Stability check using the example from previous page ~
Timber of strength classification C22 (see page 115):
Modulus of elasticity, 6500 N/mm^2 minimum.
Grade stress in compression parallel to the grain, 7.5 N/mm^2.
Grade stress ratio = 6500...7.5 = 867
The grade stress and slenderness ratios are used to provide a
modification factor (K 12 ) for the compression parallel to the grain.
The following table shows some factors adapted from BS 5268-2:
By interpolation, a grade stress of 867 and a slenderness ratio of
20 indicates that 7.5 N/mm^2 is multiplied by 0.57.
Applied stress should be7.5¾0.57 = 4.275 N/mm^2.
Applied stress = axial load...prop section area
= 19650 N...(300¾225 mm) = 0.291 N/mm^2
0.291 N/mm^2 is well within the allowable stress of 4.275 N/mm^2 ,
therefore 300¾225 mm props are satisfactory.
Ref. BS 5268-2: Structural use of timber. Code of practice for
permissible stress design, materials and workmanship.
Effective length/breadth of section (slenderness ratio)
3 6 12 18 24 30 36 42 48
Grade
stress
ratio
400 0.95 0.90 0.74 0.51 0.34 0.23 0.17 0.11 0.10
600 0.95 0.90 0.77 0.58 0.41 0.29 0.21 0.16 0.13
800 0.95 0.90 0.78 0.63 0.48 0.36 0.26 0.21 0.16
1000 0.95 0.90 0.79 0.66 0.52 0.41 0.30 0.24 0.19
1200 0.95 0.90 0.80 0.68 0.56 0.44 0.34 0.27 0.22
1400 0.95 0.90 0.80 0.69 0.58 0.47 0.37 0.30 0.24
1600 0.95 0.90 0.81 0.70 0.60 0.49 0.40 0.32 0.27
1800 0.95 0.90 0.81 0.71 0.61 0.51 0.42 0.34 0.29
2000 0.95 0.90 0.81 0.71 0.62 0.52 0.44 0.36 0.31Design of Temporary Vertical Supports and Struts