A STATE-OF-THE-ART REVIEW ON COLD- FORMED STEEL ROOF … · Figure 2 The example of the roof truss...
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International Journal of Civil Engineering and Technology (IJCIET)
Volume 9, Issue 9, September 2018, pp. 746–758, Article ID: IJCIET_09_09_071
Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=9&IType=9
ISSN Print: 0976-6308 and ISSN Online: 0976-6316
©IAEME Publication Scopus Indexed
A STATE-OF-THE-ART REVIEW ON COLD-
FORMED STEEL ROOF TRUSS SYSTEM
Mohd Syahrul Hisyam Mohd Sani, Fadhluhartini Muftah
Faculty of Civil Engineering, Universiti Teknologi Mara, Cawangan Pahang,
Kampus Jengka, 26400 Bandar Jengka, Pahang
Cher Siang Tan
Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor
ABSTRACT
The roof truss is an important structure to keep a building safe, protect it from
rain and sunshine, and protect home appliances and equipment inside it. The stability
of the roof truss system, either by using timber or steel, is necessary to study and
investigate. The roof truss material, member section, configuration, connection or
fastener, end support and span must be checked. Nowadays, cold-formed steel (CFS)
with numerous advantages is selected as chord and web truss. However, the failure of
CFS, such as buckling, must be revised to ensure that the CFS roof truss is stable.
From the observations of previous research studies, the failure of the CFS roof truss
has been broadly discussed and to determine the causes of stability. The effect of CFS
truss mainly fails or become unstable due to a slender section of the truss, especially
on the compression member. Intentionally, the slender section of the top chord must
be replaced with a short section or changed to become a curved section. The idea is to
certify the local buckling failure of the top chord and prevent the nearest section from
failing. Finally, the guideline for checking the stability of the CFS roof truss was
established to give an experience for engineers.
Key words: stability, cold-formed steel, roof truss system
Cite this Article: Mohd Syahrul Hisyam Mohd Sani, Fadhluhartini Muftah,
Cher Siang Tan, A State-of-the-Art Review on Cold-Formed Steel Roof Truss System.
International Journal of Civil Engineering and Technology, 9(9), 2018, pp. 746-758.
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=9&IType=9
1. INTRODUCTION
Roof truss is a structure constructed to protect residential houses, buildings and commercial
buildings from rain, wind and sunshine. Technically, roof truss sustains the roofs, floors and
internal loading. Internal loading covers electrical and mechanical services, water storage and
ceilings. Roof truss provides safety, long span, lightweight, support for considerable loads and
reduce deformation as compared to other structural elements. Generally, roof truss is
triangular in shape and act in tension and compression load along the truss member.
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The roof truss is usually made from steel, timber or a combination of both materials. Roof
truss consists of the bottom chord, top chord, web, fastener and support, which is known as a
roof truss system. A roof truss system chord and web are available in various shapes and size,
which can be customised according to structural design and drawing. They are built as two-
dimension (2D) truss or three-dimension (3D) truss. A 2D truss is established in two axes and
used for small buildings with low-load capacity. The 3D truss is recognized as a space truss
and utilised for big buildings with high load capacity, such as a stadium, airport, auditorium
and factory. The steel space truss is connected between linear straight members that are made
of roller hollow shape or CFS section with an end connector to form a 3D structure (Bezerra
et al., 2009). Bondok and Salim (2017) mentioned that a roof truss system made of CFS is
classified as a versatile element. They are more lightweight, have economical values,
versatility and load resistance as shown in Figure 1.
Figure 1 The end connector of node system (Bezerra et al., 2009)
Space trusses are built by using steel tubular bars or rods that are connected with nodes.
Caglayan and Yuksel (2008) stated that the space truss is known as a lightweight rigid
structure that has interlocking struts in a special pattern. The nodes show the significant
function for structural safety, weight and cost of space structure (Alinia and Kashizadeh,
2006). Alinia and Kashizadeh (2006) reported that the space trusses have high value of
indeterminacy degree and multiple redundancies, and thus with the 3D section it provides an
additional limit of safety and prohibit sudden collapse if one or more sections is locally failed
. This happens when the overall loading that is applied to the space truss is still below the
service load. In addition, Alinia and Kashizadeh (2006) reported that the space truss did not
affect the flexibility of the substructure section when subjected to elevated temperature. There
are many researchers who studied the steel space trusses, such as Bezerra et al. (2009), Kaveh
and Talatahari (2009).
