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1
Light Steel Framing structures for buildings
Diogo José Martins Rego Instituto Superior Técnico, Technical University of Lisbon, Lisbon, Portugal
Abstract: The structures of buildings that use the Light Steel Framing (LSF) constructive method stand out as a possible
constructive system for the future, since it effectively addresses and resolves the ecological faults existent in other
methods. One of the objectives of this dissertation was to examine, study and evaluate several parameters related to
thermal performance, acoustics, fire resistance and sustainability, because these are decisive factors when selecting a
construction method. Comparing the construction method in study with the traditional masonry and reinforced concrete
solution, it was found that the LSF method presents (i) excellent thermal performance, (ii) good acoustical performance,
(iii) excellent durability, sustainability and eco-efficiency indicators. The main objective of this thesis consisted in the
development and analysis of a structural model of a LSF building, studying the behaviour of the structure in conjunction
with the OSB siding panels and the calculation techniques related with the system, prescribed in EN1993. Modelling the
structure in SAP2000 allowed the assessment of the best techniques for computational modelling of certain structural
elements and behaviours, predominantly the diagonal straps and the diaphragm effect of OSB. It was then concluded, that
this effect should be considered in the structural model due to its influence on the seismic behaviour. Subsequently, the
critical design values were analysed in the main structural elements, in order to proceed with its design. Given the
complexity of the design process of Class 4 steel sections, this was presented concisely through flowcharts that
summarize the requirements of EN1993. Because of the extension of the obtained results, charts which summarize the
design values were developed, depending on the type of structural element and section in analysis, so that they can be
used in the future for a more efficient pre-design.
Keywords: Light Steel Framing (LSF), Oriented Strand Board (OSB), EN1993, diaphragm effect, class 4 cross sections.
1. INTRODUCTION
Nowadays, more than ever, the search for
sustainability and eco-efficiency is present in all
types of industry, and construction is no exception.
From the energy consumed in every household to
the energy used in the production of the
construction materials, everything must be taken
into account. The true challenge consists in
analysing the costs from a life cycle perspective,
more than just thinking about the initial investment.
When one can save energy and water, expand the
durability and still increase the productivity, the
sustainability characteristics of the project and the
materials are very easy to justify (Pinheiro, 2003).
Light Steel Framing, commonly termed LSF,
stands out as a possible constructive system for
the future. It effectively addresses and resolves the
ecological faults in other construction methods.
The first objective of this dissertation was to study
and evaluate several parameters related to thermal
performance, acoustics, fire resistance and
sustainability, since these are decisive factors
when selecting a construction method.
1
Assessment of Bridge Behaviour Due to the Passage of High Speed Trains
André Filipe Biscaya Semedo Pereira da Graça Instituto Superior Técnico, Technical University of Lisbon, Lisbon, Portugal
Abstract: The railway bridges, due to the passage of high speed trains, can suffer high resonance phenomena that cause, for example, ballast instability, endangering the railway line. Therefore, the main goal of this paper
was to develop a simplified but efficient MATLAB model able to reproduce and analyze the dynamic response of
bridges to the passage of high speed trains, in order to obtain static (bending moments, shear forces and reactions) and cinematic (accelerations, displacements and rotations) envelopes along the bridge. This methodology offers the capacity, in a short period of time, of studying a large amount of different structural solutions. Implementation took place using a continuous beam model subjected to moving point loads, which was based on the determination of the free vibration modes, generated semi-analytically, considering simple supports model, or by finite elements, using spring supports model. Thereafter, the dynamic equilibrium equation was solved, using the Newmark-β method. For different tested solutions, the suitability of acceleration peak criterion, proposed in EN1991-2, was analyzed. Furthermore, extensive parametric studies were performed, which contributed to raise the sensibility to results, particularly for continuous bridges, the less studied type. With this purpose, the influence on the bridge response of the number of internal spans and the length relationship between the external span and the internal span was investigated, generating abacus and indications of conceiving. In this study, it was only used the box type prefabricated girders. Finally, studies were accomplished about the influence of foundation deformability on vertical acceleration of bridges, which allowed determining the minimum stiffness that obligates to consider foundation deformability in models and verifying the main response differences provoked by the stiffness relationship between piers and abutments.
