Detailed Member Designs

1. Scope

This section documents how the editor evaluates the strength and stability capacities of individual members (beams, joists, studs, posts/columns) for residential and light-commercial buildings. It covers bending, shear, axial compression/tension, and deflection checks for sawn lumber, LVL, glulam, and other timber products. Steel and concrete member designs are handled in separate modules.  

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The focus here is on ASD capacity checks per the National Design Specification (NDS) for Wood Construction, with adjustment factors applied for load duration, moisture, size, stability, and system effects.

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Governing Codes & References

• ‍NDS 2018 – National Design Specification for Wood Construction (AWC).  ‍
• NDS Supplement (2018) – Design Values for Wood Construction, Tables 4A/4B for species and grade properties.  

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2. Design Assumptions

• ‍Member Isolation – Each beam/column is designed independently; no redistribution of forces is assumed.  ‍
• Line Element Idealization – Members are treated as prismatic, homogeneous elements with cross-section properties per actual dressed lumber dimensions.  ‍
• Load Application – Loads applied at centroid; torsion ignored unless eccentricity exceeds tolerance.  ‍
• Support Conditions – Beams assumed simply supported unless modeled otherwise; columns/posts assumed pinned at both ends unless user specifies fixity.  ‍
• Buckling Restraint – Studs/posts embedded in sheathing panels are considered laterally braced in the weak axis unless the user disables.  ‍
• Serviceability Default - ‍
  
  • Bending deflection: floors $$L/360$$ , roofs $$L/240$$ (or $$L/36$$ with brittle finishes).
  • Axial compression: slenderness limit  $$\frac{L_e}{d} \leq 50 \quad \text{(per NDS §3.7)}$$‍
• Adjustment Factors – All applicable NDS factors are applied: load duration ($C_D$), moisture ($C_M$), temperature ($C_t$), size ($C_F$), repetitive member ($C_r$), stability ($C_P, C_L, C_T$), etc.  

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3. Design Methodology

Material Properties

Base design values $$(F_b, F_t, F_v, F_c, F_{c\perp}, E, E_{min}$$ are drawn from the NDS Supplement tables for the selected species/grade.  

Adjusted design values are computed as:

$$F' = F \cdot C_D \cdot C_M \cdot C_t \cdot C_F \cdot \ldots$$

The software maintains a transparent breakdown of each factor applied.

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Beam/Joist (Bending & Shear)

‍Bending Capacity

$$M_n = F'_b \cdot S$$

where $$$$ = section modulus.

Shear Capacity

$$V_n = F'_v \cdot \frac{A}{3}$$

where $$$$ = cross-section area.  

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Deflection Check (uniform load, simple span)

$$\Delta = \frac{5wL^4}{384EI}$$

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Column/Post (Compression & Buckling)

Axial Capacity

$$P_c = F'_c \cdot A \cdot C_P$$

where $$C_$$ = column stability factor per NDS §3.7.1.  

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Perpendicular Bearing

$$P_{c\perp} = F'_{c\perp} \cdot A_b$$

where $$A_$$ = bearing area.  

Effective slenderness ratio is computed with end-fixity assumptions:

$$\frac{L_e}{d}$$

Overall column capacity is taken as:

$$P = \min(P_c, \; P_{c\perp})$$

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Tension Members

Axial Tension Capacity

$$P_t = F'_t \cdot A$$

Stability factors are not applied unless compression reversal is expected.

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Built-Up Members

Multiple plies (e.g., 2-ply 2×4 post) are modeled as acting together with full composite action unless the user disables.  

Net area reduction for fastener holes is not applied unless user inputs connection detailing.  

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