent to 25mm - 60mm at full scale. In real life

bamboo diameters up to 100mm are common (Janssen, 1995).

• Galvanised wire of diameter 1mm, sourced from Mackays Hardware of Cambridge. Dowling

(2006) used galvanised wire of 2mm diameter in tests of 1:2 scale wall units, indicating that

1mm is of the correct order of magnitude for testing 1:5 scale models. Guidelines on the use

of internal wire reinforcement suggest that 2mm - 3mm is readily available in the field

(Nienhuys, 1999). Provided the wire has sufficient tensile strength to avoid snapping under

the loading generated, then the exact diameter is not critical.

Bamboo spacing = 100mm in Test 6

20

Section 5: Behaviour of single walls

As discussed in Section 4.1, two single unreinforced model walls were tested in-plane with the

motion of the shaking table. These tests would check that the in-plane walls (and therefore also

the more vulnerable out-of-plane walls) of unreinforced corner joint models tested later would

fail within the capacity of the shaking table. The failure mechanisms of the in-plane walls

should also be realistic compared to typical real-life mechanisms.

Dimensions of the walls are described in Section 4.2. The following tests were performed (all

accelerations referred to within the text of this section are the nominal calculated accelerations

of the shaking table, as discussed in Section 5.1):

Test 1 (13/11/06):

• Single wall in-plane. 30% sand in mortar (Hostun sand, undried), model dried for 4 days.

• Cracks began to open at 0.39g, total collapse at 1.05g.

Test 2 (20/11/06):

• Single wall in-plane. 40% sand in mortar (Hostun sand, undried), model dried for 4 days.

• Total collapse at 0.91g, no cracks observed at accelerations lower than collapse acceleration.

The next sections summarise the visual observations for the in-plane single wall tests, and then

summarise the relevance of the results in this stage of the project. No useable instrumentation

data was recorded for the in-plane single wall tests: for Test 1 the necessary equipment was

unavailable and for Test 2 there was an error in recording the data.

5.1. Visual observations

After each earthquake, the damage observed was recorded qualitatively, by digital camera

images and high speed video camera. Damage was also categorised based on the classification

system described by IAEE (2004), summarised in Table 5.1. This was adopted to provide a

consistent form of comparison between the effects of different earthquakes on different

models. The same classification system was used throughout the project, both for single in-

plane walls and corner joint models.


21

Table 5.1. Categorisation of damage to buildings (from IAEE, 2004)

Damage category Description of extent of damage 0 No damage No damage. 1 Slight non-structural damage Thin cracks in plaster; falling of plaster in limited parts.

2 Slight structural damage Small cracks in walls; falling of large plaster pieces over wide areas; damage to non-structural parts e.g. chimneys.

3 Moderate structural damage Large and deep cracks in walls; widespread cracking of walls and columns; fall of chimney etc.

4 Severe structural damage Gaps occur in walls; inner and outer walls collapse; failure of ties to parts of buildings, 50% of structural elements fail; demolition or extensive restoration needed.

5 Collapse A large part or whole of the building collapses.

Table 5.2. Summary of Test 1: Single wall in-plane with 30% Hostun sand in mortar. Nominal maximum displacement of table (mm)

Nominal frequency (Hz)

Nominal calculated acceleration of table (m/s2)

Observations Damage category

2 4.00 0.13g No damage observed 0 6 4.00 0.39g Cracks begin to open up 2 8 4.00 0.52g No further cracks 2

11 4.90 1.05g ‘Rocking’ motion of wall on base followed by total collapse initiated with large diagonal cracks

5

Table 5.3. Summary of Test 2: Single wall in-plane with 40% Hostun sand in mortar. Nominal maximum displacement of table (mm)

Nominal frequency (Hz)

Nominal calculated acceleration of table (m/s2)


3 4.02 0.20g No damage observed 0 6 4.02 0.40g No damage observed 0 6 4.50 0.49g No damage observed 0 8 4.50 0.65g No damage observed 0

10 4.75 0.91g

‘Rocking’ motion of wall on base (less severe rocking than Test 1) followed by total collapse initiated with large diagonal cracks

5

In Table 5.2 and 5.3, the displacements listed are the nominal displacements measured directly

from the shaking table crank radius. The frequencies listed are the nominal frequencies

displayed on the control box. The accelerations are the nominal accelerations calculated from

the nominal displacement and frequency. When instrumentation was used to record the actual


22

displacements and frequencies from Test 3 onwards it was found that these were consistently

higher than the nominal values (discussed in Section 6.2). However the nominal values were

sufficient for Test 1 and Test 2 since detailed analysis of the single walls was not required.

Figures 5.1 and 5.2 show images of the collapse of the single in-plane walls in Test 1 and Test

2. The initial rocking motion and the formation of the diagonal cracks are visible.

(a) (b)

Figure 5.1(a). Test 1: Rocking occurring as right-hand end of wall ‘lifts off’, 1.05g.

Figure 5.1(b). Test 1: Major diagonal cracks form before total collapse, 1.05g.

(a) (b)

Figure 5.2(a). Test 2: Rocking motion seems to initiate damage and diagonal cracks, 0.91g.

Figure 5.2(b). Test 2: Diagonal failure planes visible after collapse (highlighted), 0.91g.

The rocking motion observed in Test 1 was reduced in Test 2 by the use of bolts in the wooden

base which extended into the base of the wall. The diagonal failure planes observed suggested

that realistic collapse mechanisms occurred after the initial rocking effect.

‘Lift off’ due to rocking

Diagonal failure planes


23

(a) (b)

Figure 5.3(a). Test 1: Significant suction still holding gravel to mortar after testing.

Figure 5.3(b). Test 2: Mortar drier due to extra sand content, but some suction still present.

