TB-102 - Information About Duration of Load

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TECHNICAL BULLETIN
TB-102
November 2014 (Expires 11/2016)
Information About Duration of Load
Introduction
The contents of this technical bulletin consist principally of text quoted from Appendix B of the National Design Specification for
Wood Construction®, ANSI/AF&PA, NDS-2012, October 2012, American Forest & Paper Association.
Adjustment of Reference Design Values for Load Duration (NDS Section B.1)
Load duration reflects the unique ability of wood to support higher stresses for short periods of time, as well as lower stresses for
extended periods of time. To account for this phenomenon, applicable mechanical properties of wood are adjusted using a load
duration factor consistent with the intended application.
Normal Load Duration (NDS Section B.1.1)
Normal load duration contemplates fully stressing a member to its allowable design value by the application of the full normal
design load for a cumulative duration of approximately 10 years, and/or the application of 90% of the full design load continuously
throughout the remainder of the life of the structure, without encroaching on the factor of safety. No adjustment of design values is
required for normal load duration.
Other Load Durations (NDS Section B.1.2)
Tabulated design values for normal load duration shall be multiplied by load duration factors, CD, for other load durations. Load
duration factors do not apply to modulus of elasticity design values, E, nor to compression design values perpendicular-to-grain, Fc⊥,
based on a deformation limit.
• When the member is fully stressed to the allowable design value by application of the full design load permanently or for a
cumulative total of more than 10 years, tabulated design values for normal load duration (except E and Fc⊥, based on a
deformation limit) shall be multiplied by the load duration factor, CD = 0.90.
• Likewise, when the duration of the full design load does not exceed the following durations, tabulated design values for
normal load duration (except E and Fc⊥, based on a deformation limit) shall be multiplied by the following load duration
factors:
Load Duration
2 Months
7 Days
10 Minutes
Impact
CD
1.15
1.25
1.6
2.0
• The 2-month load duration factor, CD = 1.15, is applicable to design snow loads based on ASCE 7. Other load duration
factors shall be permitted where such adjustments are referenced to the duration of the design snow load in the specific
location being considered.
• The 10-minute load duration factor, CD = 1.6, is applicable to design earthquake loads and design wind loads based on
ASCE 7.
• The impact load duration factor, CD = 2.0, shall not apply when the member has been pressure-treated with waterborne
preservatives to the heavy retentions required for "marine" use, nor when the member has been pressure-treated with fireretardant chemicals. The impact load duration factor shall not apply to connection design values. For questions regarding
applications that may require the use of preservative treated Trus Joist® structural composite lumber products, which
treated product is best for a particular application, or a list of treatments that are compatible with Trus Joist® products,
please consult your Weyerhaeuser representative or refer to the Preservative Treatment Guide (Reorder TJ-1020).
Combination of Loads of Different Durations (NDS Section B.2)
When loads of different durations are applied simultaneously to members that have full lateral support to prevent buckling, the
design of structural members and connections shall be based on the critical load combination determined from the following
procedures:
*
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TECHNICAL BULLETIN
TB-102
November 2014 (Expires 11/2016)
• Determine the magnitude of each load that will occur on a structural member and accumulate subtotals of combinations of
these loads. Design loads that are established by applicable building codes and standards may include load combination
factors to adjust for probability of simultaneous occurrence of various loads (see section B.4). Such load combination factors
should be included in the load combination subtotals.
• Divide each subtotal by the load duration factor, CD, for the shortest duration load in the combination of loads under
consideration.
Shortest Duration of
Load in the Combination
of Loads
Permanent
Normal
Two Months
Seven Days
Ten Minutes
Impact
Load Duration Factor,
CD
0.9
1.0
1.15
1.25
1.6
2.0
• The largest value thus obtained indicates the critical load combination to be used in designing the structural member or
connection.
Example: Determine the critical load combination for a structural member subjected to the following loads:
Load Description
Dead Load, DL
Live Load, LL
Snow Load, SL
Wind Load, WL
Method to Determine Loads
Established by summing the weights of all the permanent structural components
Established by applicable building code or design standard
Established applicable building code or design standard
Established applicable building code or design standard Table Data
The actual stress due to any combination of the above loads shall be less than or equal to the allowable design value
modified by the load duration factor, CD, for the shortest duration load in that combination of loads:
Actual Stress Due to
Load Combination:
D
Governing Load Duration Factor,
CD
≤ (0.9) x (Design Value)
≤ (1.0) x (Design Value)
D+L
D+W
≤ (1.6) x (Design Value)
D+L+S
≤ (1.15) x (Design Value)
D+L+W
≤ (1.6) x (Design Value)
D+S+W
≤ (1.6) x (Design Value)
D+L+S+W
≤ (1.6) x (Design Value)
The equations above may be specified by the applicable building code and shall be checked as required. Load combination
factors specified by the applicable building code or standard should be included in the above equations.
All Weyerhaeuser Structural Frame Software handles load duration analysis in this matter.
Mechanical Connections (NDS Section B.3)
Load duration factors, CD, less than or equal to 1.6 apply to design values for connections, except when connection capacity is
based on design of metal parts.
Load Combination Reduction Factors (NDS Section B.4)
Reductions in total design load for certain combinations of loads account for the reduced probability of simultaneous occurrence of
the various design loads. Load duration factors, CD, account for the relationship between wood strength and time under load. Load
duration factors, CD, are independent of load combination reduction factors, and both may be used in design calculations.
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