Columns

Columns, or posts, carry axial compressive loads from beams and sometimes other components down to the foundation or other supporting component below. The capacity of of columns is governed by stability as much as by strength. Three of the more common materials used for columns in residential and smaller buildings are sawn lumber wood posts, steel jack posts, and CMU (concrete block) piers.

Euler buckling describes the tendency of a column to suddenly deflect laterally under compressive load before the material itself fails. The critical buckling load depends on the column’s cross section, length, end conditions, and material stiffness. In structural engineering, a column is considered either slender or short. Slender columns have higher length to cross section, and are controlled by buckling. In this case, capacity is inversely proportional to the square of the effective length, meaning that even small increases in unbraced length can significantly reduce load capacity. This makes stability, rather than crushing strength of the column material, the governing failure mode for many columns.

Sawn lumber wood posts are widely used due to their availability, relatively low cost, and ease of installation. Wood has relatively low stiffness compared to steel, which makes it more susceptible to buckling at shorter unbraced lengths. Additionally, wood is anisotropic and has natural defects such as knots, which further reduce its capacity. To mitigate buckling, wood posts are often laterally braced by framing elements like walls or sheathing. A typical SYP #2 4×4 wood post, 8 feet in length and unbraced, can support approximately 6000 lbf. Buckling controls, and the capacity more than doubles if the post is properly braced at mid-height. The required strength of the lateral brace is usually between 1% to 2% of the braced column’s axial load.

Steel jack posts, often used as adjustable temporary or supplemental supports, offer much higher material strength and stiffness. Their smaller cross-sectional dimensions, however, can make them quite slender. As a result, despite steel’s superior compressive strength, jack posts can be sensitive to Euler buckling if not properly braced. The effective length factor plays a critical role in this case as end conditions such as pinned or fixed connections influence buckling capacity. Fixed connections, such as a steel post embedded in concrete, result in more capacity. In many cases, ensuring adequate lateral restraint or reducing the unsupported height is more important than increasing material strength. Typical steel strength is 36000 lbf per square inch, but 50 ksi is also common.

CMU block piers are also often used to support vertical gravity loads. They are typically constructed by stacking the blocks in mortared bed joints, or about 3/8 inch of mortar between each horizontal course of block. Sometimes the CMU cells are reinforced with steel and grouted. Concrete has relatively low tensile but high compressive strength, usually between 2500 to 5000 psi. However, CMU blocks are not assigned that much compressive strength. Individual block strength requirement is commonly 1900 psi, but only 1500 psi is allowed after a CMU wall or pier is constructed. Additionally, ungrouted CMU piers are usually assumed to have pinned end conditions, which further lowers their critical buckling load.

ADDITIONAL RESOURCES:

National Design Specifications for Wood Construction

Timber Construction Manual

AISC Steel Construction Manual

ACI 530