Physical Properties

Janka Hardness:

This is a measurement of how hard the timber is and is a good indication to its resistance to wear.  It is determined by measuring how much force is required to imbed an 11.18mm diameter steel ball halfway into the timber.  Be careful when comparing from different sources as some measure the force in different units (newtons, pound-force, kilogram-force, etc).

Modulus of Rupture / Bending Strength:

This can be thought of as the short-term load capacity of a beam when the load is applied slowly.  It represents how much weight the timber can hold before failing.

Modulus of Elasticity / Stiffness:

This can be thought of as how much timber will move under a heavy load, or by how much it will deflect its load.  This can be more important than the Modulus of Rupture, because although a timber beam may be able to handle a heavy load, if its deflects excessively it might cause lining or surrounding materials to crack or the timber beam could even set up vibrations in the structure when subject to transient loads.

Maximum Crushing Strength:

Also known as the 'Compression Strength Parallel to Grain'.  This indicates the strength attained under a load slowly added parallel to the grain.  It can indicate the suitability for use as short columns.

Strength Group Unseasoned:

Rather than list multiple strength properties for each timber, a serious of strength groups have been defined.  Unseasoned (green) timber can be classified into seven strength groups, from S7 (weakest) into S1 (strongest).  Each strength group has minimum requirements as detailed in the tables below.  Sometimes when there is not enough data available to rate a timber, a provisional strength grouping may be assigned based on the timbers density.

Strength Group Seasoned:

As explained above, timbers have been grouped into strength groups for simplicity.  Seasoned (12% moisture content) timber is classified into eight strength groups ranging from SD8 (weakest) to SD1 (strongest).  Minimum requirements are detailed in the tables below.

Compression Strength Perpendicular to Grain:

This is the maximum accross-the-grain stress of a short duration that can be applied to a plate covering a portion of the timber surface without damaging the timber.

Shear Strength Parallel to Grain:

Is the strength attained when the applied force cause the timber to fail by the sliding of one part upon another across the grain.

Minimum values for strength groups for green timber: