Design methods and basis for timber structures

Design methods for timber structures

1. Working stress (permissible stress) method (IS 883:1994, used in Nepal and India): the stresses produced by service loads must not exceed the safe permissible stresses of the species and grade. It is simple and is the method used in this course.
2. Limit state method: partial safety factors on loads and material strength, with checks for strength and serviceability, as used in Eurocode 5 and other modern codes; it is more rational, but IS 883 does not use it.
3. Load factor / ultimate strength approach and tests on prototypes (IS 883 allows design by calculation or by prototype testing).

Design basis (IS 883:1994)

• Permissible stresses from Table 1 for the species (Grade I values), modified as follows: Select grade $\times1.16$, Grade II $\times0.84$; low-durability species used outside $\times0.8$; slope of grain ($K_1$), duration of load ($K_2$, e.g. 1.33 for wind), form factors ($K_3$ to $K_6$), bearing length ($K_7$).
• Loads from IS 875 (dead, imposed, wind, snow); wind and seismic loads are not combined.
• Beams: check bending, horizontal shear, bearing and deflection (limits span/360 with brittle finishes and span/240 otherwise, with the dead load doubled for initially dry timber); minimum width 50 mm or span/50; depth not more than 3 times the width unless laterally stiffened.
• Columns: classified as short, intermediate or long by the slenderness $S/d$; $S/d \le 50$ for solid columns; combined bending and axial compression is checked by $f_{ac}/f_c + f_{ab}/f_b \le 1$.
• Tension members: net section; notches may not remove more than one quarter of the section.
• Moisture content, grading and preservation as per IS 287, IS 3629 and IS 401.