Bezerra et al. (2009) reported the relation between roof truss span and height is referred to
the chord and web arrangements, end support conditions, loading and also rigidity of the
linking system. CFS roof truss span for residential houses and small buildings is reported
from 7.0 m to 15.0 m, while more than 15.0 m is classified as wide-span trusses (Mohammad
et al., 2012).
Piroglu and Ozakgul (2016) stated that building with large span structures, such as airport
terminals and shopping mall are dealt with flat or low-sloped roof system by using
lightweight steel. The spatially truss arch is another type of truss that be used for long span
buildings. Normally, the spatially truss arch is constructed from three surfaces of the web and
is connected by a diagonal and dividing section to support the three surfaces. The diagonal
and dividing section acts as a lattice configuration. Guo et al. (2013) studied the out-of-plane
inelastic strength and design of the spatially truss circular arch. Meanwhile Han et al. (2015)
studied the failure mechanism of the steel arch truss due to earthquake.
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Truss moment frames are another type of truss that combined between the truss and frame.
Pekcan et al. (2009) have established the innovative concept of truss moment frame by using
energy dissipating devices. This concept acts as diagonal internal braces in the truss moment
frame. Bondok and Salim (2017) reported that the American Iron and Steel Institute (AISI)
and Light Gauge Steel Engineers Association have published a complete documentation of
design guide for CFS truss in 1995 but the information of ultimate strength, failure mode
shape and end connection still raises the question.
The main objective of the study is to define the roof truss system parts and other issues
that perform the stability of the roof truss. The second objective is to check and review the
previous study on CFS roof truss and provide a few comments about it. Finally, the new ideas
and recommendations for constructing the stability of the CFS roof truss is established.
2. ROOF TRUSS SYSTEM MATERIAL, MEMBER SECTIONS AND
CONFIGURATION
Nowadays, the roof truss system is shifted from timber to steel and lastly reformed from hot-
rolled steel for CFS. This is because the timber usages have affected environmental issues and
provided air pollution. Next, the sustainable development and green technology principles
could not be achieved in construction activity. CFS is popular as compared to hot-rolled steel
because of its advantages, such as lightweight, easy fabrication, easy erection and corrosion
resistant.
The roof truss member section is divided into two categories. The first category is for light
structural buildings and second category is for heavy structural buildings. The selection of
roof truss member sections is important to avoid failure due to section selection, depending on
the internal load, ease of connection between members, ease of installation on support, and
comply with the architectural demands. Besides, the out-of-plane buckling and uplift loading
resistance must also be considered for light structural buildings, in which the member section
is by using a channel, which is the small size of rectangular or circle hollow section, zed,
angle or a combination. Utilisation of unsymmetrical or singly symmetrical section is allowed
in roof truss because small span of trusses and light load capacity. The unsymmetrical
member section is tended to twist or buckle if the loads are transversely applied on it. As for
the second category, normally the selection of member section for longer span and heavy load
is the symmetrical member section and the bigger size hollow section, either rectangular or
circle. For example, member section which is I-section, built-up, boxed-up and hollow.
Hollow sections are more reliable for longer span because of their structural effectiveness and
thermal resistance. Ezeagu et al. (2012) assured that the selection of the suitable roof truss
depends on the failure strength, especially deflection and load bearing capacity.
The types of roof truss configuration with respect to construction are pitched truss,
bowstring truss, flat truss, Pratt truss, Howe truss, warren truss, k-truss, Fink truss, scissor
truss, as shown in Figure2. In general, the Pratt truss, Howe truss, warren truss and k-truss are
suitable for bridge structure. Roof truss parts are shown in Figure 2.
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Figure 2 The example of the roof truss configuration (Truss Craft, 2014)
Figure 3 Roof truss system parts
3. ROOF TRUSS CONNECTION/ FASTENERS
Normally, the roof truss system is designed as fasteners by using bolts or welded connection.