Keywords: high speed railway bridges, dynamic analysis, moving loads, parametric analysis, acceleration, eurocode, foundation flexibility
1. INTRODUCTION In modern societies, transports play a crucial role in economic growth and globalization, by allowing people and goods traveling. Among the other transports, High speed trains can be more competitive for distances about 100/200km and 400/600km, comparing with terrestrial and aero transportation, respectively[1]. However, with the increase of high speed trains usage, some
resonance problems were identified (e.g. ballast liquefaction due to high accelerations produced by the passage of the trains[2]), being essential to evaluate the bridge response due to their passage. The bridges safety studies should be performed through dynamic analyses that allow the determination of accelerations and displacements (required to compute static parameters). Full dynamic analyses gives more accurate results, which are important if the designer intends
2
The main goal of this work, however, lies in the
analysis of the structural performance of such
method. In order to accomplish such objective, this
thesis studied the modelling, analysis and design
of a 4-storey building using the LSF structural
system. The modelling of the structure in SAP2000
allowed the assessment of the best techniques for
computational modelling of certain structural
elements and behaviours, predominantly the
diagonal straps and the diaphragm effect of OSB.
The design procedures of the class 4 steel
sections and the consideration of the multiple
types of buckling, although very extensive and
complex, was defined and implemented to all the
necessary sections.
2. LSF SYSTEM
2.1. Wall assemblies
In order to accurately analyse the performance of
the LSF wall assemblies, it was compared to the
masonry wall system. Figure 1 and 2 display the
schemes for each one of the considered external
walls.
Figure 1 - Masonry wall scheme: 1 - plastering mortar; 2 -
masonry; 3 - air; 4 - XPS board; 5 - masonry; 6 - plastering
mortar.
Figure 2 - LSF wall scheme: 1 - plastering mortar; 2 - ETICS; 3
- OSB board; 4 - C150 steel section; 5 - mineral wool; 6 - C150
steel section; 7 - OSB board; 8 - Gypsum board.
The first characteristic that differentiates the two
types is their weight, defining the LSF solution as a
lightweight wall and the masonry as a heavy wall.
Table 1 and 2 present the calculation and result of
the weight of the two wall classes.
Table 1 – Masonry wall mass calculation.
Material Thickness Density Mass/m2
(mm) (kg/m3) (kg/m2) Plastering
Mortar 20 500 10
Masonry 110 1400 154 Air 20 - -
XPS board 40 40 1,6 Masonry 150 1400 210
Plastering Mortar 20 500 10
Total 360 385,6
Table 2 – LSF wall mass calculation.
Material Thickness Density Mass/m2
(mm) (kg/m3) (kg/m2) Gypsum board 15 1000 15
OSB board 12 700 8,4 Air 15 - -
Mineral wool 120 135 16,2 Air 15 - -
OSB board 12 700 8,4 ETICS 60 35 2,1 Total 249 50,1
As shown in the previous tables, the weight of a
masonry wall is around 7,5 times greater than the
LSF, affecting significantly the total mass of the
structure.
3
2.1.1. Thermal performance
The main objective of the external walls, combined
with the roofing, is to establish a barrier between
the external and internal environments, in such
way that the internal environment can be
maintained within certain comfort conditions. Such
walls need to meet certain demands: be stable and
durable; establish an obstacle against wind, rain,
solar radiation, heat, noise, fire, insects and even
humans. The external walls compose most of the
exterior area of the building and it is there that the
majority of the thermal exchanges occur.
