Lamination or solid wood?

[Jim Mackell]

I need to replace some broken floor supports in a 100 year old trolley car. The originals are about 72 inches long by 3 inches wide by 7 inches tall. Not sure if they were oak or ash as there's not a whole lot left.

I can laminate some 8/4 ash and come up with the required size. Or I can laminate some 3/4 inch marine plywood and come up with the required size,

These supports have a tremendous amount of stress placed on them, is there any way of determining which lamination would be stronger? Strength is key here, even more than curatorial accuracy.

Thanks for any input.

[Peter Pedisich]

Jim,

Hi, I'm neither an engineer or architect, but I use 18, 24, and 27mm Multiply (russian birch ply) often.
With the plywood lamination, in roughly half the beam thickness the grain is going in the wrong direction. I would think this would be weaker than the solid wood by a good amount, especially in the relatively small 7" dimension relative to the 6' length.

I'm intersted in the answer...hopefully someone in the right profession will weigh in here. (not me!)

[Henry Ambrose]

The laminated solid wood of will be stronger (and stiffer) than the original parts.
Be sure to use a glue that does not creep - one that is recommended for laminating.
West System epoxy would be a good choice. The ash you have should be good too.
Joint and plane each piece for the best result.

[John TenEyck]

The amount of flex you will be in a beam is related to it's Young's Modulus, or stiffness. Here is some info. I found comparing the Young's Modulus of plywood to solid wood:


Wood

Modulus of Elasticity
(in million pounds per square inch)

density
(in pounds per cubic feet)

weight of 4x8 sheet
1/2" thick (in pounds)
MDF
0.53
48
75-85
Oak
1.55
38
60-70
Pine
1.3
29
45-50
Plywood
1.2
33
45-55



And some more on various species of wood:


Common Name/Material
Latin name/source
Young's Modulus, N/m2
Young's Modulus, psi
Specific Gravity
White Ash
Fraxinus americana
1.22x1010
1.77x106
0.60
Yellow Birch
Betula alleghaniensis
1.39x1010
2.01x106
0.62
Butternut
Juglans cinera
8.14x109
1.18x106
0.38
Western red cedar
Thuja plicata
7.58x109
1.1x106
0.32
Black Cherry
Prunus serotina
1.22x1010
1.77x106
0.50
Douglas fir
Pseudotsuga menziesii
1.34x1010
1.95x106
0.48
Shagbark Hickory
Carya ovata
1.49x1010
2.16x106
0.72
Hawaiian koa
Acacia koa
9.31x109
1.35x106
0.4
Red Maple
Acer rubrum
1.13x1010
1.64x106
0.54
Honduras Mahogany
Swietenia macrophylla
1.04x1010
1.51x106
0.52
Black Locust
Robinia pseudoacacia
1.41x1010
2.05x106
0.69
Redwood
Sequoia sempervirens
9.24x109
1.34x106
0.40
White Oak
Quercus alba
1.23x1010
1.78x106
0.68
Englemann spruce
Picea engelmannii
8.96x109
1.3x106
0.35
Sitka spruce
Picea sitchensis
1.08x1010
1.57x106
0.40
Black Walnut
Juglans nigra
1.16x1010
1.68x106
0.55
Ebony
?
1.38x1010
2.00x106
0.65
Graphite carbon / epoxy composite
Moses, Inc. (http://www.mosesgraphite.com/)
6.9x1010
10.4x106
~1.46
What's not explained is whether the E-mod values for plywood were measured parallel or perpendicular to the normal use axis of the product, but I suspect it would have been perpendicular. If so, the values could be substantially higher when turned up on edge, as it would be in your application. But from the data shown, plywood is generally not as stiff as most woods, nor is it as strong. That being the case, solid wood would be a better choice, with either oak or ash having about the same stiffness. Of those two, white oak would be the better choice if you expect water exposure to be an issue. Also, plastic resin glue would be a good choice for glue since it is waterproof and has low creep. Hope this helps. John

[David Gutierrez]

you might want to look at LVL - laminated veneer lumber. it is generally stronger for a given depth than solid lumber. The properties of the material can be looked up online. This comes stanrdard in 1.75" and 3.5" widths. if you google LVL the GP web site can give you properties for this but i am sure they will be equal to oak or ash in strength.

[Jim Matthews]

Finewoodworking posted a video of Gary Bennett building a trestle table. That incorporates some large laminations, including a full length spar that was built for strength.
The video briefly mentions grain orientation for this purpose, around 1:50.

It's cool, really. (http://www.finewoodworking.com/SkillsAndTechniques/SkillsAndTechniquesarticle.aspx?id=34053)

[John TenEyck]

I looked up the Young's modulus of LVL, and it's around 1.8 x 10^6 psi, just about the same as ash and oak, so it would be a good choice as well.

[Jeff Duncan]

Why not use solid wood? I don't see the need to do any laminating for a 3" beam, that's a readily available size and would be my first choice.

good luck,
JeffD

[Stephen Cherry]

Modulus of electicity is not streangth, it's a measurement of the force required to bend. Completely different concept.

[John TenEyck]

Modulus of electicity is not streangth, it's a measurement of the force required to bend. Completely different concept.


And sorta important for a beam.

[Stephen Cherry]

And sorta important for a beam.

This is very true, but you can't assume that you want the stiffest possible beam to fit into an existing structure. The reason for this is that the stiffer structural element will take more of the load because, due to the elastic modului, a structure needs to flex somewhat to carry a load.

Just a side note- several glues are now sold with low elastic modulus listed as a benefit. An example of this is 3m 5200.

[Scott T Smith]

Jim, I have some oak in stock that would allow you to make those beams as solid units. Send me an e-mail if you'd like to pursue. Thx. Scott

[John TenEyck]

This is very true, but you can't assume that you want the stiffest possible beam to fit into an existing structure. The reason for this is that the stiffer structural element will take more of the load because, due to the elastic modului, a structure needs to flex somewhat to carry a load.

Just a side note- several glues are now sold with low elastic modulus listed as a benefit. An example of this is 3m 5200.

You're right, Steve, but I wasn't assuming the stiffest material was best. I was looking to see what the E-mod. was of the wood the OP thought the original beam was, and when someone mentioned LVL I looked that up as well. Since they all are similar they all should work about equally as well. I didn't bother to consider MOR (strength), because if the original beam was oak or ash then those same woods (and certainly LVL) would be strong enough for the new beam, assuming it has the same cross section.

Low E-mod glues would be a good choice for some applications, like where you have materials with coef. of thermal expansion that are widely different, or similar applications. For a laminated beam, the best glue is probably one where the E-mod is similar to or higher than the substrate