Unlike solid-sawn lumber, the mechanical properties of some materials are greatly affected by ambient temperatures and solar-radiation heating, as depicted by the photograph. However, while the engineering design properties of solid-sawn lumber are not affected by ambient temperatures, they can be affected by some elevated and sustained temperatures when used in special applications. The purpose of this discussion is to review the temperature conditions that require special consideration and applicable adjustments to the published design values for lumber.
The National Design Specification for Wood Construction (NDS) is the building code referenced standard for wood construction in the US and it includes provisions for adjusting design stresses of lumber when used in different applications. A free view-only edition of NDS is available from the AWC website.
NDS-2015 Section 2.3.3 and Table 2.3.3 cover the subject of temperature effects on lumber [see PDF or View in Full Issue]. Review of the NDS table reveals the wood science of the issue, notably interesting is the fact that the in-service moisture content (MC) further reduces the tabulated design values in four of the six properties listed and used in structural designs.
The role of MC and temperature was made clear to me as a graduate student at Purdue University when I learned that timber columns in a “cooling tower” had collapsed. Being a cooling tower, the timber support columns were kept wet and hot, thus negatively impacting the design properties of a timber column.
Truss Applications
Regarding attics, Table 2.3.3 (viewed alone) might suggest typical attics require a Ct adjustment on design stresses; however, this is not the case for a typical attic as explained by Section 2.3.3:
2.3.3. Temperature Factor, Ct
Reference design values shall be multiplied by the temperature factors, Ct, in Table 2.3.3 for structural members that will experience sustained exposure to elevated temperatures up to 150ºF (see Appendix C).
The key word in Section 2.3.3 is “sustained.” As discussed in NDS Appendix C, wood has the ability to tolerate elevated temperatures up to 150°F on an intermittent basis. Specifically, from NDS Appendix C:
Up to 150°F, the immediate effect is reversible. The member will recover essentially all its strength when the temperature is reduced to normal. Prolonged heating to temperatures above 150°F can cause a permanent loss of strength.
Possible Exceptions Requiring Ct
Of course, some truss applications may not be typical with respect to temperature thus consideration by the design professionals is needed. One example might be the attic of a laundromat over a bank of clothes dryers. For this case, it may reasonable to assume that the heating is “sustained” above the adjustment threshold of 100°F in NDS Table 2.3.3.
The project Building Designer or General Contractor should have knowledge of the expected in-use environment of the trusses to be designed and installed. The truss applications that trigger the temperature factors, Ct less than 1.0 in NDS Table 2.3.3, are probably atypical; however, when the Ct factors are required, they are a substantial and a significant aspect of the truss design process.
Frank Woeste, P. E., is Professor Emeritus, Virginia Tech University and a wood construction consultant. He can be reached by e-mail: fwoeste@vt.edu.