LARGE GLUED-LAMINATED
TIMBER BEAMS WITH TWO
GRADES OF TENSION
LAMINATIONS
U.S.D.A. FOREST SERVICE
RESEARCH PAPER
FPL 113
SEPTEMBER 1969
U.S. Department of Agriculture
Forest Service
Forest Products Laboratory
Madison, Wis.
SUMMARY
Bending strength characteristics of large glued-laminated timber
beams were appraised as a basis for more precise design. Current design criteria defining strength characteristics of gluedlaminated construction were developed from data on relatively
small members, with essentially no data on large beams. Present
extensive use of large beams emphasizes the importance of reevaluation of the design criteria for glued-laminated construction
specifically as they relate to large members.
A total of 26 beams, one-half of Douglas-fir and one-half of
southern pine, were evaluated. Twenty beams were 5-1/4 inches
wide, 24-3/8 inches deep, and 40 feet long. Six beams were 9 inches
wide, 31-1/2 inches deep, and 50 feet long. The beams were manufactured according to the current AITC and lumber industry
specifications for glued-laminated construction, using lumber
grade combinations with a listed bending stress rating of 2600
p.s.i. Tension laminations on the 50-foot-long beams and on onehalf of the 40-foot-long beams were graded according to AITC 301
requirements. The remaining 40-foot-long beams had tension
laminations selected according to additional requirements which
further restricted size of knots and amount of grain deviation,
Boards used for the “midlength” region of all tension laminations
were carefully selected to represent near minimum quality for the
grade.
The beams were tested to failure in bending. Eleven of the beam
failures were associated with end joints while 15 initiated in the
wood. The data indicated that (1) the design stress for beams with
minimum 301 grade tension laminations should be less than
2,600 p.s.i.; (2) beams with tension laminations selected according
to the additional requirements could be safely designed for 2,600
p.s.i. stress, provided the amount of low-strength pith-associated
wood in the critical tension laminations is limited and adequate end
joints are used; and (3) the strength and quality of end joints need
to be further evaluated.
These data will aid in the development of more exacting design
criteria for large glued-laminated members.
LARGE GLUED-LAMINATED
TIMBER BEAMS WITH TWO
GRADES OF TENSION
LAMINATIONS
1
By BILLY BOHANNAN, Engineer
and R. C. MOODY, Engineer
Forest Products Laboratory 2
Forest Service
U.S. Department of Agriculture
INTRODUCTION
Glued-laminated timber is now one of the major
structural building materials. Its size and shape
are almost unlimited. Its reliability is verified
by the good performance and long service life of
large structural timber members. To insure
continued reliability and competitive position for
large glued-laminated members, the design criteria must engender confidence in the strength
of glued-laminated construction, Conversely, the
criteria should not produce beams that are
greatly overdesigned, thereby resulting in a
“waste of strength.”
The glued-laminated wood industry in the
United States has built on research started in
the Forest Products Laboratory in the early
1930’s. Many of the design criteria defining
strength characteristics of glued-laminated con1This
research was conducted
Construct ion (AITC).
2
3
Maintained
at
Madison,
in
Wis., in
struction were developed in the 1940’s. This
early research was conducted on relatively small
members, up to 12 inches in depth, with essentially no large beams being evaluated. The large
timber members now being manufactured, and
the possibilities of even larger members, have
caused both producers and consumers to realize
the importance of re-evaluating the design criteria
of glued-laminated construction. Since about 1960,
research at the Laboratory, in cooperation with
industry associations, has been directed toward
more exacting data that would provide a more
reliable basis for the design of large gluedlaminated timber members.
Many data have been obtained from flexure
tests of laminated beams 12 inches or less in
3
depth (6,10,12,18); however, similar data on
large beams are limited. Only a few beams over
30 inches deep have been tested. The strengths
of three structural grade Douglas-fir beams,
cooperation with the American
cooperation
with
the
University
Institute of
of
Timber
Wisconsin.
Underlined numbers in parentheses refer to Literature Cited on page 26 of this
research paper.
31-1/2 inches deep, tested by the Laboratory in
1966 (8) were lower than were predicted by the
then-current design criteria. Based on the observed failures and strength data from large
beam tests, on prestressing of wood beams (6),
and on tensile strength of structural lumber (11),
L ab or at or y and industry representatives concluded that more importance should be placed on
the grading of tension laminations for gluedlaminated timber members.
A small part of Bohannan's research on prestressed wood beams (6) was directed toward
evaluating the importance of the tension lamination, He found that addition of a clear straightgrained tension lamination had a pronounced effect
on modulus of rupture of 10.5-inch-deep laminated
beams made of L-3 grade lumber, The addition
of one 1-1/2-inch-thick clear lamination representing 14 percent of the beam depth increased
the modulus of rupture 32 percent, and one 9/16inch-thick clear lamination representing 5 percent of the beam depth increased the modulus of
rupture 23 percent, These data indicate a potential
for significant improvement in flexural strength
of structural glued-laminated beams by giving
special attention to a small portion of the beam-the outer few tension laminations.
The American Institute of Timber Construction
developed and published specifications for tension
laminations in 1967 (3). These tension laminations,
called the 301 grade, are required on all beams
of a certain stress rating and size in order to
meet AITC certification specifications. This 301
grade is not necessarily clear and straightgrained, but rather is a structural grade. The
most significant improvement in the specifications for this tension lamination grade over
specifications for other structural grades is
limitation on localized grain deviation. This has
not been previously considered in lumber grading.
The study discussed herein provides data to
appraise the strength of Douglas-fir and southern pine glued-laminated timber beams. Sixteen
beams were manufactured according to current
AITC and lumber association specifications for
structural glued-laminated timber members and
10 beams were manufactured according to specifications developed by AITC and FPL representatives for this research study,
BEAM DESIGN,
MATERIALS,
AND FABRICATION
Description of Beams
A total of 26 beams were evaluated. One-half
of the beams were fabricated from Coast-type
Douglas-fir lumber and the other half from southern pine. Twenty beams were 5-1/4 inches wide,
24-3/8 inches deep, and 40 feet long. These
beams contained fifteen 1-5/8-inch-thick laminaions. Six beams were 9 inches wide, 31-1/2 inches
deep, and 50 feet long and contained twenty-one
1-1/2-inch-thick laminations.
Beams were manufactured using lumber grade
combinations with a listed bending stress rating
of 2,600 p.s.i. The Douglas-fir beams were Combination A as given in “Standard Specifications for
Structural Glued Laminated Douglas-Fir (Coast
Region) Timber” (17). The southern pine beams
were Combination A-2 as given in 'Standard
Specifications for Structural Glued Laminated
Southern Pine Timber" (14). These beams are
described in table 1.