Sometimes for a short span, the recommended connection is a self-drilling screw or self-
tapping screw. Testing of all connections is established by shear and pullout testing to
determine the connection performance. Some examples of bolt connection that were reformed
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are self-piercing rivets, where Rosette tube joint and press-joining are used. Normally, bolted
connections are utilised for medium and large span trusses.
Recently, new techniques for the connections of CFS were proposed, for example self-
piercing rivets, rosettes and mechanical clinching (Pedreshchi and Sinha, 2008). Another
connection that was lately introduced is the Howick Rivet Connector (HRC) and is purposely
designed for CFS section. This connector is used by comprising a hollow rivet from one
section to another section and through the flange by clamping them together (Mathieson et al.,
2016). Some examples of connection are normally inspired from other mechanical
engineering, such as automotive engineering and industrial manufacture. So with the
development of new techniques and improvisation of the traditional method for the
connections, all these are created to enhance quality, fabrication and installation speed and
provide a safe environment. Additionally, the connector that is selected must be attributable to
fast assemblage, easy to dismantle, expand and transport with reduced costs (Bezerra et al.,
2009).
Lennon et al. (1999) explained several factors that must be considered when selecting the
connection techniques. They are the local environmental effects, connection cost, strength,
available in generic analysis and design method, manoeuvrability of apparatus and reliable
with quality control methods (Lennon et al., 1999). Bolted connections in CFS roof truss
systems with rigid jointed behaviour are not suitable because deformability of the joint
attributed to bearing failure. This is because the CFS with thin section is associated with the
elongation in hole, slippage and bolt tilting (Zaharia and Dubina, 2006). All connections must
be checked and their behaviour determined by shear and pull out testing.
End connection of the roof truss system is also an important issue that must be determined
to avoid failure. Bondok and Salim (2017) reported that premature failure occurred when the
end connection was not properly designed. Sivapathasundaram and Mahendran (2017)
reported that the premature failure of a roof truss connection or batten existed when the high
uplift load by wind is acted upon the roof truss system. There are two end connections that are
always studied, such as fixed and pinned end connections. The fixed end connection, which is
fully restrained, is classified a stronger end connection and is always utilised in the load
bearing wall.
4. ROOF TRUSS SYSTEM FAILURE
Roof truss failures normally occur due to deterioration, overload, improper and poor design,
inadequate material, inappropriate construction, structural integrity losses, workmanship error
and unexpected weather. Structural integrity is dealt with overall structure ability to sustain
and support a designed load, such as self-weight of structure and other load without any
failure and collapse. A structural integrity loss means that the structure has lost its load-
carrying capacity or its component. In addition, the material that was used in the structure had
also influenced the structural integrity loss when the material is stressed further than its
strength limit. Mechanical behaviour or structural performance of the material, such as
strength, hardness, elasticity, durability and weight must be first considered and finally
examined for suitable thickness, size and shape that can support the load for a long period.
Kozlowski et al. (2004) have stated the most common reason for the steel structure
collapse and failure, especially if the roof truss is the instability factor. Improper buckling
length evaluation of the upper chord in the roof truss design will influence instability problem
(Kozlowski et al., 2004). Piroglu et al., 2014 reported that the roof truss failure in industrial
hall building in Marmara Region, Turki was caused by improper fabrication and did not
follow the basic design principles for the support assemblies. Guo et al. (2015) reported that
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one of the failures of a steel truss building occurs due to bending failure because the carrying
capacity steel column and trusses are not adequate, as shown in Figure 4.
Figure 4 The roof truss failure due to bending (Guo et al., 2015)
Caglayan and Yuksel (2008) reported that the reason for the Mero space truss roof
structure collapse in eastern Turkey was because the snow load intensity and a number of
mistakes from the elastic design of the truss was underestimated.
5. STABILITY OF STEEL ROOF TRUSS SYSTEM
The slenderness of CFS sections is an important issue that is determined and checked to
ensure the trusses stability is tenable when subjected to compressive loads (Wood and Dawe,
2006). Besides, the roof truss stiffness is larger in a plane than out of plane so that it should be
braced to resist lateral deformation for the overall structure and twisting of the slender section
(Krajewski and Iwicki, 2015). EC3 is required to brace against the lateral stability of the roof
truss compressed member.