Subsequently, the meticulous study of thermal
performance is fundamental to reduce the energy
consumption, making the building more
sustainable and eco-efficient (Mateus, 2004). The
thermal conductivity (λ) of each material, in
conjunction with its thickness, allowed the
calculation of the total thermal resistance and thus
the thermal conductivity of both the studied walls,
as presented in Table 3 and 4. Table 3 - Thermal resistance for common masonry wall
Material Thermal
conductivity Thickness Thermal resistance
λ (W/m °C) t (mm) R (m2 °C/W) Plastering
Mortar 1,15 20 0,02
Masonry 0,36 150 0,42 XPS board 0,033 40 1,21 Masonry 0,38 110 0,29
Plastering Mortar 1,15 20 0,02
Total - 340 1,95
Table 4 - Thermal resistance for LSF wall.
Material Thermal
conductivity Thickness Thermal resistance
λ (W/m °C) t (mm) R (m2 °C/W) ETICS 0,034 50 1,47 OSB 0,12 12 0,10
Mineral wool 0,045 120 2,67 OSB 0,12 12 0,10
Gypsum board 0,19 15 0,08 Total - 209 4,42
Thus, it is possible to conclude that masonry walls
have a thermal conductivity coefficient λ=0,51
W/m2 °C and LSF walls λ=0,23 W/m2 °C. This
substantial difference results in significant thermal
efficiency, reducing the amount of energy required
to maintain a house within the comfort zone.
Moreover, this coefficient does not take into
account the thermal bridges, which are
significantly reduced in the LSF system due to the
usage of External thermal insulation composite
system (ETICS).
2.1.2. Durability
As denoted before, the durability of the materials
used in any given construction influence the
sustainability of the project. To achieve great
performance in this matter, hot-dip galvanising
process is used, which involves dipping steel in
almost pure molten zinc.
Zinc coatings provide a barrier that prevents
oxygen, moisture and other atmospheric pollutants
from reaching the steel. Furthermore, zinc is a
reactive metal and, on exposure to the
atmosphere, a complex mixture of zinc compounds
forms readily on a galvanised surface. As many of
the products formed are partially soluble in water,
the zinc is consumed over a period of time in any
damp location. Galvanising has the advantage
that, when the encapsulation is breached (for
example at cut edges or drilled holes, or when the
zinc has been eroded away locally) significant
corrosion of the steel substrate will not necessarily
occur. This is because zinc in close proximity to
the exposed steel will still corrode preferentially,
acting as a consumable anode in an electro-
chemical cell (i.e. it protects the steel cathodically)
(Lawson et al., 2010). The normal standard has
been 275 g/m2 (i.e. a surface thickness of about 20
4
mm) designated as G275. According to studies
performed in different environments, as long as the
galvanized steel is protected by the other wall
components, its loss rate does not exceed 0,3
g/m2 per year. For G275 galvanising, it follows that
the design life is at least 230 years (Lawson et al.,
2010).
2.1.3. Sustainability
In conclusion, the sustainability and eco-efficiency
of the LSF wall assemblies can be summarized by:
• Lightweight walls, which result in savings
in the foundations, transport to site and
handling in site;
• Reduced wall thickness, providing
maximized interior area and optimizing the
transport to site;
• Superior thermal performance, boosting
the reduction of energy consumption
required to maintain comfort;
• Durable and recyclable materials, which
guarantee a greater market value in the
end of its design life.
•
3. STRUCTURAL MODELING
In order to evaluate the structural performance of
the Light Steel Framing system, 3 computational
models were created and subjected to numerical
analysis within SAP2000. The models differ in
bracing system (considering, or not, the diaphragm
effect of the OSB boards) and beam support
condition (continuous or simply supported).
3.1. Architectural conception
The model was based in a 4-storey residential
building with an implementation area of 225 m2, as
shown in Figure 3.
Table 5 summarizes the characteristics of the
analysed models according to the aforementioned
objectives. Table 5 - Models characteristics.
Model Considering
OSB
Studs
spacing
Support
condition
#1 No 0,60 m Simply
supported
#2 No 0,60 m Continuous
#3 Yes 0,60 m Continuous
QUARTO QUARTOW.C.
