1 Laboratory Three: Parallam Beam Deflection Lab Group - 1 st Mondays, Late: Jesse Bertrand, Ryan Carmichael, Anne Krikorian, Noah Marks, Ann Murray Report by Ryan Carmichael and Anne Krikorian E6 Laboratory Report – Submitted 12 May 2008 Department of Engineering, Swarthmore College Abstract : In this laboratory, we determined six different values for the Elastic Flexural Modulus of a 4-by- 10 (100” x 3.50” x 9.46”) Parallam wood-composite test beam. To accomplish this, we loaded the beam at 1/3 span with 1200 psi in five load increments in both the upright (9.46 inch side vertical) and flat (9.46 inch side horizontal) orientations. We then used three different least- square methods (utilizing Matlab and Kaleidagraph) on the data for each orientation to fit the data, resulting in the following: E: Upright Orientation E: Flat Orientation Units 10 3 ksi 10 3 ksi Method One 0.981 ± 0.100 1.880 ± 0.046 Method Two 1.253 ± 0.198 2.080 ± 0.083 Method Three 1.065 ± 0.247 1.881 ± 0.106
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Laboratory Three: Parallam Beam Deflection
Lab Group - 1st
Mondays, Late:
Jesse Bertrand, Ryan Carmichael, Anne Krikorian, Noah Marks, Ann Murray
Report by Ryan Carmichael and Anne Krikorian
E6 Laboratory Report – Submitted 12 May 2008
Department of Engineering, Swarthmore College
Abstract:
In this laboratory, we determined six different values for the Elastic Flexural Modulus of a 4-by-
10 (100” x 3.50” x 9.46”) Parallam wood-composite test beam. To accomplish this, we loaded
the beam at 1/3 span with 1200 psi in five load increments in both the upright (9.46 inch side
vertical) and flat (9.46 inch side horizontal) orientations. We then used three different least-
square methods (utilizing Matlab and Kaleidagraph) on the data for each orientation to fit the
data, resulting in the following:
E: Upright Orientation E: Flat Orientation
Units 103 ksi 10
3 ksi
Method One 0.981 ± 0.100
1.880 ± 0.046
Method Two 1.253 ± 0.198
2.080 ± 0.083
Method Three 1.065 ± 0.247 1.881 ± 0.106
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Purpose:
The purpose of this lab is to determine the flexural elastic modulus of a Parallam wood-
composite beam by examining its behavior when simply supported and under flexural stress, and
to analyze deflection data using different least-squares methods to fit theoretical deflection
curves.
Theory:
In theory, a beam’s deflection can be mapped by the governing equation of beam flexure:
EI d2y/dx
2 = M(x), where E is the elastic modulus, I is the second moment of inertia about the
neutral axis of the beam (the value of which changes significantly according to orientation), y is
deflection, and M(x) is bending moment in the beam. This equation requires that several
assumptions be made about the beam:
1) Geometric Assumption: the beam must be a straight, prismatic member with at least one
axis of symmetry.
2) Material assumption: the beam must be linear, elastic, isotropic, and homogeneous, and
the modulus of elasticity in tension must equal the modulus of elasticity in compression.
3) Loading Assumption: the beam must be loaded in pure moment in a plane of symmetry.
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4) Deformation Assumption: plane sections before bending must remain in plane after
bending.
Making these assumptions, we can apply the general equation for beam flexure to our
experiment. Assuming we are using point loads or can model our setup with point loads, we can
then use singularity functions to determine that the bending moment of the beam is:
2/3 P*x – P <x-L/3>1
Where P is the load applied with the UTM, L is the length of the beam, and x is the
distance from the origin (defined as the end closest to the applied load). From this we get:
M(x) = EI d2y/dx
2 = 2/3 P*x – P <x-L/3>
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.
To solve for the constants we need to make two more assumptions: that when x=0 and
when x=L there will be no deflection (i.e. y=0). Using these assumptions, we can plug into our
previous equation and use algebra to determine that c1 = -5PL2/81 and that c2 = 0. This gives us:
Taking an integral of both sides
with respect to x yields:
where c1 is a constant. Taking
another derivative yields:
where c2 is a constant. Rearranging
we get:
EI dy/dx = P/3 * x2 – P/2 * <x-L/3>
2 +c1
y * EI = P/9 * x3 – P/6 * <x-L/3>
3 +c1x + c2
y = Px3/9EI – P/6EI * <x-L/3>
3 +c1x/EI + c2/EI
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y = P/EI (x3/9 - <x-L/3>
3 /6 – 5L
2x/81)
This is the theoretical beam deflection equation for the lab.
