A seismic vulnerability study establishes how prepared an existing building is for the earthquakes the current code requires. The NSR-10 governs it in chapter A.10: it compares the forces and drifts the structure should withstand today with its actual strength and stiffness, and expresses the result as a fraction of what a new building would have (A.10.5.1).
When does the NSR-10 require it?
When the building is extended, its structure is modified, its use changes, or it is strengthened or repaired after an earthquake (A.10.1.3). A change of use counts even if the works are minor: going from housing to retail, or to a higher occupancy group, requires an assessment of its effects under gravity loads and, above all, under earthquakes (A.10.1.3.2). The code also allows the same criteria to be used to diagnose any building built before it took effect (A.10.1.3.3).
What does the study include?
- Background information: the original drawings and calculations, if they exist, compared with what was built; construction quality and state of repair; local failures, deflections, corrosion of the reinforcement and settlement; and any history of earthquakes, fires or remodeling (A.10.2.1).
- Rating of the structural system: the quality of the original design and construction, and the current condition, are rated good, fair or poor (A.10.2.2). That rating reduces the strength credited to the structure.
- Analysis: an elastic model of the structure and its foundation under the equivalent loads of the current code (A.10.1.4, stages 4 and 5).
- Overstress index: the ratio between the force the code requires and the effective strength of each member (A.10.4.3.1).
- Flexibility index: the ratio between the calculated drifts and those allowed by chapter A.6 (A.10.4.3.5).
How are the results read?
The inverse of the overall overstress index expresses the vulnerability as a fraction of the strength of a new building, and the inverse of the flexibility index as a fraction of its stiffness (A.10.5.1). An overstress index of 2 means the structure has half the strength the code would require of a new building. Those indices decide whether a retrofit is needed, and where.
What tests are done?
Concrete cores, rebound hammer, ferroscan to locate the reinforcement, carbonation depth and chloride content. The tests confirm what the visual inspection suggests and feed into the rating of the structure's condition.
What is a "peritaje estructural" (forensic structural assessment)?
That is the usual name for the technical assessment of a structure to establish its condition or the cause of damage, often for a claim, a purchase or a court case. It may include a vulnerability study, but it asks a different question: what happened and why, rather than how far the structure falls short of the code.
What if the building is a heritage building?
For buildings declared historic heritage, with severe restrictions on reaching the safety of a new building, the NSR-10 allows a lower level of seismic safety, justified by the engineer, accepted by the owner and recorded before a notary, except in occupancy groups III and IV (A.10.9.2.5). In those cases the limited-safety ground motions are used, with each municipality's Ae parameter (A.10.3), which you can look up in the seismic hazard tool.
See the assessment and retrofit service, the note on existing buildings and the guide when is a crack dangerous?.
Source: NSR-10, Title A, chapter A.10.
Frequently asked questions
What is a seismic vulnerability study?
It is the assessment of an existing building against the earthquakes the current code requires. The NSR-10 governs it in chapter A.10 and expresses the result as a fraction of the strength and stiffness of a new building.
When is it mandatory?
When the building is extended, its structure is modified, its use changes, or it is strengthened or repaired after an earthquake (A.10.1.3).
What does an overstress index greater than 1 mean?
That at least one member receives, under the forces of the current code, more than its effective strength can carry. The inverse of the overall index shows what fraction of a new building's strength the structure has (A.10.5.1).
