Thursday, July 21, 2011

How strong is "good lumber" in compression?

Compression Failure of a short timber column

To check the safety and serviceability of wood used as columns, posts and truss members, the compressive strength of the wooden elements must be known. "Good lumber" which is available in the Philippines are usually used but their properties ae unknown. Hence, this research was conducted.

Mechanical Properties in Compression of  Commercially Available Lumber in Metro Manila
Liang Ta Chen, Francis F. Ebanos and Mark Justin K. Kung

Lumber in particular vary in strength depending on different parameters such as specie, dry density, slope of grain and others. Stress grading is used to be able to use lumber’s mechanical properties. However, many lumberyards do not have stress grading for the lumber they sell. In the survey conducted by the group on lumberyards in Metro Manila, Philippines, 81% of these lumberyards have little awareness or knowledge on what kind of wood they are selling wherein they do not know the specie of the lumber they were selling or the specie of their lumber was known but were mixed and could not be identified. The common species of these so-called “good lumber” were tangile, lauan, miranthe and saba while its source are almost half imported and half local.

This research aims to determine the mechanical properties in compression of commercially available lumber used as a structural member commonly referred to as "good lumber" in Metro Manila, Philippines. Compression members are usually used as post or truss members in roof trusses. 

Lumber that were tested had varying properties. The moisture content value of good lumber ranged from 11% to 62 % while the density ranged from 319 to 729 kg/m3. The compressive strength and modulus of elasticity of “good lumber” had high coefficient of variance which means it is very distributed. Compressive strength ranged from 3.996 to 51.386 MPa while modulus of elasticity ranged from 816.4 to 7921.3 MPa. The allowable compressive strength when ASTM D6570 is imposed is 7.372 MPa which is similar to the specie Malugai of the medium strength group with 63 % stress grade in the NSCP having an allowable compressive strength of 7.35 Mpa. On the other hand, the  modulus of elasticity when ASTM D6570 is imposed is 3510 MPa which is similar to the specie Bayok of the moderately low strength group with 63 % stress grade in the NSCP having an allowable modulus of elasticity of 3740 MPa. Tests of long columns also showed that Equation from Section 617 of NSCP holds true and showed that the values computed are much smaller than the actual failure stresses. The mechanical properties of  compression members obtained from this study may guide structural designers in their design computations.
Buckling of a long timber column

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