Understanding Accidental Torsion in Seismic Design: 6 Reasons to Consider

If you’ve been designing buildings for even a short while, you’ve probably heard this line:

“My building is symmetrical, so torsion won’t be a problem.”

Unfortunately, earthquakes don’t care about your symmetry.

Just like we discussed in the “Why There Are 5 Seismic Zones in India” article, real buildings behave far differently from the beautiful, idealised models we have in ETABS or STAAD.

One of the biggest (and most underestimated) uncertainties in seismic behaviour is torsion.

And no—not the intentional torsion arising from eccentricity between centre of mass and centre of stiffness. We are talking about the accidental torsion that codes force you to consider.

Let’s break down the 6 real-world reasons why accidental torsion is not just a code formality, but a serious engineering requirement.

Uncertainty in Stiffness Distribution

Your structural model is not perfect—it’s just your best attempt at representing it.

In reality:

• Columns crack differently but we take a flat value as per codes

• Beams deflect differently as the loads are varying

• Joints aren’t as rigid as assumed

• Reinforcement placement isn’t perfect as considered in design

• Boundary conditions vary due to construction tolerances

  
  

Even a few percentage points of variation in stiffness can shift the stiffness centre. That shift introduces torsion automatically.

The moment stiffness is not uniform, torsion appears. And stiffness is never perfectly uniform.

Uncertainty in Mass Distribution

If stiffness is unpredictable, mass is even worse.

• Storage concentrated on one side

• Water tanks partially filled

• Heavy machinery moved

• Architectural changes during construction

• Construction stage loading

• Furniture clustering in one zone

  
  

Your ETABS model may assume a perfectly distributed mass. Reality does not.

Mass asymmetry = Torsion. It’s that simple.

Earthquake Forces Don’t Come in Neat Directions

You may load EQX and EQY separately because the software allows it.

But earthquakes don’t travel along your building axes. They arrive at any angle they want.

Even a perfectly symmetrical building can experience torsion when the seismic force direction is not aligned with the principal axes.

This directional variability is one of the biggest creators of accidental torsion—and you cannot “model” it accurately.

Rotational Ground Motion Exists (Even If You Don’t Model It)

This is one engineers rarely think about.

Ground motion is not purely translational. It has a rotational component too.

Instruments typically measure only translation, so most designers forget rotation exists. But in reality, the ground twists, and that twisting imparts rotational acceleration to your building.

When the base rotates and the building resists, the result is torsion—even if the building is perfectly symmetric.

Influence of Non-Structural Components in Seismic Design

Your analysis model probably does not include:

• Masonry infill

• Partition walls

• Cladding

• Staircase walls

• Lift walls in partial contact

  
  

But onsite, these elements stiffen some regions more than others.

A single masonry wall in one bay can increase stiffness drastically compared to its neighbouring bay.

The outcome?

Unintended stiffness irregularity → accidental torsion.

If your model ignores these components, your analysis is already missing half the building.

Uneven Damage Changes the Stiffness Mid-Earthquake

This is something codes are very aware of.

During an earthquake:

• Cracking is not uniform

• Yielding happens at different levels in different elements

• Damage in one zone reduces stiffness

• The building becomes uneven halfway through shaking

A building that starts symmetric may become asymmetric within seconds.

Suddenly, the building begins to twist—not because it was designed badly, but because damage is never “balanced.”

This time-dependent asymmetry is the very reason codes insist on accidental torsion.

The Practical Takeaway

This is why every major seismic code—including IS 1893—tells you to explicitly account for accidental torsion.

The simplest way?

Assume a % accidental eccentricity of the centre of mass relative to the centre of stiffness.

This is the code’s way of telling you:

“There are uncertainties you cannot predict, so design for them.”

Final Thoughts

Ignoring accidental torsion is like ignoring half the earthquake story.

You may not “see” torsion in your analysis output. You may not “feel” its effects in your symmetric ETABS model.

But the ground motion will bring it out—especially when you least expect it.

✅ Always include accidental torsion in your design

✅ Never blindly trust symmetry or software models

✅ Structural consulting is not just modelling—it’s dealing with uncertainties

If you want more engineers to understand this, share this blog with your team or juniors. Seismic safety improves when we stop assuming perfection and start designing for reality.