Every December, Schaefer’s Cincinnati office eagerly awaits the upcoming holiday season. To spread holiday cheer, one particular quad of workstations always stands out with its cheerful candy cane + string light display. Lights are strung between two candy canes, effectively creating an inviting doorway into the workspace.
One morning, we arose to a fright (we returned to the office to find a tangled mess of lights on the floor). Our colleagues quickly repaired it and went about their day, only to find the lights on the floor again the next morning.
Before we take a page out of National Lampoon’s Christmas Vacation, this is really a simple structural engineering problem.
Every holiday season, we have clients reach out about hanging string lights around + from their structures. Whether the lights are hanging around our workstations or 30 ft in the air crisscrossing Main Street, the physics is the same.
Let’s break it down.
The physics behind hanging string lights
When hanging a string of lights, the smile-shape it forms is known as a catenary curve. This natural shape occurs when a flexible chain or string is suspended by its ends and acted on by gravity. The primary force in the string of lights is tension. This force acts along the length of the string, pulling it taut between the two anchor points. The tension force has both horizontal and vertical components. The weight of the lights creates a downward force at the candy cane anchorage, while the tension in the string pulls the lights inward. If these forces are not properly managed, the decorations might end up on the floor.
In our case, the horizontal component of the tension was what pulled in on the candy cane display and caused it to fall. The combination of this force and not having a great place to tape the candy cane to the cubicle walls led to the candy canes tipping over in the night.
How to hang string lights correctly
- Know your variables – they’ll impact the tension force. Heavier lights will create a higher tension (+ lateral) force to maintain equilibrium within the system. Adding ornaments or additional decorations will also increase the weight, further impacting the tension.
- The distance between support points is also crucial. If the same length of lights is strung across a longer span, there will be less sag in the curve. With less sag, the horizontal component of the force actually increases, which requires stronger anchorage to keep the decorations from falling. This is because a longer span requires more tension to keep the lights taut.
- Additionally, the height and position of the anchor points can influence the stability of the lights. You’ll want string lights to be at least eight ft above the ground. If the anchor points are not at the same height, the catenary curve will be uneven, potentially leading to one side being more heavily loaded than the other. This imbalance can cause the lights to fall more easily.
- Think about the aesthetics of your anchor points. For example, in transforming a roadway into a lively pedestrian-friendly street, engineers might install sturdy columns on either side, anchored into the ground or into concrete-filled planters. These columns support the string lights, creating a charming, well-lit space for community events. This setup not only enhances the aesthetic appeal, but contributes to safety + durability.
By understanding these forces and their interactions, better solutions can be designed and implemented to keep holiday decorations securely in place. This knowledge is not only applicable to festive decorations but also to day-to-day applications, such as suspension bridges, fire curtains + overhead power lines.
String lights are probably not something that you think you’d need a structural engineer for, but if you’re hanging them in a public setting, it’s worth running the numbers.