Figure 5.3 shows that in both Test 1 and 2 there was suction present in the mortar, indicating

that the mortar had not fully dried out before testing. In soil that has not dried fully, there will

be negative pore pressures (suction) which gradually dissipate as the soil dries out, until pore

pressure is zero when the soil has fully dried. Negative pore pressure causes an increase in the

strength of the soil since the soil shear strength depends on effective stress, where:

effective stress (σv’) = total stress (σv) - pore pressure (u) (5.1)

From equation (5.1), it can be seen that negative pore pressure (suction) during drying causes a

higher effective stress during drying than the final effective stress after drying. This causes a

higher strength in the mortar than desired while drying is still occurring. A similar problem had

been noted by Tetley (2006). Suction could therefore be reduced either by reducing the clay

content and increasing the sand content of the mortar (since the sand was coarser than the clay

and so a higher sand content would allow faster drainage), or by increasing the model’s drying

time. As discussed in Section 4.3, it was decided after Test 2 that the fine Hostun sand used in

Tests 1 and 2 should be replaced in subsequent tests by coarser building sand, to allow faster

drainage and therefore effectively increase the drying time.


24

5.2. Summary of single wall results

• The high failure acceleration of Test 1 (30% sand in mortar) and the observation of suction

remaining in the mortar after testing demonstrated the need to increase the sand content of

the mortar for subsequent tests. This would achieve a drier mortar which would fail at an

acceleration not near the acceleration capacity of the shaking table.

• The high failure acceleration of Test 2 (40% sand in mortar) compared to an equivalent

test by Tetley (2006) and the observation of some suction still remaining in the mortar (albeit

less than in Test 1) indicated that coarse building sand should be used where possible in

subsequent tests, instead of fine Hostun sand, to allow faster drainage and achieve a drier

mortar.

• The initial rocking motion in Test 1 and Test 2 before failure suggested that longer bolts

fixed into the base of the wall should be used in subsequent tests to minimise this effect and

enable observations to be made of the wall failure mechanism rather than any wall-base

interaction.

• The diagonal shear cracking observed in Test 1 and Test 2 indicated that typical real-life

collapse mechanisms for in-plane walls occurred after the initial rocking effect.

25

Section 6: Behaviour of ‘L’-shaped corner wall joints

As discussed in Section 4.1, ‘L’-shaped corner wall joints were tested (with and without

structural improvements) to observe behaviour of the corner and the out-of-plane wall section,

typically the two most vulnerable parts of adobe buildings.

Firstly, two corner wall joints were tested without any structural improvements. The aim of

these tests was to confirm that typical real-life collapse mechanisms were observed in the

models and to provide a benchmark with which to compare later models which had structural

improvements. The collapse mechanisms anticipated, as shown in Section 2.1, were

overturning of the transverse wall, vertical cracking at the corner joint, shear failure of the in-

plane wall, or a combination of these.

Dimensions of the walls are described in Section 4.2. All accelerations referred to within the

text are the calculated accelerations of the shaking table, as discussed in Section 6.1, except

where comparisons are made to previous research on the shaking table which only recorded

nominal accelerations. The following tests were performed:

Test 3 (01/12/06):

• ‘L’-shaped joint. 40% sand in mortar (building sand, undried), model dried for 4 days.

• First cracks observed at 0.56g, transverse wall began to oscillate at 0.84g.

• Total collapse of transverse wall at 1.02g, total collapse of in-plane wall at 1.12g.

Test 4 (12/01/07):

• ‘L’-shaped joint. 40% sand in mortar (building sand, undried), model dried for 37 days.

• First cracks observed and transverse wall began to oscillate at 0.29g.

• Total collapse of transverse wall at 0.47g, total collapse of in-plane wall at 0.95g.


The same system of observing, recording and categorising damage was used as for previous

tests, as explained in Section 5.1. In Table 6.1 and 6.2, nominal accelerations and calculated

accelerations are both shown. Nominal accelerations are presented to allow comparison with

Tetley (2006) who used nominal accelerations based on nominal displacement and frequency,


26

using the same shaking table. Calculated accelerations based on the measured displacement

and frequency are presented because analysis of the LVDT and accelerometer data showed that

the measured displacement and measured frequency were consistently greater than the nominal

values (discussed further in Section 6.2).

Table 6.1. Summary of Test 3:

‘L’-shaped joint with 40% sand in mortar (building sand, undried), model dried for 4 days. Maximum displacement of table (mm)

Frequency (Hz)

Nominal acceleration of table (m/s2)

Calculated acceleration of table (m/s2)


2.50 4.30 0.13g 0.19g No damage observed 0 4.75 4.30 0.26g 0.35g No damage observed 0

7.50 4.30 0.39g 0.56g Some small pieces of mortar came away from the surface 1-2

9.10 4.30 0.52g 0.68g Some pieces of mortar came away; small cracks formed 2

9.10 4.80 0.65g 0.84g Small oscillation of transverse wall; deep crack near base 3

10.95 4.80 0.73g 1.02g Failure of transverse wall by overturning, initiated by near-vertical crack at joint

4-5

12.10 4.80 0.81g 1.12g Failure of in-plane wall, initiated by diagonal crack near the base

5

Table 6.2. Summary of Test 4:

‘L’-shaped joint with 40% sand in mortar (building sand, undried), model dried for 37 days. Maximum displacement of table (mm)

Frequency (Hz)

Nominal acceleration of table (m/s2)



2.85 5.00 0.18g 0.29g Oscillation of transverse wall; deep crack from near base of transverse wall to top of joint

3

4.70 5.00 0.36g 0.47g Failure of transverse wall by overturning, initiated by near-vertical crack at joint

4-5

6.65 5.00 0.53g 0.67g No further visible effect on remaining in-plane wall 4-5

9.40* 5.00 0.71g 0.95g Failure of in-plane wall, initiated by diagonal crack 5

*Estimated value because the LVDT data was not useable due to wall collapse onto the LVDT.


27

Figure 6.1 and Figure 6.2 show images taken from the digital camera video of the collapse

mechanisms of the transverse and in-plane wall failures in Test 3 and Test 4. The observed

failure mechanisms (overturning of the transverse wall initiated by vertical cracking at the joint

and collapse of the in-plane wall initiated by diagonal failure planes) seemed realistic.

(a) (b)

Figure 6.1(a). Test 3: Point of failure of transverse wall by overturning, 1.02g (nominal

acceleration 0.73g).

Figure 6.1(b). Test 3: Point of failure of in-plane wall, initiated by diagonal cracking, 1.12g

(nominal acceleration 0.81g).

(a) (b)

Figure 6.2(a). Test 4: Point of failure of transverse wall by overturning, 0.47g (nominal

acceleration 0.36g).

