Lumber has ability to handle higher stresses under shorter periods of time. This characteristic is known as Load Duration Factor. Load Duration Factor typically ranges from 0.9 to 1.6 and is based on the amount of cumulative time the load is applied to the structure. Per the National Design Specification® (NDS®) for Wood Construction, normal load duration represents a load that fully stresses a member to its allowable design value by the application of the full design load for a cumulative duration of approximately ten years. Since wood has the property of carrying substantially greater maximum loads for short durations than for long durations of loading, reference design values for normal load duration shall be multiplied by load duration factor, CD, for other durations of load (except modulus of elasticity design values, E, and compression design values perpendicular to grain, FCꓕ). [For all images, See PDF or View in Full Issue.]
Dead loads are permanent, and they are expected to act on a structure for the life of that structure. Under dead loads only the allowable stress of the wood must be reduced by 10 percent.
The ten years load duration factor is typically used for the floor application. It is relative to “normal” loading, CD = 1.0, there is neither an increase nor decrease in the allowable stress.
The two months load duration, CD = 1.15, is applicable to design snow loads based on Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE/SEI 7-22). Wood can safely resist a 15-percent increase in its maximum allowable load as long as the total accumulated time under this load does not exceed two months.
The seven days load duration, CD = 1.25, is applicable to the typical roof live load (not snow): construction workers on the roof, walking on the roof during repairs, etc.
The ten minutes load duration factor, CD = 1.6, is applicable to design earthquake loads and design wind loads based on ASCE 7. This means wood can safely handle a 60-percent increase in its maximum allowable load if the total accumulated time under this load does not exceed 10 minutes.
Why is it important to understand? A designer who understands this concept will be able to make a better choice during the design process, for example, not using a CD = 1.25 (DOL – duration of load) for roof truss design in areas of heavy snow load or using DOL (CD) = 1.15 for roof trusses that will be used as a deck.
After truss loading conditions are selected, the software will run through all the load combinations as required by the building code and will automatically apply the load duration factor associated with the shortest duration load in each combination of loads (per NDS 2018 Edition, Section 2.3.2.2). For example, in load combination dead load (CD = 0.9) + snow load (CD = 1.15) + wind load (CD = 1.6), the actual stress due to combination of all these three loads can be adjusted by the load duration factor CD = 1.6.
Per NDS, load duration factor greater than 1.6 shall not apply to structural members pressure- treated with water-borne preservatives, or fire-retardant chemicals. The impact load duration factor shall not be used in the design of connections or wood structural panels.
For additional information, or if you have questions, please contact the MiTek Engineering department.