David Hubbell is a Senior Engineer on Entuitive’s Bridge team. David completed his undergrad at University of Toronto, then attended Princeton University to complete a Masters of Science in Engineering before returning to the University of Toronto to complete a PhD.  David is an award-winning bridge engineer, leads some of Entuitive’s most complex projects, sits on Entuitive’s Technical Board, and is a member of the Structural Analysis Working Group for the new CSA S7 Pedestrian, Cycling, and Multiuse Bridge Design Guideline.

In this article, David shares seven rules of thumb that help him achieve technical excellence on his projects.

1. Bound the problem

Even in the best of circumstances, there is uncertainty everywhere in our work. And we never work in the best of circumstances. More often than not, we start with incomplete information and need to get to a conclusive analysis as soon as possible. So “Technical Excellence” doesn’t mean “Precision Above All”. It often means arriving quickly to the point of being able to say something like “it can be no worse than X, and X is OK”, so that everyone can get on with the project. The limit-states design approach that we use all the time consists of comparing an upper-bound expected load to a lower-bound strength, so we use this “rule of thumb” in a fundamental way on a daily basis, but here’s two higher-level examples.

A fairly routine request we get is to assess a bridge for loading due to some piece of construction equipment – for example, a mobile crane or an articulated dump truck. Often a request like this needs to be addressed on short notice in response to some unforseen circumstance that has arisen on site. Completing a full moving-load analysis of the structure, and checking the design of all components would be prohibitively time consuming, so we start with a simpler check (see Rule of Thumb #4), comparing the moving load response envelopes for the construction equipment against the corresponding response envelopes for the design truck. If it can be shown that the loading from the equipment is no worse than the loading from the design truck, we can be confident that the bridge can sustain the unusual loading without needing a more detailed analysis.

A less routine but very clear example arose when I was tasked with assessing a bridge girder that had been struck and visibly cracked by a piece of construction equipment. Of course it is impossible to know the magnitude of the true dynamic load that would have been applied to the girder, but we were able to match the extent of the observed cracking to a nonlinear analysis of the girder under increasing lateral load. In this way, we could determine an upper-bound on the load the girder experienced and then assess the structure for that equivalent static load. In the end, I was able to show that the girder had not suffered any loss of strength, and could be made fit for continued service by filling the cracks to restore its durability. Avoiding the need to replace the girder was a huge benefit to both the contractor and the bridge owner.

2. Let computers do what they’re good at so you can focus on what you’re good at

The idea of technical wizardry in bridge design might bring to mind a fully automated design process that you can imagine reading about in a sci-fi novel: “…at the click of a button, the form-finding algorithm leapt into action, and in mere seconds it completed all the relevant design checks on thousands of prototypes sampled from across the design space… each feasible design was assessed through a multi-dimensional evaluation matrix and an optimal bridge design materialized on a set of pdf drawings ready for final review and signatures!”

I don’t think this utopia (or dystopia, depending on your point of view) will ever be realized. Every project and every client has a different set of economic, aesthetic, social, and environmental opportunities and constraints, and it’s only through thoughtful and active engagement between humans (listening, discussion, debate, brainstorming…) that a bridge design can be achieved that will be right for its owner and right for its community.

That said, computers sure are good at crunching numbers, and there’s plenty of ways for them to help. A good example is our recent work on the Gardiner Expressway rehabilitation in Toronto. This project involved the prefabrication and installation of nearly 400 unique “supermodules” consisting of two steel girders with a precast concrete deck slab. The geometry of each panel was dictated by the road profile, the crossfall, the location of construction joints, the skew of the supports, and the expected immediate and long-term deflections of the superstructure – i.e. a lot of numbers to crunch. Our core team of three engineers and one technician was able to successfully respond to the challenges of the project only because we found ways to delegate much of the computational heavy-lifting to our computers. This allowed us to be focused on understanding and working through the puzzles that inevitably arise over the course of a large project – helping our client to find value rather than being swamped by the technical “busy-work”.

Lifting out a superstructure panel on the Gardiner Expressway Rehabilitation project

3. Nothing is simple…

Very often what seems like a simple thing will reveal depths of complexity once you start to scratch at the surface. An example is a recent set of construction engineering projects I completed to allow reinforcing cages for caissons to be lifted from horizontal and rotated to vertical before lowering into the ground. While it might seem like a simple operation that is undertaken regularly on job sites, it can be quite complex for large, slender, or heavily-reinforced caissons. Two cranes must be used, with carefully planned rigging, and the cage must be able to carry its own weight to the support points at every angle of rotation between horizontal and vertical. In some cases, the caisson is attached to a structural steel spine which serves to support the rebar when it is horizontal and is removed once the cage is hanging vertically. In our projects we were able to design a system of bar clamps and internal bracing which enabled the longitudinal bars to act compositely with each other, eliminating the need for a temporary spine. This involved digging up an academic study on the stiffness and strength of various types of rebar ties (it varies noticeably depending on the level of experience of the person doing the tying) and working through the elliptic geometry of the rotating rigging to determine the loads applied to the cage.

