Timber Design

Basic Formulas

Bending Stress
fb=McI
 

Horizontal Shear Stress
fv=VQIb
 

Formula for Spacing of Bolts and Nails
s=RIVQ=Rq

 

Notching of Beams Formulas

Rectangular beams notched at points of support on the tension side:
Fv=3V2bd(dd)
 

Beams notched at points of support on the tension side
Fv=3V2An(ddn)
 

Beams notched at point of support on the compression side
V=23Fvb[de(dd)d]

 

Allowable Bending Stress

Slenderness Factor
Cs=Ledb2
 

Short Unbraced Beam (Cs ≤ 10)
Fb=Fb
 

Intermediate Unbraced Column (10 < CsCk)
Fb=Fb[113(CsCk)4]
 

Ck=0.811EFb
 

Long Unbraced Beam For (Ck < Cs < 50)
Fb=0.438ECs2

 

Allowable Compression at an Angle to Grain

Jacoby's Formula
Pn=Psin2ϕ+Qcos2ϕ
 

Hankinson's Formula
Pn=PQPsin2θ+Qcos2θ

 

Allowable Compressive Stress for Columns

Short column (Le/d ≤ 11)
Fc=Fc
 

Intermediate column (11 < Le/d < K)
Fc=Fc[113(Le/dK)4]
 

K=0.671EFc
 

Long column (Le/dK)
Fc=0.30E(Le/d)2

 

Combined Flexure and Axial Stress

Flexure and Axial Tension
ftFt+fbFb1.0
 

fbftFb1.0
 

Flexure and Axial Compression
fcFc+fbFbJfc1.0
 

J=Le/d11K11
 

K=0.671EFc

 

Wind Load for the Design of Purlins

Wind pressure perpendicular to vertical surface
P=0.0000473v2
 

Wind pressure normal to the roof (Duchemins Formula)
Pn=2Psinθ1+sin2θ