Development of Long Span Bamboo Trusses

Lucas Hogan, Graham Charles Archer

Architectural Engineering, California Polytechnic San Luis Obispo, USA

Abstract

In response to the increasing depletion rate of our natural resources, the design

community is utilizing new and creative construction techniques and materials

for building systems. One such solution is the use of bamboo in long span

trusses. This paper details ongoing research to facilitate practical, low tech,

inexpensive long span bamboo trusses.

This research has developed a new connection that involves filling a portion

of the hollow bamboo with mortar, embedding rebar in that mortar and welding

these bamboo members to steel gusset plates to create a truss. This connection

also allows these trusses to be built cost effectively. Other than the required

welding the construction of these trusses is relatively simple and requires no

skilled labour. This research is currently being considered in the design of a

poly-technical college in the district of Samé in Tanzania which requires roof

trusses of 12m spans.

Keywords: bamboo, truss, connection, sustainable, construction

1 Introduction

As the world population continues to grow we are utilizing our building

resources at an ever increasing rate. Thus the need for alternative building

materials grows as well. This need is growing even faster in developing

countries. Where infrastructure such as clinics and schools are needed the 3-

12m roof span becomes difficult to construct due to the high cost of steel and

concrete. One alternative is the use of bamboo roof trusses to span these

distances. Bamboo grows natively on five continents and grows in some of the

poorest parts of the world. It can also be cultivated in dense crops and harvested

only after six years as opposed to over twenty years for conventional timber.

Also, all the fibers in a culm grow parallel providing straight members that are

very efficient at carrying axial load [Liese 1992]. Thus by being both a local and

plentiful material bamboo becomes a very cost effective building material

[Rodriguez-Camilloni 2009] for many of the parts of the world that have

difficulty in obtaining steel and concrete.

While bamboo is a viable building material [van der Lugt 1992], it has

limited use because it is difficult to connect the members together. This paper

discusses the development of a new truss connection that can be constructed

using local materials and unskilled labor in remote areas of the world at a low

cost.

2 Current Bamboo Connections

While bamboo has been used for centuries, the traditional methods of lashing

bamboo together (See Figure 1) [Bambus 2002] are not appropriate for the

design of long span trusses. First there is little predictability in the connection

for the quality is highly dependent on the laborer. Secondly, quality control is

difficult to obtain. These lashed connections also don’t fully utilize the full

strength of bamboo member. Because they rely solely on friction, the load

transfer between members is limited and thus structures require more members

to do the same job that one could if it were well connected. Finally complex

geometries with many members framing into one node or three dimensional

space frames are difficult if not impossible to construct.

Modern connections have been proposed [Huybers 1990] [Bambus 2002] to

solve these problems such as the bolted connection created by Shoei Yoh (Figure

2) and the steel wire connection by Renzo Piano (Figure 3). These connections

solve the issues of complex geometries by joining the members at a central hub.

However these connections have some challenges. While they provide a

standardized connection throughout a project, the hubs are proprietary, and they

are not readily available. The need to purchase, order, and transport these hubs

limits their flexibility. In addition, these connections require puncture of the

bamboo walls. Since all fibers in a bamboo culm run parallel once a bolt is

placed through it and the connection loaded in tension, the bolt acts like a wedge

and splits the bamboo. Also the puncture allows moisture to enter the culm and

accelerate decay. Based on the proprietary nature of the hubs, their installation

requirements, and the desire to develop cost effective, simple connections, the

research focused on an alternate connection type to eliminate these challenges.

Figure 1: Traditional Bamboo Connection

[Bambus 2002]

Finally a connection has been used by Simón Velez [Rorbach 2001]

[Rodriguez-Camilloni 2009] for many projects. This connection requires filling

several hollow cells of the bamboo with concrete and embedding a threaded rod

(Figure 4). With this connection Velez has obtained spans up to 30 meters in

some of his structures [Kries 2000]. While this is a proven the connection it

Figure 2: Bolted Connection by Shoei Yoh [ Bambus 2002]

Figure 3: Steel Wire Connection by Renzo Piano

[ Bambus RWTH 2002]

requires shaping the end of the member framing into the connection and pressure

grouting. This requires a high degree of skilled labor throughout the building of

the trusses, thus making it difficult to utilize local labor on projects in remote

villages. This research thus focused on a connection type that could utilize

mostly unskilled labor in the fabrication of the bamboo trusses.