Figure 6.2(b). Test 4: Point of failure of in-plane wall, initiated by diagonal cracking,

highlighted, 0.95g (nominal acceleration 0.71g).

Overturning

Overturning Diagonal cracking

Diagonal cracking


28

Collapse in Test 3 still occurred too near to the maximum acceleration of the shaking table and

at a much higher acceleration (nominal acceleration 0.73g for the transverse wall) than in the

equivalent test by Tetley (2006), in which the transverse wall collapsed at nominal acceleration

0.32g. It was observed that there was still suction in the mortar after Test 3, as discussed

previously in Section 5.1. This led to the decision to allow an extended period of drying time

(37 days) for the model in Test 4 compared to the previous tests (4 days). The difference in

mortar dryness due to this extra time is visible in Figure 6.3.

(a) (b)

Figure 6.3(a). Test 3, model dried for 4 days: Gravel still held in mortar by suction after

failure of transverse wall.

Figure 6.3(b). Test 4, model dried for 37 days: Mortar drier with much less suction after

failure of transverse wall.

The nominal collapse acceleration (0.36g) of the transverse wall in Test 4 was very similar to

the nominal collapse acceleration (0.32g) of the wall tested by Tetley (2006) which dried for 4

days but used pre-dried sand. This suggested that it is necessary to use a model without suction

present as a benchmark, and that the level of drying needed can be achieved either by pre-

drying the sand or by leaving the whole model to dry for an extended period. After Test 4, the

moisture content of a sample of mortar set aside during construction and permitted to dry under

the same conditions as the wall was measured as 0.80%, three days after testing. In subsequent

tests, there was not sufficient time available to allow a drying period of 30+ days. However, the

sand could be pre-dried in an oven. The measurement of moisture content of the mortar from

the model in Test 4 could provide a benchmark for moisture content in subsequent tests which

used pre-dried sand. Provided the moisture contents were similar, comparisons of the effect of

structural improvements could then be made. This is discussed further in Section 7.2.


29

6.2. Instrumentation data recorded

For Test 3 and 4, an LVDT and accelerometers were placed on the shaking table and the model

at the positions shown on Figure 3.1, Section 3.3. An example of the LVDT and accelerometer

data recorded is shown in Figure 6.4, which displays the measured displacement and

acceleration of the shaking table in the first earthquake of Test 4.

0 1 2 3 4 5 6 7 8 9-3

0

3

time, s

disp

lace

men

t, m

m

0 1 2 3 4 5 6 7 8 9-0.5

0

0.5

time, s

acce

lera

tion,

g

(a)tabledisplacement(LVDT)

(b)tableacceleration(Ac1)

Figure 6.4(a). Measured displacement of shaking table (LVDT) during Test 4, Earthquake A

(note the difference with the nominal displacement, 2mm).

Figure 6.4(b). Measured acceleration of shaking table (Ac1) during Test 4, Earthquake A (note

the difference with the calculated acceleration, 0.29g).

Full analysis of the LVDT and accelerometer data showed that on average the measured

displacement was about 20% greater than the nominal displacement (measured on the shaking

table crank) and the measured frequency was about 3% - 8% greater than the nominal

frequency (recorded from the shaking table control box). Therefore from Test 3 onwards,

accelerations quoted in the text are the accelerations calculated from the measured

displacement and frequency, instead of the nominal accelerations as used in Test 1 and 2.

From the full set of accelerometer data, MatLab was then used to produce graphs comparing

the acceleration at each position for each earthquake in Test 3 and 4. An example of the graphs

produced comparing the accelerations for the first earthquake of Test 4 is shown in Figure 6.5.

This graph clearly shows the increasing acceleration of the transverse wall compared to the


30

other positions, due to the cracking and oscillation of the transverse wall. There is greater noise

evident in the accelerometer data from the transverse wall, which may cause the difference in

acceleration to be exaggerated. However, it is still clear from the graphs that the acceleration of

the transverse wall increases throughout the test as cracking develops, and that the acceleration

of the top of the transverse wall is higher than the acceleration of the lower part of the wall.

0 1 2 3 4 5 6 7 8 9-1

0

1

0 1 2 3 4 5 6 7 8 9-1

0

1

0 1 2 3 4 5 6 7 8 9-1

0

1

0 1 2 3 4 5 6 7 8 9-1

0

1

acce

lera

tion,

g

0 1 2 3 4 5 6 7 8 9-1

0

1

0 1 2 3 4 5 6 7 8 9-1

0

1

0 1 2 3 4 5 6 7 8 9-1

0

1

time, s Figure 6.5. Measured accelerations during Test 4, Earthquake 1:

(a) Acceleration of shaking table (‘ground’) [Ac1].

(b) Acceleration of in-plane wall, corner, lower position [Ac2].

(c) Acceleration of in-plane wall, corner, upper position [Ac3].

(d) Acceleration of in-plane wall, free end, lower position [Ac4].

(e) Acceleration of in-plane wall, free end, upper position [Ac5].

(f) Acceleration of transverse wall, lower position [Ac6].

(g) Acceleration of transverse wall, upper position [Ac7].

(a)

(b)

(c)

(d)

(e)

(f)

(g)


31

In order to summarise the main information visible in each graph into a form that would allow

easy comparison between positions on the model, MatLab was used to find the mean of the

peak accelerations of the different positions during each earthquake1. The ratios of the

measured accelerations at the different positions on the model to the measured accelerations on

the shaking table were also calculated (i.e. the acceleration amplication factors). An example of

a complete set of these results, for Test 3, is shown in Table 6.3 and 6.4.

Table 6.3. Test 3: Mean of the peak measured accelerations of all different positions. Acceleration of table (m/s2)

Acceleration of in-plane wall, corner (m/s2)

Acceleration of in-plane wall, free end (m/s2)

Acceleration of transvserse wall (m/s2)

Earth- quake ref.


