Lateral Stability

Posted by in Structural terms | February 21, 2012

Lateral instability is a relatively common cause of structural failures, because it is often forgotten or misunderstood.  Lateral instability is a result of inadequate lateral stability.  Simply defined, lateral stability is the property of an object to develop forces or to have forces imposed upon it that restore it to or maintain its original condition (position).  A laterally unstable structure or structural member is able to twist, buckle sideways, or fall over.

One example of lateral instability is the twisting or the sideways buckling of a yardstick or a 2×4 set on its narrow edge as a beam when a heavy object is placed on it.  Another example of lateral instability is an egg placed on its end or a slender masonry (brick and/or block) wall: if either of these lean in any direction, their own weight causes them to fall to the side.

Lateral stability is commonly provided for a beam, a column, a steel or wood frame, or a wall with lateral bracing.  Lateral bracing is the structural component that prevents the beam or column from twisting or buckling sideways or the structural component that prevents the steel frame, wood frame, or wall from falling over.

In the case of a beam, the top of the beam is often laterally braced by joists (open web steel joists that look like trusses or I- or C-shaped member) that frame into the side of it or by attaching the floor deck or roof deck to the top of the beam.  A column is commonly braced in the lateral direction with the beams that frame into its sides.  The lateral bracing for a steel frame, a wood frame, or a masonry wall is usually provide intermittently by the floor deck or near its top by the roof deck in combination with diagonal bracing and/or cross walls.  Lateral bracing prevents a steel frame, a wood frame, or a masonry wall from falling over under its own weight or during exposure to an impact or wind pressures.

It is unusual for a beam, a column, a steel or wood frame, or a wall in a completed structure to be inadequately braced in the lateral direction because they are usually intentionally or inadvertently braced by the adjacent structural components previously mentioned.  However, lateral instability is not uncommon during the construction or demolition phases.  During these phases lateral stability must be considered and temporary lateral bracing provided where necessary.  Lateral bracing is commonly provided during the unstable phases with temporary guy wires, diagonal braces, X-bracing, bracing to a nearby completed structure, etc.

The occurrences of structural failures, injuries, and loss of life over the decades due to lateral instability has prompted the preparation and publishing of recommendations, guidelines, and/or requirements in industry standards, building codes, and/or OSHA safety rules.  Standards and codes providing recommendations and guidelines and building codes providing requirements are available through non-governmental organizations for the construction and engineering industry.  Safety rules are available through the labor departments of the state and/or federal governments.