3 The New Connection

This research aims to develop a new bamboo connection that overcomes some of

the shortcomings with current connection types that limit implementation in

construction of trusses in developing areas. The following criteria were used in

the development of the new connection. Since bamboo is very strong when

loaded axially [Chung 2002] [Yu 2003] the new connection was developed to

resist only the axial tension and compression forces and not increase resistance

to bending. This is consistent with the loading of the truss in which the truss

members experience axial loads and any bending is small and accidental.

Second, the new connection was to be ecologically friendly and avoid the use of

synthetic materials. The ideology of using a sustainable material such as

bamboo would be undermined if there was extensive use of highly processed

materials in the connection. Highly processed materials were also avoided in an

effort to keep the connection cost effective. The cost effective criterion was

developed due to the limited supply and high cost of building materials such as

steel and concrete in the remote areas were the use of these trusses is being

proposed. Finally the connection must be able to be constructed with a minimal

amount skilled labour. The philosophy behind this is that members of a village

could build these trusses with minimal training and provide safe buildings using

local labour and resources.

Figure 4: Velez Connection, Threaded Rod Embedded in Mortar [Rorbach 2001]

The new connection involves embedding a common steel reinforcing bar

(rebar) into a mortar filled bamboo culm and fillet welding several of these

members to a steel gusset plate. The inner surface of the bamboo is roughened

to provide a bond between the mortar and the bamboo while avoiding puncturing

the member. A common steel hose clamp is also placed at the end of the member

to provide confinement of the bamboo and prevent splitting of the bamboo

(Figure 7). Because the rebar is embedded in mortar, the load is transferred

evenly across the member’s cross section and can transfer high axial loads to the

bamboo. Finally, the incorporation of the steel gusset plate makes the bamboo

easy to connect in any configuration desired (Figure 6).

Figure 7: Section of Connection

Figure 5: Proposed Bamboo Truss

Figure 6: Steel Gusset Plate (Top Center of Figure 1)

To construct this connection a worker would cut the bamboo culm allowing

8-15cm to the first node. Then the worker would punch out the first internal

node to provide approximately a 30cm hollow section in the bamboo. This

removal of the node can be done easily with a hammer and a piece of rebar.

Next to provide a positive connection between the mortar and the bamboo, the

inner surface of the bamboo would be roughened by constructing gouges of

approximately 1-3mm deep, spaced at 6mm are made for the first 3-5 diameters

of the bamboo member. This can be achieved by running a drill bit along the

inside of the bamboo (Figure 7 and Figure 8)

Next a mortar comprised of sand, water, and cement is mixed and placed in

the hollow ends of the bamboo. It is important that this mix be fluid enough to

flow easily into the small spaces and avoid voids yet be stiff enough to provide

the desired strength. The rebar is then embedded into the mortar and the side of

the bamboo is tapped repeatedly to vibrate the mortar and eliminate any voids.

A hose clamp is then place at the end of the member and the member is allowed

to cure for 28 days. Once cured, the member can be welded to the gusset plate

and the truss erected.

Figure 8: Roughening the inner

surface with drill press

Figure 9: Completed Scouring

Figure 10: Completed Specimen

The connection functions as follows. First the load is developed in the gusset

plate where the roof framing is attached. Then the load is transferred to the rebar

via welds and then from the rebar to the mortar inside the bamboo. The mortar

transfers load to the bamboo by friction and bearing on the deformations (Figure

7). As the deformations resist the pullout of the mortar, hoop stresses are

induced in the bamboo. A hose clamp is placed at the end of the member where

these hoop stresses are largest. This provides confinement and reduces the loss

in capacity as the bamboo splits.

4 Connection Benefits

The main benefits of this connection is its simplicity which results in it being

very cost effective and allows it to be used in remote areas of the world.