Then, to ease calculations, we make the previous equation non-dimensional by
multiplying both sides by EI/PL3, which yields:
yEI/PL3 = (x/L)
3/9 - <x/L – 1/3>
3/6 – 5/81 (x/L)
We define this dimensionless quantity as: (x/L)3/9 - <x/L – 1/3>
3/6 – 5/81 (x/L) = !theoretical
where: !theoretical = f(x/L)
Similarly, we define: ymeasured * EI/PL3 = !measured.
If the beam were to behave as a theoretical beam, then !theoretical would equal !measured.
E is defined as the slope of the stress-strain curve in the elastic region. However, there is
no perfect way to measure stress and strain in the loaded beam. As a result, to determine E one
must make some assumptions. For methods one and two the assumption made is that !theoretical =
!measured. This is done because !measured can only be calculated if the value of E is known (if E is
unknown, then the equation ymeasured * EI/PL3 = !measured has two unknowns and is thus
unsolvable). For method one this assumption is used to write this equation:
f(x/L) = E (I ymeasured/PL3).
Manipulating this equation gives an equation in the form: P = E (I ymeasured/f(x/L)L3)
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This equation is in the form of y=mx, the form of a line. Thus, if it is plotted
P versus (I ymeasured/f(x/L)L3)
then the slope of the line will be E. In method two, the same assumption is made, resulting in the
formula:
E = f(x/L)PL3/ ymeasuredI
From this formula E can be calculated on a point by point basis and then the values can be
averaged.
Method three approaches the problem in a different way. Instead of assuming that
!theoretical = !measured, a new quantity V was defined as:
V = !theoretical - !measured
Then we make a guess for the E value and solve for the rms error, defined as:
rms = sqr(1/n * sum(V2))
where V represents the difference between theoretical and measured deflection for every data
point at a certain E value, and n is the total number of V values (5 loads * 4 locations = n = 20).
The rms error is then plotted against the many guessed E values, and the point on the graph
where the rms error is minimized is determined to best the best value of E for method three.
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Procedure:
In the lab, we tested a simply supported Parallam beam (nominal dimensions: 4 by 10) in
two orientations while loaded in flexural stress from the UTM (setup shown in figure 1). The
beam’s dimensions were 100 inches span by 3.50 inches thick by 9.46 inches deep. Our two
orientations were with the 9.46 inch side vertical (the ‘upright’ orientation) and then with the
9.46 inch side horizontal (the ‘flat’ orientation). For each orientation, we applied an approximate
point load by placing a roller between the UTM and the beam at the point L/3 on the span. (In
fact, as the roller comes into contact with a small area of the beam and not a single point,
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describing it as a ‘point load’ is not quite accurate.) We applied the load in five increments: 240,
480, 720, 960, and 1200 psi. At each of the load increments, we measured deflection at three
points: L/4, L/2, and 3L/4 (the UTM recorded deflection at L/3). We also observed the deflection
and the location of maximum deflection, and calculated values of I (the second moment of
inertia) for each orientation.
Outside of lab, we used three methods to determine E. As discussed in the theory, method
one consisted of plotting the load P (lb) versus the quantity Iymeasured/f(x/L)L3 (in
2). The slope of
this graph was the first value for E. Both Matlab and KaleidaGraph were used in this process.
Utilizing the same theory as method one, method two used the equation E = f(x/L)PL3/ Iymeasured
to solve for E for each individual point with each load. The resulting values of E were then
averaged to determine the best value of E for method two. The average was found using Matlab
and the error using KaleidaGraph. Method three (also as discussed in the theory section) plots
rms error against many guessed E values. The best value of E (for method three) was found by
determining where the rms error was minimized. This process was done entirely in Matlab.
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Results:
E: Upright Orientation E: Flat Orientation
Units 103 ksi 10
3 ksi
Method One 0.981 ± 0.100
1.880 ± 0.046
Method Two 1.253 ± 0.198
2.080 ± 0.083
Method Three 1.065 ± 0.247 1.881 ± 0.106
Discussion:
The values of E that we determined for each orientation were very close in value. The
values for the upright beam all fall within error of each other, while for the flat orientation one
value (while still very close) was just outside of the error of the other two, which are nearly
identical.
