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American Wood Council

What is the difference between 2012 NDS 3.5 and 2012 IBC Table 1604.3 Footnote d for adjusting long-term deflection of wood members?


The provisions for calculating bending deflection in the National Design Specification (NDS®) for Wood Construction Section 3.5 increase the long-term deflection with a creep factor, Kcr.  The creep factor ranges from 1.5 - 2.0. These NDS design provisions relate specifically to estimating total deflection including the effects of long-term loading.

The deflection limit for the D+L 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 structural members that are dry at time of installation and used under dry conditions, the creep component of the long-term deflection can be estimated as the immediate dead load deflection resulting from 0.5D.  For wood structural members at all other moisture conditions, the creep component of the long-term deflection is permitted to be estimated as the immediate dead load deflection resulting from D. These assumptions are consistent with the creep component of long-term deflection in the NDS Section 3.5.

Deflection limits for the D+L load combination in IBC Table 1604.3, were taken from the legacy Uniform Building Code (UBC) deflection limits.  However, the intent of the UBC limits was not brought forward with the provisions.  The original intent of these provisions was to limit the total deflection based on the combination of live load deflection and the creep component of the dead load deflection.  As a result, there have been several prior code cycle modifications to these provisions to re-instate the original intent, such as the addition of footnote “g” for steel structural members which effectively excludes steel from checking for the creep component of dead load deflection.  As currently written and formatted, the D+L deflection provision can be misinterpreted to suggest that the total deflection due to dead load, D, including both the immediate and creep components of the dead load deflection, should be used with the deflection limit in this column. Additionally, use of 0.5D in footnote “d” is potentially non-conservative without clarification that the 0.5D load reduction approach is only applicable to calculating theD+L deflection for use with the deflection limits in IBC Table 1604.3.  As a result, AWC has submitted a change to Table 1604.3 footnote “d” to clarify these points and to make the 2015 IBC Table 1604.3 provisions consistent with the provisions in NDS 3.5.2 for long-term loading, with the stated intent in the UBC, and with similar provisions in ACI 318 as described in the ACI 318 Commentary.

 

 
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