PREFACE - mowhs.gov.bt · Winter monsoon Figure 1. Summer monsoon and Winter monsoon in the South...

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Houses in Bhutan has become subject to damages due to frequent windstorms in the recent history.

The damages have particularly been to the roofs of rural houses across the country. The major events

of damages due to windstorm in April 2011 and December 2013 in which roofs of over thousands

of houses were damaged and more so recently, damages due to windstorm of April 2015 in which

over 700 houses were affected across the country calls for an urgent and necessary intervention.

Necessity for an intervention is further substantiated by the economic loss of the country due to

the huge amount paid in compensation for reconstruction and repair of damages. For instance,

the Royal Insurance Corporation of Bhutan Limited paid a total compensation amount of more

than Nu. 45 Million for the damages caused by windstorm in the years 2013, 2014 and 2015.

The houses affected so far by windstorm are found in the rural houses and it can be gathered with

some level of confidence that the weaknesses lie within the traditional roofing system. The traditional

roofing system is relatively more susceptible to impacts of windstorm hazard than that of the modern

structurally sound truss systems.

This guideline provides various measures to be incorporated to improve and strengthen

the traditional roof truss system to make it more resilient to windstorm while keeping the

traditional architectural components, features, and the naturally appealing appearance intact.

Towards reducing the vulnerability of the roofs of rural houses to damages caused by

windstorm, and with sincere anticipation that the rural home owners will adopt and implement

the doable and practical recommendations provided herein, the Ministry of Works and Human

Settlement is pleased to bring forth this “Windstorm Resilient Roofing System Guideline -

2017”. This document should serve as a ready one-stop guideline to all engineers, architects,

artisans and builders to build a more windstorm resilient roofing system for rural houses.

The guideline will be reviewed and updated periodically as and when better methods

applicable to our context are available. We would likewise appreciate valuable

comments and suggestions on this guideline for consideration in its next edition.

Secretary

Ministry of Works and Human Settlement

PREFACE

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ACKNOWLEDGEMENT

This Windstorm Resilient Roofing System Training Guideline 2017 is prepared under National

Adaptation Programmes of Action (NAPA II ) project funded by Least Developed Country Fund

(LDCF). Accordingly, the Ministry of Works and Human Settlement would like to thank the NAPA

II for the support.

The Ministry would also like to thank the Technical Core Group members for their valuable inputs in

refining and developing the Guideline. The Technical Core Group members are:

Ms. Phuntsho Wangmo, Department of Culture, MoHCA

Mr. Karma Dupchuk, Gelephu Thromde

Mr. Sonam Tobgay, Department of Engineering Services, MoWHS

Mr. Yeshi Lotay, Department of Disaster Management, MoHCA

Mr. Jigme Dorji, Thimphu Thromde

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SCOPE

This GUIDELINE broadly provides information about the different types of roof damages, their causes

and the mitigation measures that can be incorporated during the construction and reconstruction

of houses to reduce future windstorm damage while preserving the traditional roof architecture.

Recommendations made in this guideline are based on damage surveys, field visits and good practices

from both skilled artisans and professionals. This guideline will also serve as a basis for future

scientific research on Windstorm Resilient Roofing System.

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CONTENTTypes of Storm ………………………………………………………………………….. 1Effect of Wind on Building …………………………………………………………….. 4History of Windstorm in Bhutan ……………………………………………………...... 41.1. Types of Roofs in Bhutan ……………………………………………………….

A. Jabzhi Roof ………………………………………………………….....B. Jamthok Roof …………………………………………………………...C. Drangim Roof …………………………………………………………..D. Chenkhep (Lean to Roof) …………………………………………........

1.2. Roofing Materials ................................................................................................

911121213

142. Failure Patterns Observed ………………………………………………............

2.1. Blown Off Type………………………………………………………….2.2. Panel Failure………………………………………………………….....2.3. Peeling Failure Type ……………………………………………………2.4. Roof Overhang Failure ………………………………………………….2.5. Support Failure ………………………………………………………….

161617171819

3. Causes …………………………………………………………………..............3.1. Roofing Style of Bhutan ………………………………………………...3.2. Poor Workmanship ………………………………………………….......3.3. Lack of Maintenance ……………………………………………………3.4. Roof Pitch ………………………………………………………………3.5. Other Causes …………………………………………………………...

202021212222

4. Recommendations ………………………………………………………………4.1. Planning Aspect …………………………………………………………4.2. Creation of Continuous Load Path ……………………………………..4.3. Securing the Connections of a Roof Truss ……………………………..4.4. Connection between the Truss and Wall ………………………………..4.5. Connections between the Truss and gable Wall ………………………...4.6. Open Truss System …………………………………………………......4.7. Steel Truss ……………………………………………………………....4.8. Securing of CGI Sheet ………………………………………………….4.9. Strengthening of Overhangs …………………………………………….4.10. Roofing Types …………………………………………………………...4.11. Use of Tie downs ……………………………………………………......4.12. Other General Considerations …………………………………………..

23232526283032353638394041

5. Annexture A: Survey form for data collection ………………………………….Annexture B: Timber Joinery ..............................................................................

4248

6. References …………………………………………………………………….... 64

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TYPES OF STORM

Storm is the disturbed state of the atmospheric body characterized by significant disruptions to the

normal conditions which causes severe weather conditions such as windstorm, hail, thunderstorm,

heavy precipitation (snowstorm, rainstorm), cyclones and tornadoes, or wind transporting some

substance through the atmosphere as in a dust storm, blizzard and sandstorm. Storms can be detrimental

to human lives and property.

Different types of storms are outlined below:

a. Windstorm: A storm marked by powerful wind blows which may or may not be accompanied

by precipitation. Windstorms may last ranging from few minutes to hours (or even several

days) and cause damages to structures. Strong winds may carry dust and stones and cause dust

storms and sandstorms in dry climates.

b. Thunderstorm: A thunderstorm is a type of storm strong enough to generate lightning and

thunder and is generally accompanied by heavy precipitation. A thunderstorm is formed due

to a combined presence of i) moisture to form clouds and rain, ii) unstable air (relatively warm

air) that rises rapidly and iii) lifts such as due to fronts such as sea breezes and mountains.

Thunderstorm is triggered when high levels of condensation form in a volume of unstable air

and the consequent heat energy generates powerful rising air currents that brushes against the

concurrently cool descending air currents.

c. Hailstorm: It is a storm in which strong and powerful wind blows are accompanied by fall of round

or irregular lumps of ice called hailstones. Hailstones are formed during severe thunderstorms

when the water caught in strong upward moving wind freezes and eventually falls to the ground.

