Future Forward Interview: Assessing and Mitigating Structural Performance
Christopher Higgins is a professor of structural engineering in the School of Civil and Construction Engineering at Oregon State University (OSU), where he also directs the Structural Engineering Research Laboratory. Informed Infrastructure Editor Matt Ball spoke with Higgins about his work assessing and mitigating underperforming bridges.
I2: What is the name of your research lab, and what types of problems are you tackling?
Higgins: I’m at Oregon State University in the school of Civil and Construction Engineering, and my field is structural engineering. Our lab is named the very bland Structural Engineering Research Laboratory. It allows us to investigate structural components and pieces of buildings under all types of different simulated loading conditions. The types of loads we’ve looked at included the classics such as seismic and wind, but with our well-known wave research laboratory, we also look at water effects such as breaking waves and ponding that leads to the collapse of roof structures. We do some of the loading investigations that a lot of other places don’t have the capability of doing.
I2: Waves are certainly a hot topic now with all the changes in the coastal zone. What types of structures are you looking at?
Higgins: If you talk to the ocean people, their idea of a structure is very different than mine. They think in terms of a jetty and the like. We do real structures, such as bridges. We don’t focus much on the offshore, such as oil-production rigs. We’re focused more in the nearshore or onshore structures affected by waves, particularly structures impacted by hurricanes or tsunamis. That includes structural components and scale models of buildings and bridges.
I2: You’re putting these through stress tests and simulated scenarios as well as load tests?
Higgins: I’ll give you an example: one of the things we do that we’ve taken from the earthquake community. Just as they would do a shaking table and a reduced-scale model and understand how it responds under dynamic shaking, and then build a full-scale model and subject it to the forces they measured from the shaking table but at full scale from the small scale.
In our long-wave flume, for example, we build a 1:10 scale model of a highway bridge that is subjected to hurricane wave loading. We measure the forces applied on that structure and build a full-scale prototype of the connection of that bridge to the substructure. We then subject that connection to the forces and force effects we measured on our structures’ laboratory strong floor, taking our small-scale measurements and blowing those up to full scale. We look at how that full-scale component would have responded to those loads. We call that hybrid simulation.
I think we’re the first in the world to take it from small-scale modeling up to a full-scale structural response under wave-load conditions.
I2: Is it tricky to extrapolate small to full scale?
Higgins: There are scaling issues at really small scales, because water starts to act like honey. The bigger the scale you can produce, the less the scaling effect. We try to do large-scale models, and we have one of the largest facilities to be able to do that. On the structure side, we try to do that only at full scale to get away from the scaling effects, particularly when we’re working with concrete, for example.
I2: Do you investigate failures as part of your work?
Higgins: The type of work I do isn’t on new design as much as it is on evaluation, inspection and retrofitting of existing infrastructure so it can continue to be serviceable. Often it’s not failure, but performance problems.
We worked on the biggest research program that Oregon DOT (ODOT) ever funded, which started in 2003. It was a $1.4 million research project over a two-year period of time. The ODOT has a lot of 1950s-era reinforced concrete bridges in inventory, because they were a darling of the agency at the time. Once you have a design and designers that are comfortable, you often stick with that and adapt it to different locations.
We have a large population of these types of bridges, and, in 1950, we were transitioning to rebar—it hasn’t always looked like it does today. From 1920 to 1950, there were a variety of proprietary types of bars. You’d have all kinds of deformation patterns and shapes to get bond enhancement and be able to get the stresses in and out of the bars. As a designer, you didn’t know what kind of steel was going to show up, so they would add a lot of details to anchor the bars—hooking them and bending them out of the tension zone and anchoring them in the compression zone.
If you look at a design with the same span with the same girder size prior to 1950, they all have these well-detailed bars. Then we came up with the standardized form bars, and early tests on bond strengths showed good deformations to get the force in and out of the bars. The code changes and these new standardized ASTM bars with a deformation pattern that is identical to what we have today, and now we can just cut them off with straight-bar terminations.
