Eng. Dr. Ian Khan-Kernahan BSc, MSc, BSc, PhD, REng, FAPETT
A Chat with Engineer Dr. Ian Khan-Kernahan , APETT’s prestigious Career of Excellence Awardee
Engineer Dr. Ian Khan-Kernahan is a distinguished civil engineer whose career spans over four decades in professional practice, academia, and consultancy. He holds degrees in Mathematics from Queen Mary, University of London, and in Civil Engineering from The University of the West Indies, where he later earned his PhD and served as Lecturer in Civil and Environmental Engineering for more than 30 years. As Director of Trinstruct Services Limited, he has contributed extensively to bridge and structural design across the Caribbean—having designed nearly 100 bridges, including the landmark Naparima-Mayaro crossing of the Ortoire River and the restoration of the historic Marianne suspension bridge. His professional service also extends to major housing and infrastructure projects that have improved community connectivity and access. A respected researcher and author, Dr. Khan-Kernahan continues to advance knowledge in structural and bridge engineering, making him a truly worthy recipient of APETT’s Career of Excellence Award.
Q1. Congratulations on receiving APETT’s Career of Excellence Award for Exceptional Contribution to the Engineering Profession. How did you feel when you learned you were selected for this prestigious honour?
I was pleasantly surprised. Indeed, I am very grateful that bridge engineering was acknowledged by the Awards Committee.
Q2. Looking back over your 45-year career, what personal or professional milestones stand out most in your memory?
I was employed at the Min. of Works when Prof. Phelps recruited me to join the staff of the Civil Eng. Dept. at UWI. Eleven years later I earned a Ph.D. for developing a computational method (semi-grillage) of analysing bridge decks. That was a personal milestone.
At a professional level, my involvement in the MOWT Roads and Bridges Rehabilitation Programme (Year 3, 2002) gave me the greatest satisfaction. A team of two, Eng. Tim Stiff and myself, inspected 96 bridges. Of these, forty (40) were selected for total reconstruction and one for partial reconstruction. I was the structural design engineer for these bridges. The detailed designs were finished in 18 months. Looking back, this was probably my finest moment as a bridge engineer. And I earned recognition as a specialist bridge engineer. A fuller account of the programme is described in the West Indies Journal of Engineering (WIJE) paper: “Assessment and Reconstruction of Bridges in Trinidad and Tobago (2013)”.
Q3. What initially drew you to engineering after starting your academic journey in mathematics, and how did that transition shape your engineering outlook?
Growing up, I was always good with my hands. At home, I dabbled in plumbing, carpentry, painting and tiling. In school, I excelled in applied mathematics which is essentially the study of statics, kinematics and dynamics. When I studied mathematics at university (Queen Mary, University of London), I took mostly applied courses: advanced mathematical methods, relativity and quantum mechanics, for example.
I taught mathematics at St. James Secondary School for two years. It was interesting at first but my initial enthusiasm gradually waned; moreover, the salary was not terribly good. Some of my former schoolmates who were now engineers painted a glowing picture of the local job market—challenging work, great salaries and perks. That’s when I decided to pursue engineering at the UWI.
Q4. You’ve designed over 90 bridges across Trinidad and Tobago and the Caribbean, including the remarkable Naparima-Mayaro Bridge. What do bridges represent to you, beyond their structural function?
Bridges are structures that provide the very expensive foundation of the roadway that they support. A bridge crosses obstacles like rivers, chasms or even other roads, for example. It provides social connectivity. In this way the bridge engineer does social engineering by keeping communities together and allowing them to access and enjoy their surroundings.
Q5. Could you share some of the technical or environmental challenges involved in restoring the historic Marianne Suspension Bridge, and what it meant to you personally to work on such a landmark?
The Marianne River bridge was originally designed as a single span unstiffened suspension bridge. The wire rope (flexible cable) changes its form under moving loads, light vehicles, creating large vibrations—hence the name Spring Bridge. First, a Mabey & Johnson bridge was installed upstream of this bridge to cater for vehicular traffic. Work then began on preparing a full rehabilitation scheme for upgrading, as far as possible, the Spring Bridge superstructure to modern engineering standards. In particular, it was agreed that the bridge should be stiffened without significantly altering the appearance and diminishing the character of the bridge.
