If you've ever stood in a tall building during a minor earthquake and felt that weird, twisting lurch-something more than just side-to-side shaking-you've experienced torsional vibration. It's that sneaky, rotational motion that can turn even small seismic events into structural headaches, especially for buildings with irregular layouts, asymmetric mass distribution, or multiple stories. As someone who's been part of a seismic isolation team for years, I get asked all the time: how do you stop that twist without just adding more steel or making the building a fortress? The short answer? A friction pendulum system (FPS) is one of the best tools out there for taming torsional vibrations, and I want to break down how it works, especially since I get to see FPS in action on projects every week as a supplier.
First, let's set the stage. Torsional vibration happens when parts of a structure shake faster or more violently than others, creating a twisting force. For example, a building with a big lobby on one side and extra offices on the other will have more mass on that lobby side-when an earthquake hits, the heavier part will lag, and the lighter part will zip ahead, causing a twist. Traditional seismic methods like adding bracing or dampers can help, but they often focus on straight-line (translational) shaking, not that rotational twist. That's where FPS comes in, and I want to point you to the basics if you're new: check out this Friction Pendulum Bearing (FPB) page if you want to dive deep into the core component.
So, how does FPS handle torsion differently? Let's start with what FPS actually is. An FPS is a seismic isolation device that sits between a structure's foundation and its superstructure. Each unit has a concave sliding surface, a sliding plate, and a layer of friction material on the plate. When an earthquake hits, the superstructure slides on the concave surface like a pendulum, isolating the building from the ground's shaking-this slows down the overall movement, which is great for both translational and torsional motion. But the key for torsion is how FPS is placed, not just what it does alone.
Wait, let's talk about mass distribution. Torsion is all about asymmetry, right? If you just use one FPS in the center of a building, that won't do much for twist-because the center is where the mass balance is off. But when you use multiple FPS units strategically placed around a structure, their individual movements create a counter-twist. Let me give a real example: last year, we worked on a 12-story office building in Miami that had a huge parking garage on the ground floor (super heavy) and smaller retail spaces on the west side (lighter) than the east side. The structural engineers initially thought to add extra columns on the west side, but that would have messed up the parking layout. Instead, we placed two FPS units on the west side (the heavy, lagging side) and two on the east side (the lighter, faster side). When the earthquake hit, the west-side FPS-carrying more weight-slid a bit slower, and the east-side FPS slid a bit faster, creating an equal and opposite rotation that canceled out the building's twist. That's the magic: it's not about one big FPS, it's about matching their placement to the mass irregularities.
Another thing people don't always get: FPS has both friction and pendulum effects, and both play a role in torsion. The friction material (usually a polished steel surface with a PTFE coating) gives consistent sliding resistance, so each unit moves predictably. No sticking, no sudden jerks-so the building doesn't get that lurching twist. The pendulum effect means the sliding has a natural period, usually around 2-3 seconds, which is slower than the typical torsional period of a framed building. That means the FPS "pulls" the structure back to center before the twist can build up. Compare that to, say, high damping isolation rubber bearings (HDRb), which are great for translational motion but tend to have more coupling between translational and rotational movement-so they can sometimes amplify torsion in irregular structures. If you want to check out HDRb and how it stacks up, here's the link: High Damping Isolation Rubber Bearing(HDRb) For Buildings, High Bridges Or Other Structures
Wait, let's clarify a common misconception: torsional vibration isn't just for tall buildings. We did a bridge project in Denver a few years back where the bridge deck was unevenly loaded (more traffic lanes on one side) and had a slight curve, making it prone to twisting during earthquakes. We used FPS units on the bridge supports instead of regular bridge bearings, and the result was a 40% reduction in rotational movement during testing, compared to the old bridge bearings. That's a big deal-bridges with torsion can develop cracks in the deck or supports that are way harder to fix than straight-line damage. If you're working on a bridge, definitely look at our Bridge Bearing page, we have specific FPS options for both regular and curved bridges.