[Ac1] lower [Ac2]

upper [Ac3]

lower [Ac4]

upper [Ac5]

lower [Ac6]

upper [Ac7]

1 0.19g 0.24g 0.26g 0.26g 0.29g 0.31g 0.26g 0.33g 2 0.35g 0.51g 0.53g 0.55g 0.55g 0.58g 0.57g 0.71g 3 0.56g 0.72g 0.73g 0.80g 0.74g 0.81g 0.81g 1.05g 4 0.68g 0.73g 0.70g 0.73g 0.72g 0.75g 0.71g 0.92g 5 0.84g 1.27g 1.13g 1.65g 1.16g 1.59g 1.52g 1.69g 6 1.02g --- --- --- 1.29g 1.38g --- --- 7 1.12g --- --- --- --- --- --- ---

Table 6.4. Test 3: Ratios of the measured accelerations at all different positions on the model

to the measured accelerations on the shaking table (acceleration amplication factors). Acceleration of table (m/s2)

Amplication factor of in-plane wall, corner

Amplication factor of in-plane wall, free end

Amplication factor of transvserse wall

Earth- quake ref.


[Ac1] lower [Ac2]

upper [Ac3]

lower [Ac4]

upper [Ac5]

lower [Ac6]

upper [Ac7]

1 0.19g 0.24g 1.08 1.10 1.20 1.28 1.08 1.37 2 0.35g 0.51g 1.03 1.07 1.07 1.12 1.10 1.38 3 0.56g 0.72g 1.01 1.12 1.03 1.13 1.13 1.45 4 0.68g 0.73g 0.97 1.00 0.99 1.03 0.98 1.26 5 0.84g 1.27g 0.88 1.29 0.91 1.25 1.19 1.33 6 1.02g --- --- --- --- --- --- --- 7 1.12g --- --- --- --- --- --- ---

Mean amplification factor: 0.99 1.12 1.04 1.16 1.10 1.36

It was noted for Test 3 and Test 4, and also found in Tests 5-7, that the recorded mean peak

acceleration of the shaking table was on average 1.7 times the calculated acceleration. It is

likely that this is because the frequencies used in this project are all 5Hz or below, a frequency

range in which this model of accelerometer is known to suffer inaccuracy and noise. Noise was

still present in the acceleration data even after filtering using MatLab, so all data presented here

1 All MatLab code used throughout this project was written by Ulas Cilingir, PhD student at Cambridge University, to whom the author is very grateful for his assistance.


32

is unfiltered. However, since the difference between the recorded acceleration and calculated

acceleration was consistent it was concluded that the results were still valid for comparisons

between different wall positions, earthquakes and tests.

For the remainder of the report, only the data from the three key positions of the shaking table

(accelerometer ‘Ac1’), the top of the in-plane wall at the corner (accelerometer ‘Ac3’) and the

top of the transverse wall (accelerometer ‘Ac7’) is presented. This is because the main points

of interest are the possibility of any differences in behaviour between the transverse wall and

the in-plane wall at the corner, and any differences between points on the walls and the table

(‘ground’) acceleration. The accelerometers nearer the tops of the walls are more likely to

show such differences if they exist, as in the example shown for Test 3 in Table 6.4.

Table 6.5. Test 4: Summary of peak measured accelerations and acceleration amplication

factors of key positions. Earth- quake ref.


Acceleration of table [Ac1] (m/s2)

Acceleration of in-plane wall, corner, upper [Ac3] (m/s2)

Amplication factor of in-plane wall, corner, upper

Acceleration of transverse wall, upper [Ac7] (m/s2)

Amplication factor of transverse wall, upper

A 0.29g 0.34g 0.41g 1.20 0.79g 2.31 B 0.47g 0.61g --- --- C 0.67g --- --- --- D 0.85g --- --- ---

[ ‘---’ indicates accelerometer data was not useable due to collapse of part or all of the model ]

6.2.1. Observations on accelerometer data

It can be noted in Test 3 (from Table 6.4) that for each earthquake, the acceleration of the top

of the in-plane wall is on average 1.12 to 1.16 times the acceleration of the shaking table, and

the acceleration of the top of the transverse wall is on average 1.36 times the table acceleration.

There is amplification even for the first two earthquakes, where no oscillation or damage was

visible. This suggests that there may be differences in behaviour of different parts of the model

even when there is no visible change.

In Test 4, collapse of parts of the model prevented useable data being obtained after the first

earthquake so comparisons can not be made with any confidence.


33

6.3. Summary of results for ‘L’-shaped corner wall joints

• The observed failure mechanisms in Test 3 and Test 4 (overturning of the transverse wall

initiated by vertical cracking at the joint and collapse of the in-plane wall initiated by

diagonal failure planes) were typical real-life collapse mechanisms.

• The high failure acceleration of Test 3 (model dried for 4 days) and the observation of

suction remaining in the mortar after testing demonstrated the need to achieve a drier mortar

which would fail at an acceleration not too near the acceleration capacity of the shaking

table.

• Test 4 (model dried for 37 days) experienced a very similar failure acceleration compared

to an equivalent test by Tetley (2006) which used pre-dried sand. This indicated that the

level of drying necessary to avoid suction in the mortar can be achieved either by pre-drying

the sand or by leaving the whole model to dry for an extended period. The moisture content

measurement of Test 4 provided a benchmark to check that subsequent models had a similar

dryness whichever drying method was used.

• The use of a wooden support for the transverse wall was successful in preventing rotation of

the models.

• The accelerations measured in Test 3 and Test 4 demonstrated that there may be amplified

accelerations of the transverse wall compared to the ground (shaking table) acceleration,

even during earthquakes when there is no visible change in the behaviour of the model.

34

Section 7: Behaviour of ‘L’-shaped corner wall joints with improvements

In Test 4, discussed in Section 6, an unimproved corner wall joint model was developed with:

performance similar to previous research using the same shaking table; failure at accelerations

well below the shaking table capacity; and realistic collapse mechanisms. Tests 5-7 were

performed on three further models with different structural improvements which could be

compared to the benchmark of Test 4. Dimensions of the walls and material composition were

equivalent to the unimproved joint in Test 4 (‘L’-shaped joints, with 40% building sand in the

mortar, but with sand pre-dried and the model dried for 4 days). The following tests were

performed (all accelerations referred to in the text are calculated accelerations of the table):

Test 5 (26/01/07):

• Reinforced with external vertical bamboo on both sides of wall at 100mm spacing and

external horizontal wire at 180mm spacing. Bamboo joined through wall with wire.