SALA
COZINHA
SALASALA
SALA
QUARTO QUARTO
QUARTOQUARTOQUARTOQUARTO
W.C.
W.C.W.C.
COZINHA COZINHA
COZINHA
HALL
SOBE
JARDIMJARDIM
SOBE
A.S. A.S.
A.S. A.S.
9.4002.000
9.400
3.500
2.475
2.475
3.500 4.650
1.325
1.325
4.650
1.200
5.450 2.750 2.000 2.750 5.450
1.200
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Figure 3 - Architectural floor plans (dimensions in meters).
5
3.2. Floors
Using the Releases option of SAP2000 the beams
where modelled to the abovementioned support
conditions. This allowed studying the bending
moment distribution that, depending on the length
limitation of the steel profiles, can help determine
the optimized and economic solution. Figure 1 and
2 show the floor model achieved in SAP2000, for
the simply supported beams and continuous
beams, respectively.
Figure 4 - Simply supported beams floor (left) and Continuously
supported beams floor (right)
3.3. Walls
3.3.1. Studs and diagonal straps
Both the studs and the diagonal straps were
considered simply supported elements, having no
moment restraint in both ends. Since the diagonal
straps are tension only members, but the Tension /
Compression limits defined in SAP2000 work
exclusively in non-linear analysis (implying
pushover analysis for seismic evaluation, which
was beyond this thesis scope), a different
approach was adopted. Only half the elements
were modelled (see Figure 5), that is, instead of
two diagonal straps for each braced frame, only
one diagonal strap was placed, because it works
equally in tension and compression, thus
reproducing its real rigidity.
Figure 5 - Modeling of X-bracing diagonals.
3.3.2. Diaphragm effect modelling
In order to correctly model the diaphragm effect of
the Oriented Strand Board (OSB) boards, two
different experimental tests were studied and used
to calibrate the design. Both studies used 12 mm
OSB panels and fasteners spaced 15 cm apart
along the board edge and 30 cm along the
intermediate studs. The first shear wall, tested by
AISI (AISI, 1997), was 2,44 x 12,2 m (as shown in
Figure 6) and was studied with different types of
openings, although the test used to calibrate the
model had no openings (Wall 1 in Figure 6).
Figure 6 - OSB panel test configuration (AISI, 1997)
The results, presented in Figure 7, display
pronounced rigidity and a linear behaviour in the
initial phase of the test.
Figure 7 - Force/displacement chart (AISI, 1997)
Similarly, the test performed by Tian (Tian et al.,
2004), which used a 2,45 x 1,25 m panel (as
INOVAÇÕES TECNOLÓGICAS 75
12,2m, com fechamento em placas de OSB de 12mm fixadas com parafusos espaçados a
cada 10 centímetros no perímetro externo das placas e a cada 20 centímetros nos apoios das
placas com os montantes intermediários em uma das faces e fechamento em gesso
acartonado na face oposta. A FIG. 3.10 apresenta as configurações geométricas dos painéis
ensaiados.
FIGURA 3.10 – Configurações dos painéis ensaiados AISI (1997)
Em tal trabalho são avaliadas diferentes configurações dos painéis, com diferentes relações
entre aberturas. A carga lateral foi aplicada aos painéis por meio de atuadores hidráulicos
posicionados no canto direito superior dos painéis. Os deslocamentos foram obtidos por
meio da instalação de três transdutores de deslocamento (DTs). Um para medir o
deslocamento do topo dos painéis, um segundo para medir o deslocamento no topo da guia
9
Three linear variable differential transformers (LVDT) were used to measure thedisplacement of the specimens during the test. LVDT #1 measured the horizontaldisplacement of the top of the wall. LVDT #2 measured the horizontal displacement, or slip,of the bottom track of the specimen. LVDT #3 was used to measure the uplift of the endstuds relative to the foundation.
All tests were one directional, displacing the top of the wall to a maximum of six inches over aten minute period. Data from the load cell and 3 LVDTs were collected 1 time per second.Each of the four wall configurations was tested once. Items of interest are ultimate shearload capacity, stiffness, and failure modes of the walls. Load-displacement curves wereplotted for each of the wall specimens to better understand and compare the behavior of thewalls during the test.
RESULTS
Force-Displacement Response
The response of all shear wall specimens to the monotonic loading history are shown in theforce-displacement curves of Figure 4. Initial response to load was linear and wascharacterized by large stiffness. The peak load, as well as the corresponding displacement,was gathered directly from the data. These loads and displacements are listed in Table 3.The equation developed by Sugiyama and Matsumoto conservatively predicts the ultimatecapacity of the steel-framed specimens (Figure 5). Figure 5 suggests the relation betweensheathing area ratio and peak load more closely follows the equation F = r/(2-r) for thesheathing area ratios tested. Additional testing should be conducted to confirm this finding.
FIGURE 4Force-Displacement Response
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
0.0 1.0 2.0 3.0 4.0 5.0 6.0Interstory Drift (in.)
Load
(lbs
)
WALL 1
WALL 2A
WALL 4WALL 2B
6
shown in Figure 8), generated a
force/displacement chart (shown in Figure 9). The
studied panel (A-2) corresponds to the OSB-3
steps curve.
Figure 8 - OSB panel test configuration (Tian et al., 2004)
Figure 9 - Force/displacement chart (Tian et al., 2004)
Because the studs have almost no influence in the
shear resistance of the walls, they were omitted
from the model. Based on a shell element with a
15 cm mesh (spacing of the fasteners), the effect
of the fasteners on the panel was simulated by
restraining the rotation around X and Z, and the
displacement in Y in every node that corresponded
to a fastener, as shown in Figure 10.
Figure 10 - Models for AISI test (left) and for Tian test (right)
Using the abovementioned charts and this two
SAP2000 models, it was concluded that the latter
represented the structural behaviour of the panels
in the linear phase. Therefore, all the OSB panels
with no openings considered in the building can be
regarded as shear walls and divided in 4
configurations, as presented in Figure 11.
Figure 11 - Panels considered in the structure.
Two similar methods were used to simulate the
panels’ rigidity, both leading to the same results.
The first method consisted of using an equivalent
diagonal of OSB (Smith, 1966), whereas the
second used steel (Inoue, 2004). Being R the
tension force of a given diagonal due to a
horizontal force on the top of the panel, d the
length of such diagonal, Ea the elasticity modulus
of steel and Δd the length variation of the diagonal,
the area of the equivalent steel rod (A) is given by:
𝐴 =
𝑅.𝑑𝐸! .∆𝑑
(1)
Applying such logic to the 4 panels considered, the
equivalent diameter is as shown in Table 6. Table 6 - Equivalent diameter calculation summary.
Panel type
H L R d Δd D (m) (m) (kN) (mm) (mm) (mm)
1 2,8 1,2 2,54 3046,3 0,21 15,1 2 2,8 1,8 1,85 3328,7 0,15 15,9 3 2,8 2,4 1,54 3687,8 0,12 17,0 4 2,8 3,0 1,37 4103,7 0,10 18,4
+0,8
-1,2
-0,8
-0,5
-1,2
-0,8
Vento 90º
DA
B
A
B
E
Panel 1 - 1,2m x 2,8m
Panel 2 - 1,8m x 2,8m
Panel 3 - 2,4m x 2,8m
Panel 4 - 3,0m x 2,8m
Caption:
1
A
2
4
6
7
B C D E
3
5
Panel 1 - 1,2m x 2,8m
Panel 2 - 1,8m x 2,8m
Panel 3 - 2,4m x 2,8m
Panel 4 - 3,0m x 2,8m
Caption:
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7
3.4. Behaviour factor
This building was to be placed in Lisbon in a type
C soil. The definition of the behaviour factor for
LSF buildings is not clear in the EN1998. The
structure can be classified as a frame with
concentric diagonal bracings (dissipative zones in
tension diagonals only), when neglecting the OSB
panels, or as a frame with concentric V-bracings
(dissipative zones in tension and compression
diagonals) when considering the OSB panels’
rigidity, as show in Figure 12. In this dissertation,
since the OSB panels were to be considered, the
behavior factor prescribed in the EN1998 is q=2,0.
