Torsion is a fundamental aspect of structural behaviour. In asymmetric buildings, it becomes a governing factor. It alters load paths, stresses, and deformation.
Torsion challenges conventional thinking in structural engineering. It disrupts predictability. It destabilizes well-proportioned frames. Yet, it often hides behind compliant plans and seemingly rational layouts. The issue arises subtly. An asymmetry creeps in through eccentric mass or stiffness. The result becomes distortion—twist—about the vertical axis.
Buildings resist forces best when they act through their centre. But forces rarely oblige. Irregular configurations or non-uniform lateral systems distort the load path. They shift demand away from intended alignments. Once lateral loads act eccentrically, torsional moments develop. These moments may dominate behaviour, particularly in tall structures or seismic zones.
This rotational effect complicates design. Torsion is not a secondary load path—it is a critical one. The rotation affects floor diaphragms, columns, and bracing members. Without careful analysis, torsion can render a structure inefficient or dangerous. Thus, asymmetric buildings need deliberate engineering strategies. Predicting torsional response early saves lives and reduces costs.
What Causes Structural Torsion?
Torsion begins when lateral loads act off-center from a building’s lateral stiffness. The geometry may be uneven. The stiffness may be misaligned. The mass distribution may be irregular. All these result in eccentricity. Once this offset exists, the applied lateral load causes not only translation but also rotation. The twisting response must be resisted. That resistance comes from the lateral force-resisting system.
Architectural freedom often introduces asymmetry. Setbacks, recesses, or atria shift mass. Irregular shear walls concentrate stiffness. Diaphragms may span unevenly. When structure and architecture do not align, torsion finds a path. This effect becomes pronounced when diaphragms remain flexible or when vertical elements lack continuity. Even small eccentricities can cause large rotation under dynamic excitation.
Asymmetry need not be dramatic. A simple L-shaped plan can introduce measurable torsion. Open ground floors or uneven column spacing also contribute. Often, the imbalance hides until a dynamic load reveals it. Then, unanticipated forces emerge. The system rotates. Stress redistributes. Weak links crack.
The Mechanics of Torsional Response
Once lateral force meets eccentricity, the structure rotates. This generates torsional moments in the horizontal plane. These moments distribute based on stiffness. Stiffer elements resist more. Flexible elements yield. As the structure twists, load paths lengthen. Shear lags. Bending increases. The stress pattern deviates from pure translation.
At floor level, the diaphragm becomes a transfer plane. It distributes the combined lateral and torsional effects to vertical elements. The diaphragm’s stiffness—especially in concrete or steel decks—affects this distribution. In flexible diaphragms, rotation becomes unconstrained. In rigid diaphragms, torsional equilibrium is better controlled.
The resulting shear forces on individual elements increase with distance from the centre of rotation. This means outer columns or walls experience more force. The internal distribution shifts, often doubling demand compared to pure translation. Therefore, torsion amplifies internal actions. Engineers must capture this effect during design.
Codes provide guidelines. Most require accidental eccentricity. This accounts for uncertainty in mass and stiffness distribution. By shifting lateral loads by a small percentage, the analysis simulates unexpected twist. The resulting design actions protect against overlooked asymmetry. But this does not replace sound layout or good proportioning.
Design Implications of Torsion
Torsion alters how we size elements. A column near the perimeter may face loads twice as large as one near the centre. Braced frames must resist combined axial, shear, and torsional effects. Connections require rotational stiffness. Floor systems must maintain diaphragm integrity. Everything rotates. Everything resists.
If ignored, torsion causes progressive damage. Cracks appear at corners. Columns buckle under uneven loading. Pounding occurs between adjacent units. Vibration modes change unexpectedly. In seismic conditions, torsion becomes especially dangerous. It concentrates drift on one side, triggering soft storey collapse.
Engineers must evaluate torsional demand early. The layout stage provides opportunity. Balance mass and stiffness. Symmetry helps, but so does uniformity. Place shear walls strategically. Avoid large plan irregularities. Keep setbacks minimal. Distribute bracing evenly.
Torsion-resistant design is not about eliminating asymmetry. It is about managing its effects. Use analysis models that capture three-dimensional response. Model diaphragms correctly. Include accidental eccentricities. Evaluate drift and rotation. Size members for amplified forces. Consider nonlinearities.
Mitigating Torsional Effects in Practice
Structural mitigation begins at planning. A symmetrical building rarely suffers major torsion. But when asymmetry becomes inevitable, balance remains the key. Engineers must align stiffness centres with mass centres. This reduces primary eccentricity. For example, placing concrete cores strategically can improve alignment.
