<![CDATA[Rooftop water tanks are near-universal in Indonesian multi-story buildings, yet the mass of stored water is frequently omitted from the seismic mass definition applied in structural design. Whether this omission is consequential depends on the seismic hazard of the site, and hazard across the Indonesian archipelago varies widely between regions. This study examines how the inclusion of rooftop operational water mass alters the code-compliance status of a fixed 7-story reinforced concrete Special Moment Resisting Frame (SMRF) when assessed at three locations of increasing hazard: Yogyakarta, Kupang, and Atambua. A three-dimensional model was constructed in ETABS Version 21 and subjected to modal response spectrum analysis in accordance with SNI 1726:2019, with output post-processed in MATLAB R2023b. Two operational states were compared: an empty tank (baseline) and a full 18,000 kg tank (governing condition). Design base shear increased by 13.8%, 14.6%, and 15.9% at Yogyakarta, Kupang, and Atambua, respectively. Maximum inter-story drift ratios rose from 1.65% to 1.95% at Yogyakarta, from 2.10% to 2.55% at Kupang, and from 2.40% to 2.85% at Atambua, against an allowable limit of 2.00%. The most consequential finding is that drift exceedances in Kupang and Atambua pre-exist the full-tank condition; operational mass enlarges an existing deficit rather than creating one. Operational mass acts as an aggravating factor in high-hazard zones. Furthermore, while element capacity checks were beyond the scope of this linear analysis, the systemic failure to satisfy drift limits indicates a fundamental stiffness deficiency, strongly suggesting that member proportioning transferred from moderate-hazard Java is inadequate for eastern Indonesia.]]>
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