Functional loss of C- joists due to large stiffened web openings: Experiments

Abstract

Cold-Formed Steel (CFS) C-Joists (joists) support floors and roofs. Knowledge of the structural behavior (local, distortional, and, global buckling modes) of CFS joists has matured and been codified over the last three decades. Research has also advanced estimating the influence of web openings on structural strength focusing on local buckling. However, insufficient experimental data is available for the global behavior of joists with large stiffened web openings (openings) that are increasingly becoming popular to allow the passage of utilities. Deflection is often the controlling joist design parameter, especially for large spans. Reported herein are results of experiments conducted by loading a large number of joists with openings deeper than half of the web height and wider than the opening's depth. It is demonstrated herein how an equivalent moment of inertia and allowable load for joists can be obtained. Joist test specimens cover multiple combinations of web heights, thicknesses, sizes and intervals of openings, and, joist spans. The functional loss (loss) is measured in terms of the increase of flexural deflection due to openings. The loss was found to be negligible for some combinations and as high as 35 % for some others. Closer spacing and larger openings result in greater loss; and, unexpectedly, shorter spans also increase the loss. Research presented herein underscores the need, and offers experimental data, for validating computer models, updating national design standards, estimating loss due to similar punchout geometries for computing the effective moment of inertia, and, ultimately, the allowable load for CFS joists.