Error in our values comes partially from universal measurement error, and from flaws
and inconsistencies in the beam (i.e., a non-isotropic and non-homogeneous beam); these types
of error have a global influence on our results. Other major sources of error are method-specific.
In method one, there is error from fitting a line to a set of data that is not precisely linear. As a
result, we took our method one values from Kaleidagraph, which is more specifically graphing
software and which provides a curve fitting error. We also used Matlab values as a check of
accuracy. In method two, error came from the variance in the E value of each data point. For this
method, we used Kaleidagraph simply to determine error (having calculated the values in
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Matlab), taking the standard deviation as representative of the variance. In method three, error
comes from the lack of a perfect fit of a deflection graph to our data; our E value minimized the
error between predicted and actual deflection, which was then represented as rms error. In all of
the methods, we weighted each data point equally (this will be discussed more thoroughly later
in this section).
Interesting to note is the difference in the value of E for each orientation. This is partially
a result of the composition of the beam, as, upon inspection, the grain of the wood is
pronouncedly evident (see Appendix F). We expect the material that the beam is composed of to
behave more rigidly when loaded to parallel the grain (in the upright position), and to bend more
easily when loaded perpendicular to the grain (in the flat position). The grain of the wood is
largely the result of the beam’s construction, as it is fabricated from strips of wood bound with
glue and pressed until formed: this method of construction results in a major difference in
stiffness, according to orientation. However, although the material does perform more rigidly
according to grain orientation, the difference in the value of I made a more significant impact on
our final values of E and, thus, the beam behaved more rigidly in the flat orientation, where the I
value was significantly smaller.
Another point of interest was the location-specific variation in E: upon examination of
the graph for method one (in appendix B), this variation becomes apparent. The data points
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collected at L/4 and 3L/4 are the two points closest to the left at each load; the slope of the line
formed by connecting these points is steeper than the rest, which means that the resulting E value
is higher. The data points from L/3 appear at the far right at each level of load, and when
connected have a lower slope and therefore smaller E value. The data points from L/2 are in the
middle, and have a slightly less steep slope than at L/4 and 3L/4, and thus a slightly smaller E
value.
It should be noted that the E value that differed most from the other values is the E value
at L/3, which was the value determined in part by the deflection measured by the UTM. The
difference in measuring methods may be the cause of this. It is possible, for instance, that the
UTM measurements include deflection from the compression of the beam itself where the load
was applied, or alternately that the measurements from the UTM were more accurate than those
we found by manually reading gauges. However, as we do not know whether those values are
more or less accurate than the others, we have no definite reason to weight it differently one way
or the other and, thus, weight all four locations equally.
Having found values for E, we then decided that the best value for each orientation was
that determined with method three. This is because the method three values are the median
values for each orientation, and they do not rely on the assumption that we based methods one
and two on. Numerically, this yields the following values: for the upright orientation,
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(1.065 ± 0.247) *103 ksi, and for the flat orientation, (1.881 ± 0.106) * 10
3 ksi.
Conclusion:
The results of this beam testing determined six values of E for the beam, three in the
upright orientation and three in the flat orientation. Of the E values, those for each orientation
were very similar to the other E values for the same orientation; from these values, the method
three values were deemed most reliable. Furthermore, if only one E value were to be chosen to
tell a consumer, it would be (1.881 ± 0.106) * 103 ksi, the value from method three for the flat
orientation. This choice was made with the assumption that the consumer would know that it was
only the E value for one orientation, and the flat value chosen because the flat orientation seems
a more likely choice for flexural loading, and also because it is the more appealing value and so
more competitive on the market.
References:
Figure 1 adapted from Zachary Eichenwald, Swarthmore College, May 2007.
Matlab created with assistance from Noah Marks et al.
Appendices H and I are the intellectual property of E. Carr Everbach, Swarthmore College, May
2008.
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List of Appendices: A……………………………………………. Lab Data
B……………………………………………. KaleidaGraph Graphs for Method One
C……………………………………………. Matlab Graphs for Method One
D……………………………………………. Matlab Graphs for Method Three