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Load Duration Factor for Wood

Load Duration Factor for Wood

Load Duration Factor, or LDF, is based on wood’s ability to recover after a reasonable load has been applied for a given time. Wood is a stiff material but it is not completely rigid. Wood will flex under load, and once load has been removed, wood member will rebound or spring back to its original shape (if load was not excessive or applied for too long). LDF is relevant in regular service load cases expected to act on a structure and member in consideration must resist without failure. LDF does not address loads overstressing a member to breakage point.

Some loads are expected to act on a structure for short time periods, such as wind and seismic loads whose duration would normally be measurable in seconds. Other loads, such as snow, might last at least three months, depending on geography. Dead loads are permanent and they are expected to act on a structure for its life. Load Duration Factors allow us to increase wood’s load carrying capacity based on how long a load is expected to act on a structure—shorter time period, higher allowed increase.

Load Duration Factors are applied to member capacity or resistance, not to loads on member. Only those capacities or resistances related to wood’s ability to recover from a load are subject to LDF adjustments, i.e. moment and shear. Bearing capacity is not adjusted for LDF for IBC (International Building Code). Instantaneous deflection is also not affected by LDF because we are measuring how far a member flexes, not for how long. There is a deflection analysis type called Creep Analysis, it checks long term deflection beyond instantaneous deflections due to heavy loads acting on a member for long time periods. Our software does not analyze this deflection type because North American building codes do not require creep analysis. 

Load Duration Factors are tightly integrated into load combinations used to load a member, i.e. if a load combination has both dead and wind loads then load duration factor for this load combination will be higher value defined by wind. Even though a load combination may have more permanent loads, by default load combination will apply shortest acting load’s LDF. Our software will run through all load combinations and their LDFs as required by building codes and will pick one producing highest stresses as critical load combination for a particular design check.

IBC allows engineers to choose what load duration factor (CD) can be applied with a given load. Table below shows LDF default values and ranges our software allows.

Default LDF Range Loading
CD = 0.90 Dead load only
CD = 1.00 0.90 – 1.00 Floor and Heavy Snow Load
CD = 1.15 1.00 – 1.15 Snow Load
CD = 1.25 1.00 – 1.25 Roof Load, Construction Load
CD = 1.60 1.00 – 1.60 Wind Load, Seismic Load

Notice our software defaults to the highest possible factor (liberal, but correct most times). For example, a structure for an area with heavy snow loads lasting in place for half a year, it would be reasonable to reduce your snow LDF to 1.00. Always consult your local building officials when in doubt.