As loads and volumes increased on these 1950s-era bridges, that ended up producing big diagonal cracks. That’s always scary to engineers, because we recognize those diagonal failures as non-ductile—they will occur all of a sudden.
That’s the problem ODOT was facing. We did a lot of work looking at all the different details and properties on full-scale bridge girders that we designed to look like these 1950s bridges with the same as-designed details. We worked to understand how they fail, the different failure modes, and we looked at high- and low-cycle fatigue, different spacing of reinforcing bar, and different cutoff details.
We had 44 full-scale specimens and a lot of analytical work; we developed methods to assess these bridges. We looked at the loading conditions in the state of Oregon, which are different than other states as we allow fairly heavy loads compared to national standards. We developed our own load factors based on traffic within our state, and when we compared all of those things, we came up with ways to keep bridges in service that they thought they would have to replace.
It’s difficult to quantify the return on investment on research. ODOT estimated they saved $500 million from this work, because there were a lot of bridges that didn’t have to get torn down. The new tools allowed us to assess the remaining capacity of these bridges.
I2: For the existing cracks, was there a suggested remediation as well?
Higgins: After that work, we’ve done almost every type of repair approach that you could imagine. We’ve done such things as surface-bonded carbon fiber using what’s called near-surface-mounted carbon fiber. Surface mounting means you cut grooves in the concrete skin, being careful not to cut any of the underlying steel, and then you glue in what’s called a pre-cured carbon fiber bar with high-strength structural epoxy. We’ve done embedded high-strength steel. We’ve done external steel. Now we’re looking at titanium.
I2: Many of these materials are new, certainly at this scale. How do you explore the application of these materials?
Higgins: Our view is looking at the structural properties in terms of its mechanical strength, stress/strain behavior, bonding behavior, and getting stresses in and out of the material. Then we look at how the macro structural element performs and build that up to system performance. We build from the pieces up, but not from the microstructure of the material up. We’re not so much interested in the grain size and chemistry of the material as much as the mechanical performance.
I2: Is it difficult to source these newer materials?
Higgins: Cost is the major concern for everyone in the industry—first cost, even. We have the first titanium-reinforced bridge in the world, which crosses I-84, a major east/west transportation corridor. That bridge, based on my calculations, was very close to collapsing onto I-84. ODOT did emergency shoring on that bridge and then put in a plan to strengthen it.
We provided two alternatives. One was carbon fiber, and the other was titanium, based on the research we were doing at the time. Titanium came up 30 percent cheaper than the carbon-fiber alternative, even though carbon fiber has been in the market a lot longer.
Most people assume titanium is very expensive. From my perspective, I’m not looking at a cost per pound of the material, I’m looking at a cost for performance. When you think about the performance you can gain from the materials, you can go beyond what tradition would tell you.
Titanium has features that make it very attractive. It has high strength, but it comes with ductility. I can use traditional methods to fabricate it; I can bend it in a rebar bender, and I can shear it in a rebar shearer; and I can use it as I would carbon fiber, except that it’s ductile carbon fiber. So I can get 11 percent elongation, for example, and at the very high strength of 140 kilopound per square inch (ksi). Because I can produce a mechanical hook, just by bending the end of the bar, I can often make use of those material properties, where I often wouldn’t with carbon fiber.
I2: Does the ability to use common tools and skills factor heavily into the price?
Higgins: Any time you have a first, nobody knows what the costs are. That’s the hardest thing in terms of “breaking the mold” and trying something new. Then people want to know the specification for it, and we have to make up the specification. We’re not just pulling it out of a hat, we have to create it. So there are challenges to new approaches and new materials.
Fortunately, we were doing the research on full-scale models in the laboratory, and on that bridge we retrofitted, we built full-scale replicas of those girders with the repair technique that we were proposing. We demonstrated that we could generate much more strength than what was required to put the bridge back in service.
I2: In terms of the software and modeling that you do, does that provide the means to communicate these approaches to the broader structural engineering community?
