You have modelled the building, assigned the loads, defined the combinations, and clicked "Run Analysis." The progress bar completes. Green checkmarks appear. ETABS displays a beautiful colourful contour plot of moments, shears, and axial forces. Everything looks… finished.
It is not.
The coloured contour is not your design. It is raw data — the internal forces your structure would experience if your model perfectly represents reality, if your assumptions are correct, and if your input is flawless. None of those "ifs" are guaranteed.
Understanding what the numbers mean, where they come from, and what they imply for your design decisions is the core of structural engineering practice. This article walks through the critical ETABS outputs, explains what each represents, and highlights what to look for — and what to question.
1. Axial Forces, Bending Moments, and Shear: The Three-Force Picture
Every structural element in your model resists three primary internal forces:
| Force | Symbol | ETABS Label | What It Represents |
|---|---|---|---|
| Axial Force | P | P | Compression or tension along the member axis |
| Major Axis Moment | M3 | M3 | Bending about the local 3-axis (typically strong axis) |
| Minor Axis Moment | M2 | M2 | Bending about the local 2-axis (typically weak axis) |
| Shear Force | V2, V3 | V2, V3 | Transverse force perpendicular to the member axis |
Understanding the Sign Convention
ETABS uses a local coordinate system for each frame element. The local 1-axis runs along the length of the member (from start joint to end joint). The 2-axis and 3-axis are perpendicular to it.
- M3 is the moment about the strong axis — for a typical beam, this is the moment that causes sagging or hogging in the vertical plane. This is the primary design moment for beams.
- M2 is the moment about the weak axis — for a typical beam, this is usually small unless there is significant lateral load.
- P is positive in compression. A beam with axial compression is rare in gravity-only frames but common in seismic frames with brace systems.
What to Look For
- Beams: Focus on M3 (strong-axis moment). For a uniformly loaded simply supported beam, the moment diagram should be a smooth parabola with maximum at mid-span. For continuous beams, expect hogging moments at supports and sagging moments at mid-span.
- Columns: Focus on P (axial) combined with M2 and M3 (biaxial bending). A column under pure axial load is rare — nearly every column carries some moment, especially in seismic frames.
- Shear Diagrams: Should mirror the moment diagram (shear is the derivative of moment). Peak shear occurs at supports, zero shear at points of maximum moment.
2. Storey Drift: Is Your Building Too Flexible?
Storey drift is the relative horizontal displacement between two consecutive floors divided by the storey height. It is the single most important indicator of whether your building is stiff enough to resist lateral loads.
The Code Limits (IS 1893:2016 Clause 7.11.1)
| Building Type | Storey Drift Limit |
|---|---|
| RC moment-resisting frame | 0.004 × h (where h = storey height in mm) |
| RC shear wall frame hybrid | 0.004 × h |
For a typical 3.0 m storey height:
Δlimit = 0.004 × 3000 = 12 mm
How to Read the Drift Table in ETABS
Navigate to Display → Show Tables → Analysis Results → Story Drifts. The table will show: Load Case/Combo, Storey, Drift (X or Y), and Drift Ratio (Drift ÷ Storey height).
Interpreting Abnormal Drift Patterns
| Pattern | Likely Cause | Action |
|---|---|---|
| Sudden spike at ground storey | Soft storey (open ground storey) | Add shear walls or increase column sizes |
| Gradual increase with height | Normal behaviour for moment frames | Verify limits are met |
| Large torsion component | Irregular plan shape or asymmetric layout | Reassess centre of mass vs centre of rigidity |
| One direction significantly higher | Asymmetric column arrangement | Adjust column orientations or add walls |
| Very low drift (< 0.0005) | Over-stiff model (check property modifiers) | Verify cracked section modifiers are applied |
3. Mode Shapes and Modal Participation: The DNA of Your Building
Every building has natural ways it "wants" to move. These are called mode shapes. The frequency (or period) of each mode tells you how fast the building vibrates in that pattern. The first few modes carry the most energy and dominate the seismic response.