The only change here was that one tension
lamination, meeting the near-minimum requirements as discussed in the following section, was
substituted for one outer tension lamination for
each beam,
Tension Laminations
All the 50-foot beams and one-half of the 40foot beams had tension laminations which were
graded according to AITC 301-67 requirements
(3). One tension lamination meeting the requirements summarized below is now required by
AITC on beams 16-1/4 to 32-1/2 inches in depth
1. Growth rate requirements shall apply to the
full length and pieces of exceptionally light weight
shall be excluded.
2. Knots shall not occupy more than one-quarter
of the cross section.
3.
2
The general slope of grain shall not exceed
T a bl e 1 . - - D e s c r i p t i o n a n d g r a d e c o m b i n a t i o n s f o r t h e s o u t h e r n p i n e a n d D o u g l a s - f i r b e a m s
:
Combination
and species
:
:
:
:
: Number of : Allowable : Number and grade of
Slope of grain
: laminations : extreme :
laminations
: ----------------------: f i b e r :- - - - - - - - - - - - - - - - - - - - - - - - - - : Outer : Next : Balance
: s t r e s s : O u t e r : I n t e r -: I n n e r : 10
: 10
:
: mediate : zone : p e r c e n t : p e r c e n t :
: zone :
--------------: ------------ :--------- -- --------:------- : ------- :-------:-------:------:
: P.s.i. :
:
:
:
:
:
:
A- 2
:
Sou t h e r n p i n e : 14 to 21
A
D o u g l a s- f i r
A
Douglas - f i r
:
:
:
zone
:
2,600
:
:
:
:
: 1:12
: 1:8
:
:
:
:
:
2,600
: 2 L-1
: 2 L-2 :
L-3 : 1:14
: 1:12
: 1:8
: 21 or more :
2,600
: 3 L-1
: 2 L-2 :
L-3
: 1:12
: 1:8
:
:
:
: 2 No. 2D : No. 2 : No. 2 : 1:14
9 to 20
1 in 16 except where more restrictive slope of
grain is required by the standard laminating
specification.
:
:
: 1:14
with a slope of grain 1 in 16 or flatter between
them and the edge of the piece,)
4. Areas of local grain deviation steeper than
1 in 16 individually or in cornbination with
knots shall not occupy more than one-eighth of
the cross section if located closer than 1/2 inch
from the edge of the piece,
4. Areas of local grain deviation steeper than 1
in 16 shall not occupy more than one-third of the
cross section.
5. Knots shall not occur within two knot diameters of the finger joints,
5. Strength-reducing characteristics of maximum size must be at least 4 feet apart,
The remaining one-half of the 40-foot beams
had tension laminations graded according to the
following requirements developed for these test
beams by AITC and FPL representatives and
hereafter referred to as “301+”:
6. A 1-foot length of a lamination shall be considered as a cross section.
1. Growth rate requirements shall apply to the
full length and pieces of exceptionally light
weight shall be excluded.
7. The general slope of grain shall not exceed
1 in 16 except where more restrictive slope of
grain is required by the standard laminating
specification.
2. Knots shall not occupy more than one-fifth
of the cross section.
8. Knots shall not occur within two knot diameters of the finger joints,
3. Two-thirds of the cross section of the laminations must be free of strength-reducing characteristics and must have 1 in 16 or flatter
slope of grain. (Knots plus associated localized
grain deviation, or knots plus associated localized grain deviation plus localized grain deviation not associated with knot, or localized grain
deviation not associated with a knot may occupy
up to one-third of the cross section, provided
these characteristics are not on the edge of the
piece and there is at least 1/2 inch of clear wood
The tension laminations were generally made
up of three boards, a midlength board and two
end boards finger-jointed to it. The midlength
board of the tension lamination was selected by
both industry and Laboratory personnel to have
at least one near-maximum allowable strengthreducing characteristic permitted by the above
specifications, Tension laminations in the zone
of maximum bending moment were therefore
near minimum quality permitted by the grade.
Laminations were chosen in which knots were at
3
various locations across the width of the board
and, generally, the worst face of the boards was
used as the outer face of the beam Laminations
were not selected to have minimum specific
gravity; rather the midlength tension laminations
were selected to have specific gravities representative of the species but exceptionally heavy
or exceptionally lightweight boards were not used.
The volumetric size of knots and grain deviation was used in grading the tension laminations.
Selection of Material
Efforts were made to use lumber representative of that for each species. To obtain some
randomization of the lumber, the Douglas-fir
for the 40-foot-long beams was obtained from
production lumber over a 5-consecutive-day period in which the lumber of the correct size was
being processed through a commercial lamination
plant. For the 50-foot beams, the lumber was
obtained on 2 consecutive days. An approximately
equal amount of lumber was obtained on each of
the days,
The southern pine lumber for the 40-foot-long
beams was obtained from five different lumber
shipments received at a commercial laminating
plant. Four suppliers were represented. For the
50-foot beams, the lumber was obtained from
two shipments. Again, an approximately equal
amount was obtained from each shipment.
The reason for the different methods of obtaining the material was due to the procedures
used at the two commercial laminating plants
where the beams were fabricated. One plant buys
green lumber and processes it through their own
dry kilns. The other plant purchases kiln-dried
lumber.
A supply of lumber equal to about 2-1/2 times
the amount necessary to make the beams was
accumulated. As the lumber of each species
was obtained, each board was numbered consecutively. Generally, odd-numbered pieces were
used in the beams, which further randomized the
selection of material.
Nondestructive Evaluation of Lumber
All boards were evaluated by representatives
4
Equipment
described
in
product
information
of
of lumber associations, the laminating industry,
and the Laboratory. The nondestructive evaluations included rechecking visual grades, determining weight, moisture content, and modulus of
elasticity of each board, and measuring size and
location of knots in the middle one-half length of
each lamination,
The lumber was grade-marked when it was
obtained. After all lumber was accumulated, it
was checked by an inspector from the appropriate
grading association to assure that it was of the
proper grades. Moisture content was measured
with a power-loss type meter at three locations
along board length. The modulus of elasticity of
all boards for each species was determined
using a vibration technique. 4
After the boards were end-jointed into fulllength laminations, the size and location of all
knots and other major strength-reducing characteristics in the middle one-half of the length
of the laminations were recorded. For all except
the tension laminations, for which the selection
procedure was described earlier, only the knots
on the face of the lamination away from the neutral
axis of the completed beam were measured. The
size of each knot was measured perpendicular to
the edge of the board. An estimate was made of
the effective size of spike knots, The width of the
knots and of areas of grain deviation steeper than
1 in 16 was measured on both faces of the tension
laminations,
Except for the visual grading, none of the nondestructive evaluations had any bearing on the
assembly of the beams. The nondestructive evaluations were made to get basic information that
might be useful in subsequent analysis.
