Is Designing a Building Structure for Strength Alone Enough?

One day in June 2000, a new pedestrian bridge over London’s River Thames opened to public attention. Built from steel tubes and cables, it was simple and elegant, designed and built by the renowned structural engineering firm Arup. It linked Tate Modern with St Paul’s Cathedral, bringing them into a dialogue across eight hundred years of history. The bridge was also built to celebrate the new millennium, hence its name: the Millennium Bridge, London.

Figure 1: London’s Millennium Bridge. (Image source: the internet.)

Unexpectedly, on the very first day the bridge opened, nearly 90,000 people crossed it, with more than 2,000 on it at once, causing it to sway violently. Even though the number of people allowed onto the bridge was restricted over the next two days, the vibrations did not lessen, so the Millennium Bridge was closed. It did not reopen until 2002, after numerous damping devices had been added.

“Human-Induced Vibration” in Lightweight Structures

Simply put, the reason was that once a lightweight structure was used, walking crowds could induce excessive bridge vibration. Under certain conditions, a lighter bridge mass means greater vibration acceleration. Although such vibration would not damage the bridge—in other words, its strength was adequate—it frightened the crowd, making serviceability a major problem. People generally walk at a frequency of around 2 Hz. If a bridge’s frequency is below 8 Hz, resonance must also be considered. The Millennium Bridge became a classic engineering failure case, propelling the emerging research topic of human-induced vibration into prominence. Such problems rarely occurred on older bridges, largely because their self-weight was much greater than that of today’s bridges.

“Lightweight Structures Must Also Consider Vibration Comfort”

Throughout the history of construction, humans have tried in every possible way to overcome the influence of gravity and build higher, longer, and larger structures. Over the past few decades, the strength of structural materials has increased and spans have grown.Engineers once thought they could rest easy after solving the strength problem, but vibration forced them to think again.

Similar issues of vibration comfort also arise in floor design. Older floors had short spans and high self-weight. With the incorporation of steel components, spans have become increasingly large. Although self-weight is still considerable, floor frequencies have decreased, making vibrations caused by people walking more prominent. Some floors use lightweight steel, and a person’s weight can amount to one-fifth of the floor’s self-weight or more, producing a larger vibration response. Strength is assured, but vibration remains bothersome.

“Let Brick and Stone Become What They Want to Be”

Finally, we should understand that there is no perfect material. Every construction material has strengths and weaknesses; there is no solution that works once and for all. A good designer understands these advantages and disadvantages, then designs accordingly. Writing this, I suddenly recall Louis Kahn’s words: let brick and stone become what they want to be. The same idea applies to all construction materials.