While most hails are small and virtually harmless, some hailstones can be greater than 2 inches in

diameter and can cause much damage and injuries.

d. Tropical cyclone: Tropical cyclones are large revolving vortices in the atmosphere where fierce

winds spiral anticlockwise inwards in the lower levels while above 8-9 kms the wind direction is

opposite to the lower levels. It has a relatively cloud free central region called the “eye” where the

pressure is the lowest and the temperature is the highest compared to the surroundings. The eye

is surrounded by an “eyewall”, a deep thick cloudy zone of maximum wind speed and thus the

most dangerous. Tropical cyclones form in the oceans if the conditions in the area are favourable.

It bears different names such as tropical depression, tropical storm, hurricane and typhoon.

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e. Ice storm: Ice storms are one of the most dangerous forms of winter storm. The vapour in the

atmosphere condenses and precipitates snow which melts into rain when it meets a warm air front.

The falling rain droplets subsequently hit a cold layer of air below of sub-freezing temperature

and supercool. The supercooled drops form ice upon impact on ground or anything below freezing

temperature.

f. Snowstorm: It is simply a storm with heavy fall of snow accumulating at a rate of more than 5

centimeters per hour together with blows of strong and powerful winds.

h. Blizzard: A blizzard is a severe snowstorm accompanied by very strong winds of speed 56 km/h

which last for 3 hours or more in general. The considerable snowfall reduces visibility to less than

quarter of a mile and creates very cold conditions with temperatures below 0˚ C.

j. Firestorm: Firestorms are conflagrations which attain such intensity that they create and sustain

their own wind systems. It occurs during large fire events which may be either due to natural

or man-made causes. Fire events such as bushfires, forest fires and wildfires may be either due

to natural causes or man-made while man-made fire events are those such as detonations of

explosives.

k. Tornado: A tornado is a localized, violently destructive rotating column of air occurring over

land and travels at high speeds. Tornadoes are characterized by a typical dark, long and

funnel-shaped cloud extending toward the ground. The strength of tornadoes are rated in Fujita

(F), Enhanced Fujita (EF) and TORRO (T) Scale. While tornadoes form all over the world, the

interior of the United States is the most prone area, especially throughout the Tornado Alley.

Of the different types of storm, Bhutan is susceptible to windstorm, thunderstorm, hailstorm, tropical

cyclone and ice storm of which windstorm has been found to be the most common destructive agent

from the available past records.

During the change in season, Bhutan is subjected to strong wind occurrences due to the pressure

difference in the north and south zones. In the winter, the high pressure zone is created in the northern

Himalayan region while low pressure zone is created in the Indian Ocean; thus the resulting pressure

difference brings cold strong winds from the north to south and vice-versa in summer as shown in

Figure 1(a) and Figure 1(b) respectively.

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a. Summer monsoon

b. Winter monsoon

Figure 1. Summer monsoon and Winter monsoon in the South Asia region

Source: www.pearsonhighered.com

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EFFECT OF WIND ON BUILDING

HISTORY OF WINDSTORM IN BHUTAN

Figure 2 shows a generalized effect of wind forces on a building.

The arrows pointing towards the house indicate the pushing action of wind on the surface while

the arrows pointing away from the house indicate the pulling action of wind on the surface.

The wind force impact every surface of the building, not only the surface facing

the wind, and in many cases, the pulling forces of wind are more critical than the

push as is evident from the loss of roof coverings in many severe wind events.

Due to the global climate change,

recent climatic variations has been

unpredictable. Bhutan has been

experiencing number of windstorms

which caused extensive damages

around the country. The inherent

weaknesses in the roofing style

clubbed with the severe windstorms

have caused a lot of damages to the

roofs of houses around the country.

Of the many windstorms that affected Bhutan, the windstorms of 2011 and 2013 were the most

devastating. A total of 2,598 roofs were damaged in 2011 windstorm of which 2,424 were traditional

house roofs. The 2013 windstorm damaged 1017 traditional roofs in rural areas. The details of the

Figure 2. Wind force effect on building

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damages during 2011 and 2013

windstorm are as depicted in Table 2

and Table 3 respectively. From the

two tables, it can be concluded that

the roofs that are most susceptible

to damage during windstorm are

traditional roofs as compared to

the modern roof truss. As per the

data provided by RICBL, a total

compensation of Nu. 45,158,760/-

has been provided for the damages

caused by the windstorm that struck

Bhutan in 2013, 2014 and 2015. The

details of the compensation provided

by RICBL is shown in Table 4.

Figure 3. Roofs damaged by 2013 windstormSource: DDM, MoHCA

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Table 1. History of windstorms that affected Bhutan (Media Archive and DDM)Source: Media Archive and Department of Disaster Management

Sl. No. Windstorm Events Affected Regions (Dzongkhags) Roof Damages

1 18 March 2010 Samdrup Jongkhar 18 houses

2 30 March 2010 Samdrup Jongkhar & Pemagatshel

95 rural houses & public buildings

3 April 2011 17 Dzongkhags 2598 rural houses & public buildings

4 March-April 2012 Zhemgang, Wangdue Phodrang, Punakha & Haa

236 rural houses & public buildings

5 April 2013 Punakha 21 structures

6 September 2013 Zhemgang 24 rural houses & public buildings

7 December 2013 Bumthang, Chhukha, Dagana, Gasa, Haa, Lhuentse, Paro, Punakha, Samtse, Trashigang, Trashi Yangtse, Thimphu & Wangdue Phodrang

1093 rural houses, public buildings & structures

8 March 2014 Dagana

Mongar, Samdrup Jongkhar, Sarpang, Trashigang & Zhemgang

108 rural houses & public buildings

9 2014 Samtse 126 rural houses & public buildings

10 April 2015 9 Dzongkhags 792 houses (Approximate)

11 March 2016 Dagana, Mongar & Pemagatshel 18 houses

12 April 2016 Wangdue Phodrang, Trongsa, Tsirang & Thimphu

47 houses

13 17 March, 2017 Punakha 318 houses, 4 lhakhangs, 2 schools

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Table 2. 2011 windstorm damages to the rural house roofs in seventeen Dzongkhags in BhutanSource: Department of Disaster Management, MoCHA

SI. No.

Name of Dzongkhag

Rural Home Roof

Monastery Roof Stupa

School/NFE Roof

BHU/ORC Roof

RNR Centre Roof

Village Head Office

RBP Outpost Roof

Total

1 Gasa 3 3

2 Thimphu 11 3 1 1 16

3 Paro 137 6 1 1 145

4 Haa 28 4 1 1 1 35

5 Wangdue Phodrang

17 3 20

6 Chukha 59 3 4 66

7 Tsirang 34 34

8 Dagana 170 4 7 1 1 2 185

9 Trongsa 2 2

10 Zhemgang 300 21 9 5 3 1 339

11 Mongar 378 6 8 4 1 1 398

12 Trashigang 401 7 13 2 423

13 Trashi Yangtse

200 5 1 1 1 3 211

14 Pema Gatshel

433 9 6 5 1 454

15 Sarpang 161 2 1 5 169

16 Samtse 1 1 1 3

17 Samdrup Jongkhar

89 3 2 1 95

Total 2424 77 4 59 21 6 4 3 2598

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SI.