Manufacture of Beams
The Douglas-fir and southern pine beams were
manufactured in the summer of 1968 at two commercial laminating plants, each of which specialized in one of these species. All beams were
manufactured to conform to U.S. Commercial
Standard CS-253-63 for “Structural Glued Laminated Lumber” (15). The tension laminations were
specially selected as previously discussed, but
all other laminations were randomly positioned
within the beams in accordance with the required
grade combination.
Irvington,
4
Portland,
Oreg.
Commercial production finger-type end joints
meeting the requirements of the AITC qualification tests (1) were used. The end joints in the
tension laminations were at least 5.4 feet from
the midlength of the 40-foot-long beams and 6.5
feet from midlength of the 50-foot-beams. As will
be noted later, they were at least 1.4 feet outside
of the area under maximum bending moment
during test. The primary objective of the tests was
to evaluate the effect of grade of the tension
lamination on the strength of the beams. For
this reason, the end joints were purposely placed
where they would not be in the most highly stressed
areas of the beams when tested.
Beams were manufactured with a phenolresorcinol adhesive, After gluing, the beams
were surfaced to either a 5-1/4- or 9-inch width
and cut to the proper length. They were shipped
in accordance with commercial practice to Madison, Wis., where they were stored indoors prior
to testing, The beams were numbered as follows:
Beam description
Tension
lamination
grade
Beam No.
40- foot
DougIas-fir
301
1 to
40- foot
Douglas-fir
301+
6 to 10
40- foot
southern pine
301
11 to 15
40-foot
southern pine
301+
16 to 20
50- foot
DougIas-fir
301
21 to 23
50-foot
southern pine
301
24 to 26
Sixty-foot-long steel beams were used to extend
the 10-foot base of the million-pound-capacity
testing machine at the Forest Products Laboratory. By extending the base of the test machine
with steel beams, the load imposed on the wood
beams could be measured directly on the weighing
platform of the test machine. This test method
differed slightly from that generally used in
testing structural members--since the supports
were somewhat elastic, The 60-foot steel beam
supports were cantilevered from the 10-foot-long
base of the test machine and deflected slightly
during beam test. Thus, the loading somewhat
resembled a dead-load test because the load was
maintained approximately constant as maximum
load was attained, Once failure in a wood beam
was initiated, the energy in the deflected steel
beams acted to cause complete failure without
load drop-off as the first failure occurred,
Each wood beam was laterally supported 40
inches each side of midlength, as shown in
figure 3. Each lateral support consisted of 3inch-diameter steel pipes anchored to the columns
of the test machine and two wood blocks. One
wood block moved with the test beam as it deflected and the other block was fastened to the
steel pipes. Polyethylene film was used between
the two wood blocks to reduce the frictional force
as the beam deflected,
At 19 feet each side of midlength, or directly
over the supports of the 40-foot-long beams and
5 feet inside the supports of the 50-foot-long
beams, the top of each beam was braced to
prevent lateral movement, The bracing at one
end of a 40-foot beam is shown-to the far right
in figure 2.
Beams were loaded with two symmetrically
placed concentrated loads. A preload was applied
to the beams to assure all loading and support
points and gages were properly alined and fully
contacted. The loading and testing conditions were:
5
40-foot
beams
50-foot
beams
2,000
4,000
38
48
Distance between load
points (ft.)
8
10
Rates of loading
(in. per min.)
1.0
Conditions
RESEARCH METHODS
Preload (Ib.)
Load span ( f t . )
Equipment
The test method conformed to requirements of
ASTM D 198 (5). The method of loading is shown
schematically in figure 1, with a general overall
view of a 40-foot beam during test in figure 2.
Loading was continuous to failure.
5
1 .25
Figure 1.--Loading diagram for testing 40- and 50-foot-long glued laminated wood beams.
For the 40-foot beams, L and a were 38 and 8 feet, respectively, and for the 50-foot
beams, 48 and 10 feet, respectively.
M 136 732
The 10-foot-long base of the m i l l i o n Figure 2.--General view of beam during test.
pound capacity testing machine was extended with two 60-foot-long steel beams. A
steel loading beam was used to apply loads 4 feet each side of the midlength of the
40-foot-long beams and 5 feet each side of midlength of the 50-foot-long beams.
M 135 616
FPL 113
6
Figure 4.--Yoke deflectometer mounted at
midlength and supported on a 7-foot span
used to measure deflection within the
constant moment section of each gluedlaminated wood beam. Dial gage was read
to the nearest 0.001 inch using a telescope and mirror. Def lect omet er was
removed from beam at about 50 percent of
maximum expected load. The wood block at
the right was used to laterally support
the specimen and moved with the beam
during
test.
Figure 3.--Lateral supports located 40 inches
each side of midlength of beams. The 3inch diameter pipes were anchored to
columns of test machine. The wood block
touching the beam moved with the beam and
polyethylene film was used between wood
blocks to reduce frictional force as beam
deflected during test.
M 136 522
scribed in ASTM D 198 (5). Readings were taken,
to failure, at 2,000-pound increments of load on
the 40-foot beams and 4,000 pound increments
on the 50-foot beams.
Lateral loads which developed during loading
were determined from the strain gages mounted
on four 3-inch pipes. Conversions of strain
readings to loads were made using actual calibration data for the 3-inch pipe and strain gage
combinations.
Failure details in each beam were sketched,
and each failed beam was photographed, When
failure was attributable to a specific area of the
beam, this area was cut from the beam for further inspection.
Data Obtained
The dimensions, weight, and moisture content
of the beams were determined prior to test,
Moisture content was determined by averaging
readings of a power-loss moisture meter, taken
at random positions of the beams, at every 5foot interval along the length.
Data taken during tests included maximum load,
centerline deflections between supports, centerline deflections of the beam over a 7-foot span
between load points, and strains on the four
lateral support pipes at the compression face of
the beams, Deflections between load points were
measured on one side of the beam with a yoke
deflectometer (fig. 4) as described in ASTM D
198 (5). Deflection data were taken at 2,000pound increments of load up to 18,000 pounds
total load for the 40-foot-beams and at 4,000pound increments up to 36,000 pounds total load
for the 50-foot beams, At these loads, the yoke was
removed.
