This article examines a misunderstood topic: functional redundancy in structural design. Not redundancy in the traditional sense—where backup systems prevent collapse—but in the ability of a structure to continue serving its intended purpose even when compromised.
Modern structures carry an unspoken expectation: they should stand, serve, and endure. When buildings don’t fall, the industry often declares success. Engineer’s close files, architects move on, and clients begin occupation. Yet not all success is meaningful. Some structures remain standing but become dysfunctional the moment a small part fails, becomes inaccessible, or degrades slightly. Their failure is not collapse—but irrelevance. This is the invisible breakdown no inspection captures, and no certificate anticipates.
Structural engineers often walk away from completed projects with a clear conscience. If design loads were met and detailing followed codes, what more is there to check? But this mindset fosters a blind spot. The building may be strong, but is it smart? Resilient? Adaptable? These questions explore more than code compliance. They ask whether the structure will continue functioning when hit by the unplanned. Unfortunately, many structures today aren’t designed to survive usefulness loss—only structural failure.
This article examines a misunderstood topic: functional redundancy in structural design. Not redundancy in the traditional sense—where backup systems prevent collapse—but in the ability of a structure to continue serving its intended purpose even when compromised. This difference separates buildings that remain operational after disruption from those that don’t fail but cannot serve. We explore why this is often neglected, what it looks like in real-world scenarios, and how better design decisions can change this.
Failure Without Collapse: A Growing Problem
Most structural engineering training prepares you to avoid collapse. You’re taught to run ultimate limit state checks, verify deflections, prevent punching shear, and select reinforcement accordingly. But very little time is spent on what happens when one system underperforms—not catastrophically, but enough to limit building use. Consider a footbridge with perfectly safe load capacity that vibrates uncomfortably under normal use. It hasn’t failed structurally. But if pedestrians avoid it, hasn’t it failed functionally?
Buildings face the same fate. A cantilevered balcony that allows water ingress may meet strength checks. But if it leaks, rusts, and stains walls, it eventually gets sealed off. Now the building loses usable area. Multiply that across roof terraces, basements, and mezzanines, and you have a structure that’s technically sound but gradually shedding its utility. This decay of usability rarely gets flagged during design. It doesn’t trigger alerts. But it renders the structure ineffective.
Functional redundancy in structural design is the key concept missing here. When a slab serves both as a structural element and a waterproofing surface, it carries dual burdens. If cracking ruins waterproofing but leaves structural strength intact, has the slab failed? For code, no. For the client, yes. This mismatch between structural adequacy and practical performance leads to buildings that can’t function, even though they still stand.
The Illusion of Full Function
Engineers often assume that if design assumptions hold, the structure will function indefinitely. But reality isn’t as predictable. Take underground tanks integrated below foundations. Their concrete shell may remain sound, but a failed joint or misaligned access point makes them useless. Retrofitting becomes impossible due to structural constraints above. The tank didn’t crack. It didn’t leak. But it failed its purpose.
Another example: elevated slabs with embedded pipe sleeves. If rebar clashes during casting and sleeves shift, pipework may no longer pass through. The slab is structurally fine. But now services reroute externally, compromising building aesthetics and maintainability. Clients often live with it. Engineers rarely revisit it. But functional redundancy in structural design was never considered. The failure didn’t show up in any load combination. It just appeared on site, during commissioning.
Even basic columns can expose this flaw. In one mid-rise project, perimeter columns were detailed too close to curtain wall mullions. Installation proceeded, but later, water-proofing membranes couldn’t wrap around adequately. The columns were too close to the edge. They held loads perfectly. But over time, water seeped through the perimeter. Within three years, internal finishes showed signs of rot. Again, the structure didn’t fail. But it became dysfunctional.
Design Decisions That Breed Vulnerability
What causes this lack of functional resilience? Often, it begins in early design stages. Architects may over-rely on space without understanding structural impact. Structural engineers try to accommodate without requesting alternatives. Services engineers are consulted late. Details are rushed. Elements share functions without separation of risk. This blending creates single points of failure that paralyze function if disturbed.
A typical case involves podium decks. Many serve as parking, gardens, or even building entrances. These decks must resist loads, water, and movement. Designers sometimes use post-tensioned slabs to control thickness. But waterproofing systems, if not properly detailed or sequenced, can’t keep up. One puncture during construction creates years of headaches. If the deck leaks, the parking becomes unsafe. Retail spaces below face water damage. The structure doesn’t fail. But no one wants to use it.
Functional redundancy in structural design would ask: what happens when this deck leaks? Can drainage layers isolate the problem? Are there backup membranes? Can users still access key spaces? These questions rarely appear in traditional structural workflows, which focus on capacities—not consequences.
Code Isn’t the Enemy—But It Isn’t Enough
Some may argue that building codes protect against this. But they don’t. Codes regulate strength, stability, and sometimes serviceability. They rarely speak to continued function after non-critical failure. Even Eurocode EN 1990 mentions robustness but stops short of prescribing functional continuity. This creates a regulatory blind spot. Engineers meet codes, but still design buildings that fall short of real-world demands.
For instance, a multi-story hospital in West Africa met all wind, seismic, and serviceability requirements. But slab deflection exceeded partition tolerance, causing cracks in blockwork. HVAC ducts had to be re-routed. Patients were moved. The building didn’t fall. But parts became uninhabitable. Engineers had passed all checks. Yet the building failed its users. A simple redesign with deeper beams or cambered slabs could have preserved function.
Even critical infrastructure isn’t exempt. Consider telecom towers mounted on rooftops. If access ladders are poorly detailed, and water ponds on roof slabs due to minor settlement, inspections get deferred. Over time, lack of access prevents repairs. Eventually, the tower might be removed—not due to collapse risk, but due to loss of function. Here too, functional redundancy in structural design was missing.
A Better Mindset for Structural Engineers
What should engineers do differently? First, shift mindset from “What will make it stand?” to “What will keep it working?” Ask how every component behaves under misuse, partial failure, or delayed maintenance. Don’t just verify ULS and SLS. Probe scenarios that don’t break the building—but break the building’s use.
Second, separate functions where possible. Don’t rely on one slab to carry loads, resist fire, stay waterproof, and anchor services. Each added role increases exposure. When one fails, all suffer. Instead, design structural systems to withstand local compromises without disabling overall operation.
Third, document non-load-related risks clearly. If a basement wall loses its waterproofing due to cracking, will the structure stay safe? Probably. But will the space remain usable? Probably not. Clients rarely know the difference until it’s too late. Drawings should highlight these zones. Reports should speak to these realities.
Finally, advocate for design reviews that include operational continuity—not just safety margins. Invite facilities managers into design conversations. Ask, “If this fails slightly, what happens?” This is the essence of functional redundancy in structural design—not as luxury, but as discipline.
See: Structural Redundancy and Progressive Collapse Prevention
Conclusion
Structural engineering is no longer just about holding loads. It’s about holding purpose. Buildings must not only stand but serve. As more projects combine complexity, mixed-use demands, and tight budgets, the margin for failure—of any kind—shrinks. Yet failure isn’t always visible. Sometimes, it lives quietly in spaces no one uses, slabs that leak, joints that shift, and pipes that never connect.
The future belongs to engineers who design for more than resistance. It belongs to those who ask: what happens when this goes wrong—not to the building, but to the people who rely on it? Only then will we close the gap between structural success and functional reality.