No.

Name of

Dzongkhag

Rural Home Roof

Monastery

Roof

Stupa School/NFE Roof

BHU/ORC Roof

RNR Centre Roof

Village Head Office

RBP Outpost Roof

Grand

Total

1 Gasa 72 3 1 1 77

2 Thimphu 13 13

3 Paro 310 25 4 2 341

4 Haa 229 8 4 2 1 244

5 Wangdue Phodrang

12 3 1 16

6 Chukha 37 2 39

7 Dagana 31 31

8 Trashigang 46 2 48

9 Trashi Yangtse

2 2

10 Samtse 15 15

11 Bumthang 15 4 19

12 Lhuentse 5 5

13 Punakha 230 8 1 2 2 243

Total 1017 53 12 8 3 1093

Period No of Households Compensation (Nu.)

1-4-2013 to 31-12-2013 785 14,635,400

1-1-2014 to 31-12-2014 637 11,974,244

1-1-2015 to 31-6-2015 1088 18,549,116

Total 2510 45,158,760

Table 3. 2013 windstorm damages to rural house roofs in the thirteen Dzongkhags in Bhutan

Table 4. Compensation provided by RICBL for windstorm damages in past three years

Source: Department of Disaster Management, MoCHA

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1.1. TYPES OF ROOFS IN BHUTAN

Roofs play an extremely significant part in the characterization of traditional Bhutanese architecture

and is therefore one of the most important elements in traditional Bhutanese architecture. A very

noticeable design aspect of the typical Bhutanese roof is the elevation of the roof high above the

building (often in layers) to form what is often called the “flying roof”. This type of roof appears to

float above the building. This design thus allowed the roof to protect the building from rain and sun

while allowing cool breezes to flow freely through the attic space. This meant that the attic space

provided a very useful and practical space for storing and drying vegetables, fodder and other produce

from farms and gardens around a house.

In addition to protecting the building from external environmental elements, in the traditional

Bhutanese practice, the roof element additionally played an important role in defining the hierarchy

and significance of buildings and their status.

There are four main types of roof design in traditional Bhutanese architecture. These are the following:

A. Jabzhi Roof

B. Jamthok Roof

C. Drangim Roof

D. Chenkhep Roof

Within the Thobthang, a traditional practice of entitlement of architecture, the Jabzhi roof is the roof

of the highest level. The simple gable roof and the layered gable roof style known as Jamthok roof is

the most common one found in traditional Bhutanese architecture.

For buildings of very high status such as the Utse of a Dzong, palace or for Lhakhangs, a square

hipped roof called Jabzhi (four corner roof) with Sertog is used to signify their importance in the

community.

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Jamthok

Gungchhen Gungchung

ShathungDhingriLhiuchung

Dingri Langna

Lungzey

GhaShari

Shari Jugshing

Shinglep Dangchung

Drangim Roof

Chenkhep Roof

Figure 4. Traditional Bhutanese house with traditional roof

Figure 5. Details of a typical gable roof commonly used in traditional Bhutanese house

Illustration: Bishnu Pradhan, EARRD, DES, MoWHS

Illustration: Bishnu Pradhan, EARRD, DES, MoWHS

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Sertog

Jabzhi Roof

Jabzhi Roof

Main Jabzhi Roof

Figure 7. Jabzhi roof

Figure 6. Chholo

Illustration: Yeshey Jamtsho, CDRD, DHS, MoWHS

Jabzhi roof is a square hipped roof with four prominent corners. These roofs were used in buildings

of high status such as the Utse of a Dzong or palace or over the building housing the main altar of a

Lhakhang. Jabzhi roof can be designed with just one roof layer or with several layers of roof to form

a pagoda style roof. The roofing material for the Jabzhi roof over these prominent places are usually

made of metal plated with gold. The eaves of the roof are then ringed with a metal curtain embellished

with decorative and sacred iconographic carvings known as Chuzha Chulo. Where gold was not

available, the Jabzhi roof was painted in yellow colour to symbolize its sacred and high status. The

four sides of a Jabzhi roof is capped off usually by the heads of auspicious animals like the garuda

or dragon or by the scared carved sculpture known as Chuju Patra. These caps are made of brass or

copper which are usually gold plated.

A. JABZHI ROOF

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The Jamthok roof in traditional Bhutanese architecture consists of a smaller gable roof laid over a

longer gable roof in layers one over the other. This allowed upper layer of the roof to be raised further

up to create a much more spacious area in the central area of the attic. In the traditional practice of

Thobthang or entitlement in architecture, this layered Jamthok roof is said to be second in rank to the

Jabzhi roof.

The upper layer of the Jamthok roof did not traditionally have windows under it and was left open

to allow for air to flow through. However, in some cases, where the attic area was converted into a

habitable space, the space under the upper layer of the Jamthok roof was framed into a horizontal line

of clerestory windows on two sides and often decorated with Horzhu and Pem Choetse. The Jamthok

roof when left open without windows is also often called a Lungo Roof.

The Drangim roof in traditional Bhutanese architecture consists of a gable roof of the same length laid

over a lower level gable roof. This allowed upper layer of the roof to be raised further up to create a

much more spacious area in the attic space. The upper layer of the Drangim roof did not traditionally

have windows under it and was left open to allow for air to flow through.

B. JAMTHOK ROOF

C. DRANGIM ROOF

Figure 8. Traditional Bhutanese house with Jamthok roof Illustration: Bishnu Pradhan, EARRD, DES, MoWHS

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The Chenkhep roof is a simple traditional lean-to-roof that is usually installed to provide additional

protection to a cantilevering Rabsel. It is laid at a lower level under the main roof of the building.

D. CHENKHEP ROOF

Figure 9. Traditional Bhutanese house with Drangim roof

Figure 10. Traditional Bhutanese house with Chenkhep roof

Illustration: Bishnu Pradhan, EARRD, DES, MoWHS

Illustration: Bishnu Pradhan, EARRD, DES, MoWHS

Chenkhep Roof

Drangim Roof

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1.2. ROOFING MATERIALSUntil the early 1960s, most Bhutanese traditional roofing materials were wooden shingles and slate

in the eastern, central and western part of Bhutan whereas in the southern part, bamboo mats, leaves

and straw were used. The roofing materials were selected based on the climatic condition and material

available in the surrounding.

Commonly used roofing materials are :

• CGI sheets

• Shingleps

• Bamboo mats and thatched roof

• Others.

CGI Sheets

The commonly used roofing material now a days in rural areas is CGI sheets owing to its affordability,

durability and less maintenance.

Shingleps

In olden days, Shingleps were the most commonly used roofing material. From the report compiled

by Yeshi Lotey (2013), the change in the roofing materials used in the rural areas in Bhutan started in

the early 1980s, and now a days, most of the houses in rural areas are roofed with CGI sheets.

