N D S 2018 3 3 Bending Members Flexure
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  4. N D S 2018 3 3 Bending Members Flexure

NDS 2018 Section 3.3 Bending Member - Flexure

The scope of this module is to calculate the sawn lumber beam bending capacity. The building code subsections required include:

  • NDS 2018 Section 3.2 Bending Members - General
  • NDS 2018 Section 3.3 Bending Members - Flexure
  • NDS 2018 Section 4 Sawn Lumber
  • NDS 2018 Supplement Table 4B and 4D Adjustment Factors and Reference Design Values

Assumptions and limitations

  • ASD method
  • Douglas Fir only
  • Rectangular section, not notched or tapered
  • Beam stability requirements in NDS 2018 Section 4.4.1.2 must be met
  • Beam sizes limited to 6”, 8”, 10”, 12”, 14”

Design Codes

  • AWC-NDS (2018), National Design Specification (NDS) for Wood Construction 2018 Edition. American Wood Council, Leesburg, VA.
  • AWC-NDS (2021), National Design Specification (NDS) Supplement: Design Values for Wood Construction 2018 Edition. American Wood Council, Leesburg, VA.

Inputs parameters

Design Geometry Input

  • b[in]: Rectangular dimension 1.
  • d[in]: Rectangular dimension 2.

Material Properties Input

Take Fb from Reference Design Values

Bending reference design value

  • \(F_b\) : Bending design value

Wood adjustment factors

Take these parameters from Adjustment Values

  • \(C_D \) : Load duration factor. Recommended value: 1 for D+L combination. Refer to (2018 NDS, 2018) - Section 2.3.2

  • \(C_M \) : Wet service factor. Recommended value: 1 for moisture content less than 19% for an extended time period. Refer to (2018 NDS, 2018) - Section 4.3.3

  • \(C_t \) : Temperature factor. Recommended value: 1 for not exposed to temperatures larger than 150 F. Refer to (2018 NDS, 2018) - Section 4.3.4

  • \(C_L \) : Beam Stability Factor. Assumed value:1 since the beam meets the requirements of NDS 2018 Section 4.4.1.2. Refer to (2018 NDS, 2018) - Section 3.3.3.8

  • \(C_F \) : Size factor. Recommended value: 1 to be conservative. Refer to (2018 NDS, 2018) - Section 4.3.6

  • \(C_fu \) : Flat-use factor. Assumed value:1. Refer to (2018 NDS, 2018) - Section 4.3.7

  • \(C_i \) : Incising factor. Recommended value: 1 for not incised. Refer to (2018 NDS, 2018) - Table 4.3.8

  • \(C_r \) : Repetitive member factor. Recommended value: 1 to be conservative. Refer to (2018 NDS, 2018) - Section 4.3.9

Calculations

Calculate the section modulus

S [in3] is calculated a:

  • \( S = \frac{b \cdot d^2}{6} \)

Adjusted bending capacity

Adjusted bending capacity \( f'_b [psi] \) is calculated as:

  • \( F'_{b} = F_b \cdot C_D \cdot C_M \cdot C_t \cdot C_L \cdot C_F \cdot C_fu \cdot C_i \cdot C_r \)

Bending Strength

The bending strength \( M_{n} [kip-ft] \) is calculated as:

  • \( M_{n} = F'_{b} \cdot S\)

Output Results

  • Bending Strength, [kip-ft]

References

2018 NDS. (2018). American Wood Council. https://awc.org/publications/2018-nds/

2018 NDS Supplement. (2021). American Wood Council. https://awc.org/publications/2018-nds-supplement/

Breyer, D. E., Cobeen, K. E., & Martin, Z. (2017). [Design of Wood Structures ASD/LRFD Eighth Edition]. https://shop.iccsafe.org/design-of-wood-structures-asd-lrfd-eighth-edition.html

Validation

The validation was performed using Example 6.18 from (Breyer et al., 2017).