Monday, July 11, 2011

Good Lumber: How "good" are you as a structural beam?

Many low cost houses used wood as structural beams for joists in floor framing systems and for purlins in roof truss systems. When you design a beam, you must know the allowable strength properties for shear and flexure and the moduus of elasticity to check whether safety and serviceability requirements are satisfied. But when you buy wood in lumberyards in the Philippines, the wood specie is not usually known. The lumberyards simply label wood used as structural members as "good lumber" and obviously the strength properties are unknown. To address this problem of a civil engineer, the following research was conducted by DLSU undergraduate students. Here is the abstract of the thesis. A paper will be presented soon in conferences in the Philippines.

Mechanical Properties on Flexure and Shear of Commercially Available Timber Beams in the Philippines
Earl Marvin B. De Guzman, Michael Stephen C. Go, Katrina C. Tengki

Commercially available wood used as structural members are commonly referred to as “good lumber.” Good lumber consists mostly of imported lumber, and those of lesser known or unknown local species. With the wood species not clearly specified, there is a need to determine the mechanical properties of good lumber.

This study aims to determine the variation of strength properties of timber beams classified as good lumber. Standard laboratory tests were conducted to determine the range of values of the properties of good lumber such as moisture content, specific gravity, modulus of elasticity and the bending and shear strength of timber beams. The mechanical properties of good lumber were obtained through a series of laboratory tests that simulate the conditions for the loading schemes specified in the ASTM manual, on beams of nominal cross section 2”x4”. The laboratory tests results showed that good lumber has a moisture content ranging from 12-82% with 25% as average value, a relative density ranging from 0.236-0.743, and an average modulus of elasticity of 8.15GPa. The modulus of rupture ranged from 5.7 to 63.5MPa, while the shear stress at failure ranged from 0.60 to 4.15MPa. For structural design purposes, a reduction factor of 2.1 based on ASTM Standards was applied and the allowable flexural strength obtained falls within the range 11.0-18.0MPa, while the allowable shear strength obtained is within 0.77 to 1.34MPa. Comparing the strength properties to the timber species in the NSCP 2001/2010, it was found that good lumber in general would have values within the ranges for medium and moderately low strength group for 63% stress grades. With the information on the variation of strengths of good lumber obtained from the study, structural designers would be guided on appropriate allowable stresses to be used in the design of structural members made of wood such as purlins and joists.
The student researchers, Go, Tengki & De Guzman received certificates for being a finalist for the 2011 Gold Thesis Award for the Structural Engineering Division

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