Figure 11. Bhutanese house with CGI roofing

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Bamboo mats and Thatched Roof

Bamboo mats and thatched roofs are mostly found in southern parts of Bhutan because of its

availability and suitability against the local climatic conditions.

Others

In some parts of Bhutan, we can still find slates being used as roofing materials. Some even use

tarpaulins for roofing huts. In recent times, krill roofing is gaining popularity in urban areas.

Figure 12. Bhutanese house with shinglep roofing

Figure 13. Bhutanese house with thatched roofing

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2. FAILURE PATTERNS OBSERVED

2.1. BLOWN OFF FAILURE

Based on the desktop study conducted on the data available on the past windstorm events and field

assessment report from both DES and DDM on windstorm, the failure patterns observed could be

broadly categorized as following:

1. Blown Off Failure

2. Panel Failure

3. Peeling Failure

4. Roof Overhang Failure

5. Support Failure

Roof in which more than 50% of the trusses are lifted up are categorized as blown off type. This type

of damages are caused mainly due to absence of engineered truss system. The truss system includes

designed section of tie beam, principle rafter, common rafter, purlin and laths with connection details.

The lack of connection between tie beam and truss props to the walls/columns also leads to this type

of failure. Usually the roof systems which are simply supported on the walls are at a higher risk of

being blown off.

Figure 14. Blown off roofs

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2. FAILURE PATTERNS OBSERVED

2.1. BLOWN OFF FAILURE

2.2. PANEL FAILURE

2.3. PEELING FAILURE

The failure in which a portion of the roof mainly the CGI sheets/ shingles, reapers (tshim) and purlins

are blown off is categorized as panel failure. This failure usually arises when there is inadequate

connection of the CGI sheets/shingles, reapers and purlins to the truss system. Use of undersized

reapers and purlins in the truss system also leads to this kind of failure.

The failure in which the whole or part of roof coverings are peeled off but the underneath truss

members remain undamaged are classified as peeling of roofs.

This failure arises because of large spacing provided between the purlins resulting in large surface

area for the wind pressure to act upon. Improper and inadequate connection of roof coverings to

purlin and use of wire nails in place of J hooks and screw nails also sums up to this failure.

Use of very thin sheets, improper overlapping of coverings/claddings and lack of proper fittings

especially at large roof overhangs are also found to be the cause for this failure.

Figure 15. Panel failure

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2.4. ROOF OVERHANG FAILURE

2.5. SUPPORT FAILURE

Roof failures which results from an excessive projection of the purlins and trusses from the supports

are classified as roof overhang failure. When the purlins protrude too much from the supports, they

create large space for the upward wind pressure. The economization of truss member sizes and

improper extension of truss members for patio weakens the roof overhang leading to this type of

failure. As per Bhutan Building Rules 2002, the normally adopted roof overhang is 1.5m.

Figure 16. Peeling failure

Figure 17. Roof overhang failurePhoto: Bishnu Pradhan and Karma Tenzin, EARRD, DES, MoWHS

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2.5. SUPPORT FAILURE

Type of failure in which the entire or part of roof is damaged due to failure of support are classified

as support failure.

This failure arises due to provision of inadequate support sizes and improper connection between the

support and the roof floor.

Figure 18. Damage caused due to construction of patio roof as one structure

Figure 19. Support failure

Illustrations: Bishnu Pradhan, EARRD, DES, MoWHS

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3. CAUSESIn the preceding chapters, various roof failure patterns due to windstorm observed are described.

The factors causing these failures, as per the various international publications studied and from field

observations, are summarized below:

1) Roofing Style of Bhutan

2) Poor Workmanship and Techniques

3) Lack of Maintenance

4) Pitch of the Roof

5) Other Causes

3.1. ROOFING STYLE OF BHUTAN

In the traditional construction system, the roof systems are usually simply supported and gravity load

based. This makes the roof inherently vulnerable to windstorm damages. The rationale behind this

traditional design is to contain damage to the roof and protect the walls and main structure during

windstorm.

The transition in roofing material from shingles to CGI sheet has been observed in the past decade.

New roofing materials are used for the same truss design and roof configuration which could be one

of the reasons for roof failure during windstorm.

Figure 20. Simply supported truss provided in rural housesPhoto: Yeshey Lotay, DDM, MoHCA

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3. CAUSES

3.1. ROOFING STYLE OF BHUTAN

3.2. POOR WORKMANSHIP

3.3. LACK OF MAINTENANCE

Poor workmanship is observed to be one major cause for the failure of roof. Poor workmanship in

various truss joinery and connections and improvisation at site without any design basis tend to fail

during windstorms.

Lack of periodic maintenance is also a main cause of roof failure during windstorms. Material strength

deteriorates mainly due to exposure to moisture and insect attack in absence of routine maintenance.

Rusting of CGI sheets and screw could also lead to roof failure during windstorm. Furthermore, aging

is also a natural phenomenon which makes the roof weak.

Figure 21. Some examples of poor work quality

Figure 22. Failure due to lack of maintenance

Photo: Bishnu Pradhan and Karma Tenzin, EARRD, DES, MoWHS

Photo: Bishnu Pradhan and Karma Tenzin, EARRD, DES, MoWHS

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3.4. ROOF PITCH

3.5. OTHER CAUSES

4.1. PLANNING ASPECT

Roof pitch has an important bearing on the wind pressure developed on the roof. As per IS: 875 part

III external pressure coefficients is maximum for roof angle of 10 degrees when the wind angle is 0°

and maximum for flat roofs when the wind angle is 90 degrees. Generally in Bhutan, the roof angle

is constructed at an angle of 10-15 degrees.

When wind enters from one face there should be an exit on the other side. But when the exit path is

closed, the wind from the face of the entry creates uplift pressure due to which the roofs are blown

off. The roof also fails due to suction force that is created by the windstorm while trying to balance

the pressure between outside low pressure and inside high pressure.

Figure 23. Effects of Wind PressurePhoto: Bishnu Pradhan, EARRD, DES, MoWHS

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4.1. PLANNING ASPECT

Site condition plays a very important role in the wind safety of a building. Building sited in exposed

areas are most vulnerable and so the following precautions must be taken into account:

The area behind a mound or a hillock should be preferred for providing natural shielding. Similarly,

a row of trees planted upwind also acts as a shield but to avoid the damage from broken trees, a

distance of about 1.5 times the height of tree needs to be maintained.

i. Shape/Plan of the Building

Building should be regular, simple and symmetrical. Regular shaped buildings like circular, square

or rectangular are more stable than irregular buildings with zigzag plan, having empty pockets, as the

latter is more prone to wind damages

Figure 24. Location of building for natural shielding

4. RECOMMENDATIONSBased on the failure patterns observed and the desktop study that was carried out, the doable

recommendations are as below.

