= 5. CHAPTER 5 — LOADS
= 5.3 — Load factors and combinations
== 5.3.1 Required strength U shall be at least equal to the
effects of factored loads in Table 5.3.1 , with exceptions and
additions in 5.3.3 through 5.3.13.
Table 5.3.1—Load combinations
= R5.3 — Load factors and combinations
== R5.3.1 The required strength U is expressed in terms of
factored loads. Factored loads are the loads specified in the
general building code multiplied by appropriate load factors.
If the load effects such as internal forces and moments are
linearly related to the loads, the required strength U may be
expressed in terms of load effects multiplied by the appropriate
load factors with the identical result. If the load effects are
nonlinearly related to the loads, such as frame P-delta effects
(Rogowsky and Wight 2010), the loads are factored before
determining the load effects. Typical practice for foundation
design is discussed in R13.2.6.1. Nonlinear finite element
analysis using factored load cases is discussed in R6.9.3.
The factor assigned to each load is influenced by the
degree of accuracy to which the load effect usually can be
calculated and the variation that might be expected in the
load during the lifetime of the structure. Dead loads, because
they are more accurately determined and less variable, are
assigned a lower load factor than live loads. Load factors
also account for variability in the structural analysis used to
calculate moments and shears.
The Code gives load factors for specific combinations of
loads. In assigning factors to combinations of loading, some
consideration is given to the probability of simultaneous
occurrence. While most of the usual combinations of loadings
are included, it should not be assumed that all cases are
covered.
Due regard is to be given to the sign (positive or negative)
in determining U for combinations of loadings, as one
type of loading may produce effects of opposite sense to that
produced by another type. The load combinations with 0.9D
are included for the case where a higher dead load reduces
the effects of other loads. The loading case may also be critical
for tension-controlled column sections. In such a case,
a reduction in compressive axial load or development of
tension with or without an increase in moment may result in
a critical load combination.
Table R5.2.2—Correlation between seismic-related terminology in model codes
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== R5.3.1 Continuation
Consideration should be given to various combinations of
loading to determine the most critical design condition. This
is particularly true when strength is dependent on more than
one load effect, such as strength for combined flexure and
axial load or shear strength in members with axial load.
If unusual circumstances require greater reliance on the
strength of particular members than circumstances encountered
in usual practice, some reduction in the stipulated
strength reduction factors ϕ or increase in the stipulated load
factors may be appropriate for such members.
Rain load R in Eq. (5.3.1b), (5.3.1c), and (5.3.1d) should
account for all likely accumulations of water. Roofs should be
designed with sufficient slope or camber to ensure adequate
drainage accounting for any long-term deflection of the roof
due to the dead loads. If deflection of roof members may
result in ponding of water accompanied by increased deflection
and additional ponding, the design should ensure that
this process is self-limiting.
Model building codes and design load references refer
to earthquake forces at the strength level, and the corresponding
load factor is 1.0 (ASCE/SEI 7; BOCA 1999; SBC
1999; UBC (ICBO 1997); 2018 IBC). In the absence of a
general building code that prescribes strength level earthquake
effects, a higher load factor on E would be required.
The load effect E in model building codes and design load
reference standards includes the effect of both horizontal and
vertical ground motions (as Eh and Ev, respectively). The
effect for vertical ground motions is applied as an addition
to or subtraction from the dead load effect (D), and it applies
to all structural elements, whether part of the seismic forceresisting
system or not, unless specifically excluded by the
general building code.
== 5.3.2 The effect of one or more loads not acting simultaneously
shall be investigated.
== 5.3.3 The load factor on live load L in Eq. (5.3.1c),
(5.3.1d), and (5.3.1e) shall be permitted to be reduced to 0.5
except for (a), (b), or (c):
(a) Garages
(b) Areas occupied as places of public assembly
(c) Areas where L is greater than 4.8 kN/m2
== 5.3.4 If applicable, L shall include (a) through (f):
(a) Concentrated live loads
(b) Vehicular loads
(c) Crane loads
(d) Loads on hand rails, guardrails, and vehicular barrier
(e) Impact effects
(f) Vibration effects
== R5.3.3 The load modification factor in this provision is
different than the live load reductions based on the loaded
area that may be allowed in the general building code. The
live load reduction, based on loaded area, adjusts the nominal
live load (L0 in ASCE/SEI 7) to L. The live load reduction, as
specified in the general building code, can be used in combination
with the 0.5 load factor specified in this provision.
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PART 2: LOADS & ANALYSIS 63
5 Loads
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== 5.3.5 If wind load W is provided at service-level loads, 1.6W
shall be used in place of 1.0W in Eq. (5.3.1d) and (5.3.1f), and
0.8W shall be used in place of 0.5W in Eq. (5.3.1c).
== 5.3.6 The structural effects of forces due to restraint of
volume change and differential settlement T shall be considered
in combination with other loads if the effects of T can
adversely affect structural safety or performance. The load
factor for T shall be established considering the uncertainty
associated with the likely magnitude of T, the probability
that the maximum effect of T will occur simultaneously with
other applied loads, and the potential adverse consequences
if the effect of T is greater than assumed. The load factor on
T shall not have a value less than 1.0.
