The effects of reducing design drift limits: evidence from the field and nonlinear dynamic analyses
This study combines field data from over 1,800 buildings with nonlinear dynamic analyses to demonstrate that reducing design drift limits to 1.0% significantly decreases the risk of collapse and severe damage to both structural and nonstructural components without compromising drift capacity.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
When the ground shakes during an earthquake, buildings do not simply stand still or fall over; they sway. Engineers measure this side-to-side movement as a drift, which is the distance a floor moves relative to the one below it. For decades, building codes have set limits on how much this drift is allowed to grow, operating on the assumption that if a building is designed to bend a certain amount without breaking, it will survive. The prevailing wisdom has been that making a building stiffer to reduce this sway might be too expensive or could cause other problems, such as shaking the contents inside too violently. However, a new study challenges this long-held assumption, suggesting that the safest way to protect both a building and the people and equipment inside it is to design structures that are significantly stiffer than current standards require, provided they are built with enough internal strength to handle the stress.
The researchers behind this work, based at ETH Zurich and the University of Canterbury, set out to test whether tightening these drift limits actually improves safety. They approached the question from two angles: by looking at what happened to thousands of real buildings after past earthquakes, and by creating detailed computer models of new buildings designed to different stiffness standards. In the real world, they gathered data from over 1,800 reinforced concrete buildings surveyed after sixteen different earthquakes around the globe. They focused on a simple physical characteristic: how much wall and column space a building had relative to its floor area. In engineering terms, a higher density of these vertical supports means the building is stiffer and sways less. By comparing this density to the damage reports, they found a clear pattern. Buildings with more walls and columns, and therefore less sway, suffered far fewer catastrophic failures. Specifically, structures with a higher density of vertical supports were roughly ten times less likely to suffer severe damage or collapse compared to those with fewer supports.
The team also looked at the often-overlooked damage to the inside of buildings, such as broken ceilings, damaged medical equipment, and ruined partitions. They analyzed reports from 111 hospitals in Chile following a major earthquake. The results were striking: hospitals built with flexible frames, which sway more, were ten times more likely to suffer severe damage to their non-structural components than hospitals built with stiff concrete walls. This finding directly counters a common fear that stiffer buildings shake their contents too violently. The data showed that the increased stability of the stiff buildings actually protected the delicate equipment and interior finishes better than the swaying frames did.
To understand exactly why this happens and to predict future performance, the researchers then turned to computer simulations. They designed sixteen different reinforced concrete frame buildings and sixteen reinforced concrete wall-frame buildings. Each set was designed to meet four different drift limits, ranging from a very flexible 2.5 percent down to a very stiff 1.0 percent. Crucially, they ensured that every single building was detailed with strong internal steel reinforcement, meaning that even the stiffest buildings had the same ability to bend and absorb energy without breaking as the most flexible ones. They then subjected these virtual structures to simulated earthquake forces, including extremely rare, powerful shaking that might occur once every 2,500 years.
The simulations confirmed the trends seen in the real-world data. When the design limit for sway was reduced from 2.5 percent to 1.0 percent, the risk of the building collapsing during a massive earthquake dropped by a factor of ten. The stiffer buildings also experienced less severe damage to their structural elements and their floors. Furthermore, the study found that making the buildings stiffer did not increase the acceleration forces felt by the contents inside, a concern that had previously discouraged engineers from pursuing stiffer designs. Instead, the stiffer buildings maintained their integrity and kept the floors more stable.
The study concludes that reducing the allowable design drift limit to 1.0 percent is a simple and effective way to dramatically improve seismic safety. This approach would significantly lower the risk of collapse and severe structural damage while also protecting the valuable contents and non-structural elements within the building. The evidence suggests that the perceived trade-off between stiffness and safety is a misconception; by designing buildings to be stiffer without compromising their internal strength, engineers can create structures that are far more resilient to the violent shaking of the earth.
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