Most owners come to us with one question: will this building hold up in an earthquake? The honest answer is that nobody knows until the building has been measured and analysed. In Türkiye the procedure for doing that is set out in Chapter 15 of the Turkish Building Earthquake Code, TBDY 2018, which covers the assessment of existing buildings under earthquake loading and the design of their strengthening.
This article walks through what the engineer actually does during a seismic performance assessment, in the order it happens. It is written for building owners and facility managers who need to understand the process well enough to commission it, read the report and act on it.
1. Reading whatever paperwork exists
The first day is spent with documents, if there are any. We ask for the original structural drawings, the architectural set, the licence file from the municipality, any as built records, soil reports and anything from past repairs or extensions. Some of this will be missing. For older buildings the drawings often cannot be found at all, or the ones in the file differ from what was built.
That is not a dead end. TBDY 2018 anticipates it. The code ties the amount of site investigation to how much reliable information you already have, so a building with a full drawing set needs less surveying than one with none. What the documents give us at this stage is a starting hypothesis about the structural system: where the columns and shear walls are, what the floor system is, how the foundation was designed. Everything in that hypothesis is checked on site.
2. The measured survey
The survey, the röleve in Turkish practice, is where the assessment stops being about drawings and starts being about the building. An engineer walks every floor and records the structural system as it exists today: the position and dimensions of every column, beam and wall, the storey heights, the slab type and thickness, openings, stair cores and any element that has been added, removed or altered since construction.
Ground floor shops that removed a wall for a wider display window, a mezzanine nobody applied for, a basement partition that turned out to be load bearing: these are the things the survey exists to find. The survey also records condition. Cracks are mapped and their width and direction noted. Corrosion staining, spalled cover, water damage and settlement signs go into the record with photographs.
When the survey is done, we compare it to the drawings. Where they agree, the drawings become usable evidence. Where they disagree, the survey wins.
3. Material testing
A concrete column drawn as a certain grade on paper may have been cast at a quite different strength. The only way to know is to test. TBDY 2018 requires the existing material strengths to be established from the building itself, and the scope of testing follows from the information level being targeted.
Core samples
Concrete strength is measured on core samples, the karot. A core drill removes a cylinder of concrete from a column or wall, the hole is patched, and the cylinder is tested to failure in a laboratory. Cores are taken from a spread of elements across the building rather than from one convenient spot, because the concrete in a building is rarely uniform. The results give the strength value that goes into the analysis, and they also tell us how variable the concrete is, which matters for how much confidence the code lets us place in the model.
Reinforcement scanning
The reinforcement is located with a cover meter or a ground penetrating radar scan. This tells us the number and spacing of the bars, the stirrup spacing and the cover depth without breaking the concrete. On a sample of elements the cover is opened locally so the bar diameter and surface type can be confirmed by eye and, where needed, a coupon can be cut for tensile testing. Stirrup spacing deserves attention because it governs how much a column can deform before it loses its capacity, and older buildings often have wider spacing than today's practice.
Ground conditions
If no reliable soil report exists, a geotechnical investigation is commissioned. The site class feeds directly into the earthquake demand the building will be checked against, so it is not something to estimate from a neighbouring plot.
4. Settling the information level
Once the survey and testing are complete, the engineer decides which information level the building has reached. TBDY 2018 distinguishes between a limited and a comprehensive level of information about an existing building, and it assigns an information level coefficient to each. That coefficient enters the analysis and reduces the capacity the engineer is allowed to count on. Less investigation means a lower coefficient, which means a more conservative result.
This is a point worth understanding as an owner. Skimping on the investigation does not make the building look better. It makes the analysis assume the worst about the parts you did not check, and a building can fail its assessment on paper for lack of information rather than lack of strength. The report states which level was reached and why.
5. Building the analysis model
With the geometry from the survey, the strengths from the tests and the information level fixed, the building is modelled in structural analysis software. Every column, beam, wall and slab goes in with its measured dimensions and its measured reinforcement, not its designed reinforcement. Loads are assigned from the actual use of each floor.