Sometimes, the stability of the truss is also related to imperfection factor. The
imperfection factor is classified into three types,which are changes in joint coordinates,
imperfections in member length and member straightness (Peek, 1993). An important thing
which must be considered while designing and fabricating the CFS as roof truss structure, is
local and overall stability CFS section (Semko and Prokhorenko, 2013)
Purlin must be located on the roof truss system at least a bracing in 6 m span that is
regarded to CFS structure technical specifications (GB50018-2002) (Guo et al., 2015). Purlin
reacts as bending members when roof truss is subjected to wind loads, either uplift or downlift
load or sometimes both loads, is applied on it. This issue is one of the factors that influenced
the stability of the roof truss.
6. LATERAL-TORSIONAL BUCKLING
Normally, CFS members are subjected to fail due to local buckling, distortional buckling and
flexural-torsional buckling or sometimes in combination failure. There are several factors that
must be considered, such as the type of the truss chord load, the distance between the lateral
chord bracings, stiffness of the lateral bracings and type of lacings with their rigidity
(Jankowska-Sandberg and Kolodziej, 2013). Jankowska-Sandberg and Kolodziej (2013) have
reported the buckling length of compression chord is smaller than location and distance
between the lateral bracing of about 30% – 80 %. Awaludin et al. (2015) stated that the
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buckling member failure of the CFS could be solved by using timber laminas as a slot in
material to form a composite member.
7. COLD-FORMED STEEL AS ROOF TRUSS MEMBER
Lack of understanding and structural information of connections for truss made by CFS has
led to the design of inefficient connections which reduces the ability of the truss to compete in
the construction industry (Wood and Dawe, 2006). Mathieson et al. (2016) have explained
that the CFS truss shows a rise in cost with the installation of a connector activity. CFS
structures are broadly used in construction due to their advantages, such as high strength, cost
effective and easy to use with (Tian et al., 2015). With a lot of advantages, as compared to
hot-rolled and timber structure, CFS becomes more popular. Examples of CFS advantages are
thin in thickness, corrosion resistance, anti-termites, anti-fungal, easy installation, easy
fabrication, easy transportation, less on maintenance and lightweight. Dawe et al. (2010)
reported that CFS with high strength to weight ratio is important to transport, handle and
install the CFS on-site and establish as an alternative material in roof truss. Normally, CFS
roof truss with the channel section as truss members are proposed as either inline or offset
truss design (Dawe et. al, 2010). Inline truss design is projected by all truss members, which
include web and chord in a single plane while offset truss design is established in more than a
single plane. Channel section as top chord member in roof truss is created by the bending
effects in continuous member that happened to the in-plane eccentricities of the connection
(Ibrahim et al. 1998). Besides, CFS structure is demonstrated by the reduction of labour cost
and fast erection (Zaharia and Dubina, 2006). Parent et al. (2007) reported on the prediction
of the design method for critical load of compression web truss members with periodically
shifting section properties.
8. COLD-FORMED STEEL FAILURE
The roof truss can fail due to unpredicted extreme weather events, poor construction and
inadequate design subjected to normal load conditions (Piroglu et al., 2014). CFS as a
structural element in roof truss is subjected to fail due to buckling, including global buckling,
local buckling, distortional buckling and web crippling. A slender section of CFS is exposed
to failure due to global buckling while the shorter section is ineffective in crushing. Buckling
phenomena that happened in CFS will lead to unexpected failure and lastly a sudden failure
without any warning. Cheng et al., 2013 did an analytical study of the CFS channel beam
section that is subjected to combine compression and bending at major and minor axes. The
flexural buckling and lateral-torsional buckling failure were investigated when the channel
was subjected to both actions.
Dubina and Zaharia (1997) explained in detail that the bearing deformation is caused by
the connection between bolt and thin plate, such as CFS like local buckling on the section and
the bolt slippage. Besides, Sivapathasundaram and Mahendran (2017) have studied about the
premature localised pull-through failures which existed on the batten to truss connections.
9. LITERATURE REVIEW OF COLD-FORMED STEEL ROOF TRUSS
SYSTEM
Wood and Dawe (2006) tested and analysed the full scale roof truss specimens that were built
by a span of 6,096 mm, roof pitch 4:12 and overhang span of 500 mm. The self-drilling
screws and four types of section, with different cross-section, are used in the testing. They
were constructed inverted in the roof truss specimens and loads were applied at every joint of
the bottom chord. Mohammad et al. (2012) reported on the testing of wide-span CFS roof
truss with 25 m of length and analysed its structural performance. Below is the example of the
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study that has been investigated and Table 1 represents the information and review analysis of
the study.