Bamboo can be grown in extremely dense groves. A hectare of bamboo can

produce 22 to 44 metric tons of usable members per year, a yield 25 percent

higher than a timber grove. This translates to a 20m x 20m piece of land

producing enough bamboo in a five year period to build two houses measuring

8m x 8m. Because bamboo propagates via its root system it does not require

replanting after harvesting which will provide sustainable plantations [Kries,

2000]. Also, because bamboo species suitable for construction are grown all

around the world it is a locally available material for many locations. This

reduces transportation costs and provides stimulus to local economies. Both its

rapid growth and local availability make bamboo an inexpensive construction

material for many locations worldwide. In addition the developed connection

requires very little of the truss to be made of expensive materials such as steel

and concrete with less than 25 kg of steel including gusset plates and 75 kg of

concrete for a 12m span.

These bamboo trusses can be easily constructed using local unskilled labor.

The cultivation of bamboo requires no new skills, and the mixing of mortar,

preparation of members and the setting of the reinforcement can all be done

assembly line fashion and require only minimal training. Welding is the only

skilled task that the creation of the trusses requires. This is not likely to pose a

problem because welders who can construct the simple fillet welds required in

the connection are available even in remote parts of the world. One example of

this is the East African working in a village outside of Nairobi pictured in Figure

11. The use of welded connections not only provides redundancy and a joining

method that has a measurable quality, but it also provides potential for regular

high paying jobs for welders in the area and the stimulus for technical education.

Finally this connection provides for a strong lightweight truss that can be

prefabricated on the ground and lifted into place much easier than a steel

alternative. This makes construction practices safer and quicker by allowing

most of the work to be done on the ground level and reduce the number of

labourers required to lift the trusses.

5 Proof of Concept Test Results

The proposed connection was tested in tension (See Figure 15) to determine the

failure method and capacity of the connection. In the first experiment a standard

concrete mix of cement, water, fine aggregate, and coarse aggregate was used.

Specimens were prepared in a variety of ways to improve the bond between the

bamboo and concrete. The stiff mix and large aggregate was very difficult to

consolidate and large voids occurred. The specimens were allowed to cure for

28 days and once tested yielded bond stresses of 87.8 kPa to 527 kPa. These

bond stresses translate to pullout capacities ranging from .40 kN to 2.4 kN.

Several observations were made. First the aggregate size was too large for

the concrete to fill the deformations created. Second, there was a large degree of

splitting that occurred in the bamboo during curing. This occurred because the

hydration of concrete pulled moisture out of the bamboo and caused the inner

diameter to shrink faster than the outer diameter which pulled the bamboo fibers

apart. In addition, when tested the failure mode for the connection was the

splitting of the bamboo and pull out of the concrete from the bamboo. This

suggests that if confinement were to be improved, the connection capacity would

increase.

Figure 11: East African Welder

[Bates 2007]

A second experiment was run to refine some of the issues encountered in the

first experiment. The concrete mix became a mortar of three parts sand, one part

cement, and one part water. While this mix flowed well into the deformations in

the bamboo it was far too wet and pooling occurred. This caused the mortar in

the bamboo to shrink drastically, up to one inch out of six longitudinally. It also

provided a weak mortar. The interior surface of the test specimens were then

prepared in varying ways. Specimens left smooth were denoted with an “S” in

the specimen name (Figure 12). Those where the inner surface was roughened

with a drill bit are denoted with a “DR” (Figure 13). Finally those denoted with

an “ES” had the inner surface coated with epoxy and sand bonded to the epoxy

(Figure 14). The mortar was then poured into the bamboo specimen and rebar

embedded. The specimens were again tested in tension and the pullout

capacities and maximum bond stress between the bamboo and the mortar are

shown below in Table 1.