Centerline deflections between supports were
measured using a wire deflectometer, as de-
Minor Tests
Immediately after failure of each beam, a section about 1 foot long was cut from within the
center 8 feet of each beam, Clear wood specimens,
about 4 by 4 by 1-1/2 inches, were then cut from
each lamination to determine their specific gravity
and moisture content. Specific gravity was based
on volume at time of test and ovendry weight.
Moisture content was calculated from the weight
loss during ovendrying and the ovendry weight.
7
PRESENTATION
A N D ANALYSIS
OF DATA
The specific gravity of each beam is given in
columns 6 and 7 of tables 2, 3, and 4. The average
specific gravity of all beams was approximately
0.48 for the Douglas-fir and approximately 0.49
for the southern pine. This is the approximate
average clear wood specific gravity for Coasttype Douglas-fir and slightly less than the average
clear wood specific gravity of southern pine
(table 12 of Wood Handbook(16 )). Specific gravity
and moisture content data for each lamination
located near midlength of the beams are given in
Appendix I.
The location and comments on failures are given
in column 15, tables 2, 3, and 4. These will be
discussed in detail in a following section of this
paper.
Flexural Strength Data
Data obtained from the flexure tests of the 26
glued-laminated timber beams are given in tables
2, 3, and 4. Data as determined from test results
are given in columns 8 through 11. Results of
moisture content determinations made both prior
and immediately following test show that the
Douglas-fir beams had about 7 to 8 percent
moisture content when tested, while the southern
pine beams had a moisture content near 13 percent. These moisture contents are given in columns 4 and 5. The modulus of rupture and modulus of elasticity data in columns 8, 9, and 10 were
adjusted to a 12 percent moisture content using
the following formulas:
MOR
MOE
where
12
= MOR (1.020)
M
12
= MOE (1.015)
M
MOR
MOR
12
M
M-12
M-12
Tension Laminations
Results of the nondestructive evaluation of the
tension laminations are given in table 5. Size
and location of major strength-reducing characteristics are given in tables 6, 7, and 8. In the
tables, any knot whose associated grain deviation
occurred at the edge of the board is classified
as an edge knot. Examples of boards used for
the midlength of the tension laminations for both
Douglas-fir and southern pine beams are shown
in Appendix II.
The boards positioned at the midlength of
the tension laminations were selected to contain
a strength-reducing characteristic of near maximum size as permitted in either the 301 or 301+
grades. As the boards were selected from a given
supply of lumber, it was not always possible to
obtain a strength-reducing characteristic of maximum size in all pieces, but most all boards
selected for the 40-foot-long beams would represent a lower line 301 or 301+ grade,
It was more difficult to select 2- by 10-inch
boards for the tension laminations of the 50foot-long beams that contained near-maximum
strength-reducing characteristics. The maximum
permitted knots are less frequent in this size
material, and they were not attainable in the
given supply of lumber. AS a result, the tension
laminations for the 50-foot Douglas-fir beams
Nos. 21 and 23 and southern pine beam No, 26
would be a minimum 301+ grade rather than 301
grade as intended.
(1)
(2)
= Modulus of rupture adjusted
to 12 percent moisture content
= Modulus of rupture from test
M = Moisture content at test (col.
4, tables 2, 3, and 4)
MOE
MOE
12
M
= Modulus of elasticity adjusted
to 12 percent moisture content
= Modulus of elasticity from test
These percentage changes are approximately
equal to the seasoning effects for sawn structural
lumber given in ASTM D 245-68 T (4) and as
found by Gerhards (13). The data adjusted to 12
percent moisture content are given in colums 12,
13, and 14 of tables 2, 3, and 4,
FPL 113
8
Table
2.--Results
of
bending
tests
on
40-foot
laminated
beams
with
301
tension
laminations
Table
3.--Results
of
bending
tests
of
50-foot
laminated
beams
with
301
tension
laminations
Table 4.- - Results of bending tests on 40-foot laminated beams with 301+ tension laminations
Table
of
5. - -Specific
gravity
tension
and
modulus
laminations
of
elasticity
of
selected
boards
used
for
center
portion
T a b l e 6 .--D e s c r i p t i o n o f m a j o r s t r e n g t h-r e d u c i n g c h a r a c t e r i s t i c s i n c e n t e r 1 0 f e e t o f
301
t e n s i o n l a m i n a t i o n s f o r 4 0-f o o t b e a m s
FPL 113
14
Table
8 .--D e s c r i p t i o n o f m a j o r s t r e n g t h-r e d u c i n g c h a r a c t e r i s t i c s i n c e n t e r 1 0 f e e t o f
301+
t e n s i o n l a m i n a t i o n s f o r 4 0-f o o t b e a m s
As shown in tables 6, 7, and 8, the average
percentages of knots and of clear straight-grained
sections of wood show that an approximate
equivalent grade of boards, as determined visually, was used for the tension lamination for both
species.
three types: (1) splintering tension failure of tension lamination not necessarily associated with
any major strength-reducing characteristic; (2)
failure of first or second tension lamination at a
major strength-reducing characteristic; or (3)
failure of end joints in first or second tension
laminations. Failure of beam 9 (fig. 5) was apparently a combined bending and shearing failure
initiated at steep localized grain deviation associated with spike knots in the fifth and sixth
laminations on the tension side.
Three of the failures were of the above type 1,
11 of type 2, and 11 of type 3. Strength-reducing
characteristics in the second laminations could
have been partial cause for the failure of seven
beams. Failure in the second lamination of beam 21
was observed at about 80 percent of ultimate load.
Brief comments on failures are given in column 15
of tables 2, 3, and 4. Examples of failures at major
strength-reducing characteristics are shown in
figures 6, 7, and 8 and at end joints are shown in
figures 9, 10, and 11.
Locations of failures in the beams are shown in
Appendix 11, along with modulus of elasticity and
specific gravity information on the boards in the
midlength region of the outer three or four tension
laminations.
Beam No, 5 (fig. 7) failed due to cross grain in
the tension lamination. The general slope of
grain throughout the selected tension lamination
was 1 in 16, which just met the grading specifica-
Description of Failures
Failures were generally sudden and complete,
with cracks propagating throughout the midsection
of the beams. This type of failure was probably
due in part to the test method used but would be
typical of that resulting from service-applied
loads. Each cantilevered end of the 60-foot steel
beam supports deflected during beam test, and
thus represented a source of energy which is
believed to have acted to completely fail the wood
beams once failure was initiated, With the sudden
failure, it was not always possible to exactly
determine the principal cause of failure. The
following comments represent the best estimate
of causes of failures, based on close observations
during tests and evaluation of failed sections after
tests:
A slight amount of compression failure was
noted under load points after testing of a few
beams, but generally the compression laminations were free of visible damage. Except for one
beam, the beam failures were on the tension side
of the beam and could be classified as one of
Figure
5.--Failure of beam 9 was attributed to spike knots in laminations 5 and 6 about
There was no
1 foot apart .Beam then split to end of beam in a shear-type failure.
damage to either the first or second tension lamination throughout the length of the
beam.
