Hey everyone, let’s talk about something I get asked a lot here at our seismic isolation shop: retrofitting existing structures with Tension Resistant Friction Pendulum Bearings (TRFPBs). If you’re a structural engineer, building owner, or anyone involved in upgrading old infrastructure, you know how tricky it can be to bring a decades-old structure up to modern seismic standards. TRFPBs are a game-changer here because they handle tension (super important for bridges, parking garages, and mid-rise buildings where uplift from earthquakes used to wreck regular bearings) but I’ve seen a lot of folks mix up retrofit steps with new installations. Let me break this down like I would with a customer on the phone—no jargon overload, just real stuff we’ve learned from retrofits over the last 12 years.
First off, let’s make sure we’re on the same page: TRFPBs aren’t your standard bridge bearing, linear natural rubber bearing, Lead-core Rubber Bearings (LRB), or High Damping Rubber Bearing (HDRB). Wait, no—wait, actually, those other bearings have their place, but TRFPBs fill a gap when traditional friction pendulum bearings (FPB) or rubber bearings can’t resist uplift. Traditional FPBs work great for lateral movement, but they lose their grip if the structure tries to lift off the ground during a quake. TRFPBs add that tension resistance, which is non-negotiable for retrofits where the original design didn’t account for strong vertical seismic forces. That’s why when we’re talking retrofits, we start with something most people skip: a full structure health audit.
Step 1: Pre-Retrofit Assessment (The Groundwork You Can’t Skip)
You wouldn’t put new tires on a car without checking the alignment and frame, right? Same here. A lot of teams rush into ordering TRFPBs because they heard they work for seismic issues, but they miss critical details that make the retrofit fail. First, we do a site survey: check the existing foundation’s condition—cracks, soil bearing capacity, any past repairs. Then, we run a dynamic analysis specific to the structure. For example, if it’s a 40-year-old bridge, we need to see how the original piers and abutments handle the TRFPB’s unique forces. We also test the current bearing capacity of the structure’s superstructure (the part above the bearings) because TRFPBs add a different load path.
Wait, here’s a common mistake: some engineers try to retrofit with standard FPBs without accounting for tension. We had a customer last year who did that on a parking structure in Southern California, and during a small test quake simulation, the bearings lifted, causing 2 inches of horizontal shift and a cracked beam. Switching to TRFPBs fixed that, but it cost them an extra 3 weeks and a change order. Moral of the story: the audit has to include seismic vertical force data, not just lateral. Also, we compare the structure’s current performance to ASCE 7-16 (that’s the standard for seismic loads in the US) because old structures often don’t meet modern uplift requirements.
Once the audit is done, we narrow down the TRFPB specs. Size matters here. A small single-story building might need a TRFPB with a 100-ton tension capacity, but a bridge over a river might need 500 tons. We also consider the sliding surface material—some retrofits prefer PTFE-coated surfaces for low friction, but if the structure has heavy dead loads, we might go with a composite material to reduce wear. This is where knowing the other bearing types helps: if the structure only has minor seismic issues (no uplift risk), we might suggest a different option, but if uplift is a problem, TRFPBs are non-negotiable. I always tell customers to use our resources—like the info on our Friction Pendulum Bearing page—to compare, but if tension is on the table, don’t hesitate.
Step 2: Existing Bearing Removal (Gentle Does It)
Now, the actual work. First, we have to remove the old bearings, but we can’t just jack the structure up willy-nilly. That’s a huge red flag. When retrofitting, the structure is already stressed from age and past loads, so jacking has to be synchronized. We use hydraulic jacks with a monitoring system that checks the load on each jack every second—no uneven lifting, which would cause more cracks in beams or foundations.
For structures with multiple bearings (like a bridge with 12 bearings), we remove bearings one at a time, not all at once. Why? Because removing all bearings at once would leave the structure unsupported, and even a small shake from a nearby truck could cause it to shift. We also support the structure with temporary shoring during this process. The shoring has to be designed to handle the same seismic loads as the new TRFPBs—we can’t just use 2x4s here. We recommend working with a structural engineer who’s familiar with seismic retrofits for this step, because a bad jacking job can set the whole project back months.
Another thing: old bearings often have corrosion or debris around them, especially if it’s a bridge or a structure in a coastal area. We have to clean the bearing seats thoroughly before installing the new TRFPB. If there’s rust on the seat, we grind it down, then apply a corrosion-resistant coating. If we skip this, the new bearing won’t seat properly, and the friction performance will drop over time. We learned this the hard way on a project in Florida—we didn’t clean the seat enough, and a year later, the TRFPB’s sliding surface was worn unevenly. Now we add a 4-hour cleaning step during removal, no exceptions.
Step 3: TRFPB Installation (Precision Is Everything)
When it comes to putting in the TRFPBs, installation is way different from new construction. New construction can align bearings with laser levels, but existing structures have slight unevenness in the foundation and superstructure. We adjust for that by using shim packs made of steel or high-damping rubber, depending on the load. The goal is to make sure the TRFPB is level within 1/32 of an inch—any more and the friction performance will be off, especially during lateral movement.
Wait, let’s talk about tension resistance specifically. TRFPBs have a tension mechanism (usually a cable or a spring assembly) that connects the superstructure to the foundation. We have to make sure this mechanism is pre-tensioned correctly—too much and it will pull too hard during small shifts, too little and it won’t engage during a quake. We use a torque wrench calibrated to our specs, and we test the pre-tension before finalizing the installation. We also add a seal around the TRFPB to keep out water, dust, and debris—super important for retrofits because old structures often have gaps that let in contaminants.
