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

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

Saturday, July 9, 2011

An Optimum Mix Design of High Strength Concrete using Genetic Algorithms

High-strength concrete (HSC) is a highly complex and evolving construction material. Careful selection of constituent materials must be employed to successfully proportion HSC mixtures. A guide for proportioning HSC by the American Concrete Institute (ACI) is available but the guide provides only a general idea of the proportions of the various components for HSC production. Presently, batching companies produce different trial mixes using the ACI guide and some trial and error considering the observed effect of each constituent material to the strength development of HSC in order to attain a target concrete strength. This method, however, requires plenty of mix design experimentation that is costly and time consuming. Through the years, trial mixes of HSC of various strengths have been compiled by batching companies. These trial mix data may be useful in deriving optimum mix designs of HSC.

This study explored the use of genetic algorithms (GA) in deriving optimum mix designs for HSC using data collected from a batching company. Three hundred ninety-six (396) HSC trial mixtures were analyzed to derive empirical equations for strength and slump which were adapted as GA fitness functions. The GA program generated optimum preliminary designs for concrete strengths in the range of 7,000 psi (48 MPA) to 10,000 psi (69 MPa) depending on the type of sand and whether silica fume is present or not. In-situ adjustments for the dosage of admixture and amount of water were applied to the GA preliminary mix designs to account for the moisture content and absorption of the aggregates. These mix designs were verified by implementing the mixture with in-situ adjustments and testing the concrete cylinders for compressive strength. The target values for strength and slump were obtained. Cost comparison also showed that the GA-HSC mix designs yielded lower material cost than the mix designs provided by the company indicating a near optimal and more economical mix design.
This is the undergraduate thesis of Iris Mae M. Malabatuan, Bertrand B. Teodosio and Analyn C. Yee Concepcion at Department of Civil Engineering, De La Salle University, Manila. CE faculty, Engr. Alden Paul Balili, who also completed his MSCE thesis on GA with application to RC Frames was a co-adviser of the group and his GA program was used in the thesis. The thesis group is a finalist for the 2011 Gold Thesis Award for the Structural Engineering Division.

The group members and the advisers wish to express their gratitude to D. M. Consunji, Inc. and its current president, Mr. Jorge A. Consunji, for granting the request to obtain concrete mix design data.