Buckling Stiffness Factor Explained:
Used primarily in Sawn Lumber Design the buckling stiffness factor is used to recognize the contribution of plywood sheathing to the buckling resistance of compression truss chords (in the plane of chord depth) when subject to the following conditions:
- The truss chord is 2"x4" or smaller
- Combined flexure and axial compression.
- Dry service condition
- Attached to a 3/8" or thicker plywood sheathing nailed to the narrow face according to code required roof sheathing fastener schedules [1].
Note: If all the previous conditions are not met, then the buckling stiffness factor can be taken as unity (1.0).
Buckling Stiffness Factors (CT):
1) if le ≤ 96 inches:
C_T = {1+ {K_M*l_e\over K_T*E}}
where:
le = Effective Column Length? = Ke*l
KM = 2300 (wood seasoned to 19% moisture content or less at the time of plywood attachment)
or 1200 (unseasoned or partially seasoned wood at time of plywood attachment)
KT = 1 - 1.645(COVE) = 0.59 (visually graded lumber), 0.75 (machine evaluated lumber),
0.82 (products with COVE ≤ 0.11)
COVE = Wood Coefficients of Variation? in the Modulus of Elasticity.
E = Modulus of Elasticity (see table 4a in NDS Specifications [1])
KM = 2300 (wood seasoned to 19% moisture content or less at the time of plywood attachment)
or 1200 (unseasoned or partially seasoned wood at time of plywood attachment)
KT = 1 - 1.645(COVE) = 0.59 (visually graded lumber), 0.75 (machine evaluated lumber),
0.82 (products with COVE ≤ 0.11)
COVE = Wood Coefficients of Variation? in the Modulus of Elasticity.
E = Modulus of Elasticity (see table 4a in NDS Specifications [1])
2) if le > 96 inches:
If le > 96", CT should be calculated assuming le = 96" and use the same procedure described above.
Buckling Stiffness Factor Notes:
- This factor should not be applied to deflection calculations.[2]
References:
- American Forest and Paper Association, "National Design Specification for Wood Construction", 2005
- Breyer, D., Fridley, K., Pollock, D., & Cobeen, K., "Design of Wood Structures - ASD", 2003