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3.3.2 - Flexural Design Equations
The calculation of actual bending stress for sawn lumber, glulam and SCL members is given in 3.3.2.2 as
fb = 6M / bd2
where M is the bending moment at the member cross section. The factored bending stress is compared with the factored allowable bending stress.
Refer to NDS 7 - Wood I-Joists for I-Joist bending design and PRG 320 Table A-2 and O86 8.4.3.1 - Bending Moment Resistance for CLT design.
3.3.3-Beam Stability Factor, CL
Refer to Input of Lateral Support for the input of lateral support in beam and column modes, to Lateral Support at Points of Bearing for allowable lateral support points on the member.
3.3.3.2 and 3.3.3.3: In Sizer, CL = 1.0 when the compression edge of a beam, joist, column or wall stud is fully restrained. It is the responsibility of the user to satisfy the lateral support requirements of clause 5.4.1 whenever Sizer assumes or applies a fully restrained condition.
3.3.3.4: Refer to Unsupported Length lu for an explanation of how the unsupported length lu in 3.3.3.4 is determined.
3.3.3.5: The effective length, le, is conservatively calculated for cantilevers under all loading conditions as:
le= 1.87 lu when lu /d < 7 (d = member depth)
le= 1.44 lu + 3d when lu /d >= 7
For all other members the footnotes of Table 3.3.3 of the NDS apply:
le= 2.06 lu when lu /d < 7
le= 1.63 lu + 3d when 7 <= lu/d < 14.3
le= 1.84 lu when lu /d > 14.3
3.3.3.6: Slenderness Ratio RB : The slenderness ratio is used to compute the lateral stability factor CL in NDS 3.3.3.8, and itself is limited to no more than 50 in 3.3.3.7. If this value is exceeded, a message appears on the screen and moment design is not calculated, showing instead a failure for slenderness ratio.
For multi-ply members, the NDS does not offer guidance on whether to use the full member width or a single ply width, so there is an option in the Design Settings for either to be selected, depending on your judgment of how well the individual members are connected.
Unpublished research has shown that nailed and bolted beams have at most 30% composite action effect in terms of resisting torsional buckling, so it is recommended to use single play width unless there is certainty that the individual members are fastened together such that they act as a unit.
3.3.3.8 Beam Stability factor CL :Where CV > 1, as can occur for SCL materials,, Sizer adjusts Fb* in the term FbE/Fb* by CV prior to calculating CL. Refer to www.awc.org/wood-design/2006/09/application-of-volume-factor-cv-for.html.