• First cracks observed and large oscillation of transverse wall at 0.45g.

• Internal collapse of transverse wall at 0.89g; large cracking of in-plane wall at 0.89g.

Test 6 (09/02/07):

• Reinforced with internal vertical bamboo at 100mm spacing and internal horizontal

wire at 180mm spacing.

• First cracks and gaps opening observed and large oscillation of transverse wall at 0.44g.

• Failure of transverse wall by overturning at 0.89g; large cracking of in-plane wall and

toppling due to collapse of transverse wall pulling the in-plane wall over at 0.89g.

Test 7 (23/02/07):

• Reinforced with external vertical bamboo on both sides of wall at 50mm spacing and

external horizontal wire at 180mm spacing. Bamboo joined through wall with wire.

• First cracks observed and large oscillation of transverse wall at 0.44g.

• Further gaps opened but most material still confined at 0.89g without catastrophic collapse.


The same system of observing, recording and categorising damage was used as for previous

tests, as explained in Section 5.1. The high speed video camera was unavailable for Test 7.

Section 7: Behaviour of ‘L’-shaped corner joints with improvements

35

Table 7.1. Summary of Test 5: Reinforced with external vertical bamboo at 100mm spacing. Earth- quake ref.

Maximum displacement of table (mm)

Frequency (Hz)



A 2.55 4.85 0.24g Some thin cracks 1

B 4.75 4.85 0.45g Large oscillation of transverse wall; large, deep cracks in transverse wall

3

C 7.00 4.85 0.66g Large gap opens near base of transverse wall 3-4

D 9.40 4.85 0.89g Internal collapse of transverse wall; large cracks in in-plane wall

4

Table 7.2. Summary of Test 6: Reinforced with internal vertical bamboo at 100mm spacing. Earth- quake ref.


Frequency (Hz)



A 2.70 4.85 0.26g No visible damage 0

B 4.70 4.85 0.44g Large oscillation of transverse wall; large gap near base of transverse wall

3-4

C 6.85 4.85 0.65g Further gaps in parts of transverse wall 3-4

D 9.35 4.85 0.89g

Failure of transverse wall by overturning; in-plane wall cracks, then topples due to transverse wall movement

5

Table 7.3. Summary of Test 7: Reinforced with external vertical bamboo at 50mm spacing. Earth- quake ref.


Frequency (Hz)



A 2.70* 4.85 0.26g* No visible damage 0

B 4.70* 4.85 0.44g* Large oscillation of transverse wall; large, deep cracks in transverse wall

3

C 6.85* 4.85 0.65g* Gaps open near base of transverse wall 3

D 9.35* 4.85 0.89g* Further gaps open but most material still confined; no catastrophic collapse

3-4

*A calibration error in the LVDT data for Test 7 made the displacement data not useable. The

measured table accelerations for Test 7 were very close to those measured in Tests 5 and 6 so

for the purposes of comparison of damage between earthquakes, the displacements and

calculated accelerations of the table in Test 7 were assumed to be the same as for Test 6.


36

Figure 7.1 shows the stages of damage experienced by the model in Test 5 (reinforced with

external vertical bamboo on both sides of wall at 100mm spacing and external horizontal wire

at 180mm spacing).

(a) (b)

Figure 7.1(a). Test 5, during Earthquake B: Deep cracks (highlighted) in transverse wall at

0.44g.

Figure 7.1(b). Test 5, after Earthquake C: Large gap near base of transverse wall at 0.66g

(Note: at the corner of the wall, small bamboo pieces are used to prevent the wire cutting into

the mortar).

(c) (d)

Figure 7.1(c). Test 5, after Earthquake D: Internal collapse of transverse wall at 0.89g.

Figure 7.1(d). Test 5, after Earthquake D: Large cracks have formed in the in-plane wall but

the wall remains confined within the bamboo at 0.89g.


37


internal vertical bamboo at 100mm spacing and internal horizontal wire at 180mm spacing).

(a) (b)

Figure 7.2(a). Test 6, after Earthquake B: Large gap near base of transverse wall at 0.44g.

Figure 7.2(b). Test 6, after Earthquake C: Further gap near top of transverse wall at 0.65g,

and accelerometers Ac2 and Ac3 have fallen off.

(c) (d)

Figure 7.2(c). Test 6, during Earthquake D: Vertical cracking at corner (highlighted) and

overturning of transverse wall (indicated by arrow) at 0.89g.

Figure 7.2(d). Test 6, during Earthquake D: Cracking of in-plane wall, then toppling of in-

plane wall (indicated by arrow), due to the effect of transverse wall still attached to in-plane

wall via the bamboo and wire at the corner, at 0.89g.

Overturning Toppling


38


external vertical bamboo on both sides of wall at 50mm spacing and external horizontal wire at

180mm spacing).

(a) (b)

Figure 7.3(a). Test 7, after Earthquake B: Cracks opening on top of transverse wall at 0.44g.

Figure 7.3(b). Test 7, after Earthquake C: Further cracks opening and sections of mortar

dislodging in transverse wall at 0.65g.

(c) (d)

Figure 7.3(c). Test 6, during Earthquake D: Deep cracking (highlighted) and large oscillation

of in-plane wall (indicated by arrow) at 0.89g, but almost all material confined.

Figure 7.3(d). Test 6, after Earthquake D: Large gap open at top of transverse wall at 0.89g.

Oscillation and cracking


39

7.2. Moisture content data recorded

To ensure that the method of pre-drying the sand was equivalent to the extended drying period

used in Test 4, moisture contents were measured of samples from each ‘batch’ of mortar made.

For practical reasons, the mortar for each model was made in 3 batches over a period of 5 days.

Therefore different layers of the wall dried for different time periods before testing. As Table

7.4 shows, the first two batches had similar moisture contents after testing to each other and to

the model in Test 4 (0.80%, as discussed in Section 6.1). However, the 3rd batch (the top 1/3 of

each wall) in each test had a higher moisture content. This may have caused greater ductility

and/or strength (due to suction, as discussed in Section 5.1) in the top part of the wall. It is not

possible to assess the significance of this effect in the performance of the models but future

research should aim for identical moisture content throughout each model tested to ensure that

material properties are consistent throughout the model.