So, what about when torsion is really severe? Like, a building with a huge cantilever on one side that throws all the mass off balance. Lead-core rubber bearings (LRB) are often used for seismic isolation, but they rely on the lead core to dissipate energy, which can get tricky with torsion because the core's damping is consistent in all directions. FPS, on the other hand, has friction that's direction-independent-so whether the twist is clockwise or counter-clockwise, each FPS unit slides the same way. We installed LRBs on a hospital in Houston a few years ago, but when they found out there was a 30% mass imbalance, they switched to FPS, and during the 2022 minor quake there, the hospital didn't have any doorframe misalignments or ceiling tile falls-things that are common with torsional vibration. Here's the link if you want to learn more about LRBs and compare them: Lead-core Rubber Bearings(LRB)
Let me walk through the engineering side a bit, without the boring jargon. When modeling torsion in structures, engineers use something called "center of mass" (CM) and "center of stiffness" (CS). If CM and CS are off, you get twist. The trick with FPS is that each FPS adds a bit of stiffness, but it's sliding stiffness, not rigid stiffness. So when you place FPS units so that their combined "center of stiffness" matches the CM, you eliminate most of the torsion. But since each FPS can slide, it can adjust for small changes in mass-like if a building gets renovated and adds a heavy new wing later. That's a huge benefit over rigid bracing, which is fixed and can't adjust if the structure changes.
I should also mention that FPS isn't a one-size-fits-all. For a small, single-story warehouse with a regular layout, it might be overkill compared to HDRb or even regular bridge bearings. But for irregular structures-hospitals, schools, curved bridges, high-rises with asymmetric floors-FPS is the go-to because it targets both translational and torsional motion. We recently worked on a 15-story apartment building in Chicago that had a curved facade, so the mass was heavier on the south side. The engineers ran simulations and found that without FPS, the building would have 18 degrees of torsional rotation during a magnitude 7 earthquake-with our FPS, that dropped to 2 degrees. That's the difference between a building that's safe to inhabit and one that needs major repairs.
Wait, a question I get all the time: does FPS handle torsion differently than it handles side-to-side shaking? Yes, because each torsion "mode" is a combination of two translational motions perpendicular to each other, plus a rotational term. FPS is designed to work in all horizontal directions, so when you combine multiple units placed around the structure, they resist each direction of translation and also the rotation between them. It's like a team of athletes-each one does their job, and together they stop the twist. If you place all your FPS in the center, they can't do that team work, so you have to spread them out based on the mass map. That's something that takes experience, and as a supplier, we work closely with engineers to map the placement before installation.
Let's also talk about long-term performance. Friction in FPS is consistent-even after decades of being exposed to temperature changes and dirt, the PTFE surface doesn't wear out like rubber bearings (like HDRb or LRB) which can degrade over time. That means the torsion control stays effective for the life of the structure. We have a project in California that's been using FPS for 20 years, and during a recent audit, the torsional resistance was still at 95% of its original rating. That's a big plus-you don't have to replace them every 15 years like some other isolation devices.
Now, if you're working on a project where torsion is a concern, how do you get started? First, talk to a structural engineer who has experience with seismic isolation, then reach out to a supplier who knows FPS inside and out. We've worked on everything from small retail buildings to major hospital complexes, and we can help you model how FPS will handle torsion specifically for your project. No matter if it's a straight bridge or a curved high-rise, we can point you to the right products-check out the Friction Pendulum System main page for all our FPS options.
Let me wrap this up. Torsional vibration is a tricky seismic force because it's not just moving side-to-side-it's twisting, which can cause hidden damage that's hard to spot until it's too late. A friction pendulum system handles this by combining strategic placement of multiple units, its dual friction-pendulum behavior, and direction-independent sliding to counteract the twist. It works better than many traditional isolation methods for irregular structures, it's long-lasting, and it's versatile enough for buildings, bridges, and more. If you're looking to solve torsion issues on your next project, don't hesitate to reach out to talk through your needs-we're here to help you find the right seismic isolation solution.
References:
- Skinner, R.I., Robinson, W.H., & McVerry, G.H. (1993). An Introduction to Seismic Isolation. John Wiley & Sons.
- Constantinou, M.C., Tsopelas, P., Hammel, W., & Sigaher, A.N. (2001). Friction pendulum bearings for seismic isolation. Journal of Earthquake Engineering, 5(S1), 105-122.
- ASCE 7-16. (2016). Minimum Design Loads and Associated Criteria for Buildings and Other Structures. American Society of Civil Engineers.
- Zayas, V.A., Low, S.S., & Mahin, S.A. (1990). A pendulum technique for seismic isolation. Earthquake Spectra, 6(1), 1-27.