A. SITE SELECTION

B. BUILDING LAYOUT

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ii. In case of construction of group of buildings, a cluster arrangement can be followed in preference

to row type.

iii. The buildings should be oriented in such a manner that the shorter span length of the wall faces

the prominent wind direction if it is known.

Figure 25. Shape of the building plan

Guideline for reconstruction of houses affected by Tsunami in Tamil Nadu

Figure 27: Orientation of building

Figure 26. Arrangement of building in group construction

Rectangle Square

L Shape U ShapeNarrow Rectangle

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4.2. CREATION OF CONTINUOUS LOAD PATH“A continuous load path is the series of building members and connections that resist loads that

act downward (gravity loads), as well as laterally and upward (wind, flood, or seismic loads).

The connections between the members are typically the point of failure (and thus most critical) in

continuous load path found in residential structures” (FEMA-DR-1432-WI).

Severe winds subject the roof to high uplift forces which are strong enough to completely remove

the entire roof system. To mitigate its vulnerability to this type of damage, the roof system has to

be adequately attached to the exterior walls of the house or anchored to the ground. . All joints

and connections—roof-to-wall, floor-to-floor, and wall-to-foundation—must be secured to create a

“continuous load path” to the building’s foundation as shown in Figure 28 below.

Figure 28. Continuous load path in a typical 2-storey wood frame residential structure. A continuous load path resists uplift and lateral loads that act on a building (FEMA 342).3-6 �����������������������������������

CHAPTER 3

FIGURE 3-2: Diagram showing a continuous loadpath for a two-story wood frame building.

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4.3. SECURING OF TRUSS

The connections of complete components of truss has to be secured so that they behave as one during

windstorm. Figure 29 below shows a typical roof truss and the connections that need to be secured.

Rafter

Strut Tie Beam

Collar Tie

Metal Strap

Gusset

Gusset

CA

B

D

If the rafters are not secured, the ridge can fall apart when strong wind passes over the roof. The ridge

can be secured by using any of the following methods:

i. Collar ties—are timbers connecting the rafters which are nailed to the sides of the rafters

ii. Gussets—are made up of steel/ plywood/ wood and used at the ridge

iii. Metal straps—providing metal straps over the top of the rafters

A. RIDGE

Figure 30. Methods of securing the ridge

Figure 29. Typical roof truss showing connections to be secured

Illustration: Jit Bdr. and Gita, EARRD, DES, MoWHS

Illustration: Bishnu Pradhan, EARRD, DES, MoWHS

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Struts

Tie Beam

Rafter

Rafter

Tie Beam

Struts

Metal Gusset

Metal Gusset

Metal Gusset

The strut and tie beam has to be secured to ensure proper functionality of the roof during windstorm.

This can be done by providing metal gussets as shown in Figure 31.

B. STRUTS AND TIE BEAM

Figure 31. Securing the tie beam and strut using metal gusset

Figure 32. Securing the rafter and struts using metal gusset

Figure 33. Securing the rafter and tie beam using metal gusset

Illustration: Jit Bdr., EARRD, DES, MoWHS

Illustration: Jit Bdr., EARRD, DES, MoWHS

Illustration: Jit Bdr., EARRD, DES, MoWHS

The rafter and strut also needs to be secured using the metal gussets as shown in Figure 32.

B. RAFTER AND STRUTS

The rafter and tie beam can be secured using gusset plates as well.

B. STRUTS AND TIE BEAM

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Figure 34. A complete truss with secured connections

Figure 35. Securing the connection between wall and truss using metal strap and vertical reinforcement

Illustration: Jit Bdr. and Gita, EARRD, DES, MoWHS

Illustration: Gita Maya Sunwar, EARRD, DES, MoWHS

4.4. CONNECTION BETWEEN THE TRUSS AND WALL

The truss to wall connection can be secured by anchoring the connection between the rafter and tie

beam to the reinforcement provided inside the walls in masonry structures as shown in Figure 35

below.

The anchoring of roof truss to masonry walls can also be accomplished through anchor bolts embedded

into roof band or by using metal strap. The roof band should not be less than 75 mm in depth.

In case of large uplift forces, the anchoring bars can be taken down to the foundation level.

StrutRafter

Tie Beam

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Illustration: Gita Maya Sunwar, EARRD, DES, MoWHS

Figure 37. Securing the connection between ekra wall and truss using metal strap with nuts and bolts.

Figure 36. Securing the connection between brick wall and truss using metal strap with nuts and bolts.

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4.5. CONNECTION BETWEEN THE TRUSS AND GABLE WALL

In some of the houses, gable walls are constructed over which roof is placed without proper

connections between the roof framing and the wall, which makes it susceptible to damage during

windstorm.

For the elimination of damage during windstorm, the ridge beam and intermediate beam has to be

fastened to the gable wall and gable band.

The gable band has to be constructed in a similar way of constructing other bands of the wall, but

it has to be inclined to the required angle for the roof formation as shown in Figure 39 below. The

provided bars of the gable band need to bend into the necessary shape to form an overlap of minimum

450mm with the eave band.

Figure 38. House with gable wall showing gable band and eave bandIllustration: Bishnu Pradhan, EARRD, DES, MoWHS

Figure 39. Connection of gable to eave bandIllustration: Bishnu Pradhan, EARRD, DES, MoWHS

Gable Band

Eave Band

Lintel Band

Eave Band

Lintel Band

Reinforcement for long wall(Max spacing 1500mm)

Reinforcement adjacent to opening

Reinforcement for gable band

Reinforcement at wall corner

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While casting gable band, weld a 250 mm long bolt of 12 mm diameter on a 100x100x5 mm MS

plate. Once the concrete becomes hard, place the timber ridge beam and intermediate beam with a

through hole over the bolt and place a washer and a nut to anchor it down.

For further strengthening the intermediate beams, provide collar ties as shown in Figure 42. In this

way the collar tie provides support to the intermediate beam and at the same time strengthens the

ridge. Collar beam can be installed between the opposite pair of the rafters.

Figure 40. Fastening of ridge beam to gable wall

Figure 41. Fastening of intermediate beam to gable wall and placement of principal rafters

Illustration: Bishnu Pradhan, EARRD, DES, MoWHS

Illustration: Bishnu Pradhan, EARRD, DES, MoWHS

8 mm dia bars at 150 mm

12 mm dia bolt100x100x5 mm MS plate

12 mm dia bars

12 mm dia barsWasher and Nut

Ridge Beam

Eave Band Gable Band

Nut and Bolt Principal Rafter

Ridge Beam

Intermediate beam

Gable Band

Eave Band

Lintel Band

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4.6. OPEN TRUSS SYSTEM

For open-type truss systems, following measures can be adopted to make it stronger and resilient to

windstorm.

Ghadhen is a wooden plank provided below the Gha for levelling and it also serves the purpose

of transferring load from above to roof floor. Continuous Ghadhen should be provided in both the

directions and it should be properly fixed with nails.