M 136 524
FPL 113
16
Figure
6.--Strength-reducing
characteristics in the midlength section and on the exposed
face of tension laminations at which failure initiated in beams 4, 7, 13, and 15. The
knot in tension lamination of beam 4 is maximum-sized 301 knot, while edge knot in
tension lamination of beam 7 represents maximum size 301+ edge knot plus grain deviation.
Failures of beams 13 and 15 were initiated at edge knots and their associated
grain
deviation in the tension lamination and then propagated to end joints where
remaining width of tension lamination failed.
M 136 516
were also graded as 301 or 301+ tension laminations, Beam No. 16 (fig. 8) failed in one of the
end boards at a small 5/8-inch-diameter knot
that had associated grain deviation with a 1 in 1
slope through the thickness of the board, This
grain deviation, which was not detected in the
grading, occupied 50 percent of the board cross
section. On this basis, the board did not meet the
tension lamination grading specification.
Failures initiated at end joints in six southern
pine beams and in five Douglas-fir beams, All of
the southern pine and two of the Douglas-fir endjoint associated failures occurred in the tension
lamination, Three end- joint-associated failures
in Douglas-fir beams occurred near midlength
in the secondlamination. All of the five end-jointassociated failures in the 40-foot southern pine
beams were in boards containing pith or pithassociated wood. No pith-associated wood was
detected in the outer laminations of the Douglasfir beams.
A question is sometimes raised about the
relative effect of edge knots as compared to
centerline knots. The data in this study are not
adequate to answer this question, but three south-
Figure 7.--Bottom of failure section from
beam 5 showing general 1 : 16 slope of
grain in tension lamination.
Failure
initiated
in grain deviation associated
with edge knot.
Slope of grain was 1:10-1:13 across full section of tension lamination at the edge knot.
Due to this
grain
deviation, the board was misgraded
and should not have been included in the
301 grade.
M 135 725
tions. The beam failed at a section where the
localized grain deviation had a slope of 1 in 10
to 1 in 13 across the full section, which was not
detected during grading. Based on this cross
grain, the board did not meet the tension lamination grade.
Tension laminations were generally made up
of three boards, the selected midlength board and
two end boards finger-jointed to it. The end boards
17
Figure 8.--Failures of beam 16 at small knot and associated grain deviation
thickness of the tension lamination; 1:1 grain deviation through thickness
occupied about 50 percent of cross section, although the knot was only 5/8
on bottom face and did not appear on top face.
Inspection of this area
indicated that the board would not meet 301+ grade.
through the
of board
inch wide
after failure
M 136 513
Figure 9.--Failure of Douglas-fir beam 2 with a 301 grade tension lamination resulted
from end joint failure in second lamination. Shown are: A, the side view of failed
beam; B, the top view of second lamination after it was cut from the failed beam; and
C, face view of the tension lamination. Failure of this type also occurred in two
M 136 515
other Douglas-fir beams, Nos. 3 and 10.
FPL 113
18
Figure 10.--Southern pine beam 12 showing failure of tension lamination at end joint.
On the face of the tension lamination, failure is across the tips of the fingers for
most of the width. Other southern pine beams failing in a similar manner were beams
M 136 514
11, 17, 19, and 20.
Figure It.--Tension laminations of 50-foot-long beams 22 (top) and 26 (bottom) showing
failure of end joints. Joints on both beams failed prior to maximum load and, at
maximum load, failure progressed to other parts of beams. Upon inspection of the
joint in beam 22 prior to test, four of the fingers appeared open. Three of these
fingers were in the failure region.
M 136 517
19
used. The following relationship,
ern pine beams failed at edge knots when the
tension lamination contained larger centerline
knots. This could indicate the edge knots have a
more detrimental effect but further research is
needed to better define their relative effect.
As the beams were loaded beyond approximately
one-half of ultimate load, the Douglas-fir beams
emitted sounds at various intervals of load until
failure. The southern pine beams did not emit as
many sounds, and a few of them were practically
noiseless until failure. It was not apparent that
the sounds become more frequent as the ultimate
load was approached,
F = 0.81
2
h + 88
where F = depth effect factor
h = depth in inches
and
as given in the Timber Construction Manual (2),
was used to adjust for depth.
The calculated value of design stress for each
beam based on these factors is given in columns 3
and 6 of table 9. The stress for five of these
beams is less than 2,600 p.s.i. The tension laminations of two of these beams, Nos. 5 and 16, did
not meet grade requirements. Failure of beam
No. 14 was due to an edge knot in the tension
laminations and cross grain in the second lamination. Beam No. 17 failed at an end joint in a tension lamination containing pith-associated wood.
Beam No. 4 failed at a centerline knot which was
the maximum permitted in the 301 grading
specifications.
Comparison of data to proposed design criteria,--The calculated stresses for the 26 beams
shown in columns 3 and 7 of table 10 represent
another analysis of the data. The stresses were
calculated by reducing the actual test modulus
of rupture for duration of load, factor of safety,
and size effect.
The duration of load adjustment used was 9/16
times 11/10 and the factor of safety adjustment
was 1.3. The size adjustment was made to correct
from test conditions to a 12-inch-deep uniformly
loaded beam having a length 21 times the beam
depth. This size-effect adjustment, which is
different than the depth-effect adjustment previously considered, was based on work by Bohannan (7, 9). The size adjustment for the 40-footlong beams was 0.919 and for the 50-foot-long
beams was 0.895.
The minimum calculated design stresses shown
in table 10 are as follows:
Discussion of Bending
Strength Data
The procedures to be used for assigning design
stresses to glued-laminated beams are now being
considered by an ASTM committee. Size effect is
one factor under consideration. The followingtwo
sections in this paper present analyses of the
data from the 26 glued-laminated beams based
on current practices using a depth-effect factor
given in the Timber Construction Manual (2) and
based on a size-effect relationship developed by
Bohannan (7,9).
These analyses are based on the modulus of
rupture values of the test beams, which are the
maximum bending moment on each beam divided
by the beam section modulus. No consideration
was given to the locations or exact modes of
failure. As is given in column 15 of tables 2, 3,
and 4, the locations of failures that initiated at
end joints were out of the most highly stressed
region of beams.
