Abstract:
This study aimed at explaining the procedure for deriving design stresses for timber
and providing engineers with some of the information which will help them in
designing timber structure. Timber is a very variable material affected by many
factors, this makes it important to determine mechanical properties by standard
methods using small clear specimens. As clear wood is not available for use, it is
important to apply all the necessary reduction factors to reach design stresses. Both
static bending and compression parallel to the grain tests were carried out according
to standard procedures. Static bending test was carried out according to
Sudanese standard no. 5175/20/2012 adopted form ISO 3133/1975 while
compression parallel to the grain was carried out according to ISO
standard procedure no. 3787/1979. The basic stresses for the two properties
were derived first by using two reduction factors to the mean ultimate stresses from
test results to cater for wood variability, safety and duration of load. The Factors
influencing strength were studied for grading the timber and assigning strength ratio
to each grade. Grade (or design) stresses were then calculated by multiplying basic
stress by the strength ratio for each grade. These Results revealed that the basic stress
for bending for gafal wood was 10.8 MPa and 13.6 MPa for compression parallel to
the grain. Grade (design) stresses in MPa were as follows:
Grade 1 Grade 2 Grade 3 Grade 4
For Bending 8.64 7.02 5.40 4.32
For compression 10.53 8.55 6.58 5.26
These results indicate that gafal wood with its low density and low strength values can
only be used for light constructions as columns and non-load bearing members in
wood frame buildings. It can also be used as raw material for many wood industries.