Also, we align the TRFPB’s sliding direction with the structure’s lateral force direction. For example, a bridge’s main span will have lateral forces along the bridge’s length, so we align the TRFPBs to slide that way. If we get this wrong, the bearing won’t handle the movement during a quake. We always use a laser alignment tool here, and cross-check with the structural analysis data from the audit. Oh, and one more thing: we don’t reuse the old anchor bolts. Even if they look good, old bolts can have metal fatigue from decades of loads. We install new high-strength anchor bolts specifically designed for TRFPBs—they’re different from the bolts used for standard bridge bearing or even HDRBs, because they have to handle tension and shear.
Step 4: Post-Installation Testing & Calibration (Don’t Skip This)
A lot of teams install the bearings and call it a day, but we always do a two-part test: static and dynamic. Static testing means we apply a load to the TRFPB to check the pre-tension, the sliding resistance, and how it sits in the seat. We use a hydraulic press to apply a load equal to 1.5 times the structure’s dead load, and we measure any movement. If the TRFPB moves more than 1/8 of an inch, we adjust the shims.
Dynamic testing is trickier. We simulate seismic movement using a shake table—some projects use a small table for individual bearings, others use a computer simulation for the whole structure. The key here is to check if the TRFPB handles both lateral movement and tension. For example, if the simulation shows a 10-inch lateral shift and 2 inches of uplift, does the tension mechanism engage smoothly, or does it bind? We had a test where a TRFPB’s tension cable bound because the installation crew didn’t align the cable correctly—we adjusted it, and it worked the second time. Now we do a 24-hour post-test inspection to make sure there’s no wear or damage.
We also do a long-term monitoring check. Retrofitted structures have to handle 20+ more years of loads, so we install small sensors on the TRFPBs to track temperature, wear, and tension over time. This helps catch issues early—like a corroded sliding surface or loosened pre-tension—before they become a problem. A lot of customers ask if this is worth it, and we tell them: we’ve had a sensor catch a corroded part 18 months after installation, which saved them $100k in repairs. That’s way cheaper than fixing a bearing during a big quake.
Step 5: Post-Retrofit Documentation & Maintenance Plan (This Is How You Keep It Working)
After installation, we give the customer a full documentation packet: audit reports, installation photos, test results, and a maintenance plan. This is super important because building codes require proof of seismic upgrades, and the maintenance plan ensures the TRFPB stays effective. The plan includes annual checks: clean the sliding surface, check tension, inspect anchor bolts. Every 5 years, we do a deeper inspection, including non-destructive testing for the tension mechanism.
Wait, let’s circle back to the other bearing types here. Sometimes, after a retrofit, a customer might wonder if they should switch to a different bearing if they have a minor seismic issue. We always recommend comparing—like, if the structure no longer has uplift risk after the retrofit, a linear natural rubber bearing or LRB might be cheaper, but if uplift is still a risk, TRFPBs are the only way. We make sure our customers understand the difference, because we want them to make the best choice for their structure, not just the most expensive.
Now, let’s talk about common myths I hear all the time. First: “TRFPBs are only for new construction.” No, we’ve done over 20 retrofits across the US, from parking garages in LA to bridges in Oregon. Second: “Retrofitting with TRFPBs will disrupt the structure too much.” We use staged installation (one bearing at a time) so the structure stays functional during the project. Most retrofits only take 4-6 weeks, depending on the size, so the customer’s revenue or traffic isn’t interrupted for long. Third: “You can install TRFPBs on any old foundation.” No, that’s why the initial audit is so important—we have to check if the foundation can handle the new loads, and if not, we can add foundation upgrades at the same time as the bearing installation.
Here’s a real example: last year, we retrofitted a 3-story office building in Seattle that was built in 1975. The original design didn’t account for vertical seismic forces, and during a 2019 small quake, some of the bearings lifted, causing a ¼-inch crack in a beam. We did the audit, found that the foundation was in good shape but needed minor reinforcement, installed TRFPBs with a 150-ton tension capacity, and tested it during a simulation that matched a M7 quake—no lift, no cracks, perfect lateral movement. The customer told us it was a lot less disruptive than they thought, and the cost was 20% less than replacing the entire superstructure.
Now, let’s wrap this up. Retrofitting with TRFPBs isn’t just “swap old bearings for new ones”—it’s a process that starts with a full audit, careful removal, precise installation, testing, and long-term maintenance. It’s not for beginners, but with the right team, it’s the most effective way to bring an old structure up to modern seismic standards, especially when uplift is a risk. If you’re working on a retrofit project and you’re not sure if TRFPBs are the right fit, don’t hesitate to reach out to our team—we’re here to walk you through the steps, compare options like bridge bearing, linear natural rubber bearing, Lead-core Rubber Bearings, High Damping Rubber Bearing, or Friction Pendulum Bearing, and help you make the best choice for your structure.
I know this was a lot, but I wanted to give you the real, on-the-ground stuff we deal with every day. Retrofits can be stressful, but when you follow these steps, you end up with a structure that’s safe for decades to come. Don’t cut corners—skip the audit, and you’ll end up paying for it later. Reach out to our team to get started on your TRFPB retrofit project.
References
- American Society of Civil Engineers. (2017). Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE 7-16).
- Kelly, J. M. (2016). Seismic Isolation: Theory and Practice. John Wiley & Sons.
- Federal Highway Administration. (2018). Seismic Retrofitting Manual for Highway Bridges. U.S. Department of Transportation.
- Skinner, R. I., Robinson, W. H., & McVerry, G. H. (2006). An Introduction to Seismic Isolation. John Wiley & Sons.