Table 7.4. Moisture content measurements for mortar batches in Tests 5-7

Batch of mortar, drying time until testing, and moisture content (%) measured 3 days after testing

Test Structural improvements

1st batch, 7-8 days 2nd batch, 6-7 days 3rd batch, 4 days

5 External bamboo (wide spacing) Not measured 0.94 2.10

6 Internal bamboo 0.95 0.95 2.35

7 External bamboo (narrow spacing) 1.28 0.91 3.22

7.3. Instrumentation data recorded

For Tests 5-7, accelerometers were placed at the positions shown on Figure 3.1, and MatLab

was used to produce graphs comparing the accelerations at these positions as for the example

in Section 6.2, Figure 6.5. As in Section 6.2, MatLab was then used to find the mean of the

peak accelerations of the different positions during each earthquake. Only the data from the

three key positions is presented. These results are summarised in Tables 7.5 to 7.7. As

discussed in Section 6.2, it was noted that the recorded acceleration of the shaking table was on

average 1.7 times the calculated acceleration, but it was concluded that the results were still

valid for comparisons between different wall positions, earthquakes and tests.


40

Table 7.5. Test 5: Reinforced with external vertical bamboo at 100mm spacing. Summary of

peak measured accelerations and acceleration amplication factors of key positions. Earth- quake ref.







A 0.24g 0.30g 0.32g 1.06 0.46g 1.53 B 0.45g 0.53g 0.59g 1.10 0.93g 1.73 C 0.66g 0.76g 0.85g 1.12 0.81g 1.07 D 0.89g 1.15g --- ---

Table 7.6. Test 6: Reinforced with internal vertical bamboo at 100mm spacing. Summary of








A 0.26g 0.30g 0.16g* 0.55* 0.46g 1.52 B 0.44g 0.59g 0.42g* 0.71* 1.28g 2.17 C 0.65g 0.85g --- 1.25g 1.48 D 0.89g --- --- ---

*There appears to be an error in the data from Ac3 for Test 6 since the values are significantly

lower than the table acceleration.

Table 7.7. Test 7: Reinforced with external vertical bamboo at 50mm spacing. Summary of








A 0.26g 0.32 0.28 0.87 0.42 1.28 B 0.44g 0.65 0.68 1.05 1.02 1.57 C 0.65g 0.87 0.81 0.94 --- D 0.89g 1.16 1.67 1.43 ---

[ ‘---’ indicates the accelerometer data was not useable due to collapse of part of the model ]


41

7.3.1. Observations on accelerometer data

It appears from the accelerometer data that in each of Tests 5-7 the top of the transverse wall

experiences greater acceleration than the shaking table and the top of the in-plane wall, even

when there is no visible oscillation of the transverse wall. It is also noted that this amplification

of accelerations between the table and the transverse wall seems to drop between Earthquakes

B and C in Tests 5 and 6 (the transverse wall data for Earthquake C in Test 7 was not useable

due to collapse of part of the wall). This may be related to the cracking behaviour of each

model: in each of Tests 5-7 the first cracking and oscillation of the transverse wall occurs

during Earthquake B. This may explain the higher accelerations experienced during this

earthquake.

In Tests 5 and 7, the acceleration of the top of the in-plane wall does not seem to experience

such an amplification factor except in Earthquake D of Test 7.

The data for in-plane wall acceleration in Test 6 appears to have a recording error since the

values are significantly lower than the table acceleration.

As discussed in Section 3.3, the accelerometers experience noise and inaccuracy in the

frequency range used here which means that detailed comparison of amplification factors is not

possible. The general observations of the amplified acceleration of the transverse wall and the

possible effects of cracking behaviour are still reasonable but it is not possible to draw more

detailed conclusions.

7.4. FFT analysis of acceleration data

It was noted from the acceleration graphs of Tests 5-7 that some of the acceleration data seems

to show evidence of frequency components other than the fundamental input frequency of the

shaking table (4.85Hz). To test this observation, Fast Fourier Transforms (FFTs) were

produced for each set of acceleration data using MatLab, to determine the relative magnitudes

of the signal content from the acceleration data at each nth harmonic frequency. An example of

the graphs produced by the FFT is shown in Figure 7.4.


42

0 5 10 15 20 25 300

1000

2000

3000

4000

5000

frequency, Hz

mag

nitu

de, g

/Hz

0 5 10 15 20 25 300

1000

2000

3000

4000

5000

frequency, Hz

mag

nitu

de, g

/Hz

0 5 10 15 20 25 300

1000

2000

3000

4000

5000

frequency, Hz

mag

nitu

de, g

/Hz

Figure 7.4(a). FFT of acceleration of shaking table (Ac1) during Test 5, Earthquake A. Figure 7.4(b). FFT of acceleration of top of in-plane wall (Ac3) during Test 5, Earthquake A. Figure 7.4(c). FFT of acceleration of top of transverse wall (Ac7) during Test 5, Earthquake A.

These graphs were produced for all earthquakes in Tests 5-7. The graphs were then used to find

the ratio of the magnitude of the signal content from the acceleration data at each nth harmonic

frequency to the magnitude of the signal content from the data at the fundamental input

frequency (4.85Hz), for each accelerometer position in each earthquake. It was assumed that

the energy in the signal content was proportional to the magnitude of the signal content

because the width of each ‘spike’ on the FFT graphs is narrow enough to assume that the

height of each spike is proportional to its area. Tables 7.8 to 7.10 show these ratios.

(a)

(b)

(c)


43

Table 7.8. Test 5: Reinforced with external vertical bamboo at 100mm spacing. Comparison of

the signal content at each nth harmonic frequency. Ratio of the magnitude of the signal content from the acceleration data at each nth harmonic frequency to the signal content from acceleration data at the fundamental input frequency (4.85Hz) Table [Ac1] Top of in-plane wall [Ac3] Top of transverse wall [Ac7]

Earth- quake ref.

3rd 2nd 3rd 4th 2nd 3rd 4th 5th A 0.03 0 0.08 0 0.09 0.32 0.04 0.24B 0.09 0 0.14 0 0 0.01 0 0C 0.05 0 0.05 0 0 0 0 0D 0.01 --- ---

Table 7.9. Test 6: Reinforced with internal vertical bamboo at 100mm spacing. Comparison of


Earth- quake ref.

