Investigating the Influence of Beam Hanger Connection Rotational Stiffness on Glulam Column Design

Abstract

The potential of using glue-laminated timber (glulam) post-and-beam frames for long spans and open spaces is increasingly being explored in North America. To control vibrations and deflections, deep glulam beams are connected to columns using various types of beam hangers, forming gravity load-resisting frames. Pre-engineered, often concealed beam hangers provide high load-carrying capacities in shear and allow for fast on-site assembly. In current design practice, a pinned condition is typically assumed for such beam-to-column joints. However, when frames deform horizontally under lateral loads, a bending moment can develop if the connection displays rotational stiffness. This moment would reduce the resistance of columns designed as axially loaded members, and therefore should be accounted for to ensure a safe design. In this study, the bending moments that arise in glulam columns connected to beams using pre-engineered beam hangers are examined to address the question of whether these columns should be designed for combined axial forces and bending moments. A finite element (FE) model is developed for a commercially available connector using the software RFEM. The model is validated using existing experimental data. Then the connection is implemented in a single-storey, two-bay frame model to investigate the moment developed at the post-and-beam joints, and its impact on the axial load-carrying capacity of columns. A parametric study is conducted with the FE model to examine the sensitivity of bending moments to beam depth and beam-to-column stiffness ratio. It is concluded that pre-engineered beam hangers induce significant moments, up to 132% of the column’s bending utilization under code level drifts, suggesting that glulam columns in post-and-beam frames should be designed for combined bending moment and axial loading. Additionally, beam hanger connections contribute significantly to the lateral stiffness of the system, which may impact the fundamental period and seismic behaviour of a structure.