A. PROVIDE CONTINUOUS GHADHEN

Figure 42. Strengthening intermediate beam by providing collar tie

Figure 43: Lhiuchung resting on continuous Gha

Illustration: Bishnu Pradhan, EARRD, DES, MoWHS

Purlin

Gha Ghadhen Lhiuchung

Intermediate BeamRidge Beam Collar Tie

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Figure 44: Connection between timber prop and tie beam

Figure 45 : Connection between Dhingri and Chholo in cement mortars

Connection of timber prop to the roof floor can be accomplished by providing the RCC tie beam or

wooden tie beam as shown in the picture below.

The anchoring of Dhingri to Chholo in cement mortar should be accomplished through anchor bolts

embedded into concrete core. The weight of participating masonry at an angle of half horizontal to 1

vertical should be more than the total uplift at the support.

B. PROVIDE CONTINUOUS GHADHEN

C. SECURING CONNECTION BETWEEN DHINGRI AND CHHOLO IN CEMENT

MORTAR

Lintel Band

Anchor Bolts

Parricioating

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The connection between Lhiuchung to Shathung and Lhiuchung and Dhingri and can be secured with

metal plate or 90 degree angle bracket as shown in the picture below.

D. SECURING CONNECTION BETWEEN LHIUCHUNG AND DHINGRI

Figure 46. Connection between Lhiuchung and Shathung and Lhiuchung and Dhingri

Illustration: Gita Maya Sunwar, EARRD, DES, MoWHS

Shathung

Lhiuchung

Metal Plate

Angle Bracket

Dhingri

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4.7. STEEL TRUSS

For open-type truss systems, following measures can be adopted to make it stronger and resilient to

windstorm.

Fan truss

Queen Truss

Pratt Truss

Flat Truss

Figure 47. Different types of steel trusses

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4.7. SECURING OF CGI SHEETThe recommended size of galvanized sheets to be used in high wind region is 24 gauge (0.63mm).

The roof sheets can be secured by fixing every two corrugation at the ridges, eave overhangs and

by fixing every three corrugation in the interior of the roofing sheet. In this way maintaining screw

pattern of the right fashion plays a vital role. Projection of sheets beyond the edge of the purlin should

be limited to a maximum length of 300mm.

The recommendations for providing screws are as follows:

a. Use proper driving screws for corrugated galvanized roof sheets.

b. The minimum depth of screws that should go into the purlins is 50 mm.

c. All holes for bolts, rivets, etc. shall be made in the crown of corrugations and shall be drilled (not

punched.

d. Use large washers under the screw heads to prevent the roof sheets from tearing when dragged

upward by high wind pressure.

Figure 48. Screw pattern for roof fittings

Figure 49. Providing laths to have more fixtures for each sheet

Illustration: Karma Tenzin, EARRD, DES, MoWHS

Illustration: Karma Tenzin, EARRD, DES, MoWHS

LathsCommon Rafter

Strut

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e. Do not fix screws within 25 mm from the end of the sheet.

f. Maximum pitch between the two screws across the width of sheet should be 375mm

g. Use of ordinary nails should be avoided.

As the corners and the roof edges are zones of higher local wind suctions, the connections of sheeting

to the truss need to be designed for the increased forces. Failure at any one of these locations could

lead progressively to complete roof failure.

A reduced spacing of 300 mm between bolts is recommended. For normal connections J bolts may be

used but for windstorm resistant connections U – bolts are recommended.

The corrugated sheeting should be properly overlapped (at least 2 & 1/2 corrugations) sidewise to

prevent water from flowing under the seam and end lap of at least 150 mm.

G.I. Nut Nut

30x30x6 gauge M.S. washer

Bolts fixed with bitumen washers

Bolt flattened and nailed to purlin with lap

Sealing washer neoprene or bituminous felt

Corrugated sheeting

Steel strip

Rubber WasherG.I. Flat Washer

G.I. Flat Washer

J Hook

Purlin

Figure 50. Properly driven screw (left) and screw with large washers (right)

Figure 51. Use of J-bolts, U-bolts and securing of high suction zone using reinforcing bands

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The sheets should overlap each other properly and while laying, the sheeting the direction in which

the wind prevails should be kept in mind. Pick the correct end of the roof as per the wind direction

to start laying the CGI sheet so that the prevailing wind is blowing over the lap joint and not into it.

Avoid stretching the width of the sheets to prevent the entry of wind and rain while installing it.

4.9. STRENGTHENING OF OVERHANGS

4.10. ROOFING TYPE

The roof overhang should be kept to a minimum as the roof projection beyond the walls experiences

high uplift. If large overhangs are provided it should be strengthened using outriggers.

The rafter span and the overhang can be adjusted based on the spacing placed along the wall span.

Overhang length can be increased from the wall by reducing rafter spacing which makes overhang

stronger against windstorm.

Overhang length of the roof from the gable end is adjusted based on the purlins and the outriggers

spacing. The strength of the overhang increases with the placement of more numbers of outriggers

with spacing maintained at closer distance.

Figure 52. Correct orientation of sheet overlap

Figure 53. Placement of outriggers

Illustration: Bishnu Pradhan, EARRD, DES, MoWHS

Illustration: Bishnu Pradhan, EARRD, DES, MoWHS

Outriggers Purlins

Sheet1 Sheet 2 Sheet 3Direction of Prevailing Wind

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4.10. ROOFING TYPE

A gabled roof can be characterized as a roof with two slopes that come together to form a ridge or peak.

A hipped roof is one that slopes upward from all sides of a building. Due to aerodynamic properties

and conventional construction techniques, most hipped roofs will perform better in windstorms than

most gabled roofs.

The intersection of the gable (triangular portion of the wall beneath the sloping roof surfaces) and end

wall is a particularly weak point of gabled roofs unless full-height stud, concrete or masonry walls

are used.

Figure 54. Construction of patio roofs as separate structureIllustration: Bishnu Pradhan, EARRD, DES, MoWHS

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4.11. USE OF TIE DOWNS

Use of tie down wires are the most common method of securing the roof adopted in rural houses. The

wires are anchored to the ground by using concrete or by tying the wire on a boulder.

Alternatively tie-down rods can also be used. Tie-down rods in timber framed construction and steel

framed elements transmit uplift forces from the roof down through the wall structure and eventually

to the ground. A regular maintenance of tie down should be carried out.

Tie Down

Concrete

Stone

Figure 55. House with gable roof (Left) and hip roof (Right)

Figure 56. Roof secured by using tie down wiresIllustration: Bishnu Pradhan, EARRD, DES, MoWHS

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4.12. OTHER GENERAL CONSIDERATIONS

• The minimum width of the beam or any flexural member shall not be less than 50 mm or 1/50 of

the span, whichever is greater.

• The depth of beam or any flexural member shall not be taken more than three times of its width

without lateral stiffening.