Reading the Modal Participation Table
Navigate to Display → Show Tables → Analysis Results → Modal Information → Modal Participating Mass Ratios. For a regular building, the first three modes should tell a clear story:
| Mode | Period (sec) | UX (%) | UY (%) | RZ (%) | Interpretation |
|---|---|---|---|---|---|
| 1 | ~0.8 | 65 | 3 | 2 | Predominantly X-translational — good |
| 2 | ~0.7 | 4 | 62 | 5 | Predominantly Y-translational — good |
| 3 | ~0.5 | 3 | 4 | 58 | Predominantly torsional — good |
| 4–6 | < 0.4 | Cumulative > 90% | Residual modes | ||
Period Magnitudes — Quick Reference
For an RCC moment frame, the empirical formula from IS 1893:2016 Clause 7.6.2 gives:
Ta = 0.075 × h0.75 (for RC moment-resisting frames)
Ta = 0.09 × h / √d (for all other buildings)
where h = total height of building in metres, d = base dimension in the direction of shaking
A simpler rule of thumb: T₁ ≈ 0.1 × N (where N = number of storeys). A 10-storey building should have a first period around 1.0 sec. If your model gives T₁ = 3.5 sec, the model is too flexible. If T₁ = 0.3 sec, the model is likely over-constrained.
If Mode 1 is torsional, stop immediately. This indicates severe plan asymmetry and will lead to non-uniform force distribution during an earthquake. Reassess the structural layout before proceeding.
4. Storey Shear: Following the Path of Lateral Load
Storey shear is the total horizontal force acting at any given floor level — the cumulative effect of seismic or wind forces pushing laterally on the building. At the base, storey shear equals the base shear.
Reading the Storey Shear Table
Navigate to Display → Show Tables → Analysis Results → Story Shear Output.
| Column | Meaning |
|---|---|
| Storey | Floor label (Base, Storey 1, Storey 2, … Roof) |
| Load Case/Combo | Which seismic or wind load case |
| X-Dir | Total shear in X at that storey (kN) |
| Y-Dir | Total shear in Y at that storey (kN) |
| % Vb | Storey shear as percentage of base shear |
Manual Cross-Check Formula
Always cross-check the ETABS base shear against the IS 1893 manual calculation:
Vb = Ah × W
Ah = (Z / 2) × (I / R) × (Sa / g)
Z = Zone factor | I = Importance factor | R = Response reduction factor
Sa/g = Spectral acceleration coefficient | W = Seismic weight of building
A discrepancy greater than 10% between the ETABS base shear and your manual calculation demands investigation before proceeding to design.
5. Column Design Forces: The Biaxial Puzzle
Columns resist axial load (P) combined with moments about both axes (M2 and M3). ETABS performs biaxial interaction design per IS 456:2000 Annex C, checking the column capacity against all load combinations.
Reading Column Design Output
Click on any column after running concrete frame design. The design output shows:
| Parameter | Meaning | What to Check |
|---|---|---|
| Governing Combo | Load combination with highest demand/capacity ratio | Should make physical sense (e.g., 1.2DL+1.2LL+1.2EQ) |
| Pu (kN) | Factored axial force | Compare with tributary area estimate |
| Mu2 (kNm) | Factored moment about 2-axis | Should be smaller than Mu3 for typical orientation |
| Mu3 (kNm) | Factored moment about 3-axis | Primary design moment — check against beam moments |
| Rebar % | Provided longitudinal reinforcement ratio | Must be between 0.8% and 4% (IS 456 Cl. 26.5.3.1) |
| DC Ratio | Demand-to-capacity ratio | Must be < 1.0; investigate if > 0.9 |
6. Beam Design Output: Reading Between the Lines
After running concrete frame design, clicking on any beam reveals the design forces and reinforcement provided.
| Parameter | Meaning | What to Check |
|---|---|---|
| Governing Combo | Critical load combination | Should include seismic if applicable |
| Mu (kNm) | Factored design moment | Compare with wL²/8 estimate |
| Vu (kN) | Factored shear force | Should be near supports, not mid-span |
| Ast,prov (mm²) | Provided steel area | Must be ≥ Ast,req and ≤ 4% of bD |
| DC Ratio | Demand/capacity ratio | < 1.0; investigate if > 0.9 |
| Stirrup spacing | Spacing of lateral ties | Must meet IS 13920 ductility requirements in seismic zones |
Steel Area Limits (IS 456:2000)
Minimum Ast = 0.85 × b × d / fy (Cl. 26.5.1.1a)
Maximum Ast = 0.04 × b × D (4% of gross cross-section)
b = beam width | d = effective depth | D = overall depth | fy = yield strength of steel
7. The Lumping Effect: Spotting Artificial Force Concentrations
Sometimes a single beam or column shows disproportionately high forces compared to adjacent similar members. This is often an artefact of meshing or load lumping, not a genuine structural response.
