Saving Structures: Why Weak Columns Matter in Earthquakes and How We Can Fix Them in Nepal

Let me tell you, living in a place like Nepal means you’re acutely aware of the ground beneath your feet. We’re smack-dab in one of the world’s most seismically active zones, thanks to those massive tectonic plates doing their slow, powerful dance. When they shift, things happen. Big things. Earthquakes like the devastating one in Gorkha in 2015 or the more recent one in Jajarkot in 2023 aren’t just news headlines here; they’re life-altering events that remind us just how vulnerable our buildings can be.

It’s heartbreaking to see the damage, and often, the structures that suffer the most are the ones built without proper engineering oversight – what we call ‘non-engineered’ buildings – or those based on standards that are simply outdated. We’ve seen countless reinforced concrete (RC) frame structures crumble, and frankly, it often comes down to some fundamental flaws, especially in the columns.

The Problem: Columns Under Pressure

We’ve observed it time and again in the aftermath of these quakes, particularly in the hilly regions of places like Far Western Nepal. As RC construction has become more popular, replacing traditional methods, we’ve unfortunately also seen a rise in buildings with significant issues. It’s not just about getting materials; it’s about how they’re used, the lack of optimized design, and honestly, a real struggle with enforcing building regulations.

Think about it: we’ve seen buildings with:

• Inadequate column sizing: Columns that are just too small for the loads they need to carry, especially lateral seismic forces.
• Insufficient reinforcement: Not enough steel bars, or bars that are too thin, or ties (the hoops around the main bars) that are too far apart.
• Soft stories: Where one floor, usually the ground floor, is much weaker or more flexible than the ones above, leading to catastrophic collapse.
• Floating columns: Columns that don’t go all the way down to the foundation, creating dangerous load paths.
• Poor material quality: Issues with the concrete mix or placement.
• Building asymmetry: Irregular shapes that twist and shake badly during a quake.

These aren’t minor hiccups; they are fundamental weaknesses that turn a potential safe haven into a hazard when the ground starts shaking. Older standards, like the initial NBC 205:1994, didn’t always account for lateral seismic stresses adequately, especially for the smaller columns commonly used (like 230x300mm or even 230x230mm). While newer codes exist, the reality on the ground is that many buildings, even four or five-story ones, are still built based on the advice of masons or technicians with limited structural design knowledge, often without proper analysis or adherence to current rules.

What the Study Did: Putting Buildings to the Test

So, we wanted to get a real handle on this. We selected three representative non-engineered RC structures from the hilly regions to analyze. We basically put them through a virtual earthquake scenario to see how their columns would perform. But we didn’t stop there. We also wanted to see if we could *fix* these issues.

We looked at three different retrofitting (that’s engineering speak for ‘fixing up’) techniques:

• Concrete Jacketing: Adding a new layer of concrete and reinforcement around the existing column to make it bigger and stronger.
• Steel Jacketing: Wrapping the column in steel plates.
• Steel Bracing: Adding diagonal steel members to the building frame to stiffen it up.

We analyzed the original buildings and then created models showing what they’d be like after applying each of these retrofitting methods to the columns. We wanted to see, scientifically, if these fixes actually made a difference.

How We Tested It: Running the Numbers

To figure out how these buildings would behave during an earthquake, we used a couple of common analysis methods: Response Spectrum Analysis (RSA) and Nonlinear Static Analysis (NSA), also known as pushover analysis. Think of RSA as checking how the building vibrates and responds to a typical earthquake pattern, assuming it behaves somewhat elastically. NSA, or pushover analysis, is like pushing the building sideways, little by little, to see how it deforms and where it starts to fail once materials behave nonlinearly – which is what happens in a real, strong quake.

