Concern has often been expressed as to the capacity of large wooden beams, girders and post columns (heavy timber) with visible horizontal cracking and their ability to handle the applied service load

Wood is an organic material rather than a manufactured material such as concrete, steel, or masonry and it tends to be less homogeneous and more susceptible to various reoccurring random defects. Lumber is generally classified according to it’s species, size, and natural variations..

Dimension Lumber consists of pieces from 2 to 4-inches thick and 2-inches or more in width (these are generally utilized as studs, joists, and rafters).

Beams & Stringers are pieces of milled lumber that is 5-inches or more in thickness with a width that is more than 2-inches greater than the thickness.

Posts & Timbers are pieces of milled lumber that’s 5" x 5" and larger where the width is not more than 2-inches greater than the thickness.

Wood is subject to three basic common defects which result in the majority of the cracking generally seen in larger solid-sawn pieces. These are shakes, checks, and splits.

Shake a lengthwise separation of the wood along the grain, usually occurring between or through the rings of annual growth. A surface shake occurs on only one surface, while a through shake extends from one side to the opposite or adjoining side of the piece.

Check a lengthwise separation of the wood along the grain which normally occurs across or through the rings of annual growth. This usually results from seasoning (drying shrinkage) of the timber.

Split a lengthwise separation of the wood due to the tearing apart of wood cells, extending through the piece from one surface to the other.

Since longitudinal cracks are more critical, when resisting tension forces acting parallel to the grain in a beam, than they are when resisting compressive forces parallel to the grain found in post columns, this discussion will be limited to the horizontal cracks seen in beams. Shear is the tendency of two equal and parallel forces acting in opposing directions to cause the adjoining surface layers of a member to slide one on the other. In a top loaded rectangular wood beam this internal stress tends to tear the beam in half longitudinally. This internal force, when acting parallel with the grain against the area of the cross-section, is known as the horizontal shear stress ( ).

Horizontal shear strength in a beam is dependant on the shearing strength of the particular species of wood and upon the extent of shake, check, and split cracking present. In a beam with long multiple cracks or which is split full length, a major portion of the shear stress is redistributed and carried by the upper and lower halves of the split member. Research has established that this "two-beam" shear action allows a cracked beam to carry loads for which it would appear to be inadequate in terms of horizontal shear.

All tabulated horizontal shear values for the various species of lumber listed in the Uniform Building Code for 3-inch and thicker lumber are established conservatively (as if the piece were split full length). These table values, used in basic design to size a beam, may in fact be increased by 100% (doubled) for members with no split, shake, or check cracking.

Nearly all beams with obvious longitudinal cracking are over designed. Therefore, there is little reason for concern about the future performance of a solid-sawn wood beam or girder exhibiting these types of cracks.

For further information about PACE Investigative Services, please contact Sherrie Henderson at 818-251-1200 or e-mail her at shenderson@hfinc.com.