What are the defects in Timber due to Natural Forces?

1.Knots: Knots are the most common defects caused due to natural forces.During the growth of a tree,branches close to the ground or lower branches die. Bases of those branches remain in the tree as the trees grow. These bases may create imperfection known as knots.
Types of Knots: Knots are of two types.
*Dead knots: The remains of damaged branches after drying out they become loose and fall out.
*Live knots: They are sound and firm. If small, are not great of a defect.
Live knots are usually not a problem as they remain firmly attached to the timber. But in dead knots, they are loosely attached and reduce strength. Knots decrease the strength of the wood and thus lower its value for structural uses. Knots cause serious defects when the load is perpendicular to the grains.
2.Twist: Twist in timber rotates the ends of the timber in opposite directions. The main reason behind this defect is twisting of the trees by the strong wind.
3.Shakes: Shakes are timber defects that occur around the annual ring or growth ring of a timber. In other words, cracks or splits in the woods are called shakes.
It may or may not be a structural problem depending upon depth and use. The main problem is aesthetic. Where the appearance is important, shakes are undesirable.
Types of shakes: Shakes can be classified into three main categories:-
*Star Shakes: This type of shake starts propagating from the bark towards the sapwood and sometimes even towards the heartwood along the lines of medullary rays. Cracks are wider on the outer edge or bark and narrower on the inside (usually sapwood, sometimes heartwood). The main reasons behind star shakes are extreme heat or frosting during the growth of the trees and rapid or uneven seasoning after cutting off the timber. Extreme heat or frost causes temperature difference, which causes shrinkage leading to the crack.
*Cup and/or Ring Shakes: Cup shakes follow the annual growth ring. It is capable to separate the growth ring partially or completely. When the crack separates the annual ring completely, it is called ring shakes. So, all ring shakes are cup shakes, but all cup shakes are not a ring shape. Excessive frost action is the main reason for this type of crack.
*Heart Shakes: Unlike star shakes, heart shakes starts propagating from the pith to the sapwood along the lines of medullary rays. Shrinkage of the interior part of the timber causes this crack.
*Rind Galls: THe meaning of rind is bark and gall is abnormal growth. So abnormal growth of the bark of the trees is called rind galls. Improper cutting of branches causes this abnormal growth. Wood from this portion of the timber lacks strength and desirable in structure.
*Upsets: Upsets in various wood indicate that the tree was subjected to crushing or compression. Improper felling of trees, heavy wind blowing during the young age of the tree these are the main reasons behind this type of defect.