A R values, and the lowest R value of

A
simple but important conclusion from the above review is that there is not a
universal agreement on the value of response modification factor, R, and in
particular, there is no reference in the codes for the R factor of systems
braced with knee-braces. More studies are required to clarify this matter

A
different structural system may be used in the orthogonal direction with
different R values, and the lowest R value of that direction shall be used in
determining loads in that orthogonal direction. Uniform Building Code, UBC 9781, and International Building Code, IBC 82, highlight that the design, installation and construction
of CFS structural and non-structural framing shall be in accordance with AISI.
Also, the R factor shall be based on ASCE 7 for the appropriate steel systems,
which are designed and detailed in accordance with the provisions of AISC.
Although UBC allows a maximum height of five storeys for steel stud wall systems
in seismic zones, provided that they comply with some specifications, IBC
limits the use of CFS systems to up to two storeys in height considering AISI
provisions. The codes restrict the thickness of CFS components to be in between
0.84 mm and 1.10 mm. According to IBC, a minimum of two studs back-to-back for
the chord member is needed and the aspect ratio of the wall system shall not
exceed 2:1. However, for some special applications, a maximum aspect ratio of
4:1 is acceptable. Moreover, studs shall be a minimum 41 mm (flange)89 mm (web)
with a 9.5 mm return lip, while minimum dimensions for tracks are 32 mm and 89
mm for flange and web, respectively. The code stipulates that bending in the
track, overall buckling in stud and pull-out of strap connection shall be
prevented. Moreover, the connection of diagonal bracing member and boundary
members shall be designed such that the full tensile strength of the member, or
O imes the prescribed seismic forces, is developed The Australian cold-formed
steel structures standard, AS/NZS 4600-05 83,
requires that when cold-formed steel members are used as the primary earthquake
resisting element, the selected response modification factor shall not be
greater than 2, unless specified otherwise. However, as Australia is located in
a low seismic zone, wind loads often dominate the design of low-rise cold-formed
steel buildings and therefore such a low value for R factor does not affect
designs. Little research attention has been paid to the evaluation of R factors
in Australia for the same reason.

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The
provisions affirm that all boundary members shall be designed to transmit the
specified induced axial forces. In addition, connections for diagonal bracing
members shall have design strength equal to or greater than the nominal tensile
strength of the members being connected, or ?0 times the
design seismic force, in which ?o is the
over-strength factor defined by the code. The pull-out resistance of screws also
shall not be used to resist seismic forces. FEMA 450 gives the nominal shear
strength for shear walls framed with cold-formed steel members based on
different sheathing materials and fastener spacing’s at panel edges. Although
the code provides the seismic response modification factors for some CFS
framing systems, it does not cover all of the many different systems currently
used in practice. As a consequence, for systems not mentioned in the code, the designer
has to use the R factor corresponding to ”Steel Systems Not Specifically
Detailed for Seismic Resistance”, which is 3. Another US standard on the cold
formed steel structures is the Technical Instructions, TI 809-0763. This code was originally Technical Instructions,
TI 809-0763. This code was originally developed
for the design and construction of cold-formed steel military constructions and
is used extensively by the US Army Corps of Engineers, USACE. The code is
primarily based on FEMA-302 80 though with
some modifications in the design load considering over-strength of straps. TI
809-07 stipulates that shear panels shall be adequately anchored at their top
and bottom to a floor diaphragm. Furthermore, when it comes to the tying of two
lateral load resisting systems together, walls in orthogonal direction shall be
anchored to the same floor diaphragm. The chords that support the vertical
component of the strap load shall be selected from a single closed (tubing)
section or built-up CFS section oriented to form a closed cross-section by means
of  intermittent welds. Although the code
provides some general recommendations for seismic design of cold-formed steel
shear walls, it mainly focuses on diagonal strap configurations. So, aseismic
response modification factor is suggested only for CFS shear panels with
diagonal strapping, which is 4. The code mentions that the R factor in the
direction under consideration at any storey shall not exceed the lowest value
for the seismic force resisting system in the same direction considered above
that storey, excluding penthouses. Other structural systems, i.e. dual systems,
may be used in combination with these CFS panels, but then the smallest R value
for all systems in the direction under consideration must be used for
determining the loads applied to the entire structure in that direction. Dual
systems must be used with caution, particularly if differences in stiffness
result in interaction effects or deformation compatibility problems.

One
of the pioneer centers working on CFS framing systems is the American Iron and
Steel Institute, AISI. The institute’s efforts in the development of
construction standards started in the 1930s and culminated in the first
publication of the AISI Specification in 1946 68.
AISI has published several standards, including the following: Standard for
Cold-Formed Steel Framing – Prescriptive Method for One and Two Family
Dwellings69; North American Specification for
the Design of Cold-Formed Steel Structural Members 70;
and a series of standards for cold-formed steel framing – General Provisions 71, Header Design 72,
Lateral Design 1, Wall Stud Design 73and Truss Design 74.
Although the design and construction of cold-formed steel structures shall comply
with the North American Specification 76 and
the General Provisions 71, seismic design
regulations have been stipulated in the Lateral Design 1
along with some design guidelines for various special shear wall types and
strap bracing 71-77. The Lateral Design
Standard does not enforce any special rule other than specifications and
general provisions for shear walls when the response modification factor is
considered as being smaller than 3 in design. However, for a response
modification factor greater than 3, some additional requirements shall apply,
mainly described for diagonal strap bracing members and anchorage of braced
wall segments that resist uplift as well as perimeter members at opening. The
alternative between R? 3, with no special requirements, or taking the
advantages of  R>3 in addition to some
essential detailing, is permitted only for the seismic design categories A–C.
In the seismic design categories D–F, using and R equal to or less than 3 is
not permitted, and the designer must use the special seismic requirements with
R greater than 3 to ensure that the system behaves properly in high seismic
regions. Eventually, the code introduces seismic response modification factors
for different basic seismic force resisting systems; however, it does not cover
all available lateral bracing systems, which are currently used in the CFS
residential industry including the knee-bracing system. National Earthquake
Hazard Reduction Program, NEHRP, is another American centers, which has
published a few seismic provisions considering CFS contexts such as FEMA 450 78 and FEMA P750 79.
They specify that the design of cold-formed carbon or low-alloy steel members
to resist seismic loads shall be in accordance with the requirements of AISI
Specifications and AISI General Provisions. However, the allowable stress and
load levels in AISI are incompatible with the force levels calculated in
accordance with FEMA provisions. Therefore, it is essential to adjust the
provisions of AISI for use with the FEMA provisions. It is mentionable that these
modifications affect only designs involving seismic loads.