3rd 2nd 3rd 4th 2nd 3rd 4th 5th A 0.06 0 0.45 0 0 0.65 0.07 0.15B 0.14 0 0.64 0 0 0.05 0 0C 0.04 --- 0 0 0 0.16D 0 --- ---



Earth- quake ref.

3rd 2nd 3rd 4th 2nd 3rd 4th 5th A 0.10 0 0.33 0 0 0.42 0.05 0.15B 0.05 0.07 0.03 0 0.18 0.06 0 0C 0.05 0.07 0.04 0 --- D 0.03 0.14 0.05 0.07 ---

Note: Values are only displayed for harmonics of non-zero content during any earthquake.

7.4.1. Observations on FFT analysis

It is clear that for Tests 5-7, apart from the fundamental input frequency, the 3rd harmonic

frequency is the most significant contribution to the magnitude of the signal content of the

acceleration data. This suggests that the 3rd harmonic frequency could also be a harmonic

frequency of the natural frequency of the models tested. The 3rd harmonic frequency seems to

have most effect during Earthquake A, the lowest intensity earthquake.


44

To test the possibility of the models having a natural frequency close to the 3rd harmonic

frequency of testing, the FFT results were further analysed for each earthquake to compare the

magnitude of the signal contents at the 1st and 3rd harmonics on the in-plane wall and transverse

wall to the signal contents at the 1st and 3rd harmonics measured on the shaking table. If there

was consistently greater amplification of the signal content at one harmonic than the other then

this would suggest that the models had a natural frequency nearer to that harmonic. These

results are shown in Tables 7.11 to 7.13.

Table 7.11. Test 5: Reinforced with external vertical bamboo at 100mm spacing. Comparison

of the amplification of signal content between the shaking table and the model. Ratio of the magnitude of the signal content from the acceleration data from positions on the model to the signal content from the data from the shaking table, at the 1st and 3rd harmonic frequency

Top of in-plane wall [Ac3] Top of transverse wall [Ac7]

Earth-quake ref.

1st 3rd 1st 3rd A 1.1 2.8 1.3 13.3 B 1.0 1.5 2.7 0.2 C 1.2 1.2 1.1 0.1 D --- ---

Table 7.12. Test 6: Reinforced with internal vertical bamboo at 100mm spacing. Comparison

of the amplification of signal content between the shaking table and the model. Ratio of the magnitude of the signal content from the acceleration data from positions on the model to the signal content from the data from the shaking table, at the 1st and 3rd harmonic frequency


Earth-quake ref.

1st 3rd 1st 3rd A 0.2 1.6 1.1 10.8 B 0.1 0.5 4.9 1.9 C --- 1.8 7.1 D --- ---


the amplification of signal content between the shaking table and the model. Ratio of the magnitude of the signal content from the acceleration data from positions on the model to the signal content from the data from the shaking table, at the 1st and 3rd harmonic frequency


Earth-quake ref.

1st 3rd 1st 3rd A 0.7 2.4 0.9 4.0 B 1.1 0.7 2.2 2.8 C 0.8 0.6 --- D 1.0 2.1 ---

Tables 7.11 to 7.13 show that there is no clear pattern in the amplifications of signal content

between the shaking table and the model at the 1st and 3rd harmonic frequencies. Therefore no

further conclusions can be drawn about the natural frequencies of the models.


45

7.5. Summary of results for ‘L’-shaped corner wall joints with improvements

• In this project the key performance measure is the capacity to resist severe structural damage

or collapse (damage category 4 or higher, equivalent to collapse of the transverse wall).

Under this measure, all three structural improvements demonstrated significantly better

performance than the benchmark unimproved corner joint in Test 4:

• In Test 4 (unimproved), the transverse wall failed during Earthquake B.

• In Test 5 (external bamboo, wide spacing), the transverse wall failed in Earthquake D,

at an acceleration about 2 times the collapse acceleration of the unimproved joint. In tests

on a similar method by Dowling (2006), the wall with external bamboo experienced its

greatest damage at an intensity about 1.7 times greater than an unreinforced wall.

Dowling measured intensity in terms of displacement not acceleration, but since in this

project displacement and acceleration were proportional, these results seem comparable.

• In Test 6 (internal bamboo), the transverse wall failed during Earthquake D, at an

acceleration about 2 times the collapse acceleration of the unimproved joint. This differs

to the most recent research by Dowling (2006), which indicated that internal bamboo

reinforcement could in fact cause extra weakness due to discontinuity in the adobe. This

problem was not evident here because gravel was used in all tests, which could be built

closely around the bamboo. Dowling noted difficulties in using adobe blocks with semi-

circles cut out for the internal bamboo tests, which caused the discontinuity problem.

• In Test 7 (external bamboo, narrow spacing), the transverse wall was damaged but

still contained during Earthquake D, the best performance of all models tested.

• All three improvement methods added to the ductility of the model. The external bamboo

also had a significant confinement effect on the adobe.

• The use of a wooden support for the transverse wall seemed successful in preventing rotation

of the models. However, in Test 7 it was noted that the transverse wall struck the support as

it oscillated. This repeated impact may have had an effect on performance which could be

analogous to the effect of an internal shear wall inadequately joined to the transverse wall.

• The accelerations measured in Tests 5-7 demonstrated that there may be amplified

accelerations of the transverse wall compared to the ground (shaking table) acceleration,

even during earthquakes when there is no visible change in the behaviour of the model.

• FFT analysis of the acceleration data showed that there were contributions to the signal

content from harmonics beyond the fundamental input frequency, most notably the 3rd

harmonic frequency. The significance of this to the results is unknown but does not appear to

invalidate conclusions made about the performance of different improvements.

46

Section 8: Conclusions

8.1. Conclusions related to simulating adobe buildings using scale models

1. Models of in-plane walls and ‘L’-shaped corner wall joints at 1:5 scale tested on a 1-g

shaking table can exhibit collapse mechanisms similar to those found in real life, provided

a suitable material composition is used to model the adobe walls.

