Building Design and Construction Handbook - Merritt - Ventech!

WOOD CONSTRUCTION 10.13
duration of about 10 years. The full design load is one that stresses a member to
its allowable design value. When the cumulative duration of the full design load
differs from 10 years, design values, except F c� for compression perpendicular to
grain and modulus of elasticity E, should be multiplied by the appropriate load
duration factor C D listed in Table 10.5.
When loads of different duration are applied to a member, C D for the load of
shortest duration should be applied to the total load. In some cases, a larger-size
member may be required when one or more of the shorter-duration loads are omitted.
Design of the member should be based on the critical load combination. If the
permanent load is equal to or less than 90% of the total combined load, the normal
load duration will control the design. Both C D and the modification permitted in
design values for load combinations may be used in design.
The duration factor for impact does not apply to connections. Load duration
factors greater than 1.6 shall not apply to structural members pressure-treated with
fire retardants or with waterborne preservatives.
10.5.2 Wet Service Factor
Sawn-lumber design values apply to lumber that will be used under dry service
conditions; that is, where moisture content (MC) of the wood will be a maximum
of 19% of the oven-dry weight, regardless of MC at time of manufacture. When
the MC of sawn lumber or timbers in service will exceed 19% for an extended
period of time, design values published in the supplement to the ‘‘National Design
Specification� for Wood Construction’’ should be multiplied by the appropriate wet
service factor listed in Table 10.6. This reduction factor for timber does not apply
to southern pine.
MC of 19% of less is generally maintained in covered structures or in members
protected from the weather, including windborne moisture. Wall and floor framing
and attached sheathing are usually considered to be such dry applications. These
dry conditions are generally associated with an average relative humidity of 80%
or less. Framing and sheathing in properly ventilated roof systems are assumed to
meet MC criteria for dry conditions of use, even though they are exposed periodically
to relative humidities exceeding 80%.
Glulam-timber design values apply when the MC in service is less than 16%,
as in most covered structures. When MC of glulam timber under service conditions
is 16% or more, design values should be multiplied by the appropriate wet service
factor C M in Table 10.6.
TABLE 10.5 Frequently Used Load Duration
Factors C D
Load duration C D Typical design loads
Permanent 0.9 Dead load
10 years 1.0 Occupancy live load
2 months 1.15 Snow load
7 days 1.25 Construction load
10 minutes 1.6 Wind or seismic load
Impact 2.0 Impact load