Table 1. Experiment 2 Wet Mix w/ Fine Aggregate

Concrete Specimen Max Load (KN) Max Bond Stress (KPa) Notes

f'c = 8625 Kpa DR_1 0.48 39.64

DR_2 1.36 112.31

DR_3 1.28 105.70

DR_4 3.53 291.41 (1) Hose Clamp / End

DR_5 1.63 134.33 (1) Hose Clamp / End

DR_6 2.08 172.13 (1) Hose Clamp / End

ES_1 3.16 260.59

ES_2 2.64 218.38

ES_3 3.60 297.29

S_1 0.24 20.19

S_2 0.53 43.68 (1) Hose Clamp / End

Figure 12: Surface Type

“S”

Figure 13: Surface Type

“DR” Figure 14: Surface Type

“E”

It can be seen that the smooth surface of the bamboo provides no usable

capacities. Thus there is little bond between the mortar and the smooth bamboo

wall. The mortar engages the bamboo through the gouged deformations. It can

also be seen that the hose clamp provides a significant increase in strength by

providing confinement to the mortar. The hose clamp forces failure in the

mortar by the shearing of concrete that filled the deformations. While many

shear failures are considered brittle, this is extremely ductile because there is a

large amount of friction that still exists between the mortar and bamboo. This

can be seen in Figure 15-16 where the specimen still maintains approximately

half of its ultimate load with over 50mm of pullout.

A final experiment was run to further define the phenomena encountered in the

first two experiments and to determine the peak capacity for this connection.

Again the mortar used was three parts sand to one part cement and just enough

water to make it workable. This provided a mix that was easily consolidated yet

had a relatively high strength. There new specimen types were also included in

this experiment to determine the minimum amount of surface preparation that

would yield acceptable capacities. These included creating four pockets around

the end of the specimen and making two scours (“PR” Type, Figure 18) or by

only making one scour around the end of the specimen (“One Ring” Type,

Figure 19). Both of these specimens used hose clamps to provide confinement.

The aim of the last new specimen type was the creation of the largest pullout

capacities. These specimens were created by making deep gouges in the bamboo

and coating the inside with epoxy and sand (“DRE” Type, Figure 20). The

specimens were allowed to cure and then were tested with the same procedures

as the first two experiments. The results are shown below in Table 2.

Figure 15: DR_6

Mortar Pullout Figure 16: DR_6 Load vs. Pullout

Table 2. Experiment 3 Stiff Mix w/ Fine Aggregate

Concrete Specimen Max Load (KN) Max Bond Stress (KPa) Notes

f'c = 16500 Kpa DR_1 3.10 255.81DR_2 0.69 57.26 (1) Hose Clamp / End, 6mm Crack Entire LengthDR_3 1.33 110.11 (2) Hose Clamp / End, 6mm Crack Entire Length

DR_4 4.33 357.85 (2) Hose Clamp / EndDR_5 4.95 408.49 (2) Hose Clamp / EndDR_6 2.37 195.99 (2) Hose Clamp / End

DR_7 5.12 422.81 (2) Hose Clamp / EndDR_8 2.24 184.98 (2) Hose Clamp / End, Bot. Conc. Prev. DamagedDR_9 4.16 343.16 (2) Hose Clamp / End

PR_1 4.21 347.94 (2) Hose Clamp / EndPR_2 3.16 261.32 (2) Hose Clamp / EndR_One Ring 3.29 271.60 (1) Hose Clamp / End

DRE_1 18.18 1501.12 (3) Hose Clamp / EndDRE_2 20.22 1669.95 (3) Hose Clamp / EndDRE_3 22.71 1875.48 (4) Hose Clamp / End

DRE_4 21.56 1780.05 (5) Hose Clamp / End

It can be seen that the combination of a stronger mix and the implementation

of hose clamps to provide confinement yielded much higher capacities,

consistently in excess of 4 kN. This translates to a consistent bond stress of over

260 kPa. The benefit of confinement is best shown in specimens DR_2 and

DR_3 in which the specimen had cracked so badly that the concrete no longer

had bond to the bamboo and it could be removed by hand. Even with no positive

connection the friction provided by the hose clamps still yielded maximum bond

stresses of 57.26 kPa and 110.1 kPa. Again the failure mode for almost all of the

specimens was pullout of the mortar. Also the specimens with minimal

deformations yielded similar capacities to the average capacity of the typical

“DR” type. Finally the “DRE” type yielded extremely high capacities, in excess

of 20 kN. The addition of four and five hose clamps on each end provided a

high confining pressure. Also the use of sand epoxyed to the inner surface

provided increased surface area for the mortar to bear on while it was resisting

pullout.