2. A suitable material composition can use gravel to simulate adobe blocks if it is not feasible

to construct individual adobe bricks. A suitable composition for the model size and shaking

table used was 60% gravel of size 30mm - 80mm and 40% mortar by weight. Mortar was

made from 60% Kaolin clay at w = 0.45 and 40% building sand by weight. This result

supports the findings of Tetley and Madabhushi (2007). It is also necessary either to leave

the completed model to dry for an extended period of time (up to 30 or more days) or to

pre-dry the sand in an oven to ensure the mortar has fully dried before testing so that there

is no suction in the mortar. If the model is made in stages, moisture content measurements

should be taken of mortar samples to ensure that the whole model has a consistent moisture

content.

3. For model walls which represent only partial sections of adobe buildings, the boundary

conditions should be artificially adjusted to simulate the effect of the continuity present in a

complete building. In particular, to simulate a continuous transverse wall in an ‘L’-shaped

model corner joint, support should be provided to the end of the transverse wall to prevent

rotation of the model. Care must be taken to avoid the support acting as an inadequately

attached shear wall to the transverse wall which can cause repeated impacts on the

transverse wall during earthquake loading.

4. Accelerometer measurements from different positions on models tested show that parts of

model walls can exhibit amplified accelerations compared to the input shaking table

(‘ground’) acceleration, even when there is no visible damage or difference in

displacement. This suggests that quantitative data is important in making detailed

observations about the behaviour of model adobe walls and that future tests should not rely

solely on qualitative observations.


47

8.2. Conclusions related to structural improvements of adobe buildings

1. External vertical bamboo reinforcement, joined through the walls of the building and tied

externally with horizontal wire, improves the seismic resistance of the building by

increasing the ductility of the walls and providing some confinement for the adobe blocks.

2. In the 1:5 scale tests on ‘L’-shaped corner joints, the capacity to resist severe structural

damage (measured as the collapse acceleration of the transverse wall) was increased by at

least a factor of 2 using external bamboo reinforcement. The effect is of a similar

magnitude to that observed in recent research on a similar technique by Dowling (2006).

Further research would be needed to assess if this factor could be expected in real life if

whole buildings were reinforced in this way.

3. The extra seismic resistance due to confinement of the adobe is dependent on the relative

sizes of the adobe blocks and the spacing of the bamboo and wire. In real life it is likely

that there would be a compromise between the amount of confinement, the materials

available, and the number of holes which could be drilled through the walls safely.

4. A combination of internal vertical bamboo reinforcement and internal horizontal wire

improves the seismic resistance of the building by increasing the ductility of the walls and

increasing the connectivity at corner joints.

5. In the 1:5 scale tests on ‘L’-shaped corner joints, the collapse acceleration of the transverse

wall was increased by a factor of 2 using internal bamboo reinforcement. Further research

would be needed to assess if this factor could be expected in real life if whole buildings

were reinforced in this way. There may be differences in the effectiveness of this method

depending on whether irregular blocks are used which can fit around the bamboo (as in this

project) or regular blocks which must be cut to fit the bamboo in and may produce

weaknesses due to discontinuity problems in the adobe (as tested by Dowling, 2006).

6. Bamboo and galvanised wire are materials already used to reinforce adobe buildings and

have sufficiently low cost and wide availability to be suitable for widespread use.

7. Internal reinforcement is only suitable for newly-built constructions. External bamboo and

wire reinforcement has the potential to be used as a retrofitting method but further research


48

is needed in the field to ensure that the technique has widespread suitability in terms of

community acceptance and ease of use. Current field research related to this topic is

ongoing in locations including El Salvador and Pakistan by Dowling and others. The author

will work with the Salvadorean Foundation for Reconstruction and Development in El

Salvador to perform a structural survey of adobe houses in order to further assess the

appropriateness of external bamboo and wire reinforcement for strengthening existing

buildings.

8. Dissemination of results in formats useful to other researchers and practitioners is essential.

The results of this project will be shared via the recently set up World Adobe Forum

(www.worldadobeforum.net).

8.3. Recommendations for future research

1. Research on small-scale models should acknowledge the limitations of such testing and

investigate potential new ways of achieving results more directly relevant to real life.

2. Further research on model walls which represent only partial sections of adobe buildings

should pay particular attention to accurately simulating appropriate boundary conditions.

3. Further research using 1:5 scale models could consider the feasibility of constructing actual

adobe blocks instead of using gravel to simulate bricks. If actual adobe blocks are used,

research could include static testing of adobe prisms as well as dynamic testing of models.

4. Research could also consider the possibility of including recognised earthquake resistant

features such as ring beams into models and investigating their interaction with other

strengthening techniques.

5. All laboratory research should consider the latest updates from field investigations

regarding the suitability of different materials and techniques and should use appropriate

methods of feeding the research results back into real-life practice.

6. Seismic resistance methods investigated should have potential for retrofitting existing

structures as well as being incorporated into new buildings.

49

Section 9: References Blondet, M., Garcia, M. and Brzev, S. (2003), Earthquake-Resistant Construction of Adobe

Buildings: A Tutorial, retrieved from www.world-housing.net. Blondet, M., Torrealva, D., Villa-Garcia, G., Ginocchio, F and Madueño, I. (2005), Using

industrial materials for the construction of safe adobe houses in seismic areas, Proceedings of SismoAdobe 2005, 16-19 May 2005, PUCP, Lima.

De Sensi, B. (2003), Terracruda, La Diffusione dell’Architettura di Terra, retrieved from www.terracruda.com/architetturadiffusione.htm.

Dowling, D.M. (2006), Seismic strengthening of adobe-mudbrick houses, Faculty of Engineering, University of Technology, Sydney.

Flores, L.E., Pacheco, M.A. and Reyes, C. (2001), Algunos estudios sobre el comportamiento y rehabilitación de la vivienda rural de adobe, CENAPRED México, IEG/03/01.

Houben, H. and Guilland, H. (1994), Earth Construction: a comprehensive guide, ITDG Publishing, London.

IAEE (2004), Guidelines for Earthquake-Resistant Non-Engineered Construction, International Association for Earthquake Engineering, Tokyo, Japan.

Janssen, J.J.A. (1995), Building with Bamboo: A Handbook, ITDG Publishing, London. Loaiza, C., Blondet, M., Ottazzi, G. (2002), World Housing Encyclopedia Report: Peru Adobe

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