Load Duration Factor

 Introduction
 

          Wood is a natural, fibrous non-homogenous material.  It is because of this that wood has several unique characteristics.  One of these is the effect of moisture content which was presented in an earlier module.  Another of these properties is duration of load.  This is the ability of wood to resist higher stresses when loads are applied for a short duration of time.  In other words, wood has a higher strength when a load is applied instantaneously than it does when the load is applied for a long period of time.  This relationship between time, strength, and load magnitude was observed as early the mid-sixteenth century.  The factors that we use  in the NDS today were developed in 1948 proceeding research done on load duration during World War II (REF).

        Duration of load is the total cumulative length of time that the full design load is applied.  For example, when assigning a duration of load factor (CD) for a snow load in allowable stress design (ASD) procedures, the CD factor is based on the total length of time that the design maximum snow load would be applied.  Tabulated design values in the NDS apply for “normal” cumulative load durations of approximately 10 years, and appropriate CD factors are applied by the designer for other cumulative load durations.
 

NDS:

        The NDS assumes that 10 years is the "normal" cumulative load duration for wood members and connections.  Therefore, a factor of CD = 1.0 applies for loads with a cumulative duration of ten years during the life of the structure.  CD = 1.0 is used for most storage loads and floor live load scenarios.  When load combinations include loads of shorter durations (e.g. - snow loads, wind loads, seismic loads), the load duration factor is selected based on the shortest duration load in the combination.

 

 Figure B1 of Appendix B (NDS)
  

LRFD

        In load and resistance factor design (LRFD) procedures, the load duration effect is addressed using a "time effect factor" (l).  The reference material properties tabulated for LRFD are based on "test" load durations of 5-20 minutes.  These properties are directly applicable for designs that resist short duration loads (e.g.- wind and seismic loads).   When LRFD load combinations are dominated by longer duration loads, material properties are adjusted downward ( l< 1.0) to account of the load duration effect in wood.

Demonstration

The following is an idealized demonstration of the load duration (or "time effect") factors and makes the following assumptions:

  1. All timber members being tested are cut from the same piece of lumber. This means that each member has the same properties.
  2. The load is applied uniformly across the members cross section. 
  3. The loads are applied at the same time to all eight members. 

The clocks below the members keep track of the total time the load has been applied.   Although the members are essentially the same, each can hold a different load for a different period of time.
 

 

 

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last modified: 18 September, 2000