Comparison of data to current design criteria.-The beams were manufactured using a lumber
grade combination with a listed bending stress
rating of 2,600 p.s.i. Therefore, test data were
compared to this value, which is the design stress
for a 12-inch-deep beam used under normal
loading conditions. For comparison with the 2,600
p.s.i. value, test data were adjusted to the same
loading conditions and size.
Adjustments were made in the test data for
duration of load, factor of safety, and for the
difference in depth of members. The duration of
load adjustment was 9/16 times 11/10 to adjust
from short-time loading test conditions to normal
loading conditions. A factor of safety of 1.3 was
FPL 113
2
h + 143
Species
20
Grade of
tension
lamination
Cause of
faiIure
DougIas-fir
301
301+
301
301+
Wood
Wood
End joint
End joint
Southern pine
301
301+
301
301+
Wood
Wood
End joint
End joint
CaIculated
design
stress
(P.s.i.)
2,140
3,050
2,590
2,700
2,050
3,810
2,490
2,220
Table
9.--Test data adjusted to design stresses for 12-inch
deep beams, for normal conditions of loading and
with a 1.3 factor of safety
binations used for each species. For these
reasons, a separate discussion for each species
is given.
Douglas-fir.--The grade combination used for
the Douglas-fir beams required an upgraded
tension lamination and an L-1 grade second
lamination. Strength-reducing characteristics of
the wood in the second lamination were not found
to be a contributing cause of failure of any of the
lower strength Douglas-fir beams.
Considering only the Douglas-fir strength values given in table 10 where wood strength con-
Southern pine beam No. 16 was not considered in
this analysis because the tension lamination was
below both 301 and 301+ grades.
301 Versus 301+
Tension Laminations
It is difficult to accurately compare the 301 and
301+ tension lamination grades because of the
limited number of tests, the different modes of
failures, and the differences in the grade com-
21
Table 10.--Test data adjusted t o design s t r e s s e s for 12-inch deep
uniformly loaded beams having a 21 to 1 span depth
r a t i o , for normal conditions of loading and with a
1.3 f a c t o r of s a f e t y
The minimum calculated stress for Douglas-fir
beams in which failure was controlled by end
joints was 2,590 p.s.i.
Southern pine.--The grade combination for the
southern pine beams required an upgraded tension
lamination and a No. 2 Dense grade second lamination, Several of the lower strength southern
pine beams failed at a location where the strengthreducing characteristics of the wood in the second
lamination may have contributed to the failure.
trolled the failures, the calculated design stress
for beams with 301 tension laminations should be
less than 2,600 p.s.i. Beam No. 4 failed at a maximum knot permitted by 301 grade specifications
and the design stress calculated from test data
is 2,140 p.s.i. The minimum calculated stress
of beams having 301+ tension laminations is
3,050 p.s.i., which indicates that beams with
301+ tension laminations could probably be safely
designed for a 2,600 p.s.i. stress.
FPL 113
22
The lowest calculated design stress in table 10
for a southern pine beam where failure was controlled by wood strength was 2,050 p.s.i. This
was for beam No. 14 which failed at a location
where there was an edge knot in the tension
lamination and grain deviation in the second
lamination. It is impossible to estimate the relative effect of each lamination on the beam strength;
but since 4 of the 13 southern pine beams failed
at a location where the strength of the second
lamination may have contributed to the cause of
failure, it is believed that the grade requirements
of the second lamination of the A-2 combination
used in this study should be studied further.
Since the strength of only one southern pine
beam with 301+ tension lamination was controlled
by wood strength, it is impossible to accurately
appraise the effect of the 301+ grade tension
lamination. However, the data indicate that beams
Table
with 301+ tension laminations could be designed
for 2,600 p.s.i. bending stress,
Six of the southern pine beams failed at end
joints, Of these six, five failures occurred at
end joints in boards containing pith or pithassociated nondense wood. It is believed that
such wood contributed to the failures and thus
should be limited in boards used as tension laminations. The other failure was in a board containing compression wood.
I /I Ratios
K G
The ratios of moment of inertia of knots to the
total moment of inertia of beams (I /I ) are
K G
given in table 11. These were calculated by the
method used by Wilson and Cottingham (18 ). Knots
within a 1-foot length were considered at the same
1 1 .- -S u m m a r y o f m a x i m u m IK /I G v a l u e s f o r l a m i n a t e d b e a m s
23
2- by 6-inch southern pine
section. Values were calculated for each 0.2 foot
increment of length across the mid half-length
of each beam.
According to knot surveys which form the basis
for present laminating standards, the maximum
I /I value expected in the 40-foot-long beams
K G
would be 0.19 and for the 50-foot-long beams
would be 0.17. None of the 50-foot beams exceeded
of 0.17, with the maximum value being
an I /I
K G
0.160. All the 40-foot Douglas-fir beams had
maximum I /I values less than 0.19; however,
K G
three of the 40-foot southern pine beams had at
least one section where I /I
exceeded 0.19.
K G
Beams 14 and 18 each had one section which
exceeded 0.19, and beam 15 had two sections
which exceeded 0.19, Beam 18 had the highest
I /I value, 0.249, which is considerably above
K G
the maximum predicted value of 0.19.
No. 1D
No. 2D
Nos. 2 and
2D mixed
2- by
L-1
L-2
L-3
FPL 113
p.s.i.
10-inch Douglas-fir
2.3 x 106
1.9 x 106
1.6 x 106
p.s.i.
p.s.i.
p.s.i.
where E = calculated modulus of elasticity of
a beam for a given grade combination
E = modulus of elasticity of the ith
1
lamination
I. = moment of inertia of the i th lamina1
tion
I = gross moment of inertia of a beam
The average modulus of elasticity of the tension
laminations was assumed to be equal to that
given for the L-1 or No. 1D grades.
A comparison of the modulus of elasticity
calculated from average modulus of elasticity
values for grades of lumber used and the values
determined from beam tests are shown in table 12.
The range of test values are within 10 percent
of the average values. Values given in table 12
are for moisture content conditions at time of
tests because the calculated values are based on
the vibration modulus of elasticity determined
6-inch DougIas-fir
2.4 x 106
2.0 x 106
1.7 x 106
1.6 x 106
These values were determined by a vibration
technique a t average moisture contents of approximately 7 to 8 percent for the Douglas-fir and
13 percent for the southern pine,
Using these average values, the moduli of
elasticity of the two different sizes and species
of beams were calculated using the transformed
section formula
The modulus of elasticity values for each beam
are given in tables 2, 3, and 4. The differences in
the two values given in columns 9 and 10 for each
beam are due primarily to deflections caused by
shear stresses, The full-span modulus of elasticity includes shear effects, while that determined
within the constant moment section is theoretically
free of shear effects. The shear deflections
theoretically represent an effect of about 4 percent
on the modulus of elasticity of beams loaded by
methods used in this study.