• All flexural members having a depth exceeding three times its width or a span exceeding fifty

times its width or both ‘shall be laterally restrained from twisting or buckling and the distance

between such restraints shall not exceed 50 times its width.

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Number of storey: One Two Three Others

Insured Not Insured

Approved structural drawing available: Yes No

Deviation of roof design from approved drawings: Yes No

RCC Stone Masonry Rammed earth

Bakal/timber/bambooEkraBricks

Workshop shed/ industrial shed/ storage sheds

Mud block

Building structure types (please tick):

Permanent structure (building life ≥ 15 yrs.) Semi-permanent structure (15 yrs. > building life ≥ 5 yrs.) Temporary structure (building life < 5 yrs.)

Construction typology:

ANNEXTURE A

SURVEY FORM FOR DATA COLLECTION

SURVEY FORM

PART A: GENERAL INFORMATION

Name of the Owner ............................................ House No ............................................Village ................................... Gewog ................................ Dzongkhag ................................Dimension of BuildingLength ................................... Width ...............................

The purpose of this survey is to take stock of existing housing design and construction

practices in the country with particular attention to roofing system and their vulnerability

to windstorms. This information will be used to revisit the design and construction

adequacy of common practices in roofing systems. It will also be resourceful in identifying

shortcomings and their appropriate improvement recommendations based on this research.

The survey shall be carried out on common building typology in the representative Gewogs.

The representative Gewogs may be selected based on the past windstorm damages in

the Dzongkhag. The survey form shall be completed as per the questionnaire set below.

If yes, specify the deviation with available evidence (copy of approved drawing and as built pictures)

................................................................................................................................................................

................................................................................................................................................................

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PART B: TECHNICAL INFORMATION

1. Location of building with respect to terrain (please tick): Exposed open terrain with few or no obstructions and in which the average height of any object surrounding the structure is less than 1.5 m.

Open terrain with well scattered obstructions having height generally between 1.5 to 10 m.

Terrain with numerous closely spaced obstructions having the size of building structures up to 10 m in height with or without a few isolated structures taller than 10 m.

a. Location of subject building with respect to obstruction: Middle of obstruction Periphery of the obstruction

b. If structure taller than 10 m present, specify the location of tall structure with respect to subject building.

Front Rear On side

Terrain with numerous high closely spaced obstructions.

Specify if closely spaced obstructions are fully or partially surrounding the subject building:………………………………………………………….. (Picture)

2. Building positioning with respect to topography: Valley Hill top Ridge Gentle slope

3. Roof Type: Gable Hip Jabzhi roof Jamthok roof Chenkhep roof Drangim roof (picture of complete roof type)Illustration of roof types please refer Note I.

4. Types of overhang: Slope overhang Horizontal overhang

5. Roof truss materials type: Timber Steel Bamboo Others

6. Truss type: Closed type Open type Lean to type (picture)Illustration of truss types please refer Note II.

7. Truss Spacing (c/c): ………

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8. Type of roofing material: Metal sheet Shingle (Shinglep) Slate Tiles (concrete, clay) Thatched roof (Bamboo, leaves, straw) Others

9. Type of opening in the attic: Fully Open Fully closed Partially closed E

W

N

S

10. Type of attic enclosing material: Shingle (Shinglep) Bamboo mat Metal sheet Ply board Wooden planks Gypsum board Wire mesh Others

11. Enclosure construction type: Inclined closed Vertical closed Others

12. Roof connections: Simply supported on Cholo (extended wall pillars) only Simply supported on cholo and timber props on the roof floor Fixed to exterior wall only Fixed to exterior wall and RCC slab Fixed to column (timber/RCC) ends Fixed to common roof beam/roof tie beam Others (Picture)

13. Truss connections: MS straps Nails Screws Welding Plywood Others

14. Connection materials used between the purlin and rafter: MS straps Nails Screws Welding Plywood Others

15. Connection materials used between reapers/purlin and roofing material: GI Screw Toe-nail J-Hook Metal wires Cane rope Others

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16. Is there stone over the roof (tshado): Yes No

17. Is there roof tie down cable/wire: Yes No

18. If YES, type of tie down material: GI wire Aluminum cable

HT wires Others

19. Whether the house was affected by windstorm in past: Yes No

20. Extent of damage: Fully blown-off Half blown-off Partly blown-off

21. Was there change in the roof structure or roofing materials during reconstruction:

Yes No

If YES, specify what was changed

……………………………………………………………………………………………………

……………………………………………………………………………………………………

………………

22. Size of opening around the hous, i.e. window opening or other opening at first floor/top

floor:

……………………………………………………………………………………………………

………

Please measure the following at site:

Attic height: …………

Overhang: …………...

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Note I: Illustration of roof types

Sertog

Drangim Roof

Tshado

Chenkhep

Jabzhi Roof

Jabzhi Roof

Main Jabzhi Roof

Figure 1. Jabzhi Roof

Figure 2. Drangim Roof

Figure 3. Chenkhep Roof

Figure 4. Jamthok Roof

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Note II: Illustration of truss types

Figure 5. Closed Truss

Figure 6. Open Truss

Figure 7. Lean-To Tuss

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Connections are one of the most important parts of any timber structure. They transfer forces between

components, anchor the superstructure to the foundation, and maintain integrity of the system in the

event of overloading.

For designing joinery, one needs the understanding of how the whole timber frame and cladding

assembly function as an integrated system to support the loads.

Most collapses that occur during extreme load events are attributable to inadequate or inappropriate

connections, and the serviceability and durability of structure can also be governed by connection.

(Snow et al. 2006).

• The geometry of the joint should have mating surfaces that allow all structural loads to be

transferred in bearing of one member against the other.

• The wood removed to create the joint should not unduly weaken either member.

• The geometry of the joint should not be altered by shrinkage of the wood and the bearing surfaces

should remain in tight contact.

Few common simple rules to keep in mind while designing the joints:

Different Types of Timber Joints

I. Halved lapped joints

Differenttypesofjoineryareusedfordifferentpurposesandatdifferentlocations.

Most commonly used joinery are:

I. Halved lapped joints

II. Mortise and Tenon joints

III. Scarf joints

IV. Tongue and groove joints

V. Housing joints

The most common uses of halved lapped joints are in wall frame construction and connection.

• Cross half may be used where internal walls intersect at a common position.

• Tee half may be used where an internal wall connects to an external wall.

ANNEXTURE BTIMBER JOINERY

III. Scarf joint

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Different Types of Timber Joints

II. Mortise and tenon joints

The mortise is a hole and the tenon is the projection on end of timber member that is inserted in

mortise. The joint may be glued, pinned or wedged to lock it in place.

Cross Half Tee Half

Dowel Rod

III. Scarf joint

A scarf joint is a method of joining two

members end to end in woodworking.

The scarf joint is used when the member

used is not available in the length required.