Redundant systems enhance performance. Adding shear walls or braced frames on both sides provides balance. If one side resists lateral loads, the other must match. Diaphragm continuity must not be compromised. Holes, openings, or soft zones weaken control over twist.
Connection detailing matters. Rigid diaphragm action depends on joint capacity. Transfer beams must engage the structure fully. Lateral-torsional stability requires stiffness in multiple directions. Thus, both horizontal and vertical integration reduce risk.
In steel structures, torsion control involves braced bays or moment frames. Braces must distribute around the perimeter, not cluster in one zone. Moment connections must resist both bending and rotation. In concrete, core walls or perimeter walls help reduce asymmetry. Flat slabs with unbalanced column grids increase vulnerability.
Torsion in Seismic Design
Seismic codes treat torsion seriously. Earthquakes cause lateral vibration. If the structure is asymmetric, it twists. This rotation creates differential displacement. One side sways more than the other. The result is non-uniform demand, potential instability, and collapse.
Codes enforce accidental torsion. This assumes unknown imperfections in mass or stiffness. The lateral load is shifted from the centre by a percentage—usually 5% to 10% of the plan dimension. This shift simulates torsion during analysis. It forces design actions to consider possible rotation.
In dynamic analysis, torsion appears as coupled modes. The fundamental mode may rotate. Higher modes amplify this effect. The response spectrum must capture both translational and torsional contributions. Buildings with significant asymmetry may require time-history analysis.
Seismic detailing reinforces torsion control. Strong column–weak beam ensures redistribution. Confinement zones resist crushing. Shear walls need boundary elements. Bracing systems must anchor into diaphragms effectively. Foundations must withstand eccentric shear. Each system must function during twist.
Failure to address torsion in seismic zones has caused catastrophic damage. Historic collapses show twisting motion leading to soft storey mechanisms. Once rotation concentrates demand, local failure escalates. Thus, engineers must identify torsional irregularities and design for them directly.
Modelling and Analysis Considerations
Torsion must be captured in structural models. This requires three-dimensional analysis. Plan irregularities, mass offsets, and stiffness concentrations all affect results. Simplified two-dimensional frames cannot identify torsional behaviour. Only a full model can simulate it accurately.
Diaphragm stiffness is crucial. Rigid diaphragms redistribute loads evenly. Flexible diaphragms allow uncontrolled rotation. Engineers must specify the correct diaphragm behaviour in models. Floor systems should be evaluated for stiffness and connectivity.
Mass eccentricity should be included. Even if geometry appears symmetric, equipment, tanks, or furniture can cause imbalance. Codes mandate accidental torsion to reflect such conditions. Engineers should simulate this directly, using shifted loads.
Nonlinear effects influence torsional response. Cracking in concrete reduces stiffness. Yielding in steel alters load paths. Time-dependent effects shift mass. These must be considered during advanced analysis. Engineers should run both elastic and inelastic studies, especially in critical zones.
Structural Robustness Against Torsion
Robust buildings resist unexpected loads. Torsion often represents such unpredictability. A robust structure distributes loads. It avoids concentration. It responds with limited damage. This demands good proportioning, redundancy, and capacity design.
In asymmetric layouts, engineers must provide multiple load paths. Each floor must transfer forces reliably. Walls and frames must resist combined loads. Ductile detailing ensures energy absorption. Foundations must prevent differential movement.
Progressive collapse often begins with overlooked torsion. One element yields. Load shifts. Another fails. The chain reaction leads to collapse. Robust design breaks this chain. It allows parts to fail without global consequence. This must be intentional, not accidental.
Torsion should also guide design checks. Engineers must assess drift, strength, stiffness, and rotation. The interaction between translation and rotation must be clear. Members must resist increased demand. Joints must maintain capacity. The system must rotate without breaking.
Conclusion
Torsion is a fundamental aspect of structural behaviour. In asymmetric buildings, it becomes a governing factor. It alters load paths, stresses, and deformation. It complicates analysis and demands greater care. But with deliberate design, its effects can be mitigated.
Good design begins with understanding. Torsion arises from eccentricity—mass or stiffness. It leads to rotation. That rotation increases demands. The building must resist. This requires layout planning, three-dimensional analysis, diaphragm integrity, and proper detailing.
Also See: Avoiding Torsion in Structures
Sources & Citations
- IS:1893 (Part 1): Criteria for Earthquake Resistant Design of Structures. Bureau of Indian Standards.
- Taranath, B.S. (2011). Structural Analysis and Design of Tall Buildings. CRC Press.
- Stafford Smith, B. & Coull, A. (1991). Tall Building Structures: Analysis and Design. Wiley.
- The Structural Engineer. (2017). Across-wind load on rectangular tall buildings
