What is Mass Source in ETABS? A Simple Explanation for Structural Engineers

Published on 2026-05-19

In ETABS, one of the most important — and most misunderstood — concepts is the Mass Source. Many engineers define loads correctly, apply combinations meticulously, and then overlook one critical setting that can silently invalidate their entire seismic analysis.

The core question: Where does ETABS get its seismic mass from? Define it incorrectly, and every response spectrum result, every modal period, and every base shear value will be wrong — even if the model looks perfectly fine on screen.

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What is Mass Source in ETABS?

Mass Source is the setting that tells ETABS how to calculate the total seismic mass of your structure. This mass feeds directly into:

• Response Spectrum Analysis (RSA) — the backbone of modern seismic design
• Time History Analysis — for dynamic loading scenarios
• Modal Analysis — determining natural periods and mode shapes
• Equivalent Static Force — base shear calculations

ETABS does not automatically "know" which loads contribute to seismic mass. You must explicitly define it.

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Why Does Mass Matter So Much?

The relationship is fundamental and unforgiving. Earthquake forces are governed by Newton's second law:

$$F = m \cdot a$$

Where:

Symbol	Meaning
$F$	Seismic force on the structure
$m$	Seismic mass (defined by Mass Source)
$a$	Ground acceleration (from response spectrum)

There is no ambiguity: wrong mass → wrong seismic force → unconservative or over-conservative design. The spectral acceleration is fixed by your site and hazard level. The only variable you control is mass.

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What Loads Should Be Included?

ETABS converts selected load cases into seismic mass using a multiplier per load case. The percentages below reflect common engineering practice and code intent:

Load Type	Typical Mass Participation	Rationale
Dead Load (DL)	100%	Always present; full contribution
Superimposed Dead (SDL)	100%	Permanent — finishes, MEP, cladding
Live Load (LL)	25% (typical)	Partial occupancy assumption
Partition Load	100% or per code	Treated as dead if permanent
Roof Live Load	0–20%	Rarely present during a seismic event

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📘 Code Reference — ASCE 7-22 Effective Seismic Weight

As per ASCE 7-22 Section 12.7.2 — Effective Seismic Weight, the effective seismic weight ($W$) of a structure must include all significant permanent and applicable live loads above the base.

This includes:

• Dead loads
• Permanent equipment and machinery
• Partition loads
• Portions of storage live loads
• Roof loads where required
• Fluids and bulk materials present during normal operation

🔹 Important ASCE 7-22 Requirements

✔ Storage Areas

A minimum of 25% of floor live load must be included in seismic weight for storage occupancies.

✔ Partition Loads

Where partitions exist (or may exist), the actual partition weight or a minimum of:

$$10 \text{ psf } \; (0.48 \text{ kN/m}^2)$$

must be included.

✔ Permanent Equipment

The total operating weight of permanent equipment must be included in seismic mass.

✔ Roof Snow Load

When flat roof snow load exceeds:

$$45 \text{ psf } \; (2.16 \text{ kN/m}^2)$$

at least 15% of the design snow load must be included.

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What Goes Wrong With an Incorrect Mass Source?

⚠️ Warning: ETABS will complete the analysis and produce plausible-looking results with no warning message. There is no error for a misconfigured Mass Source — only potentially unsafe results.

Common failure modes include:

• Incorrect base shear — directly proportional to mass error; a 20% mass error produces a 20% base shear error
• Wrong modal periods — period $T = 2\pi\sqrt{m/k}$; underestimating mass artificially shortens the period
• Distorted mode shapes — incorrect mass distribution shifts which modes activate and where
• Flawed response spectrum results — if modal mass is wrong, spectral forces are wrong
• Code non-compliance — an apparently correct model may fail peer review or independent checking

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How to Define Mass Source in ETABS

Step 1 — Open the Mass Source dialog

Navigate to:

Define → Mass Source

Step 2 — Choose the definition method

Select From Loads (recommended). This lets you assign percentage multipliers per load case, matching code requirements exactly.

Step 3 — Add load cases with multipliers

DEAD   ×  1.00   → 100% — always permanent
SDL    ×  1.00   → 100% — finishes, MEP, cladding
LIVE   ×  0.25   → 25%  — per ASCE 7-22 §12.7.2

Step 4 — Assign to analysis cases

In each Modal or Response Spectrum load case, confirm the Mass Source dropdown references the source you just defined:

Define → Load Cases → [Modal Case]
  └── Mass Source: MsSrc1  ← ensure this is set

Step 5 — Verify with mass participation report

After running analysis, check:

Display → Show Tables → Modal Participating Mass Ratios

Cumulative mass participation must reach ≥ 90% per ASCE 7. If not, add more modes until the threshold is met.

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Pre-Analysis Checklist

Before running any seismic analysis, verify the following:

• Dead loads included at 100%
• Live loads included at the code-specified fraction (check ASCE 7, UBC, or local code)
• All permanent equipment included — MEP, HVAC, façade systems
• Mass Source assigned to modal and RSA load cases (not left as default)
• Cumulative modal mass participation reaches ≥ 90%

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Summary

Mass Source is not a secondary setting — it is the foundation of every dynamic result your model produces. Define it correctly from the start, cross-reference it against your code's seismic weight provisions, and verify it with the mass participation report after every analysis run.

Wrong mass = wrong analysis. There are no shortcuts here.

For storage occupancies, ASCE 7-22 specifically requires a minimum of 25% of the floor live load to be included in the effective seismic weight.

Although this 25% requirement is explicitly mentioned for storage areas, structural engineers commonly use approximately 25% of live load for other occupancies as well when defining the ETABS mass source.

👉 This is based on engineering practice, expected occupancy during an earthquake, and realistic seismic mass representation.

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