A seismic rating is written as a percentage, which makes it look like a property of the building. It is not. It is
%NBS = what the building can take ÷ what the current standard would require of a new building on that site
The numerator is the building. The denominator is a document — and documents get revised. New Zealand went through exactly that when the National Seismic Hazard Model was revised in 2022 and raised estimated hazard across much of the country. Buildings that had not moved an inch got worse numbers.
The same thing happens when a building changes what it is used for. A higher importance level means a bigger design event, which means a bigger denominator, which means a lower percentage for the same walls.
Nothing is uploaded and nothing is stored. This page has no server: it is arithmetic running in your browser on numbers an engineer gave you, and it cannot tell you anything about a building it has never seen. It is not an assessment and it does not replace one.
1 · The number moved. The building didn't.
Put in the rating as it was assessed, then change what it is being measured against.
2 · What strengthening actually buys
A building's rating is the lowest score among its critical structural weaknesses — not an average of them. So work on anything except the governing element changes the rating by exactly nothing, and work on the governing element moves it only as far as the next one up.
What it does not know
It has not seen the building. Every number on this page came from you. An Initial Seismic Assessment is a screening tool and the guidelines say so — its %NBS is indicative, and arithmetic on an indicative number is still indicative. A Detailed Seismic Assessment is the one that carries weight, and neither is this page.
The denominator has more in it than Z and R. The site subsoil class, the near-fault factor, the structural ductility and performance factors and the period of the structure are all in there. This page moves the two that a building owner actually watches change, and holds the rest still — which is exact when the rest really did hold still and is wrong when it did not.
The return period factors here are presets, not gospel. They are the ones the loadings standard uses for the three importance levels, and every one of them is an editable field, because a standard is a document with editions and this page is not.
34% and 67% are thresholds, not physics. Below 34% a building is earthquake-prone under the Building Act and a notice follows, with a statutory timeframe that depends on the seismic risk area and whether it is a priority building — ask the territorial authority, not a web page. 67% is the level the assessment guidelines treat as acceptable. The risk does not step at either number; the law does.
An element list is not a structural model. Real assessments deal in mechanisms that interact — a diaphragm that fails changes what the walls see. Taking the minimum of a list is how a rating is reported, not how a building is analysed.