The different types of scarf joints are asfollows:

a. Simple half lap scarf jointThis type of joint is used when both parts

are supported in the full length.

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b. Splayed scarf jointThis type of joint is used when both parts are

supported in the full length

Halved and tabled joint: the table at center

adds tensile capacity and the iron bolt holds

them together.

Stop-Splayed, Under squinted and Tabled with

Wedges and Pins. The tensile capacity, torsion,

and bending strength in both directions are

greatly increased. Under squinting makes the

joint more resistant to twisting.

By drawing out the scarf, additional tables can be added to increase tensile capacity. The complexity

of this scarf precludes its use except in members under great tensile loads, as in the lower chords of

long span trusses.

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IV. Tongue and groove joint

IV. Housing joints

Tongueandgrooveisamethodoffittingsimilarobjectstogether,edgetoedge,usedmainlywith

woodflooring,panelling,andsimilarconstructions.

Tongueandgrooveisamethodoffittingsimilarobjectstogether,edgetoedge,usedmainlywith

woodflooring,panelling,andsimilarconstructions.

Through housing joint

End lap housing joint

Side housing

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A. Fixing of timber frame on the plinth wall

Uses of Different Types of Joints in Different Building Location The loads and stresses of the building need to be supported by the frame and the entire frame should

be properly connected through the joinery.

Jointsmustbechosenonthebasisofthetasktheyaretofulfil,includinglockingtheframetogether,

bearing weight and transferring forces and building loads from one member to another.

The superstructure may

be rigidly fixed into the

plinth masonry or concrete

foundation with the help of

metal bolts or MS straps.

Plinth wall should be at least

2 feet.

Timber frame should be

placed at a minimum distance

of 2 inches from the edge of

plinth wall.Floor Joist

Bu-Dhen

Concrete Fill

Vertical Post

Vertical Post

Masonry/ Concrete

MS Bolt

Sill

MS Straps

MS J Bolt

B. Plinth beam and floor joints

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B. Plinth beam and floor joints

The plinth beam ties the posts and keeps the bottom of the building from spreading and distribute

columnloadsoverthefoundation,italsostiffensandsupportsthewallsatthefirstfloor.

Floor joist is a horizontal member spanning from wall to wall or wall to beam or beam to beam and

isprovidedtosupportthefloorabove.

The connection between all these members should be such that it can keep all the structure intact and

transfer the load smoothly to the foundation.

Corner JointJoints at corner is most critical joint.

Mortiseandtenonjointareusedhereasitcombinessimplicitywitheffectiveness.

The relish on mortise beam prevent horizontal displacement of both post and adjoining beam.

Usually relish should be equal to the tenon thickness (min 2 inches). The joints between two sills

should be secured with dowel.

Tenon should be at least 2 inches thick.

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Blind mortise and tenon joint

Through mortise and tenon joint

Post

End SillLongitudinal Sill

Post

End Sill

Longitudinal Sill

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Intermediate JointTheseareconnectionswhereinteriorfloorjoistmeetseitherlongitudinalorendwallsills.Theyoftenhave

posts bearing on them.

The blind mortise and tenon joint is mostly used here.

The tenons can be wider and typically are secured with two pins or lapped half dovetail joint can be used.

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Wooden Post

Plinth Beam

Vertical Post

Min 2 Inches Thick

Floor Joist

Floor Joist

Bline mortise and tenon joint

Half lapped dovetail joint

C. Wall joint

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C. Wall joint

For wall we can do horizontal or verticalw sheathing.

For horizontal sheathing building, the supporting framework vertical nailer called studs is provided.

This stud are connected to girt plate with the barefaced mortise and tenon joint.

Barefaced mortise and tenon joint

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Barefaced mortise and tenon joint

Standard mortise and tenon joint at corner with halved joint at middle post

For vertical sheathing building, horizontal girts are framed in for support girt joinery can be barefaced

mortise and tenon or halved joint at middle post and standard mortise and tenon at corner.

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D. Truss connections

For wall we can do horizontal or verticals sheathing.

For horizontal sheathing building, the supporting framework vertical nailers called studs is provided.

This stud are connected to girt plate with the barefaced mortise and tenon joint.

Dhung

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Joist

Rafter

Tie Beam

Dhung

Wooden Column

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Rafter to plate jointStep-lap rafter seat is better for connecting

rafter to plate, it performs well in all respects

and is also fairly simple to fabricate.

The step is usually either 1.5 or 2 inches.

There can be danger of step being cut so deep

that the strength of the timber is impaired, so

another good way of supporting a rafter is by

bolting or strapping a cleat on to underside

of rafter.

Rafter

Tie Beam

Plate

Column

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Rafter joints at RidgeWhen the rafter reaches the roof peak they may be joined to each other or to the ridge beam.

When joined to each other, they are half lapped and secured with nails/pins, or mortised and pinned. These

mortise joints perform the best but require the most time to execute.

Cleated

a. Open mortise and tenon joint b. Half-lap joint

a

b

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Principal Rafter and Common Purlin JointsBest way to connect rafter and purlin is by bolting or strapping a cleat on rafter and placing purlin over it

such technique avoid cutting rafter and impairing its strength.

`

Purlin

Cleat

Rafter

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REFERENCES

1. JosephE.Minor,P.E1977:Performanceofroofingsystemsinwindstorms,InstituteforDisaster

Research,Texas Tech University, Lubbock, Texas U.S.A.

2. Guidelines for cyclone resistant construction of buildings in Gujarat,2001. Gujarat State Disaster

Management Authority, Government of Gujarat.

3. Earthquake Resistant Construction Training Manual (Stone Masonry), 2014, Engineering

Adaptation and Risk Reduction Division, Department of Engineering Services, Ministry of Works

and Human Settlement.

4. Guidelines for Reconstruction of houses affected by Tsunami in Tamil Nadu,2001. Revenue

Administration,Disastermanagement&MitigationDepartment,GovernmentofTamilNadu.

5. Agarwal.A2007.CycloneResistantBuildingArchitecture,GoI–UNDP,DisasterRiskmanagement

Programme.

6. Lotay Y 2013.Windstorm Damage Assessment and Prevention on Traditional Bhutanese Roof.

7. Lotay Y 2013.Wind induced damage to roofs “A comparative study on roofs in Bhutan and Japan.

8. IS12093-1987(Reaffirmed2002)-Codeofpracticeforlayingandfixingofslopedroofcovering

using plain and corrugated Galvanized steel sheets.

9. IS 875 (PART 3)- Code of practice for Design loads (other than earthquake) for buildings and

structures. Part 3 Wind loads.

10. Proper construction practices for Traditional Timber Framework: Division of conservation of

HeritageSites,DepartmentofCultureMinistryofHomesandCultureAffairs.

11. SobonJackA.2002.HistoricAmericanTimberJoinery:AGraphicGuide2004-08.NationalPark

Services, U.S. Department of the Interior.

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