Hello and happy Wednesday,

Today, we’ll quickly introduce the timber material properties we use to design timber elements like the compression strength, bending strength and partial safety factors.

These material properties are the fundamentals of structural design.

In this article, we’ll explain what properties are important, where we find them and how to calculate them (if needed).

So, let’s get into it.

The 12 Most Important Timber Properties

Material properties of timber are really important for structural design because they define the resistance of the material like compression or tension resistance.

The resistance or strength properties of timber are defined in different standards (but you can find them online) as there are many different types of timbers. Here are the most used timbers in structural engineering:

• Structural wood
• Glued laminated timber (Glulam)
• Cross-laminated timber (CLT)
• OSB
• Plywood
• and many more

Like concrete, timber has different strength classes, which are categorized by their bending strength. For example, a glulam element with the bending strength of 28 MPa is named a GL28h or GL28c.

Let’s look at the 12 most used timber properties.

#1 Partial safety factor γ_M

The value of the partial safety factor can be found in EN 1995-1-1 Table 2.3. This value is defined differently for the different types of timber. Here is a summary.

And for accidental loads the partial safety factor is:

γ_M.a = 1.0

Note that these values are usually defined differently in the National Annex. Make sure to check it.

The partial safety factor is used to calculate the design resistance from the characteristic resistance/strength values, which we’ll define in the next sections.

In general (for all materials) we calculate the design resistance with EN 1990 (6.6c):

R_d =R_k/γ_M

With R_k as the characteristic resistance and γ_M the partial safety factor.

But for timber structures we have another factor that we need to include to calculate the resistance: The factor k_mod.

Design value of a strenght property of a timber element is calculated according to EN 1995-1-1 (2.14):

R_d = k_mod ⋅ R_k/γ_M

We'll cover k_mod later.

#2 Service classes

The service classes are used to assign timber elements strength values in regards to their environmental conditions [EN 1995-1-1 2.3.1.3].

• Service class 1: Moisture content in the materials corresponding to 20°, relativ humidity of the surrounding air only exceeding 65% for a few weeks per year
• Service class 2: Moisture content in the materials corresponding to 20°, relativ humidity of the surrounding air only exceeding 85% for a few weeks per year
• Service class 3: Higher moisture content in the materials than in service class 2

#3 Load-duration classes

The load duration classes also have an influence on the strength parameters of the timber elements. They are characterised by the effects of a constant load acting for a certain period of time in the life of the structure [EN 1995-1-1 2.3.1.2].

Here are the different classes from EN 1995-1-1 Table 2.1:

Here are some examples of different loads for the assignment of the load duration according to EN 1995-1-1 Table 2.2.

#4 Modification factor k_mod

The modification factor kmod is the value that combines the effects of the load duration and the moisture content of the timber into one factor. Here's the table with the kmod values for the different timber materials.

#5 Bending strength f_m.g.k

f_m.g.k is the characteristic bending strength of timber. The value depends on the type of timber (Glulam, structural wood, OSB, etc.) and the strength class.

You can find the values online on manufacturer homepages or in timber books.

As said earlier, the strength class is named after the characteristic bending strength. A C24 structural wood element has a bending strength of 24 MPa.

In the end of this email I made an overview of the strength values of the different types of timber.

The bending strength is used in almost every structural design verification of timber elements because almost every element needs to be checked for bending.

#6 Compressive strength parallel to the grain f_c.g.0.k

One difference of timber compared to steel and concrete is that it has different capacities parallel and perpendicular to the span of the elements. This is due to the grain of timber.

In fact, timber is super weak perpendicular to its grain. And as a designer we need to consider this from the beginning.

f_c.g.0.k is the characteristic compressive strength parallel to its grain. Again, the value depends on the type of timber (Glulam, structural wood, OSB, etc.) and the strength class.

The compressive strength parallel to the grain needs to be checked for elements with compressive normal forces, like for example columns or beams of frames.

#7 Compressive strength perpendicular to the grain f_c.g.90.k

As I already mentined, timber is very weak perpendicular to its grain (almost by a factor of 10 compared to parallel to the grain).

Therefore we need to check compression perpendicular to the grain for every beam at supports or elements that are exposed to point loads.

These days it's not too uncommon to reinforce beams to increase the compressive strength perpendicular to the grain. I have done it in many projects.

We will look at how to do this in a future episode.

#8 Tensile strength parallel to the grain f_t.g.0.k

f_t.g.0.k is the characteristic tensile strength parallel to its grain.

Same as for compression, the tensile strength parallel to the grain is much bigger than perpendicular.

#9 Tensile strength perpendicular to the grain f_t.g.90.k

f_t.g.90.k is the characteristic tensile strength perpendicular to the grain.

The tensile strength perpendicular to the grain needs to be checked in some cases for roof elements when the wind load is greater than the dead load of the roof (often the case during construction).

#10 Shear strength f_v.g.k

f_v.g.k is the characteristic shear strength.

We need to check the shear strength of elements mainly at supports.

#11 E-modulus E_0.g.mean and E_0.g.05

The E-modulus together with geometrical properties of the cross-section define the stiffness of timber elements. Therefore the E-modulus has an effect on the internal forces of statically indeterminate systems like continuous beams and it's needed to calculate the deflection.

But the E-moduli E_0.g.mean and E_0.g.05 are both used in different design verifications. Here's what they are:

• E_0.g.mean: Mean value of modulus of elasticity (used to calculate the deflection)
• E_0.g.05: Fith percentile value of the modulus of elasticity parallel to the grain (used for the buckling verification of columns and lateral torsional buckling)

#12 Density ρ_g.k and ρ_g.mean

The density is used to calculate the self-weight (dead load) of the structure but also to calculate the stiffness and capacity of connections.

Strength Capacity Tables for the Different Timber Types

Here is an overview of the strength capacities of the different types of timber.

#1 Structural wood

#2 Glulam

#3 OSB

Final Words

These 12 timber properties are not the only ones, but the main ones we use in most design verifications. We don’t want to overcomplicate things in the beginning.

I’ll see you next Wednesday for episode #3 of the timber design series.

Cheers,

Laurin.

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