Figure 12 - Frames with: tension diagonals only (left) and
tension and compression diagonals (right)
4. STRUCTURAL ANALYSIS
4.1. Seismic analysis
The consideration of the OSB panels increases the
rigidity of the building thus reducing the
fundamental period the seismic and increasing the
design acceleration. For this reason, the
consideration of the diaphragm effect of the OSB
panels should not be ignored due to its influence
on the seismic performance of the building.
Moreover, if the OSB panels and the diagonal steel
straps are not coincident, the seismic behaviour of
the building can be even more influenced.
4.2. Design forces and displacements
The studs were analysed in two distinct parts, the
studs that only support the vertical loads (Stud
type 1) and the studs that constitute the bracing
system (Stud type 2). The most critical
combination to the latter was, as expected, the
fundamental combination with the imposed load as
leading variable. As the beam supporting
conditions were also analysed, it was also possible
to check how it influences the axial compression
force distribution. As expected, the axial
compression forces in the simply supported model
were more uniform, whereas the continuously
supported model presented higher values in the
interior supporting panels. On the other hand, the
continuously supported joists presented a more
even design moment, also as expected, as shown
in Table 7.
Table 7 - Joists design moments.
Support condition Alignment MEd+ MEd-
(kNm) (kNm)
Continuous A 1-2 and 6-7 5,0 - B 1-7 3,9 3,5 D 1-7 7,7 6,6
Simply supported
A 1-2 and 6-7 5,8 -
B 1-2 and 6-7 5,6 - 2-4 and 4-6 2,4 -
D 1-3 and 5-7 10,8 - 3-5 - -1,6
For the studs that comprised the shear wall
systems, the critical combination was the type 1
seismic action, which was predicted by the seismic
analysis of the fundamental period. The critical
design values for studs and the correspondent
storey drifts are presented in Table 8.
Table 8 - Critical design values for studs.
Stud type
OSB consideration
Axial dr,x dr,y Load combination (kN) (mm) (mm)
1 Yes 51,4 1,0 0,0 ULS -
imposed loading No 51,4 1,9 0,0
2 Yes 118,8 1,4 6,8 Type 1
seismic action No 115,1 7,7 2,9
,PSUHVVÔR�GH�
�,3
NP EN 1998-1 2010
p. 128 de 230
− contraventamentos diagonais nos quais a resistência às forças horizontais é exclusivamente assegurada pelas diagonais traccionadas, desprezando-se a contribuição das diagonais comprimidas;
− contraventamentos em V, nos quais a resistência às forças horizontais pode ser assegurada quer pelas diagonais traccionadas quer pelas comprimidas. O ponto de intersecção destas diagonais localiza-se num elemento horizontal que deve ser contínuo.
Não poderão ser utilizados contraventamentos em K, nos quais a intersecção das diagonais se localiza numa coluna (ver a Figura 6.9). (4) Para pórticos com contraventamentos excêntricos deverão utilizar-se configurações que garantem que todos os ligadores serão activos, como representado na Figura 6.4. (5) As estruturas em pêndulo invertido poderão considerar-se como sendo pórticos simples desde que a estrutura resistente aos sismos tenha mais do que uma coluna em cada plano resistente e que a condição de limitação do esforço normal: NEd < 0,3 Npl,Rd seja satisfeita em cada coluna.
a) b) c)
Figura 6.1 – Pórticos simples (zonas dissipativas nas vigas e na base das colunas). Valores por defeito para αu/α1 (ver 6.3.2(3) e o Quadro 6.2)
Figura 6.2 – Pórticos com contraventamentos diagonais centrados (zonas dissipativas unicamente nas diagonais traccionadas)
Figura 6.3 – Pórticos com contraventamentos em V centrados (zonas dissipativas nas diagonais traccionadas e comprimidas)
NP EN 1998-1 2010
p. 128 de 230
− contraventamentos diagonais nos quais a resistência às forças horizontais é exclusivamente assegurada pelas diagonais traccionadas, desprezando-se a contribuição das diagonais comprimidas;
− contraventamentos em V, nos quais a resistência às forças horizontais pode ser assegurada quer pelas diagonais traccionadas quer pelas comprimidas. O ponto de intersecção destas diagonais localiza-se num elemento horizontal que deve ser contínuo.