These analyses gave us a ton of data. We looked at things like:

• Capacity Curves: A graph showing how much sideways force the building can handle versus how much it moves at the roof level. This tells us about its strength and deformation capacity.
• Inter-story Drift and Displacement: How much each floor moves relative to the one below it, and the total movement at the top. Too much drift is a major failure sign.
• Stiffness: How rigid the building is – how much force it takes to make it move a certain amount.
• Column Design Parameters: Especially the P-M-M interaction ratio, which tells us if the combined effects of vertical load (P) and bending moments (M) are pushing the column beyond its limits. A ratio over 1 is bad news.
• Fundamental Time Period (FTP): How long it takes the building to sway back and forth once. This affects how it responds to different earthquake frequencies.
• Base Shear and Story Shear: The total sideways force at the base and the force at each floor level.
• Ductility Factor (μ) and Over-strength Coefficient (Rs): These tell us how much the building can deform *after* it starts yielding without collapsing (ductility) and how much reserve strength it has beyond the point where it first yields (over-strength). High values are generally good for seismic performance.

By comparing these metrics for the original buildings and their retrofitted versions, we could see exactly how much improvement each technique offered.

What We Found: The Power of a Good Fix

And boy, did we see some improvements! The results were pretty clear. Retrofitting significantly boosted the buildings’ ability to resist lateral loads. The SRc models (that’s the steel bracing, remember?) showed the most dramatic changes. We saw inter-story drift reduced by up to a whopping 80% in some cases! Their base shear capacity – basically, how much sideways force they could take before serious trouble – increased massively, by over 2000 kN in some models. This means they are much stiffer and stronger.

The P-M-M interaction ratios in the original, unretrofitted columns were alarming, sometimes as high as 1.557, screaming ‘danger!’ After retrofitting, these ratios plummeted, going as low as 0.148 in some cases. It’s like taking a column that was buckling under pressure and giving it a massive sigh of relief, making it much more stable.

Jacketing (both concrete and steel) also helped a lot. While generally not making the buildings quite as stiff or reducing drift as much as steel bracing, they still significantly improved strength and stability compared to the original structures. The capacity curves showed this beautifully – the retrofitted models could handle much higher forces and deform more before reaching critical states.

We also looked at how they failed. The non-retrofitted buildings often showed that classic, terrifying soft-story failure, with the ground floor columns giving out first because they were too weak or flexible. Retrofitting changed this failure pattern, pushing the failure to higher force levels and larger deformations, which gives occupants more time and increases the chance of survival.

When we looked at ductility and over-strength (μ and Rs), steel bracing again stood out, providing superior values. This means a braced building can absorb more energy and deform more safely during a severe quake compared to a jacketing-only or unretrofitted structure.

Why This Matters: Building a Safer Future

This isn’t just academic stuff about numbers and graphs. This research has real-world implications, especially in places like Nepal where seismic risk is high and many buildings are vulnerable. It shows, clearly, that we *can* significantly improve the seismic capacity of these poor-strength buildings. We can make them safer, more resilient, and ultimately, save lives.

So, what’s the takeaway? Based on our findings:

• It’s absolutely crucial to adhere to updated building codes, like the new RUD code NBC 205:2024. These codes specify minimum column sizes and reinforcement that our study supports as necessary for better performance.
• We need stricter enforcement of these codes, not just for getting permits, but for actual construction on site. This applies not only to low-rise buildings but especially to multi-story structures that are often built without proper engineering.
• Retrofitting works! Techniques like concrete jacketing, steel jacketing, and especially steel bracing, are effective ways to strengthen existing vulnerable buildings.
• Prioritizing steel bracing where feasible seems like a smart move, given its superior performance in reducing drift, increasing strength, and improving ductility and over-strength.

Of course, our study had its limitations. We used idealized analytical models, which can’t perfectly capture every detail of real-world construction, material variations, or the effect of non-structural elements like walls. The retrofitting methods were also modeled in a simplified way, not accounting for every potential challenge on site. And we used generalized seismic inputs, not site-specific ground motion data, which could affect the precise results for a particular location.

But despite these constraints, the core message is loud and clear: weak columns are a major vulnerability in RC buildings in seismic zones, and proven retrofitting techniques can make a dramatic difference in their ability to withstand earthquakes. It’s about building smarter, enforcing the rules, and fixing what’s already standing to ensure safety and resilience for communities living under the shadow of seismic risk.

Source: Springer