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Proceedings of the 
1991 INTERNATIONAL TIMBER 
ENGINEERING CONFERENCE 
	
VOLUME 2 

2 - 5 September 1991 
London, United Kingdom 
-
!

1991 INTERNATIONAL TIMBER ENGINEERING CONFERENCE LONDON 
ON THE POST-GRADE HANDLING OF STRUCTURAL WOOD MATERIALS 
John A. Akande* 
The effects of regular drying and final dressing 
were tested on 898 nominal 5-ft. (1.52-m.) long 
red oak 2 by 4's visually graded (NELMAl) green. 
Only Select-Structural (SS), No.2, and No.3 
grades were tested due to limited availability of 
No.1 and Economy (Ec) grades. Dry boards were 
dressed to standard finished dimension of 3.8- by 
8.9-cm. by 1.52-m. and regraded with the same 
NELMA specifications. Elastic modulus E of each 
board was measured after regrading to demonstrate 
that drying and final dressing affected t~e orig- 
inal wood qualities. Evidently, wood graded for 
structural applications should be procured at use 
moisture levels and target dimensions. 

1991 INTERNATIONAL TIMBER ENGINEERING CONFERENCE LONDON 
be expected after grading any structural wood material. 
MATERIALS AND METHODS 
Northern red oak logs were sawn to flitches and further 
converted to nominal 2- by 4-in. by 5-ft. (5.08- by 10.2-cm. by 
1.52-m.) wood samples. The samples were graded using NELMA 
standards at green conditions (M.C. 50 to 80%). Not many of the 
samples fell into No. 1 and Economy grades and were therefore 
not considered. Experimental stocks consisted of eight hundred 
and ninety eight samples~in the SS, No.2 and No.3 grades. The 
No. 2 grade was air-dried under laboratory conditions 
(Temp. 720F; EMC 8%) while the SS and No. 3 grades were dried in 
a Nyle dehumidifying kiln system to 10% moisture level by using 
the appropriate schedule. A WADKIN P.L.C. moulder (Model GB) was 
utilized to dress the dry boards to standard finished dimension 
of 1.5- by 3.5-in. by 5-ft. (3.8- by 8.9-cm. by 1.52-m.) after 
which the boards were regraded with the same NELMA specifications. 
After the regrading operation, elastic constant E of each board 
was measured by using a Metriguard Inc. longitudinal stress wave 
timer. This was to determine how intra-grade dynamics affected 
the ranked E distribution. Based on an elementary wave theory 
which relates the propagation speed, v, and mass density, D, of 
any isotropic, homogenous material to its elastic modulus E: 

Evidently, wood is non-homogenous and it is anisotropic. Positive 
relationship between E and propagation velocity of stress wave 
in clear wood specimens has however been verified (2). Adopting 
similar principle for a red oak material having 1.5- by 3.5-in. 
by 5-ft. (3.8- by 8.9-cm. by 1.52-m.) dimension, the following 
relationship suffices: 
D = (wt./vo1.) 
wt. = weight of the sample 
vol. = volume of the sample 
v = inch/sec.; for a unit distance, 
v = l/At, thus, v2 = 1/At2 
At = wave transit time in microseconds 
between two end-located transducers. 
INTRODUCTION
Structural wood materials are often classified into stress grades 
having different qualities or mechanical properties. Grading is 
done at any wood moisture level and is based on a premise that 
mechanical properties of lumber differ from those of '~lear wood 
due to variations in density and other visible defects like 
decay, slope of grain, presence of knots, checks, splits, wane, 
pitch pockets e.t.c. Establishment of wood structural grades 
permits orderly purchase and proper usage of this material. In 
addition to structural grades obtained through visual grading, 
the use of Machine Stress Rating (MSR) helps to relate wood 
strength to the average stiffness estimate assigned to a 
particular grade. A majority of visually graded wood are however 
dimension lumber with nominal thicknesses. This implies that such 
materials could be dressed to standard dimensions without 
significantly altering the original grade classification. It 
could be risky to accept or operate on this assumption prior to a 
thorough investigation on how and to what extent wood strength 
properties are affected by normal processing methods. The purpose 
of this work is to evaluate the residual quality or property 
modification associated with drying and regular dressing that can 
* Wood Utiliz~tion Expert, Federal Department of Forestry, 
P.M.B. 5011, Jericho, Ibadan Nigeria. 