Não poderão ser utilizados contraventamentos em K, nos quais a intersecção das diagonais se localiza numa coluna (ver a Figura 6.9). (4) Para pórticos com contraventamentos excêntricos deverão utilizar-se configurações que garantem que todos os ligadores serão activos, como representado na Figura 6.4. (5) As estruturas em pêndulo invertido poderão considerar-se como sendo pórticos simples desde que a estrutura resistente aos sismos tenha mais do que uma coluna em cada plano resistente e que a condição de limitação do esforço normal: NEd < 0,3 Npl,Rd seja satisfeita em cada coluna.
a) b) c)
Figura 6.1 – Pórticos simples (zonas dissipativas nas vigas e na base das colunas). Valores por defeito para αu/α1 (ver 6.3.2(3) e o Quadro 6.2)
Figura 6.2 – Pórticos com contraventamentos diagonais centrados (zonas dissipativas unicamente nas diagonais traccionadas)
Figura 6.3 – Pórticos com contraventamentos em V centrados (zonas dissipativas nas diagonais traccionadas e comprimidas)
8
Since the OSB panels and the diagonal steel
straps are all part of the bracing system, the critical
combination for its design was also the type 1
seismic action, as presented in Table 9.
Table 9 - Critical design values for steel straps and OSB
equivalent diagonals
Panel type
OSB consideration
Steel strap OSB diagonal
(kN) (kN) 1
Yes
17,3 12,9 2 10,7 10,9 3 16,2 14,0 4 34,3 26,7 1
No
25,0 - 2 20,9 - 3 28,8 - 4 50,1 -
The two types of connections studied were the
steel - steel connection and the steel - OSB
connection. Fort the first one, one analysed the link
between the straps and the studs, given its
importance in the structural behaviour. However,
the design value for each fastener depends on its
quantity and spacing, consequently, the design
values can be obtained by the ones presented in
Table 9. Secondly, the link between the OSB
panels and the studs was studied. Similarly to the
previous elements, the critical combination was
seismic type 1 and, as the fastener spacing was
previously defined as 15 cm in the OSB board
edge, the critical design values are presented in
Table 10.
Table 10 - Fastener design value for steel - OSB connections.
Panel type FH Fv,Ed
(kN) (kN) 1 6,7 0,87 2 5,9 0,51 3 13,6 0,88 4 26,7 1,01
5. STRUCTURAL DESIGN
5.1. Instability phenomena
The ultimate resistance of open thin walled
sections is influenced by a diverse set of instability
phenomena. As Light steel framing typically
exploits class 4 cross sections (in which local
buckling will occur before the attainment of yield
stress in one or more parts of the cross-section),
the design of LSF sections is a complex process.
The members are also subjected to distortional
and global buckling, as show in Figure 13.
Figure 13 - Instability modes of a class 4 single open section
(Adaptaded from Pinto, 2010).
Figure 13 description: a) local mode in
compression b) local mode in bending c)
distortional mode in compression d) distortional
mode in bending e) global mode in compression f)
global mode in bending.
The local buckling is taken into account by the
effective section method and the distortional and
global buckling use reduction factors, calculated
using the gross cross section, to determine the
design buckling resistance of any given member.
5.2. Studs
In order to reduce the calculation volume involved
in the stud design, two summarizing charts were
developed, one of which is presented in Figure 14.
This chart condenses the design capacity of U153
and C150 single and built-up studs.
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Not only does this chart allow a simpler design, but
it also is a good pre-design tool.