The main technical challenge was selecting details for the wire rope terminations. After researching these connections, it was decided to reuse the existing spelter sockets that anchored the main cables to the towers. Standard U-bolt clips were used to attach the suspenders (smaller diameter vertical cables) whose ends were fitted with closed swage sockets. The stiffening steel I-beam was then attached using galvanized U-bolts with sufficiently long threaded arms to allow adjustments. The contractor, Amnesty Construction, did an excellent job. I should add that the towers were also dismantled and reassembled using new components and round head bolts instead of rivets.
Q6. What advances in bridge engineering have you seen during your career, and how have these influenced your own design methods or preferences?
When I graduated from the UWI, the local industry was in a state of transition between the allowable stress method and the load and resistance factor method (LRFD) of design of structures. We used mostly manual calculations, design aids and approximate idealisations in the analysis. Nowadays, commercial software is used to rapidly model, analyse and design complex structures.
For conceptual design and preliminary analysis, I still use approximate methods of analysis. I will then use commercial analysis for a refined analysis. The actual design of elements will be done using validated spreadsheets. I perform an extensive validation of any spreadsheet that I have written. This takes time as it requires the sourcing of suitable benchmark problems by experts.
A large proportion of our bridge stock consists of reinforced concrete box culverts with spans ranging from 6m to 12m. I have used the exact stiffness matrix of a beam on elastic foundation to analyse the AASHTO equivalent 2D closed frame. The method is incorporated in a spreadsheet. I use it to rapidly design these structures. The results are validated using commercial software. It is my hope that an arrangement can be made to pass on the spreadsheet to the Ministry of Works.
Q7. You spent over three decades at UWI shaping young minds. What philosophies or teaching approaches did you find most effective in nurturing future engineers?
Well times have certainly changed. We have gone from chalk-and-talk to PowerPoint presentations in the classroom. I didn’t change. I posted the lecture notes on the net. The students were expected to read these, come to class and ask questions and annotate the notes as I developed the subject matter on the whiteboard.
Model building was introduced as part of the first-year course in design. One such exercise required groups of students to construct an unbonded prestressed beam out of Styrofoam cubes and rubber bands. Matchsticks were used as the anchors. The groups had to describe the behaviour of the model while loading it with coins during a class presentation. There were almost always very interesting discussions about how interface shear (friction) resisted the applied vertical load. The feedback from the students was generally very positive.
Q8. Your papers explore computational methods in structural engineering. How do you see the role of computation and simulation evolving in structural design today?
Structural design requires an idealization of the proposed structure—building, bridge or retaining wall. Ideally, this should start at the conceptual design stage. The goal is to create a simple mathematical model that will realistically predict the behaviour of the actual structure under the intended loads. Engineers will be guided mainly by experience and codes to select appropriate structural systems. And it is important to document the chosen idealisation in the engineering report, drawings and calculations. Everyone should be made aware of the assumptions, especially when checking a computer-based model.
Engineers must idealise materials as well. They do this by using stress-strain relationships, modulus values and strength capacities. For concrete, linear elastic models apply until cracking. For steel, bilinear idealisation represents elastic and plastic behaviour.
Soil-structure interaction is becoming more prevalent in structural models. This usually requires an expensive geotechnical investigation to assess the relevant translational and rotational stiffnesses. Engineers will have to decide whether simple text-book boundary conditions—pinned, fixed or rollers—can be used instead.
Traditional computer-based methods have transformed structural analysis landscape for the last fifty years. Software has been developed to successfully treat with various well-defined structural systems. The software is updated to reflect advances in engineering knowledge and digital hardware. Some of the software is so advanced that it relegates the user to “pushing buttons” to get an “answer”.
Artificial Intelligence (AI) systems are gradually being developed that learn from their database(s). These may soon become commonplace in areas like weld inspection and concrete cylinder tests, for example, where more reliable machine vision is used.
Q9. How important is it, in your view, for engineers to be both practitioners and contributors to technical literature, as you have been throughout your career?
I think it is very important for practitioners to share their knowledge and real-world experience. They provide insight into the construction industry and engineering practice. Writing technical journal papers can be very time consuming and demanding of rigour. Reviewers’ comments have to be addressed before the paper is accepted for publication. The peer review process is quite demanding, if not intimidating, as most academics will tell you.
The alternative is conference papers and presentations. Practitioners get the opportunity to showcase their expertise while getting real-time feedback. Case studies that show the application of new / innovative methods of tackling practical problems will keep the construction industry animated. And, in my opinion, most employers tend to support the participation of knowledgeable engineers in activities that highlight their business model.
At the end of the day, it is incumbent on engineers to find ways to share their knowledge. It is the most helpful and valuable thing that we can do.
