Optimizing Beam Hanger Placement in Mass Timber Structures

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Mass timber buildings have gained significant traction in recent years, with building code updates making it easier than ever for designers to create taller timber structures meeting prescriptive code requirements. Their popularity is further boosted by their sustainability, visual allure, and abbreviated construction timelines compared to traditional building methods. A factor contributing to these expedited schedules is the prefabricated nature of mass timber components, including cross-laminated timber (CLT) wall, floor, and roof panels, as well as glued-laminated timber (glulam) beams and columns. These elements can be efficiently assembled on-site using either custom steel or pre-engineered connectors.

Beam hangers, a type of pre-engineered connector, are often used to connect beams to columns in mass timber structures. Their placement on the beam end is influenced by three main factors: applied gravity loads, connection fire protection requirements, and seismic lateral drift limits. While wind loads do cause lateral drift, there are currently no codified limits for drift due to wind. Seismic drift limitations are established in ASCE 7, and in some cases, also subject to local building code regulations.

The ideal placement of the beam hanger with respect to the neutral axis of the beam varies among these factors, requiring designers to accommodate sometimes conflicting needs. Figure 1 illustrates optimal positions for gravity demands (below the neutral axis), fire protection (above or around the neutral axis), and lateral drift accommodation (on the neutral axis).

To reduce wood splitting risk under gravity loading, designers should place the beam hanger at the lowest practical point on the beam end while complying with geometry requirements, including prescribed end and edge distances (Fig. 1A). This positioning helps reduce stress concentrations at the lowermost fastener, which can induce cross-grain tension and lead to splitting. Reinforcement, such as self-tapping screws, can be used to minimize splitting but requires careful detailing and coordinated installation. Reinforcement is typically unnecessary if the ratio between the distance from the top fastener to the loaded edge and the beam depth exceeds 0.70, as shown in Figure 1A. This guideline is known as the “70% rule.”

Designing fire protection for mass timber connections requires understanding the demands on connections during a fire and the options for complying with fire and building code requirements. In exposed mass timber construction, where fire resistance is provided by the mass timber itself, beam hangers are often placed higher in the beam end to provide adequate wood cover at beam-to-column joints. If this placement contradicts the "70% rule," reinforcing screws can be added at the potential origins of stress development. These screws can be generally countersunk into the timber to isolate them from fire. This process involves drilling holes prior to installation and then sealing them with wood plugs (Fig. 1B). Research, such as that conducted by Létourneau-Gagnon et al. (Applied Sciences 11.8 (2021): 3579) in Canada, is currently underway to further examine the performance of self-tapping screws when exposed to fire.

When accommodating lateral drift, the ideal beam hanger placement is on the neutral axis of the section. The top and bottom edges of the beam experience the highest levels of stress and deformation. By placing the beam hanger closer to the neutral axis, the associated stresses and deformation are reduced.

A comprehensive approach is essential to achieve optimal connection performance under seismic loading, taking into account factors like gravity loading and tensile stresses perpendicular to the length of the column. If deviations from the "70% rule" occur, reinforcing screws can be installed perpendicular to the grain to mitigate adverse effects. Seismic drift introduces tension and stress in the fasteners in the column, heightening splitting risk. To counter this effect and maintain a continuous load path, additional perpendicular-to-grain reinforcing screws may be required in the column (Fig. 1C). Furthermore, if fire safety presents a concern, the previously mentioned methods to protect reinforcing screws should also be applied.

In practical applications, beams usually support CLT panel floors. These panels are attached to the underlying beams using screws, resulting in added stiffness and allowing for composite behavior between the beam and the panel system.

One effective measure to manage seismic-induced deformations at these connections involves placing the beam hanger on the neutral axis of the assembly. This positioning considers the influence of the CLT panel above, which shifts the center of rotation. Failing to account for this impact can significantly increase the prying forces exerted on the connector, potentially compromising its structural integrity during seismic events.

For simplicity’s sake, a model comprising a CLT panel attached to a beam connected to a column is used to illustrate the seismic impact, as shown in Figure 2A. In this model, the connector is positioned on the neutral axis of the beam, instead of the entire assembly.

During a seismic event, as the column undergoes racking to the left, the CLT panel imposes minimal force on the connection because there is no direct load path (Fig. 2B). In contrast, when the column racks to the right, the CLT panel bears against it, establishing a direct load path that elongates the lever arm. This scenario exacerbates the prying effect at the connection (Fig. 2C), resulting in increased tension within the assembly.

Two effective strategies exist to mitigate the stresses at beam-to-column connections during earthquakes, both of which involve isolating or eliminating the load path for these stresses.
One approach is to create a greater distance between the narrow edge of the CLT panel and the column face. This wider gap reduces bearing at the interface, thereby diminishing the panel’s impact on joint stiffness (Fig. 3A). Proper sizing of the gap or machining of the panel end is essential to fully isolate the panel under maximum drift condition.

Another approach aligns the column flush with the top face of the beam, enabling the CLT panel to extend over the column (Fig. 3B). To prevent direct load transfer between these components, the panel should be notched at the bottom, directly above the column. This design can improve the panel’s isolation from the column in response to seismic movements.

For buildings or sections where connection fire protection requirements are not mandatory, an alternative approach can be used to address drift-induced stresses at beam-to-column connections.
When fire protection regulations permit an exposed connector or do not require a bonded joint, increasing the gap between the beam and the column can effectively delay the onset of a direct load transfer between them, thereby reducing stresses caused by imposed drift (Fig. 4A). This isolating approach can improve joint performance in a drift scenario and thus should be considered during design.
In situations with an exposed connector as shown in Figure 4, the gap size matches the connector thickness (assuming no housing or cut-end detailing), which may not fully prevent bearing of the CLT panel end against the column under significant drift conditions. In cases where a smaller gap is required to meet connection fire protection requirements, bearing may take place even at lower levels of drift. In either scenario, the behavior will align with earlier discussions when drift intensifies sufficiently to close the gap and initiate bearing at the interface.

Until the gap at the interface closes, the connector’s position has minimal effects on joint stiffness or stress development; instead, it determines the joint rotation where bearing initiates. Under this condition, the connector independently supports gravity load and rotational demands, resembling a soft hinge. Reinforcement must be applied as needed to minimize the risk of wood splitting. Situations with elevated seismic risks and specific design considerations may necessitate a larger gap between the CLT panel and the column, as illustrated in Figure 4B.

Optimizing the placement of beam hangers in mass timber structures represents a critical aspect of ensuring their structural integrity and safety. This process requires a holistic approach that addresses gravity loading—a constant concern—alongside fire safety, seismic drift, or both. There are multiple approaches to mitigate seismic drift-induced stresses at beam-to-column connections, and the selection among these methods depends on the prioritization of fire protection in the overall design considerations. ■

About the Authors

Dong Han, Ph.D, is a Senior Scientific Writer with MTC Solutions. He studied advanced materials for environmental, energy, and biomedical applications and has been involved in scientific publishing for over a decade.

Lori Koch, MS, PE, is a Senior Product Engineer with MTC Solutions. She is a board member for SEAVa, serves on the NCSEA Continuing Education Committee, and is a licensed Professional Engineer in Virginia (support@mtcsolutions.com).

Max Closen, MASc, is President of MTC Solutions. After completing a timber engineering degree at the University of Applied Sciences in Rosenheim, he pursued a master’s degree at the University of British Columbia, where his research on mass timber connections contributed to their first commercial application.