These high capacities provide a proof of concept and provide usable design

capacities. The drill roughened specimens without hose clamps provided an

Figure 18: Pocket

Roughened Specimen,

“PR”

Figure 19: Single

Deformation, “R_One

Ring”

Figure 20: Gouged and

Epoxyed Specimen,

“DRE” Type

average bond stress of 128 kPa. The drill roughened specimens with hose

clamps developed and average bond stress of 254 kPa. The sand epoxyed

specimens had an average bond stress of 258 kPa. And Finally the DRE type

specimens developed an average bond stress of 1710 kPa. Since the DRE type

specimens failed by rebar pullout, this suggests that the capacity of the

connection was not fully reached. The rebar could simply be embedded further

to allow for a greater capacity. If these bond stresses of the DRE type

connection were to be extrapolated for use in 75-100mm diameter bamboo the

pullout capacity would be approximately 60 kN. This capacity is far greater than

that of the bamboo which will cause ultimate failure in the truss members and

not the connection. This provides a very predictable failure mode which allows

for safe design.

6 Implementation

Ultimately these connection bond stresses translate to the development of span to

depth ratios that can be used in truss design. These are shown in Figure 21. The

following assumptions were used in the formation of the design chart (Figure

21). The roof system considered was composed of parallel trusses spaced

between 0.6 – 2.5m apart centre-to-centre (CtoC). The span of these trusses

ranges from 2.5 – 15m. The longest member in the truss does not exceed 1.5m.

The highest ultimate capacity of the DRE type connection was used in the

development of the design chart with an applied factor of safety of 2. These

values were extrapolated linearly for varying diameters of bamboo and used to

design a truss similar to that shown in Figure 5. A design load of approximately

1 kPa was applied to the truss, and the required depth was calculated for varying

spans and spacing. Buckling of the compression member was the governed the

design of the truss. Also, for the development of Figure 21, lateral torsional

buckling was not considered. Finally, it should be noted that these are

preliminary values and should under no circumstances be used for design.

Figure 21: Span to Depth Ratios (NOTE: Preliminary Estimates Not for Design

Purposes)

From Figure 21 we can see that this connection provides for trusses that can

effectively span between 4-15m while keeping the depth below 1.5m. As the

bamboo diameter increases the surface area engaged by the mortar also increased

drastically and the failure mode is buckling of the bamboo. Since the longest

member is 1.5m the bamboo becomes very stiff, especially once the diameter is

increased. This translates in the ability to span significant distances with very

shallow trusses, saving greatly on material.

Currently this design is being considered for the use in roof trusses in the

district of Same in Tanzania to build a polytechnic college. The project calls for

local labour to build classrooms, dormitories, dining halls and other essential

buildings that require clear spans. These spans range anywhere from 6-12m both

of which could be reached with the construction techniques used in this research.

Once proven in Tanzania it would be easier to promote the use of this connection

in other parts of Africa, Eastern Asia, and Central and South America thus

providing same and cost effecting building for local peoples in those areas.

7 Conclusions and Recommendations

This research has provided a proof of concept for a bamboo connection

developed by roughening the inner surface of a bamboo member, filling a

portion with mortar, embedding rebar and welding several of these members to a

gusset plate. This connection can then be used to construct bamboo trusses that

span moderate distances while providing a safe and predictable behaviour. The

connection developed bond stress between 258 kPa – 1710 kPa which translate

to pullout capacities of up to 20 kN. These trusses also utilize a local and

renewable resource for many areas around the world and their construction is

easily accomplished using local labour thus stimulating local economies in

developing regions.

The experiments detailed in this paper are part of ongoing research to refine

the design of this connection. The ongoing work includes mitigating shrinkage

problems, varying the depth of deformations, examining the length deformations

inside the bamboo, environmental effects, and effects of heat during welding,

and compression capacity. This additional research and the research outlined in

this paper will help facilitate the future use of bamboo trusses in moderate span

buildings and provide a cost effective building solution to many parts of the

world.

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