Lifting a heavy caisson rebar cage and rotating it to vertical

4. …but always start simple

On the other hand, sometimes you know you’re up against a thorny problem, and in these cases I have found it is always best to start with the simplest possible approximation of the system and incrementally add complexity as required in order to sufficiently bound the problem (see Rule of Thumb #1). I was first introduced to Brown|Co (now part of ) through an NSERC grant to study the long-term behaviour of widened prestressed concrete bridges in collaboration with Brown|Co and 407ETR. The 407 had a number of widened bridges in their inventory that were exhibiting lateral movement at bearings, in some cases generating enough force to break the lateral guides of the bearing assemblies. There were many factors at play, including construction staging, temperature changes, concrete shrinkage and creep, strand relaxation, the curved alignment of the road, and the skew of the supports. One approach is to make a detailed model, including assumptions to account for all of these factors, run it, and then try to make sense of the results. I tried this. It was headache-inducing. The way I came to an understanding of what was going on was to break it down to a relatively simple, two-degree of freedom model that I could work with by hand (pictured below). Creep and shrinkage turned out to be the dominant effects driving the observed behaviour; given that the uncertainty in creep and shrinkage models is something like 40%, this simple model was about as accurate as the more detailed model anyway! The phenomenon, and the simplified analysis model used to understand it, was published in a paper for the 2014 Short and Medium Span Bridge Conference, and was awarded the CSCE’s P. L. Pratley award for best paper on bridge engineering.

The widened post-tensioned box girder bridge that was the focus of David’s study

The simplified frame model that allowed the bridge’s behaviour to be understood

5. Make your calculations beautiful

Early in my career I often felt sheepish about spending time formatting an excel sheet or taking the time to carefully draw a diagram with a straight edge and scale. Over time, however, I have found that efforts like these are usually well worth it in the long run. A disorganized excel sheet or hand calc is usually evidence of disorganized thinking, and the best way to sharpen up the thoughts is to clean up their output on the page. One of the first jobs I did was to assess the cracking in the deck of a bridge as part of a rapid replacement. The bridge is picked up by self-propelled modular transporters (SPMTs) at support points that are inboard of the permanent bearings, creating negative moments at locations which are in positive bending in the finished structure. Assessing the cracking in the deck that is caused by these negative moments involves a careful strain compatibility analysis of the cross section, accounting for the construction staging as well as shrinkage and creep. This was the first project where I had grappled with these effects in a calculation, and as I added pieces to the analysis, the excel sheet I was working with quickly became a mess. It was not until I took the time to format cells, organize tables, and clean up the structure of the sheet that I began to feel that I had wrestled the analysis to the ground.

Having taken the time to clearly understand and document the analysis put me in a good position to be able to efficiently undertake a similar analysis many years later with the rapid bridge replacement at Highway 427 and Finch Avenue.

6. Don’t let codes dictate design

Where would we be without design codes? In design work we rely heavily on the agreed-upon factors of safety and the definition of a sufficient level of analysis that our codes provide us. That said, technical excellence cannot be achieved without thinking and designing independently of the code. The code is the fall-back; it sets out the minimum requirements that must be met. It does not define the limit of what is possible, and it is not a guide to good engineering.

Doing a PhD drove this truth home for me. The project that I set myself was to design and validate an innovative girder for elevated transit guideway bridges using Ultra-High-Performance Fibre-Reinforced Concrete (UHPFRC). At the time, there was no design code for UHPFRC (it is now covered in a preliminary way by an informative annex in the CHBDC). The design was not validated through reference to a code, but by first-principles analysis and experimental studies. I undertook some of the first high-cycle direct-tension fatigue testing on UHPFRC specimens, the first direct measurements of the internal damping of cracked and uncracked UHPFRC, and designed and tested a shear connection for creating composite action between vacuum-infused GFRP plate and UHPFRC. The results of these material tests allowed for conservative but realistic assumptions to be made in order to validate the design in the absence of a design code.

Of course, in an academic setting it is easier to push beyond the limits of the code, but I have found myself outside the scope of our codes in the course of project work in industry as well. The joints between panels on the Gardiner Expressway rehabilitation were made with UHPFRC, and I was able create value for our client by applying my experience with the material in situations where there was no code for guidance. We can also end up outside the codes in forensic engineering work. Code equations are calibrated for design purposes and are sometimes of limited value when the task is to understand precisely what has happened with a specific structure under specific load conditions.

Schematic Cross-Section of the GFRP-UHPFRC guideway girder developed in David’s PhD research

Direct-tension fatigue test rig designed by David and used in his research

David’s PhD thesis can be accessed here: Design and Development of Glass-Fibre-Reinforced Polymer and Ultra-High-Performance Fibre-Reinforced Concrete Guideway Girders | TSpace Repository (utoronto.ca)

7. Never say “that’s just how it’s done”

I’ve had the good fortune to work under some very good engineers, both in industry and in academia, so I have only rarely had the frustrating experience of having a question met with the answer “that’s just how it’s done”. It’s true that uniqueness has no inherent value and usually comes with some risk, so “the way it’s done” is the right way to do it in most circumstances. However, it is always worthwhile to understand why it’s done that way, and how the usual approach applies or does not apply to the project at hand. Occasionally, an “outside-the-box” solution will be right for the job.

A great example of this is Highway 7/8 Pedestrian Bridge, which will begin construction in Waterloo, Ontario this Fall.The main span of the bridge is a steel through-truss – a common choice for a pedestrian bridge of this span – but it has a very unconventional feature: it has no structural steel bottom chord members. Instead, the bottom chord of the truss is formed by the concrete deck slab, which is post-tensioned in order to carry the high tensile forces from bending of the truss. This is certainly not “how it’s done” in the typical way, but in this case it allowed the clearance over the highway to be reduced, shortening the length of the access ramps at each end of the bridge by more than 30 metres. It also contributes to Montgomery Sisam Architects’ clean, minimalist, aesthetic for the design, since the usual steel framing visible on the underside of a truss bridge has been eliminated.

Architectural Rendering of Highway 7/8 Pedestrian Bridge

I’m honoured to have been trusted by clients to help with some very technically challenging work. This work demands that you stay humble and always keep looking for ways to hone your technical skills. I look forward to taking on more challenges and adding a few more rules of thumb to this list as I continue to strive for technical excellence!

To discuss David’s seven rules for technical excellence with him, reach out here.