Common Causes
- Uneven meshing: Shell elements of vastly different sizes meeting at a node create artificial force spikes.
- Point loads at unsupported locations: A line load or area load converted to equivalent point loads at mesh nodes creates spikes.
- Short stub beams: Members shorter than 300 mm connecting larger members can attract enormous axial forces.
- Offset connections: Members connected through offsets rather than direct nodal connection create lever arm effects.
How to Diagnose
- Plot the force diagram along the member length. If forces spike at a single node and drop to normal elsewhere, it is likely a meshing artefact.
- Refine the mesh by 50% and re-run. If the spike disappears or reduces dramatically, it was a meshing issue.
- Identify any member shorter than 300 mm — either remove it or replace with a rigid link.
8. Reaction Output: The Ultimate Sanity Check
The reaction output table lists the forces transferred from the structure to the supports (foundations). This is your last line of defence — if reactions don't make sense, nothing upstream is trustworthy.
Navigate to Display → Show Tables → Analysis Results → Joint Reactions.
Foundation Eccentricity Check
For each footing, calculate the resultant force eccentricity:
e = M / P
e = eccentricity | M = moment reaction at support (kNm) | P = axial reaction at support (kN)
Ensure eccentricity remains within the footing dimensions — preferably within the middle third for soils sensitive to overturning.
Equilibrium Checks
| Check | Expected Result | If It Fails |
|---|---|---|
| Sum of vertical reactions = Total gravity load | Match within ±0.1% | Missing load cases or incorrect mass source |
| Horizontal reactions under gravity-only loads | Near zero | Restrained thermal expansion or modelling error |
| Moment reactions at fixed supports | Within foundation design capacity | Revise foundation or reduce fixity assumption |
| Uplift under 0.9DL ± 1.5EQ | No uplift (or designed for it) | Foundation stability concern — check overturning |
The Output Reading Checklist
Before accepting any ETABS analysis result, run through this checklist:
Force Distribution
- Beam moments match expected shapes (parabolic for UDL, triangular for point loads)
- Column axial loads match tributary area estimates (±25%)
- Shear diagrams are mirror images of moment diagrams
- No unexplained force spikes at individual nodes
- Forces in symmetric members are approximately equal
Global Behaviour
- Fundamental period ≈ 0.1 × number of storeys (RCC frame)
- First two modes are translational, third is torsional
- Storey drift ≤ 0.004 × storey height
- Base shear ≈ 3–5% of seismic weight (Zone III, regular RCC)
- Modal mass participation ≥ 90% in each direction
- Drift pattern is smooth from base to roof (no sudden jumps)
Reactions and Equilibrium
- Sum of vertical reactions = Total gravity load (±0.1%)
- No spurious horizontal reactions under gravity-only loads
- No uplift under critical seismic combinations (or designed for it)
- Foundation eccentricities within acceptable limits (middle third)
Design Outputs
- All DC ratios < 1.0
- Governing combinations make physical sense
- Verification checks performed on a representative sample (minimum 10% of beams and columns)
"ETABS output is not a verdict. It is a conversation — a dialogue between the model and reality, filtered through your assumptions. Every number on the screen represents an assumption you made, multiplied by a solver and presented as a result."
When you read ETABS output, you are not merely checking whether DC ratios are below 1.0. You are asking: Does this building behave the way I expect it to? Are the forces flowing through the paths I intended? Is the building stiff enough, strong enough, and stable enough? If I removed the software, could I still estimate these numbers by hand?
The engineer who can answer "yes" to all four questions is not just operating software. They are practising structural engineering.
And that is the difference.
This article is part of the bypraba.com engineering encyclopedia — a free reference for practising civil and structural engineers. Explore our full suite of tools at bypraba.in.