Table 1 The review of the cold-formed roof truss from a previous study
Author Span Section Connector
and Support Pitch Loads & Overhang
Harper et al.
(1995)
6096 mm
(Fink
Truss)
Channel section
Top and bottom chord =
92.71 mm x 41.53 mm x
1.143 mm (306.13 N/mm2)
Truss web = 64.77 mm x
42.16 mm x 1.016 mm
(303.4 N/mm2)
19.05 mm self-
drilling screw 4:12
Load: Uniformed
distributed load by using
common masonry brick
Overhang: 101.6 mm
Failure
A combination failure of local buckling at the top chord above the heel connection and
extreme rotation of the top chord near the heel connection.
Prior to buckled top chord, the self-drilling screw at heel connection is pulled out and rotated.
The top chord is buckled in the area between the ridge connection and the truss web.
Review
The usage of the masonry brick as a load is not suitable because the mass of masonry brick is
not similar to each other. It is difficult to get the same mass of left and right hand sides. The
heel connections on both sides are not placed the wood block to avoid the initial local
buckling and failure of the geometry of the channel. The study has not explained about that
the load from left and right hand sides was uniform and checked the lateral movement of the
overall trusses. In construction reality load support is normally not established and the mass
must be taken.
Dawe et. al,
2010
6156 mm
(off-set
truss)
Channel (Chords & Webs)
10 hexagonal-
washer head
Self-drilling
screws
Steel plates
4:12
6:12
8:12
Load: Concentrated load
(consider as up-lift load)
Overhang: 500 mm
Failure
The failure of all specimens was reported as local buckling at the top chord adjacent to the
heel plate, either left or right side of the trusses that was nearest to point load location.
The bottom chord was initiated by the local buckling and crushing at the same part of top
chord local buckling failure or parallel to it.
Review It was good to have different pitch to check the location of local buckling and relationship
between pitch and deformation. The load applied wasnot the uniformly distributed load.
Mohammad et
al. (2012)
25 m
(length)
4.7 m
(height)
1.2 m
(width)
Channel section,
Chords = 150
(D)x66(B)x1.6(t), 365
N/mm2
Webs =74x34x0.8, 550
N/mm2
Hat section =
35x18x0.48,550
(as purlin)
4.5 mm
diameter tag-
screws
(truss member)
12 mm
diameter grade
8.8 galvanised
holding down
bolts (truss &
rigid steel
beam)
5:1
Load: Uniformed
distributed load (UDL) by
cement bags
Total load = 63.77 kN
Overhang: None
Failure
Embossed at the connection between truss member and bottom chord under compression.
Shear failure and tearing off of the tag screw at splice joint at bottom chord.
The connection on both ends at top chord is twisted and buckled.
Buckling at diagonal truss web.
Review
It was good to check the vertical deformation on bottom chord and top chord and horizontal
deformation. But the usage of the cement bag as a uniformly distributed load is not suitable
because the load was started with higher load and was unsafe for the instructor and lab
technician after failure or collapse. The failure is occurring in splice joint and on the
compression part of the top chord and truss web. The connection of the splice joint is not
sufficient and must be further studied. The horizontal deformation between left and right side
was not similar due to height and not enough lateral bracing and stiffness. The truss web
section was small and not appropriate in size as compared to chord. So the suggestion was to
construct with the same size as the chord to establish stability. The strain of the member that
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calculated the relationship between member and load is not taken.
Wood and
Dawe (2006) 6096 mm
Channel Section
Top chord
92.1(d)x40.9(b)x13.2(c)x1.
21(t)
Bottom chord & web
member
92.1(d)x40.9(b)x13.2(c)x0.
98(t)
Number 10
self-drilling
screws
4:12 Concentrated
load 500 mm
Failure
Full-scale specimens with reinforcement tied down and stiffened failed due to local buckling
at the top chord adjacent to the heel plate with total load of 16.4 kN. This failure was close to
loading part that was nearest to the end support.
