Chevron braced frames are popular among steel frame design engineers due to the convenient nature of their flexible geometry which allow for wall openings and movable workpoints along the frame beam. The connections of such configurations are typically left to a contracted connection designer who, in the past, has considered these gusseted bracing connections to be designed in ‘isolation’ apart from the frame beam.
In May 2017, Patrick Fortney, PE, SE from University of Cincinnati presented The Chevron Effect – A Paradigm Shift in Approach to Analysis for SE University. Patrick explained why the Chevron Effect has more recently been identified as an important consideration in the design of chevron braced frames.
Patrick reviewed how using the Net Vertical Force method used by EORs, and designing the connection in isolation as is done by many connection engineers, do not produce the same result when considering beam shear and moment along the length of the frame beam, as shown below.
Using the Chevron Effect Method, the EOR and connection designer have a consistent means to approach this design while considering both local and global effects of the balanced and unbalanced vertical force along the frame beam. When the sum of the vertical components of the brace force are non-zero, as it is in a mechanism analysis, the impact of the brace force on the frame beam needs to be evaluated. In this instance, both the span of the beam and the location of the workpoint become important factors in the resulting shear and moment determined by the chevron effect.
Patrick noted that when the Chevron Effect is not considered, if the actual sum of the vertical forces is relatively small, a Net Vertical Force analysis will significantly underestimate beam moment demand, and may lead to an unconservative design. When the actual summation of the vertical forces is relatively large, a Net Vertical Force analysis can be significantly conservative in regard to beam moment.
Considering the Chevron Effect in the design of the beam of a chevron braced frame will provide the most accurate design, considering both local and global effects of the balanced and unbalanced vertical force along the frame beam.
For information on the research behind the Chevron Effect, see The Chevron Effect – Useful Resources for EORs