where 
same as 

E 
E 

E 

v2 • D 
27630 x 106(wt.)(1/~t2) psi 
1598 x 106(wt.)(1/dt2) Kg/cm2 

(1) . 
(2) • 
(3) • 

1 North-east Lumber Manufacturing Association. 
2.208 

Using equation 2, E value for every test piece was obtained and 
2.209 

1991 INTERNATIONAL TIMBER t:NGINEERING CONFERENCE LONDON 
their distribution was analyzed based on the original green 
grades and final dry-dressed grades of the samples. 
RESULTS AND DISCUSSION 
Table 1 shows the percentage movements or displacement of the 
original green sample grades into final dry-dressed grades. 
Drying and final dressing changed the original wood qualities 
and defects associated with drying are independent of whether 
the woods were air dried or kiln dried. About 20% of the 
Select-Structural group was down-graded while significant grade 
movements in the upward and downward directions also occurred in 
the No.2 and No.3 categories. E values of samples that were 
dry-dressed following initial grading are somewhat erratic 
(Figure 1) when compared to E based strictly on final grades 
(Figure 2). The erratic behavior was more pronounced below the 
10th and above the 90th percentile rank but dormant at the 50th 
rank. Since structural wood materials are often designed for the 
lower 5th percentile ranking, simple alteration of the original 
character may significantly trigger a change in the lumber 
grades. To ensure reliability in design, it is recommended that 
wood graded for structural applications should be procured at 
use moisture levels and target dimensions. Alterations in 
moisture levels and specimen size may remove some surface defects, 
TABLE 1. 	Movement of grades from green to dry-dressed dimensions. 				
				dry-dressed grade distribution 	Up-grading 	Down-grading 	
Original 	Number of 			% 		-- 	% 	-- 	- 	% 	
green grade 	samples 	SS 	No.1 	No.2 	NO.3 	Ec 					
SS 	344 	80.8 	3.2 	11.6 	1.8 	2.6 	0 			19.2 	
No.2 	234 	8.1 	11.5 	64.1 	10.3 	6.0 	19.6 		16.3 	
NO.3 	320 	1.2 	1.0 	32.3 	45.3 	20.2 	34.5 		20.2 	
inadvertently modify the original lumber grades and produce 
substandard performance in structures. 
CONCLUSIONS
Knowledge of timber handling is fundamental if wood structures 
will perform &atisfactorily. Movements associated with post-grade 
handling of construction wood materials can significantly alter 
the original strength and stiffness, and thus lead to erroneous 
design. A marginal error created through careless wood handling 
2.210 
1991 INTERNATIONAL TIMBER ENGINEERING CONFERENCE LONDON 
E million psi 
31 
	'.51 	. 	. 	-: 
	'1 	~
		~--::- -- .. - 	J 
	1.5 		i 
1 
o:j§J 		i 
	o 	20 	40 	60 	80 	100 
Normalized rank •• 
Figure 1. 
E values of dry-dressed samples. 
3 f·""" ,,, 
2.51 
2 
1.5 
	.. 	~ 
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0.5 -1 	._-_... No.2 
- No.3 
O+I-----,-- __ -, 	,- 	,- 	~ 
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20 
40 	60 
Normalized rank •• 
80 
100 
Figure 2. 
E values based on final grades. 
2.211 
j
§
J

1991 INTERNATIONAL TIMBER ENGINEERING CONFERENCE LONDON 
can lead to inefficient design and unwarranted structural 
breakdown. To avoid this problem with wood structures, it is 
advisable that graded wood be purchased at use moisture levels 
and target dimensions. 
REFERENCES
1. 
AITC. Timber Construction Manual. Wiley - Interscience. 
John Wiley and Sons, New york, NY. 1985. 
Berthholf, LD. Use of elementary stress wave theory for 
prediction of dynamic strain in wood. Washington State 
Institute of Technology, Bulletin 291. WSU, pullman, WA. 
1965. 
2. 
3. 
DeBonis, AL. Development of design stresses for seven 
hardwood species groupings. Final Rep. to Penn. Bur. of 
Penn. Dep. of Envir. Res. and Dep. of Transportation. 
83p. 1988. 
Faust, TD. Grade distribution and drying degrade of sweetgum 
and yellow-poplar structural lumber. Forest Prod. J. 40(5): 
18-20. 1990. 
For. , 
4. 
problem analysis. Forest Prod. 
5.	Madsen, B. In-grade testing 
J. 28(4)142-50. 1978. 
NELMA. Standard grading rules for Northeastern lumber. 
Approved by the Board of Review, American Lumber Standard 
Comm. l19p. 1986. 
6. 
7. 
Pneuman, FC. Typical problems with structural wood d~sign 
causes and cures. Proc. of the 1988 Int. Conf. on Timber 
Engineering, Vol. 2, p 716-722. 1988. 
2.212 
1991 INTERNATIONAL TIMBER ENGINEERING CONFERENCE LONDON 
REGRESSION ANALYSIS OF MODULUS OF RUPTURE AND ELASTICITY 
RELATIONSHIP IN FEW TROPICAL HARDWOODS 
S A KANDEEL*, S A TEWFIK+ AND I E KHERALLAH* 
Some tropical hardwoods were randomly sampled for 
the estimation of modulus of rupture (MOR) and 
modulus of elasticity (MOE) relationship. Sampled 
species included: Xylopia staudtii, Nauclea ~ 
rrichii, Casuarina stricta,Casuarina cunnghamiana 
Casuarina glauca, Copaifera salikaunda,Upaca gue- 
nensis, and Araliopsis tabouvensis. T~e~st pre- 
diction models included only MOE withYR~ values 
ranging from 0.58 to 0.78 in linear models. This 
suggests a straight line relation to predict MOR 
from MOE in tropical hardwoods. The establishment 
of that relation would improve stress grading of 
tropical hardwoods. 
INTRODUCTION
Machine stress grading is based on the relationship between ulti·- 
mate bending stress and other physical properties.The relationship 
between the modulus of elasticity and ultimate sending stress pro- 
vides the basic concept of stress-grading for different types of 
stress grading machines. (9). 
According to Miller (10), a selection efficiency that is economi- 
cally acceptable can be achieved through a high tolerance (defined 
as percentage of low strength material that pass). 
'l.'he relation between modulus of elas-tieity· and the ultimate strength 
in bending in coniferous woods was found to be represented by a 
regression line. Howeuer, moisture content and growth irregulari- 
ties were found to influence the modulus of rupture to a greater 
extent (6), (14) . 
Littleford (8)concluded that a straight line was accurate for con- 
ifers. 
In a review of the research done by several universities on non- 
destructive testing; Snodgrass (13) stated that several studies 
were directed to elucidate the modulus of elasticity and modulus 
of rupture relations in a small clear wood specimens in conifers. 
* Professor and Assist. Professor of Forestry and Wood Technology, 
University of Alexandria, Alexandria, Egypt. 
+ Researcher of Forestry and Wood Technology, Research' Director 
Laboratory of Alexandria, Flour Mills Co., Egypt. 
2.213