DESIGN BASIS 55
moment, shear force, etc.) on the cross section determined
from the structural analysis and γ
i
are the corresponding
load factors that account for the uncertainties and variabili-
ties of the applied loads. The load factors are usually greater
than unity, as given in Section 3.3.2.2.
For the design of cold-formed members using carbon
and low-alloy steels, the values of φ and R
n
are given in
the main body of the North American Specification and
Appendix 1.
1.345
3.3.2.2 Nominal Loads, Loads Factors, and Load
Combinations for the LRFD Method The design provi-
sions for nominal loads and load combinations are provided
in Appendix A of the North American Specification for use
in the United States and Mexico. The following discussion
is applicable only to the LRFD method:
(a) Nominal Loads. The design requirements for nominal
loads to be used for the LRFD method are the same as
that used for the ASD method. See item (a) of Section
3.3.1.2 for the discussion based on Section A3.1 of
Appendix A of the North American Specification.
(b) Load Factors and Load Combinations for LRFD.
Section A5.1.2 of Appendix A of the North American
Specification specifies that the structure and its compo-
nents shall be designed so that design strengths equal
or exceed the effects of the factored loads and load
combinations stipulated by the applicable building
code under which the structure is designed or, in the
absence of an applicable building code, as stipulated
in the ASCE Standard, Minimum Design Loads for
Buildings and Other Structures, ASCE/SEI 7.
When the ASCE Standard is used for the LRFD method,
the following load factors and load combinations should be
considered for the strength limit state
3.201
:
1. 1.4(D + F) (3.5a)
2. 1.2(D + F + T)+ 1.6(L + H)+ 0.5(L
r
or S or R)
(3.5b)
3. 1.2D + 1.6(L
r
or S or R) + (L or 0.8W) (3.5c)
4. 1.2D + 1.6W + L + 0.5(L
r
or S or R) (3.5d)
5. 1.2D + 1.0E + L + 0.2S (3.5e)
6. 0.9D + 1.6W + 1.6H (3.5f)
7. 0.9D + 1.0E + 1.6H (3.5g)
All the symbols are defined in item (b) of Section 3.3.1.2.
For the above load combinations, exceptions are as follows:
1. The load factor on L in combinations (3), (4), and
(5) is permitted to equal 0.5 for all occupancies in
which the minimum uniformly distributed live load
L
0
in Table 4-1 of ASCE/SEI 7
3.201
is less than or
equal to 100 psf, with the exception of garages or areas
occupied as places of public assembly.
2. The load factor on H shall be set equal to zero in
combinations (6) and (7) if the structural action due
to H counteracts that due to W or E. Where lateral
earth pressure provides resistance to structural actions
from other forces, it shall not be included in H but
shall be included in the design resistance.
3. In combinations (2), (4), and (5), the load S shall be
taken as either the flat-roof snow load or the sloped-
roof snow load.
Each relevant strength limit state shall be investigated.
Effects of one or more loads not acting shall be investi-
gated. The most unfavorable effects from both wind and
earthquake loads shall be investigated, where appropriate,
but they need not be considered to act simultaneously.
For the serviceability limit state, Appendix B of Ref.
3.201 contains the suggested load combinations.
Because building codes and the ASCE Standard do
not provide load factors and load combinations for roof
and floor composite construction using cold-formed steel
deck, the following load combination is included in the
Commentary on the North American Specification for this
type of composite construction
1.346
:
1.2D
s
+ 1.6C
w
+ 1.4C (3.6)
where D
s
= weight of steel deck
C
w
= weight of wet concrete during construction
C = construction load, including equipment,
workmen, and formwork but excluding
weight of wet concrete
The above load combination provides safe construction
practices for cold-formed steel decks and panels that other-
wise may be damaged during construction. The load factor
used for the weight of wet concrete is 1.6 because delivering
methods are such that an individual sheet can be subjected
to this load. The use of a load factor of 1.4 for the construc-
tion load is comparable to the allowable strength design
method.
3.3.2.3 Design Strength φR
n
The design strength is the
available strength of a structural component or connection
to be used for design purposes. As shown in Eq. (3.4),
design strength is obtained by multiplying the nominal
strength or resistance R
n
by a reduction factor φ to
account for the uncertainties and variabilities of the nominal
strength.