The earthquake demand comes from the Turkish seismic hazard map for the building's coordinates and the site class from the ground investigation. TBDY 2018 defines several earthquake ground motion levels, from the frequent service earthquake to the very rare one, and the assessment is run against whichever level the performance target calls for.
The analysis itself can be linear or nonlinear. In the linear approach, the demand on each member is compared with its capacity and the ratio tells you how far into damage that member goes. In the nonlinear approach, the structure is pushed or shaken in the model until members yield, and the deformation each one reaches is compared with the deformation limits the code sets for each damage state. Nonlinear analysis is more work and gives a more realistic picture of what happens after the first members yield. Which method is appropriate depends on the building and on what the result has to support.
6. Performance levels and targets
TBDY 2018 describes the state of a building after an earthquake using performance levels. In ascending order of damage they are Continuous Occupancy (Kesintisiz Kullanım), Limited Damage (Sınırlı Hasar), Controlled Damage (Kontrollü Hasar) and Collapse Prevention (Göçmenin Önlenmesi). Each is defined by how much damage the individual structural members are allowed to show.
A performance target pairs one of these levels with an earthquake level. The code ties that pairing to the building's use and importance, so a building that has to keep working after an earthquake is held to a higher target than an ordinary residential block. The engineer does not choose the target freely. The code assigns it, and the analysis reports whether the building meets it.
The question is a precise one: "under the earthquake the code assigns to this building, does it stay within the damage state the code allows for it." That has a yes or no answer, and the report gives it. A general "is this building safe" is a different question, and the report does not pretend to answer it.
7. What the report says
The assessment report is the deliverable. A good one can be read at two levels. The summary states in plain terms whether the target was met and, if not, where the shortfall lies. The body of the report shows how that result was reached: the documents reviewed, the survey drawings, the test locations and results, the information level and its justification, the model assumptions, the analysis method and the member by member findings.
Where the building falls short, the report identifies which members fail and in what way. A building rarely fails uniformly. Often it is a particular storey, a particular column line or a set of short columns created by a band of windows. This detail is what makes the next step possible, because a retrofit is designed against the specific deficiency, not against the building in general.
8. What happens after the report
If the building meets its target, the report says so and the work is finished, apart from any maintenance items the survey turned up. If it does not, there are three routes and the assessment is what decides between them.
Strengthening
Conventional strengthening adds capacity where the analysis found it lacking. Column and wall jacketing, new reinforced concrete shear walls, steel bracing frames and fibre reinforced polymer wraps are the usual tools. TBDY 2018 sets out the rules for designing these interventions. The strengthened building is then analysed again to confirm it reaches the target. The disruption to the building's use during the works is a real cost and has to be weighed alongside the construction cost.
Seismic isolation
Where conventional strengthening cannot reach the target without gutting the building, or where the building's function cannot tolerate the damage that a fixed base structure would accept, base isolation is the alternative. Isolators are placed at the foundation or at a chosen storey so the ground motion is largely decoupled from the structure above. This is a separate design task with its own rules in TBDY 2018, and it suits buildings whose contents or continued operation are worth more than the structure itself.
Rebuilding
Sometimes the arithmetic does not close. If the deficiency is widespread, the concrete is poor throughout and the building has little remaining service life, the cost of bringing it to target can approach or exceed the cost of a new structure. The assessment report gives the owner the numbers to make that call with, rather than a feeling.
Commissioning an assessment
Three things make an assessment go well. Gather every document you have before the engineer arrives, including the ones you think are irrelevant. Give access to every floor, including basements, roofs and tenanted spaces, because a survey with gaps produces a report with gaps. And decide early what the report needs to support, whether that is a decision to strengthen, a transaction, a permit or an insurance question, because that shapes the analysis method and the level of investigation.
The outcome is a document that states, with evidence, what the building can carry today and what it will do in the earthquake the code assigns to it. Every decision that follows rests on those numbers.
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