Picture this: the engineer’s report says your building has “three potential deficiencies.” Should you start pricing a retrofit? Not yet. That’s a Tier 1 result, and it’s where most seismic evaluations begin.
ASCE 41 evaluations come in three tiers, three levels of detail. Each goes deeper than the last and costs more, so knowing what each one tells you saves money and worry. Below, we follow one building through all three.
First, what is ASCE 41?
ASCE 41, Seismic Evaluation and Retrofit of Existing Buildings, is a standard published by the American Society of Civil Engineers. It’s written for buildings that already exist, which is different from the codes used to design new ones. It checks a building against a performance target agreed with the owner, such as Collapse Prevention, Life Safety, or Immediate Occupancy, and it looks at both how strong the structure is and how far it can bend without failing. We use the 2017 edition, ASCE 41-17, where it suits the project.
In the Philippines, ASCE 41 is used alongside the National Structural Code of the Philippines (NSCP), not instead of it. For example, DPWH’s 2020 clarification for its “The Big One” Phase III projects called for NSCP 2015 together with ASCE 41-13 Life Safety criteria.
Follow one building through the tiers
Tap a tier to see what the engineer looks at, and what they find.
Tier 1 · Screening
The check-up
Like
A general check-up: the same list of questions for every patient.
We look at
The whole building, against checklists for its type.
Testing
Usually none. Drawings and a walk-through.
You get
A list of possible weak points, or a clean result.
Effort
Lowest
In our example: a 1980s three-storey apartment building. The checklists flag three things: the open parking floor (a possible soft storey), short columns beside a half-height wall, and an addition built tight against the main building.
Tier 2 · Deficiency-based
A closer look
Like
A specialist checking everything the check-up flagged.
We look at
The flagged weak points, and the parts they affect.
Testing
Sometimes, where the answer depends on it.
You get
Which flagged items really need action.
Effort
Moderate
In our example: the parking-floor columns fall short. The short columns pass. The joint with the addition needs a proper separation detail. Everything else stays as it is.
Tier 3 · Systematic
The full evaluation
Like
A full diagnostic scan of the whole body.
We look at
Every structural part, working together in a computer model.
Testing
Often: concrete cores and rebar scans.
You get
Exactly which members fall short, and by how much.
Effort
Highest
In our example: if the owner wants a retrofit design, plans to add a floor, or needs a higher target, we model the whole building, confirm the concrete and steel with cores and scans, and size the strengthening: in this case, jackets on the parking-floor columns.
Tier 1: the check-up
An engineer reviews the drawings (if there are any), visits the building (the standard requires at least one site visit, Section 3.2), and works through ASCE 41’s checklists for that type of building, for example a concrete frame or a building with concrete walls. The checklists ask questions like: is any storey much weaker than the one above it? Are there short columns trapped beside half-height walls? Is the building close enough to its neighbour to pound against it? (The standard has engineers look at any neighbour within 4% of the building’s height, about 40 cm for a 10-metre building; Section 3.2.5.1.) A few simple “quick check” calculations back them up.
Any item that isn’t compliant is a potential deficiency: a reason to look closer, not proof of a problem. If every item is compliant, the evaluation can stop there. ASCE 41 allows any of the three tiers to show that a building meets its target (Section 2.2.1).
A Tier 1 flag is a reason to look closer, not proof of a problem.
Worth knowing: in ASCE 41-17, the Tier 1 checklists for ordinary buildings are based on Collapse Prevention under very rare, strong shaking. (The 2013 edition used Life Safety checklists.) There is a basic configuration checklist and a structural checklist for each of the standard’s common building types, such as concrete moment frames, concrete shear walls, and concrete frames with masonry infill (Chapter 17; Table 3-1). A building whose structure doesn’t match one of those types can’t use Tier 1 or Tier 2 at all and must use Tier 3 (Section 3.2.1). Which tiers are allowed also depends on the performance target and how strong the shaking is expected to be. Metro Manila sites generally fall in the standard’s “high seismicity” category, the strictest one (Table 2-4).
Tier 2: a closer look
Tier 2 takes every flagged item and checks it properly, with calculations focused on that part of the building and the parts it affects. Some flagged items turn out to be fine. Others are confirmed as real deficiencies that need work. Because it focuses on what was flagged, an analysis of the whole building may not be needed, so Tier 2 is usually faster and cheaper than a full evaluation.
In practice
Targeting known weak points is a well-established approach. San Francisco’s soft-story program, for older wood-frame buildings with weak ground floors, accepted ASCE 41-13 as one of the methods engineers could use, and focused the work on the weak storey rather than the whole building.
Tier 3: the full evaluation
Tier 3 evaluates the whole structure. We build a computer model of the building and analyse every structural part working together, using linear or nonlinear methods as the building needs. Tier 3 also sets rules for how much information about the building must be collected (Chapter 6). Testing, such as concrete cores for actual strength and rebar scans to confirm the steel, lets the engineer use the more detailed methods and the materials’ real strengths. With less information, the standard makes the engineer assume lower strengths.
It’s the most work, but it’s also the most precise. With better information, the engineer can use fewer cautious assumptions, and a part that looked deficient in a simpler check may turn out to be adequate. Some buildings start here. A hospital wing that must stay usable after a major earthquake needs the demanding Immediate Occupancy target, and a full evaluation is common for buildings like that.
Tier 3 is also the usual route when the building itself is changing. ASCE 41 doesn’t decide when an evaluation is required; that comes from the NSCP and the building official. But a renovation, an added floor, or a change of use brings new loads or a new use, and a checklist can’t show whether the structure can carry them. In practice that means checking the new loads under the NSCP, with a full Tier 3 analysis for the seismic side. A change of use can also move the building into a different risk category, and the default target depends on the risk category (Section 2.2.1).
In practice
Davao, Philippines: researchers at the University of Mindanao evaluated a four-storey school building on its campus using ASCE 41 and the DOST-PAGASA Metro Davao earthquake model, with both linear and pushover (nonlinear) analysis. Against the Life Safety target, the linear analysis found that columns, especially on the first and top floors, didn’t meet the criteria (Cabotaje and Bernardo, 2024).
California, USA: engineers evaluating a seven-storey hospital tower that needed to reach Life Safety used a nonlinear “pushover” analysis of the whole building, the kind Tier 3 uses, specifically because it gives a more realistic picture of the forces in a major earthquake and helps keep retrofit costs down (14th World Conference on Earthquake Engineering, 2008).
Which tier might your building need?
Three quick questions for a rough idea. We’ll confirm the right tier after looking at your building.
Answer all three to see a suggestion.
Do you always go through all three?
A seismic check of a building that isn’t changing usually starts with Tier 1, if the building qualifies. If nothing is flagged, you may be done.
Flagged items go to Tier 2, which settles which ones need action.
Renovations, added floors, and changes of use usually skip the screening. The question isn’t “are there weak points?” but “can it carry the new loads or use?”, and that takes a full analysis.
Some buildings go straight to Tier 3: buildings beyond the standard’s limits for Tiers 1 and 2, buildings whose structure doesn’t match one of its common building types, and often those that need Immediate Occupancy or a retrofit design.
Some owners choose Tier 3 directly, because a simpler check would flag items a full analysis could clear, and that can mean less strengthening.
The tier, the edition of the standard, and the analysis method are chosen to suit each building and the agreed target. We’ll recommend one after looking at the drawings and the building, and explain why.
Not sure which tier your building needs? Tell us about it. We’ll recommend the right starting point.