The Aspect Effect

April 3, 2017 by Arnaud Thibonnier

I was recently asked to review a project and provide commentary to the owner/developer concerning the use of steel braces in their single-story commercial building.  The main question we needed to answer was “why does my building have structural steel bracing in it instead of wood sheathing?”

The structure consisted of Cold-Formed Steel (CFS) studs/joists for the walls and roof.  Large openings were framed with steel beams supported by steel columns, and the lateral system primarily consisted of steel braces.

We reviewed the structural and architectural documents and observed most bracing walls were penetrated by large windows.  While windows are a terrific architectural feature that take advantage of natural lighting for interior spaces, their inclusion may complicate the lateral bracing scheme.

For a single-story structure, a cost-effective way to brace the structure is by use of the sheathing (plywood etc.) that is applied to the walls of the structure.  However, per the American Iron and Steel Institute Standards (AISI – the governing body for CFS stud design in the US), the maximum aspect ratio (height/length) that a plywood or gypsum sheathed wall segment used for bracing the structure is 2:1 (refer to Table C2.1-1 and C2.1-2 in AISI S213-07/S1-09(2012)).  This means that a wall segment that is 10’ tall must have a minimum solid length of 5’ to be considered effective as a lateral bracing element.

In the subject project, most of the exterior walls were unable to meet this criterion and thus the engineer of record needed to provide another way to brace these walls; structural steel bracing.  While this bracing was structurally necessary, the addition of this framing did increase the cost of the project which was not originally anticipated.  So, if you are working on a similar project, we recommend keeping an eye on the aspect ratios and discussing with your structural engineer the implications of not meeting the aspect ratios.

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Written by Arnaud Thibonnier

Arnaud Thibonnier

Arnaud is the Director of Structural Engineering Services in our Austin office. He received his Bachelor of Science – Architectural Engineering and Master of Science – Civil Engineering from The University of Texas at Austin. He is responsible for the design of efficient and effective structural systems for various types of commercial and residential projects, specifically multi-family and mixed use projects. Arnaud has over 10 years’ experience directing various structural evaluation and construction projects. In his free time, he enjoys following college football and spending time with his two dogs.