The large deflections are occurring at loading part that started under the same total load with
other loading parts.
Review The section that be used for the chord and web member is not uniform. Concentrated load is
considered in the testing and not uniformly distributed load.
10. DISCUSSION
From the failure observations, either in concentrated load or uniformly distributed load, the
truss has become unstable and failed on the slender section of compression member,
especially at top chord. The failure occurred at the top chord in local buckling and over the
web and bottom chord. The location of failure, whether right or left hand side, is shown in
Figure 5. Some idea are existed to organise the failure is by using the bowstring truss or
avoided by using the slender section on the compression member. Bowstring truss is
recognised with a curved section at the top chord and straight section at bottom chord and
other truss members. Sometimes, the top chord section can be used as a short section to
replace the slender section that is joined by a web truss with a short top chord. Besides, the
failure due to lateral-torsional buckling of unstable trusses can be solved by using the
adequate lateral bracing. But the main important issue to make the truss stable is to first
strengthen the roof truss itself.
The use of the CFS channel as roof truss member has become a popular section that is
utilised to replace the hollow section due to economic purpose. The CFS channel with several
advantages, such as lightweight, flexible, high strength-to-weight ratio and easy to modify has
been seriously studied by a researcher and manufacturer. With changes in the shape from
normal channel section to various modification, such as channel with lipped section and
channel with intermediate stiffeners or both so that the arrangement of the CFS roof truss
member by using channel is significant to ensure the section and overall structure are stable.
Normally, to give symmetrical roof truss system, the channel must be faced in different ways,
such as bottom and top chord in a same face way and the web truss on the other face way.
Figure 5 Example of failure on the top chord
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For an established roof truss system stability, it is suitable to form it in the same
arrangement and a diagram like bending moment diagram; thus, the roof truss system
normally react as a simple supported beam with a uniformly distributed load, as shown in
Figure 6. From the observation, the roof truss is constructed by following the diagram of
bending moment that curved at the top chord. This is because the part of truss section from
support is classified as a weaken section is possible to resist compression load. Besides, the
top chord of the roof truss system is generally built in a slender section that usually fails due
to buckling.
Figure 6 The relation between the roof truss configuration and bending moment diagram
Figure 7 The guidelines of the stability of the cold-formed steel
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Hui et al., 2008 have discussed and observed the intelligent methods for the safe
monitoring of integer behaviour on a large span space truss under wind-excited through data
acquisition. This method was established for an early warning safety technique for a large
span space truss (Hui et al., 2008). These are some examples for new ideas and techniques to
provide truss safety. A lot of improvement ideas are being investigated and discussed to make
the roof truss more stable and safe.
The guideline for checking and initial investigation on the CFS roof truss stability is as
shown in Figure 7. This guideline is important for new engineers and manufacturer to ensure
the CFS roof truss is stable
11. CONCLUSIONS
It is very important to ensure the arrangement of the truss member to avoid failure between
them by constructing or fabricating in a symmetrical and stable section. This issue is solved
by forming the roof truss system in two ways of facing, a top and bottom chord is facing in
one way, and other section includes truss member by facing another way. So they form in a
different way and face to resist roof truss system failure. The selected channel must have the
lipped and intermediate web stiffener, whether one or more, to cater the local buckling failure
of the roof truss system. Additionally, the top chord must be avoided from using a slender
section, but can be replaced by using a curved or short section. This issue is proven by
checking the relationship between the roof truss systems with beam in shear force and
bending moment diagram. The simple guidelines are established to help the engineer to
checkthe cold-formed roof truss system stability.
After discussing stability in detail, the second part that must be studied for further work is
truss optimisation. Truss optimisation is about cost saving, fast erection and truss weight.
Truss optimisation has three categories that are known as size, shape and topology function
(Kaveh and Khayatazad, 2013).
REFERENCES
[1] Wood, J.V. and Dawe, J.L. (2006). Full-scale test behaviour of cold-formed steel roof
trusses. Journal of Structural Engineering 132(4), pp 616 - 623.
[2] Krajewski, M. and Iwicki, P. (2015). Analysis of Brace Stiffness Influence on Stability of
the Truss. International Journal of Applied Mechanics and Engineering 20 (1), pp 97 –
108.
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