| SECTION 1605 LOAD COMBINATIONS
1605.1 General. Buildings
and other structures and portions thereof shall be designed
to resist the load combinations specified in Section 1605.2
or 1605.3 and Chapters 18 through 23, and the special seismic
load combinations of Section 1605.4 where required by Section
12.3.3.3 or 12.10.2.1 of ASCE
7. Applicable loads shall be considered, including both earthquake
and wind, in accordance with the specified load combinations.
Each load combination shall also be investigated with one
or more of the variable loads set to zero.
1605.2 Load combinations using
strength design or load and resistance factor design.
1605.2.1 Basic load combinations.
Where strength design or load and resistance factor design
is used, structures and portions thereof shall resist the
most critical effects from the following combinations of
factored loads:
1.4 (D+F)
(Equation 16-1)
1.2(D + F + T) + 1.6(L +
H) + 0.5 (Lr or S or R) (Equation
16-2)
1.2D+ 1.6(Lr or
S or R) + (f1L or 0.8W)
(Equation 16-3)
1.2D+ 1.6W+ f1L
+ 0.5(Lr or S or R)
(Equation 16-4)
1.2D+ 1.0E+ f1L
+ f2S
(Equation 16-5)
0.9D+ 1.6W+ 1.6H
(Equation 16-6)
0.9D+ 1.0E+ 1.6H
(Equation 16-7)
f1 = 1 for floors
in places of public assembly, for live loads in excess
of 100 pounds per square foot (4.79 kN/m2),
and for parking garage live
load,and
= 0.5 for other live loads.
f2 = 0.7 for roof
configurations (such as saw tooth) that do not shed snow
off the structure, and
= 0.2 for other roof
configurations.
Exception: Where other
factored load combinations are specifically required by the
provisions of this code, such combinations shall take precedence.
1605.2.2 Other loads.
Where Fa is to be considered in the design, the
load combinations of Section 2.3.3
of ASCE 7 shall be used.
1605.3 Load combinations using
allowable stress design.
1605.3.1 Basic load combinations.
Where allowable stress design (working stress design), as
permitted by this code, is used, structures and portions
thereof shall resist the most critical effects resulting
from the following combinations of loads:
D + F
(Equation 16-8)
D+ H+ F+ L + T
(Equation 16-9)
D+ H+ F+ (Lr or
S or R)
(Equation 16-10)
D + H + F + 0.75(L + T) +
0.75 (Lr or S or R)
(Equation 16-11)
D+ H + F+ (W or 0.7E)
(Equation 16-12)
D + H + F + 0.75(W or 0.7E)
+ 0.75L + 0.75 (Lr or S or R) (Equation
16-13)
0.6D +W +H
(Equation 16-14)
0.6D+ 0.7E+H
(Equation 16-15)
Exceptions:
1.Crane hook loads need not
be combined with roof live load or with more than three-fourths
of the snow load or one-half of the wind load.
2. Flat roof snow loads of
30 psf (1.44 kN/m2) or less need not be combined
with seismic loads. Where flat roof snow loads exceed 30
psf (1.44 kN/m2), 20 percent shall be combined
with seismic loads.
1605.3.1.1 Stress increases.
Increases in allowable stresses specified in the appropriate
material chapter or the referenced standards shall not
be used with the load combinations of Section 1605.3.1,
except that a duration of load increase shall be permitted
in accordance with Chapter 23.
1605.3.1.2 Other loads.
Where Fa is to be considered in design,
the load combinations of Section 2.4.2
of ASCE 7 shall be used.
1605.3.2 Alternative basic
load combinations. In lieu of the basic load combinations
specified in Section 1605.3.1, structures and portions thereof
shall be permitted to be designed for the most critical
effects resulting from the following combinations. When
using these alternative basic load combinations that include
wind or seismic loads, allowable stresses are permitted
to be increased or load combinations reduced where permitted
by the material chapter of this code or the referenced standards.
For load combinations that include the counteracting effects
of dead and wind loads, only two-thirds of the minimum dead
load likely to be in place during a design wind event shall
be used. Where wind loads are calculated in accordance with
Chapter 6 of ASCE 7, the coefficient ω in the following
equations shall be taken as 1.3. For other wind loads, ω
shall be taken as 1. When using these alternative load combinations
to evaluate sliding, overturning and soil bearing at the
soil-structure interface, the reduction of foundation overturning
from Section 12.13.4 in ASCE
7 shall not be used. When using these alternative basic
load combinations for proportioning foundations for loadings,
which include seismic loads, the vertical seismic load effect,
Ev, in Equation 12.4-4 of ASCE 7 is permitted to be taken
equal to zero.
D+ L + (Lr or
S or R)
(Equation 16-16)
D+ L + (ω W)
(Equation 16-17)
D+ L +ω W+ S/2
(Equation 16-18)
D+ L + S +ω W/2
(Equation 16-19)
D+ L + S +E/1.4
(Equation 16-20)
0.9D+E/1.4
(Equation 16-21)
Exceptions:
1. Crane
hook loads need not be combined with roof live loads or
with more than three-fourths of the snow load or one-half
of the wind load.
2. Flat roof snow loads of
30 psf (1.44 kN/m2) or less need not be combined
with seismic loads. Where flat roof snow loads exceed 30
psf (1.44 kN/m2), 20 percent shall be combined
with seismic loads.
1605.3.2.1 Other loads.
Where F, H or T are to be considered in the design, each
applicable load shall be added to the combinations specified
in Section 1605.3.2.
1605.4 Special seismic load
combinations. For both allowable stress design and strength
design methods where specifically required by Section 1605.1
or by Chapters 18 through 23, elements and components shall
be designed to resist the forces calculated using Equation
16-22 when the effects of the seismic ground motion are additive
to gravity forces and those calculated using Equation 16-23
when the effects of the seismic ground motion counteract gravity
forces.
1.2D+ f1L +Em
(Equation 16-22)
0.9D+Em
(Equation 16-23)
where:
Em = The maximum
effect of horizontal and vertical forces as set forth
in Section 12.4.3 of ASCE
7.
f1
= 1 for floors in places of public assembly, for
live loads in excess of 100 psf (4.79 kN/m2)
and for parking garage live load, or
= 0.5 for other live loads.
1605.5 Heliports and helistops.
Heliport and helistop landing areas shall be designed
for the following loads, combined in accordance with Section
1605:
1. Dead
load, D, plus the gross weight of the helicopter, Dh,
plus snow load, S.
2. Dead
load, D, plus two single concentrated
impact loads, L, approximately 8 feet (2438 mm) apart applied
anywhere on the landing area (representing the helicopter’s
two main landing gear, whether skid type or wheeled type),
having a magnitude of 0.75 times the gross weight of the
helicopter. Both loads acting together total one-and one
half times the gross weight of the helicopter.
3. Dead
load, D, plus a uniform live load,
L, of 100 psf (4.79 kN/m2).
Exception: Landing areas
designed for helicopters with gross weights not exceeding
3,000 pounds (13.34 kN) in accordance with Items 1 and 2 shall
be permitted to be designed using a 40 psf (1.92 kN/m2)
uniform live load in Item 3, provided the landing area is
identified with a 3,000 pound (13.34 kN) weight limitation.
This 40 psf (1.92 kN/m2) uniform live load shall
not be reduced. The landing area weight limitation shall be
indicated by the numeral “3” (kips) located in the
bottom right corner of the landing area as viewed from the
primary approach path. The landing area weight limitation
shall be a minimum of 5 feet (1524 mm) in height.
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