== R5.3.5 In ASCE/SEI 7-05, wind loads are consistent with
service-level design; a wind load factor of 1.6 is appropriate
for use in Eq. (5.3.1d) and (5.3.1f) and a wind load factor
of 0.8 is appropriate for use in Eq. (5.3.1c). ASCE/SEI 7-16
prescribes wind loads for strength-level design and the wind
load factor is 1.0. Design wind speeds for strength-level
design are based on storms with mean recurrence intervals
of 300, 700, and 1700 years depending on the risk category
of the structure. The higher load factors in 5.3.5 apply where
service-level wind loads corresponding to a 50-year mean
recurrence interval are used for design.
== R5.3.6 Several strategies can be used to accommodate
movements due to volume change and differential settlement.
Restraint of such movements can cause significant member
forces and moments, such as tension in slabs and shear forces
and moments in vertical members. Forces due to T effects
are not commonly calculated and combined with other load
effects. Rather, designs rely on successful past practices
using compliant structural members and ductile connections
to accommodate differential settlement and volume change
movement while providing the needed resistance to gravity
and lateral loads. Expansion joints and construction closure
strips are used to limit volume change movements based on
the performance of similar structures. Shrinkage and temperature
reinforcement, which may exceed the required flexural
reinforcement, is commonly proportioned based on gross
concrete area rather than calculated force.
Where structural movements can lead to damage of
nonductile elements, calculation of the predicted force
should consider the inherent variability of the expected
movement and structural response.
A long-term study of the volume change behavior of
precast concrete buildings (Klein and Lindenberg 2009)
recommends procedures to account for connection stiffness,
thermal exposure, member softening due to creep, and other
factors that influence T forces.
Fintel et al. (1986) provides information on the magnitudes
of volume change effects in tall structures and recommends
procedures for including the forces resulting from
these effects in design.
== 5.3.7 If fluid load F is present, it shall be included in the
load combination equations of 5.3.1 in accordance with (a),
(b), (c), or (d):
(a) If F acts alone or adds to the effects of D, it shall be
included with a load factor of 1.4 in Eq. (5.3.1a).
(b) If F adds to the primary load, it shall be included with
a load factor of 1.2 in Eq. (5.3.1b) through (5.3.1e).
(c) If the effect of F is permanent and counteracts the
primary load, it shall be included with a load factor of 0.9
in Eq. (5.3.1g).
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== 5.3.7 Continuation
(d) If the effect of F is not permanent but, when present,
counteracts the primary load, F shall not be included in
Eq. (5.3.1a) through (5.3.1g).
== 5.3.8 If lateral earth pressure H is present, it shall be
included in the load combination equations of 5.3.1 in accordance
with (a), (b), or (c):
(a) If H acts alone or adds to the primary load effect, it
shall be included with a load factor of 1.6.
(b) If the effect of H is permanent and counteracts the
primary load effect, it shall be included with a load factor
of 0.9.
(c) If the effect of H is not permanent but, when present,
counteracts the primary load effect, H shall not be
included.
== 5.3.9 If a structure is in a flood zone, the flood loads and
the appropriate load factors and combinations of ASCE/SEI
7 shall be used.
== 5.3.10 If a structure is subjected to forces from atmospheric
ice loads, the ice loads and the appropriate load
factors and combinations of ASCE/SEI 7 shall be used.
== 5.3.11 Required strength U shall include internal load
effects due to reactions induced by prestressing with a load
factor of 1.0.
== 5.3.12 For post-tensioned anchorage zone design, a load
factor of 1.2 shall be applied to the maximum prestressing
reinforcement jacking force.
== 5.3.13 Load factors for the effects of prestressing used
with the strut-and-tie method shall be included in the load
combination equations of 5.3.1 in accordance with (a) or (b):
(a) A load factor of 1.2 shall be applied to the prestressing
effects where the prestressing effects increase the net force
in struts or ties.
(b) A load factor of 0.9 shall be applied to the prestressing
effects where the prestressing effects reduce the net force
in struts or ties.
== R5.3.8 The required load factors for lateral pressures from
soil, water in soil, and other materials, reflect their variability
and the possibility that the materials may be removed.
The commentary of ASCE/SEI 7 includes additional useful
discussion pertaining to load factors for H.
== R5.3.9 Areas subject to flooding are defined by flood
hazard maps, usually maintained by local governmental
jurisdictions.
== R5.3.10 Ice buildup on a structural member increases the
applied load and the projected area exposed to wind. ASCE/
SEI 7 provides maps of probable ice thicknesses due to
freezing rain, with concurrent 3-second gust speeds, for a
50-year return period.
== R5.3.11 For statically indeterminate structures, the
internal load effects due to reactions induced by prestressing
forces, sometimes referred to as secondary moments, can be
significant (Bondy 2003; Lin and Thornton 1972; Collins
and Mitchell 1997).
== R5.3.12 The load factor of 1.2 applied to the maximum
tendon jacking force results in a design load of about 113
percent of the specified prestressing reinforcement yield
strength, but not more than 96 percent of the nominal tensile
strength of the prestressing reinforcement. This compares
well with the maximum anchorage capacity, which is at least
95 percent of the nominal tensile strength of the prestressing
reinforcement.
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PART 2: LOADS & ANALYSIS 65
5 Loads
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