Chapter 5Permissible Stresses

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 630

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5.1 Fundamental stress values of different groups of timber are determined on small clear specimen according to accepted standards [6-3A(2)]. These values are then divided by the appropriate factors of safety to obtain the permissible stresses. Values of these factors of safety for different strength characteristics are as per the relevant table of the accepted standard [6-3A(5)].

5.2 The permissible stresses for Groups A, B and C for different locations applicable to Grade I structural timber shall be as given in Table 9 provided that the following conditions are satisfied:

a) The timbers should be of high or moderate durability and be given the suitable treatment where necessary;

b) Timber of low durability shall be used after proper preservative treatment to good practice [6-3A(6)]; and

c) The loads should be continuous and permanent and not of impact type.

5.3 The permissible stresses (excepting $E$ ) given in Table 9 shall be multiplied by the following factors to obtain the permissible stresses for other grades provided that the conditions laid down in 5.2 are satisfied:

a) For select grade timber : 1.16

b) For Grade II timber : 0.84

Table 9Minimum Permissible Stress Limits (in N/mm²) in Three Groups of Structural Timbers (For Grade I Material) (Clauses 5.2 and 5.3)

Sl No.(1)	Strength Character(2)	Location of Use(3)	Group A(4)	Group B(5)	Group C(6)
i)	Bending and tension along grain	Inside 1)	18.0	12.0	8.5
ii)	Shear 2)
	Horizontal	All Locations	1.05	0.64	0.49
	Along grain	All Locations	1.5	0.91	0.70
iii)	Compression parallel to grain	Inside 1)	11.7	7.8	4.9
iv)	Compression perpendicular to grain	Inside 1)	4.0	2.5	1.1
v)	Modulus of elasticity (×103N/mm2)	All locations and grade	12.6	9.8	5.6

PART 6 STRUCTURAL DESIGN — SECTION 3 TIMBER AND BAMBOO: 3A TIMBER

5.3.1 When low durability timbers are to be used [see 5.2(b)] on outside locations, the permissible stresses for all grades of timber, arrived at by 5.2 and 5.3 shall be multiplied by 0.80.

5.4 Modification Factors for Permissible Stresses

5.4.1 Due to Change in Slope of Grain

When the timber has not been graded and has major defects like slope of grain, knots and checks or shakes but not beyond permissible value, the permissible stress given in Table 1 shall be multiplied by modification factor $K_{1}$ for different slopes of grain as given in Table 10.

Table 10Modifications Factor K_{1} to Allow for Change in Slope of Grain (Clause 5.4.1)

Sl No.	Slope	Modification Factor ( $K_1$ )
		Strength of Beams, Joists and Ties (3)	Strength of Posts or Columns (4)
i)	1 in 10	0.80	0.74
ii)

NOTE — For intermediary slopes of grains, values of modification factor may be obtained by interpolation.

5.4.2 Due to Duration of Load

For different durations of design load, the permissible stresses given in Table 1 shall be multiplied by the modification factor $K_{2}$ given in Table 11.

NOTE — The strength properties of timber under load are time-dependent.

Table 11Modification Factor ( K_{2} ), for Change in Duration of Loading (Clause 5.4.2)

Sl No.(1)	Duration of Loading(2)	Modification Factor ( $K_2$ )(3)
i)	Continuous (Normal)	1.0
ii)	Two months	1.15
iii)	Seven days	1.25
iv)	Wind and earthquake	1.33
v)	Instantaneous or impact	2.00

5.4.2.1 The factor $K_{2}$ is applicable to modulus of elasticity when used to design timber columns, otherwise they do not apply thereto.

5.4.2.2 If there are several duration of loads (in addition to the continuous) to be considered, the modification factor shall be based on the shortest duration load in the combination, that is, the one yielding the largest

increase in the permissible stresses, provided the designed section is found adequate for a combination of other larger duration loads.

NOTE — In any structural timber design for dead loads, snow loads and wind or earthquake loads, members may be designed on the basis of total of stresses due to dead, snow and wind loads using modification factor $K_{2}=1.33$ for the permissible stresses (see Table 1) to accommodate the wind load, that is, the shortest of duration and giving the largest increase in the permissible stresses. The section thus found is checked to meet the requirements based on dead loads alone with modification factor $K_{2}=1.00$ .

5.4.2.3 Modification factor $K_{2}$ shall also be applied to allowable loads for mechanical fasteners in design of joints, when the wood and not the strength of metal determines the load capacity.