5.3. Joists
Similarly to the studs, a chart that condenses the
joists’ design was developed. Since the joists are
limited not only by the ultimate limit state (ULS) but
also by the serviceability limit state (SLS), both this
conditions are presented in the graph, as shown in
Figure 15. The loads used in such graph were the
design loads employed in the studied building.
This way, the chart provides valuable design
information not only for this example, but also to
pre-design other simply supported spans.
Figure 14 - U153 and C150 single and built-up sections stud capacity.
Figure 15 - Joists capacity for simply supported spans.
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5.4. Connections
Different steel straps were adopted depending on
the design tension, which was then used to design
the stud-strap connections. The shear strength of
the analysed fasteners (along with the bearing
strength of 1,0 and 1,5 mm plates) is presented in
Table 11 and the number and type of fasteners
required by each strap shown in Table 12.
Table 11 - Shear and bearing strength of fasteners.
Gauge Diameter Fv,Rd Fb,Rd
1,5mm plate
Fb,Rd 1mm plate
(#) (mm) (kN) (kN) (kN) 6 3,56 1,91 3,84 2,56 8 4,06 2,49 4,38 2,92
10 4,83 3,52 5,22 3,48 12 5,33 4,28 5,76 3,84 14 6,35 6,08 6,86 4,57
Table 12 - Fastener design for each steel strap.
Width Thickness Area Nt,Rd Fasteners / connection
(mm) (mm) (mm2) (kN) Quantity Gauge 60 1,0 60 16,8 6 #10
100 1,0 100 28 10 #10 85 1,5 127,5 35,7 10 #12
120 1,5 180 50,4 10 #14
6. CONCLUSION
Focusing on the results about the non-structural
parameters, one concluded that the LSF method
presents (i) excellent thermal performance, (ii)
good acoustical performance, (iii) excellent
durability, sustainability and eco-efficiency
indicators.
Relatively to structural modeling, analysis and
design, one can prompt the following general
conclusions:
• LSF computational modeling is quite
complex, given the number of elements
involved, and presents many different
challenges, mainly related to the bracing
system and diaphragm effect;
• The OSB diaphragm effect should always
be considered, since it can significantly
influence the buildings’ structural behavior;
• The presented model satisfactorily
represented the structural behavior to all
the studied combinations and provided
reasonable results for each designed
member;
• The design of the class 4 sections was
successfully accomplished, providing
concrete designing values for the studs,
joists, straps and fasteners;
• The summarizing charts condense a great
deal of valuable information and can be
used for pre-design.
7. REFERENCES
AISI. (1997). Monotonic Tests of Cold-Formed Steel Shear Walls with Openings. National Association of Home Builders (NAHB) Research Center, Marlboro, MD.
Inoue, H. (2004). Sistemas diafragma com painéis de chapa fina aplicados a edificações estruturadas em aço (Tese de douturamento). Universidade Federal do Rio de Janeiro.
Lawson R. M., Popo-Ola S. O., Way A., Heatley T., Pedreschi R. (2010). Durability of light steel framing in residential applications. Proceedings of the Institution of Civil Engineers (ICE) - Construction Materials, 163(2):109–21.
Mateus, R. (2004). Novas tecnologias construtivas com vista à sustentabilidade da construção (Dissertação de Mestrado). Universidade do Minho.
Pinheiro, M. D. (2003). Construção Sustentável – Mito ou Realidade?. VII Congresso Nacional de Engenharia do Ambiente, Lisboa.
Pinto, A. R. (2010). Estabilidade local de perfis de aço enformados a frio (Dissertação de mestrado). Instituto Superior Técnico.
Smith, B. S. (1966). Behaviour of Square Infilled Frames. Journal of Structural Division – American Society of Civil Engineers (ASCE), 92(1):381-403.
Tian, Y. S., WANG, J., LU, T. J. (2004). Racking strength and stiffness of cold-formed steel wall frames. Journal of Constructional Steel Research, 60(7):1069-1093.