Q10. Tell us about your role as the principal structural engineer on housing development projects in Trinidad and Tobago—what innovations did you apply, and what were some key lessons learned?
Most of the mass housing projects were contractor driven. For example, at Opropune Phase IV the contractor used FORSA aluminium formwork. The mock-ups of the repetitive units allowed the construction team to verify the accuracy of every detail of the design. By creating a full-scale model, we were able to eliminate errors very early in the planning stage. I visited the factory in Colombia to perform an inspection.
The formwork creates a monolithic reinforced concrete structure—walls and floors. We were able to use a load bearing shear wall system supported on a post-tensioned floor slab for the various building types.
In some instances, a concrete roof slab of the multi-storey buildings was also used. This protected the interior of the building and permitted an early start to the finishing work. A light GI roof was then added for aesthetic and drainage reasons. Flat concrete roofs can be made water-resistant, but they are prone to leaking with time.
Q11. Post-tension technology and raft foundations are critical in certain soils and geologies. What are some of the local conditions in Trinidad and Tobago that necessitate these design approaches?
It is a popular misconception that post-tensioned ground slabs are primarily used in expansive soils where the movement of the soil is significant. This movement is non- uniform and so the foundation must be suitably stiffened and reinforced to resist the differential movement. A post-tensioned slab minimizes and controls cracking. Its strength and enhanced stiffness diminish any flexural deformation.
I have used uniform thickness unbonded post-tensioned slabs-on-ground in housing developments to support buildings up to four storeys. It’s a shallow foundation that eliminates the need for trenching to place a grid of ground beams. That’s really convenient, especially in the rainy season. It was used to great advantage in Oropune where the soil is relatively compressible resulting in a low bearing capacity of around 40kN/m2. The geotechnical investigation did not indicate the likelihood of differential movement. It is worth repeating here that post-tensioned slabs are more ductile and crack resistant than conventional reinforced concrete slabs. This leads to greater durability and lower maintenance costs.
Q12. Through Trinstruct Services Limited, how are you continuing to shape civil infrastructure development in the region?
Civil engineering consulting is just one cog in the wheel. In Trinidad, we have the National Planning Authority, who should provide the guidance that shapes the development. It is my hope that the NPA will co-opt the APETT when they finally get around to publishing an up-to-date development plan. Similar sentiments apply to our regional partners.
Q13. What are the biggest engineering challenges facing infrastructure development in the Caribbean today, and how can local engineers rise to meet them?
Infrastructure development depends on the health of the regional construction industry. The construction literature generally paints a picture of stagnation and/or decline over the last few years. In Trinidad and Tobago, public sector infrastructure projects have been plagued by time and cost overruns. Indeed, there have been cases of failure during construction and, quite often, the intended service life. This suggests that insufficient attention is paid to developing proper feasibility studies and preliminary / baseline designs for assessing future scenarios (impacts) and budgetary allocations. Needless to say, the public’s confidence in the procurement process is eroded.
In Trinidad, I have seen an increasing trend to replace the FIDIC Red Book with the Yellow Book for the construction of roads and bridges. A Yellow Book contract allows the Contractor to design and build the works for a lump-sum payment. The Red Book would have required the Employer to do the designs and then hire a Contractor, who is generally paid on a measurement basis. So, under the Red Book, the Employer must carry out at least a preliminary design study before assessing the cost of the planned works.
Q14. What advice would you give to young engineers hoping to build a career as impactful and multifaceted as yours—especially those passionate about infrastructure and bridge design?
One only has to take a look at a typical job description of a civil engineer to realise that it is a multi-faceted profession. Very early specialism in one’s career can lead to narrow interests and stunted growth. It can very often, in my opinion, limit one’s value in projects that require a broader understanding of various disciplines. Bridge engineering requires, but is not limited to, knowledge of highway engineering, structural mechanics, soil mechanics, hydrology and hydraulics. This broad knowledge allowed me to seamlessly transition to doing the structural engineering for mass housing schemes.
It’s a long journey to achieve success and recognition in civil engineering. The satisfaction that awaits is worth the effort.
Q15. Finally, as you look to the future, what legacy do you hope your work will leave for both the engineering profession and the communities your projects have served?
My academic / professional contribution will be documented in the papers that I have written. The more lasting contribution to society will be all the bridges that I have designed. And in the shorter term, lots of people will have benefitted from the affordable housing units in Oropune, Goya and Santa Rosa. I hope that will be part of my legacy.