Higgins: It’s a slow process of getting this out to the broader community. Mostly it takes time. People have done some work to compare the new idea to when it actually gets broadly adopted, and the timeframe is in the neighborhood of seven years. There is a long time horizon.
Civil engineering is a mature field, and we think we know what we are doing. We hesitate to try new things because we can make traditional ways work. That’s part of what leads to the long lag time. Our application from the laboratory to the field on that titanium-reinforced bridge retrofit was less than two years. That’s very fast, and it just happened to be working on a similar retrofit problem in the lab when ODOT faced it in the field.
I2: I also understand that you’ve developed software specific to the evaluation of gusset plates. Can you speak to that research and the development of software to help others address this issue?
Higgins: The work on gusset plates came from the collapse of the I-35W Bridge in Minneapolis that raised questions regarding the performance and safety of steel-truss bridges. Departments of transportation are all required to go out and assess the gusset plates in all of their steel-truss bridges.
For many DOTs, they don’t have drawings for their bridges. It’s hard to do an evaluation if you don’t have drawings of it, so you often send out a crew with tapes to get measurements to pass along to engineers to do calculations, which is very time consuming, and you get variable results, and it’s not easy to do. If you send two people out to measure the same gusset plate, you’re going to get different answers.
We developed a way of capturing the geometry using digital photographs with traditional inspection personnel. We have a big image target that looks like a plus sign that’s magnetic and will stick on your gusset plate. It’s really a fancy ruler. When you take a picture of it, the software will find the target and rectify the image, turning it into an orthograph that is a 90-degree view from the gusset plate. Because we know the geometry of the target very precisely, it turns the pixel dimensions into inches.
You can take digital calipers in the software and get all the physical dimensions of the gusset plate. You can measure the angles of the members, you can measure the spacing of the bolts and the edge distances all off of the digital photograph. We’ve also written some software that uses image processing techniques to identify the individual fasteners and group the fasteners to the associated members. Then you can do the AASHTO strength calculations off of the digital photograph.
The software walks you through the calculations, with all measurements off of the digital picture. It is a unique tool to be able to do structural analysis off of a picture. Then, when you’re done with that, you can generate the nonlinear finite element model. We are using open-source software to do the structural analysis of the gusset plate, walking through all of the possible loading combinations if it’s tension or compression.
You’re doing an analytical test, so you don’t need to know the actual load. You go through all five connections on the gusset plate, and start the load at zero, and get the capacity of the connection. Then you sweep that around for all the different members and combinations of members.
The only thing you need off of the stress sheet are the ratio of load between members. You can get an accurate estimate of the capacity. It’s an amazing tool that takes you from no information to a very sophisticated nonlinear finite-element solution. There’s no specialized training needed.
I2: This sound like a huge step forward in assessment as well as automation. Has it gained a lot of users?
Higgins: ODOT now requires this target to be used in all of their bridge inspections; it’s written in the inspection protocol. A number of other DOTs are using it, and some are evaluating it. It’s one of those things where technology pulls rather than pushes, because it’s so much easier to do it this way than any other traditional measurement approach. LiDAR is effective, of course, but here the investment is quite small, and you’re just putting a target and putting a camera in the hands of your typical bridge inspector.
I2: That automation is impressive. Do you think we’ll see drones taking over bridge inspection work in the future?
Higgins: I don’t want drones; I like experienced people. They have a sense of scale, a sense of importance, and a sense of history and tradition. Drones don’t have those, and machine learning doesn’t have those.
I like people in the process. I’m trying to give them better, faster tools to let them collect information more rapidly and make use of that information more rapidly. I imagine you could have someone with a strong Internet connection sending their pictures to the office, and back in the office, an engineer could run the simulation, and before that inspector could move to the next connection, the analysis could be returned to them.
If the connection comes back as under strength, the inspector could be guided to check plate thickness and check for section loss or other indicators. I think this could be a real-time assessment tool, but that’s a little beyond what most DOTs are capable of right now.