Understand Creep Deflection Calculation in Structural Design | Learn about creep deflection and its impact on structural elements. Discover how to account for creep deflection in your design calculations to ensure long-term stability and performance.
Wood is a viscoelastic material, exhibiting both viscous and elastic characteristics when undergoing deformation. As a result, it suffers from creep behavior under long-term loading. Creep is the permanent sag caused by the stretching and adjusting of the wood fibers to the long-term loads on them, such as the weight of the member itself, the deck it supports, and other building components. It is a time-dependent deformation from constant stress that develops over years, and accelerates with frequent and drastic temperature and moisture content changes. Creep is an important factor to consider in the design and durability of timber structures.
Structural components must have enough stiffness to limit the deflections indicated in the IBC. For instance, members supporting brittle finishes (e.g. plaster walls) are limited to a lower allowable deflection, while members with flexible finishes are allowed a much higher allowable deflection limit. For D+L deflection, the incremental deflection is calculated and compared to the deflection limits. This includes the effects of creep but excludes instantaneous dead-load deflection, which takes place during construction and before deflection-sensitive elements are installed (e.g. glazing, masonry walls, marble flooring). For this reason, members subjected to D+L loads are allowed a larger deflection limit than those subjected to S,W,L, or Lr loads.
The deflection limit for the D+(L+Lr) load combination only applies to the deflection due to the creep component of long-term dead load deflection plus the short-term live load deflection. For wood used under dry conditions, creep deflection is estimated as half of the immediate dead load deflection. When used at all other moisture conditions, it is estimated to be the same as the immediate dead load deflection.
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The total deflection of a member is calculated as the sum of the deflection due to short term load and the immediate deflection due to long-term load, where the latter is multiplied by a time dependent factor. As mentioned above, creep is a time-dependent deformation that accelerates with frequent and drastic temperature and moisture content changes. As such, the immediate deflection due to long-term load is increased by half for dry wood, and doubled for glue laminated timber and wet wood. Since deflection limits are typically set by the IBC, which considers only the creep component of deflection, the NDS formula results in the same deflection calculation as the IBC.
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