As previously found in tests of large structural
beams (8), the load versus deflection data remained linear to failure.
The distribution of modulus of elasticity values
in the lumber used in the fabrication of beams in
this study is shown in Appendix III. The approximate average modulus of elasticity values for the
respective grades, sizes, and species of lumber
were
L-1
L-2
L-3
p.s.i.
p.s.i.
2- by 10-inch southern pine
1.9 x 106 p.s.i.
No. 1D
No. 2D
1.5 x 10 6 p.s.i.
Nos. 2 and
2D mixed
1.5 x 106 p.s.i.
Modulus of Elasticity
2- by
2.0 x 106
1.9 x 10 6
p.s.i.
p.s.i.
p.s.i.
24
Table
maximum bending load, Rather, the maximum
lateral load generally occurred at or shortly
after 50 percent of the maximum bending load
was attained. There was no observed reason why
some loads were higher than others.
12.--Comparison of calculated and average
modulus of elasticity values determined from beam tests
Table 13.--Sumary of maximum lateral loads which developed
near midlength during bending tests
on the lumber supply at the moisture content
condition just prior to beam manufacture.
Lateral Loads
The lateral loads that developed during beam
tests were determined by strain gages on the
lateral pipe supports. The measuring apparatus
was designed to give only the approximate magnitude of lateral loads. Maximum lateral loads
given in table 13 did not generally coincide with
tension laminations and adequate end joints are
used.
CONCLUSIONS
Conclusions are based on bending strength data
from five 40- and three 50-foot-long beams of
both Douglas-fir and southern pine having 301
tension laminations and five 40-foot-long beams
of each species having 301+ tension laminations:
3. The strength and quality of finger joints
need to be evaluated further,
1. The design stress for beams having a 301
grade tension lamination should be less than
2,600 p.s.i.
5. The A-2 grade combination for southern
pine glued laminated timbers should be restudied.
4. The amount of pith-associated wood allowed
in tension laminations should be limited.
6. The modulus of elasticity for the test beams
was approximately the same as a modulus of
elasticity calculated using the weighted average
modulus of elasticity values of the lumber supply
from which the beams were manufactured.
2. Data indicate that beams with 301+ grade
tension laminations could be safely designed for
2,600 p.s.i. bending stress provided the amount
of pith-associated wood is limited in critical
25
LITERATURE CITED
1.
American
1963.
10.
Institute of Timber Construction.
Inspection Manual.
AITC-200-63.
11.
2.
1966.
Timber Construction Manual. John
Wiley & Sons, Inc.
1967.
Tension laminations in structural
glued-laminated
timber
members in bending. AITC 301-67.
Society for Testing and Materials.
Establishing structural grades for
visually graded lumber. ASTM
D24.5-68 T
3.
4.
American
1968.
12.
13.
5.
1968.
6.
Static tests of timbers in structural sizes. ASTM D 198-67.
Bohannan, Billy.
1964.
Prestressed
laminated
wood
beams. U.S. Forest Serv. Res.
Pap, FPL 8. Forest Prod. Lab.,
Madison, Wis.
14.
15.
7.
1966.
Effect of size on bending strength
of wood members. U.S. Forest
Serv. Res. Pap, FPL 56. Forest
Prod. Lab., Madison, Wis.
16.
17.
8.
1966.
Flexural behavior of large gluedlaminated
beams.
U.S.
Forest
Serv. Res. Pap. FPL 72. Forest
Prod, Lab., Madison, Wis.
1968.
Structural engineering research in
wood.
J.
Struct. Div., Proc.
Amer. Soc. Civil Eng. 94(ST2):
403-416, Feb.
18.
9.
through I-5. Dimensions of each section were
approximately 4 by 4 inches by 1/2 inch. Laminations were numbered from the tension face;
No. 1 is the tension lamination, The specific
gravity was determined from volume at time of
test and ovendry weight.
APPENDIX I
The specific gravity and moisture content of
small clear sections from each lamination near
midlength of each beam are given in tables I-1
FPL 113
Curry, W. T.
1961.
Working
stresses
for structural
laminated timber.
Forest Prod.
Res. Spec. Rep. No. 15. Dep.
Scien. Indus. Res., London.
Doyle, D. V., and Markwardt, L. J.
1967.
Tension
parallel-to-grain properties of southern pine dimension lumber. U.S. Forest Serv.
Res. Pap. FPL 84. Forest Prod.
Lab., Madison, Wis.
Freas, A. D., and Selbo, M. L.
1954. Fabrication and design of gluedlaminated wood structural members, U.S. Dept. Agr. Tech. Bull.
1069.
Gerhards, C. C.
1968. Seasoning factors for modulus of
elasticity and modulus of rupture.
Forest Prod. J. XVIII(11): 27-35.
Southern Pine Inspection Bureau.
1965.
Standard specifications for structural
glued
laminated southern
pine timber. New Orleans.
U.S. Department of Commerce.
1963.
Structural glued laminated timber.
U.S. Com. Stand. CS 253-63.
U.S. Forest Products Laboratory
1955. Wood Handbook. U.S. Dep. of Agr.,
Forest Serv., Agr. Handb. 72.
West Coast Lumberman’s Association,
1962.
Standard specifications for structural
glued laminated Douglasfir (Coast Region) timber, Rev.
1963. Portland, Oreg.
Wilson, T.R.C., and Cottingham, W. S.
1952.
Tests
of glued-laminated wood
beams and columns and development of principles of design.
Forest Prod. Lab. Rep. R1687.
26
T a b l e I- 1 . - - S p ecif ic gravit y an d m oist u re content of each lamination of 40 - foot D ouglas - f i r
b eam s m ad e wit h 301 t en sion laminations 1
X
T a b l e I- 2 .- - S p e c i f i c g r a v i t y a n d m o i s t u r e c o n t e n t of each lamination of 40 - foot southern pine
b eam s m ad e wit h 301 t en sion laminations 1
27
Table
I- 3 .- - S p e c ific g r a v ity a n d mo isture content of each lamination of 50 - foot beams made with
301
tension laminations 1
T a b l e 1- 4. - -S p ecif ic gravit y an d m oist u re content of each lamination of 40 -foot D ouglas - f i r
b eam s m ad e wit h 301+ t e nsion laminations 1
X
T a b l e I- 5. - - S p ecif ic gravit y an d m oist u re content of each lamination of 40 - foot southern pine
beams made with
301+
tension laminations 1
29
APPENDIX II
describes the individual boards in terms of
modulus of elasticity, specific gravity, and moisture content.
