Understanding Floor Joists
Floor joists are horizontal members that support the floor deck and transfer loads to the foundation.
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Load Types
Dead loads (permanent) and live loads (occupancy) must be considered in joist design.
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Spacing
Common spacings are 16" and 24" on center, affecting required joist size.
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Deflection Limits
Typically L/360 for floor joists with finished ceilings below.
Technical Specifications
Wood Properties
- SPF: Fb = 875 psi
- DF: Fb = 1000 psi
- HF: Fb = 850 psi
Typical Loads
- Residential: 40 psf live
- Office: 50 psf live
- Dead load: 10-20 psf
Design Formulas
Bending Moment
M = (w × L²)/8
Where:
- M = Maximum moment
- w = Total load per foot
- L = Span length
Deflection
Δ = (5 × w × L⁴)/(384 × E × I)
Where:
- Δ = Maximum deflection
- E = Modulus of elasticity
- I = Moment of inertia