Numbers at the left end of each beam are those
of the beam and lamination.
All values for MOE, specific gravity, and
moisture content were determined prior to beam
fabrication. The modulus of elasticity was determined by a vibration technique conducted with
each board supported at its ends. The reaction
at one end was recorded and was assumed to
represent one-half the weight of each board.
Some inaccuracies are expected by this method
because of differences of specific gravities along
the length of the boards, The average specific
gravity for each board was calculated from
assumed average board dimensions and the weight
of each board corrected to an ovendry weight
using the moisture content values determined
with a power-loss type meter,
Odd-Numbered Figures
Figures II-1, II-3, II-5, II-7, II-9, and II-11
show examples of boards used for the midlength
portion of tension laminations for both Douglasfir and southern pine beams.
Even-Numbered
Figures
Figures II-2, II-4, II-6, II-8, II-10, and II-12
show properties of the bottom three or four
laminations of the indicated beam, as a means
of depicting beam failure.
The left half of these figures illustrate the location and mode of failure; that on the right
Figure II-I.--Examples of Douglas-fir 2 by 6 boards used for center portion of 301 grade
tension
laminations. Midlength was at 20 feet and the constant moment section during
test was from 16 to 24 feet on the 40-foot-long beams.
FPL 113
30
Figure II-2.--For 40-foot Douglas-fir beams with 301 grade tension laminations, the
failure (left) and lamination description (right) are shown near midlength on the
three outer tension laminations.
For each board the MOE is indicated first (in
miIIion p.s.i.), then specific gravity, and finally moisture content
(in percent).
M 136 736
Figure II-3.--Perspective views of typical southern pine 2 by 6 boards used for center
portion of 301 grade tension laminations.
Midlength of beams at 20 feet and the constant moment section during test was from 16 to 24 feet for the 40-foot-long beams.
FPL 113
32
F i g u r e I I - 4 . - - F o r4 0 - f o o t southern p i n e beams w i t h 301 grade t e n s i o n laminations,
f a i l u r e ( l e f t ) and l a m i n a t i o n d e s c r i p t i o n ( r i g h t ) a r e shown n e a r m i d l e n g t h on t h e
t h r e e o u t e r tension laminations.
F o r each board t h e MOE i s i n d i c a t e d f i r s t ( i n
m i l l i o n p.s.i.), t h e n s p e c i f i c g r a v i t y , and f i n a l l y m o i s t u r e c o n t e n t ( i n p e r c e n t ) .
M 136 735
Figure II-5.--Examples of Douglas-fir 2 by 10 boards used for center portion of 301
grade tension
laminations. Midlength was 25 feet and constant moment section during
test was from 20 to 30 feet on the 50-foot-long beams.
Figure II-6.--For 50-foot Douglas-fir beams with 301 grade tension laminations, the
failure (left) and lamination description (right) are shown near midlength on the
four outer tension laminations.
For each board the MOE is indicated first- (in
million
p.s.i.), then specific gravity, and finally moisture content (in percent).
M 136 734
FPL 113
36
Figure II-8.--For the 50-foot southern pine beams with 301 grade tension laminations,
the failure (left) and lamination description (right) are shown near midlength on the
four outer tension laminations.
For each board MOE is indicated first (in million
p.s.i.), then specific gravity, and finally moisture content (in percent).
M 136 739
FPL 113
38
Figure II-10.--For 40-foot Douglas-fir beams with 301+ grade tension laminations,
failure (left) arid lamination description (right) are shown near midlength of the
three outer tension laminations.
For each board MOE is indicated first (in million
p.s.i.), then specific gravity, and finally moisture content (in percent).
Figure II-11.--Perspective views
of
typical southern pine 2 by 6 boards used for center
portion of 301+ grade tension laminations.
Midlength of beams was at 20 feet and the
constant moment section during test was from 16 to 24 feet for the 40-foot-long beams.
FPL 113
40
Figure II-12.--For 40-foot southern pine beams with 301+ grade tension laminations,
failure (left! and lamination description (right! are shown near midlength of three
outer tension
laminations. For each board MOE is indicated
first
(in
million
p.s.i.),
then specific gravity, and finally moisture content (in percent).
M 136 737
APPENDIX III
The distribution of modulus of elasticity of the
lumber supplies used to fabricate the beams is
shown in figures III-1 through III-4. These modulus
of elasticity values were determined by a vibration
technique. The average moisture contents of the
Douglas-fir boards was about 7 to 8 percent and
of the southern pine 13 percent of the time the
modulus of elasticity was determined.
F igur e III- 1. - - D i s tri b u ti o no f m o dul us of
elas ti c i ty i n th e 2- b y 6- inch Douglas - fi r
I um b e r.
Fi gure III- 2. - - D i stri buti onof modulus of
el asti ci ty i n the 2- by 6- inch southern
pine lumber.
M 136 728
M 136 729
FPL 113
42
Fi gur e I I I- 3. - - D i s tri b u ti o no f mo d u l u s o f
elas t ic it y in th e 2- by 10 - inch Douglas - fi r
um
I ber .
Fi gure III- 4. - - D i stri buti onof modul us of
el asti ci ty i n the 2- b y I O- inch southern
pi nel umber.
M 136 731
M 136 730
43
ABOUT THE FOREST SERVICE. . . .
A s our Nation grows, people expect and need more from their
forests--more wood; more water, fish and wildlife; more recreation
and natural beauty; more special forestproducts andforage. The Forest
Service of the U.S. Department of Agriculture helps to fulfill these
expectations and needs through three major activities:
* Conducting forest and range research at over
75 locations ranging from Puerto Rico to Alaska
to Hawaii.
* Participating
cooperative
wisely use
State, local,
with all State forestry agencies in
programs to protect, improve, and
our Country's 395 million acres of
and private forest lands.
* Managing and protecting the
National Forest System.
187-million acre
The Forest Service does this by encouraging use of the new knowledge
that research scientists develop; by setting an example in managing,
under sustained yield, the National Forests and Grasslands for multiple
use purposes; and by cooperating with all States and with private
citizens in their efforts to achieve better management, protection, and
use of forest resources.
Traditionally, Forest Service people have been active members of
the communities and towns in which they live and work. They strive to
secure for all, continuous benefits from the Country's forest resources.
For more than 60 years, the Forest Service has been serving the
Nation as a leading natural resource conservation agency.
FPL 113
44
1.5-45