How to Build an Earthquake-Resistant House in Turkey

An earthquake-resistant house is not a house that comes through a major earthquake without a scratch. The goal set by the Turkish Building Earthquake Code (TBDY 2018) is more realistic: under the design earthquake the structure may crack and some members may yield, but the building must not collapse and the people inside must be able to get out. Engineers call this the life-safety performance level. Knowing that from the start helps you ask the right things of whoever builds your house.

Every new building in Turkey has had to be designed to TBDY 2018 since it came into force in January 2019. But a code is only paper. How a house actually behaves in an earthquake is mostly decided on site, while the steel is being tied and the concrete poured.

Start with the hazard map and the soil

TBDY 2018 dropped the old numbered earthquake zones. Design values (the spectral accelerations SS and S1) now come from AFAD's Turkey Earthquake Hazard Map, looked up for the exact coordinates of your plot. Two plots in the same district can end up designed for different loads.

The map is half the picture. The ground is the other half. The same earthquake shakes a house on rock very differently from one on soft clay. A geotechnical report, based on boreholes and lab tests, puts the site into a soil class from ZA to ZF, and that class scales the earthquake load up or down. The report also gives bearing capacity, groundwater level and, where relevant, a liquefaction check. You cannot get a building permit without it.

Around Şile and Ağva two situations come up again and again. On sloping plots, weathered rock and clayey fill often sit side by side, so one corner of the house bears on firm ground and another on soft ground, which leads to differential settlement. On the flat plots near the Göksu and Yeşilçay streams in Ağva, the water table is high and the soil can be sandy or silty. There, a liquefaction assessment belongs in the report.

Şile is further from the fault system under the Sea of Marmara than Istanbul's southern shore, so its mapped accelerations are lower. Lower, not zero.

Shape comes before material

A large share of earthquake damage comes from the form of a building rather than what it is made of. A plain, roughly symmetrical structure with no sudden changes between floors performs much better than a complicated one built from the same materials.

Irregularities to avoid:

• Soft storey: leaving the ground floor open for parking while the floors above are closed in with infill walls. Deformation piles up in the weak floor.
• Torsion: bunching the shear walls or stair core on one side. The building twists, and corner columns take far more than their share.
• Short columns: partly enclosing a column with a half-height wall or a ribbon window. Its free length shrinks, shear demand jumps and it fails in a brittle way. This is common in basement columns of houses built on slopes.
• Columns sitting on cantilevers: resting an upper column on a beam or cantilever to make a balcony bigger. The load path is broken.

In villa design, this is where the architecture and the structure pull against each other. Wide glazing, column-free living rooms and deep cantilevers are all possible, but each has a price: bigger sections, more shear walls, or steel strengthening. Sorting it out at the drawing stage is far cheaper than moving columns once the formwork is up.

What to check in a reinforced concrete house

The core rule is strong column, weak beam. Damage should form in the beams first, in a controlled way, while the columns stay intact. The calculation makes sure of that; the site work must not undo it.

• Concrete grade: TBDY 2018 does not accept anything below C25 in seismically designed concrete buildings. On coastal plots in Şile we use C30 or above because of chlorides in the sea air. Check the grade on each ready-mix delivery note and make sure the inspection firm takes samples at every pour.
• Rebar: B420C or B500C. Both are ductile steels that absorb energy by yielding. Plain bars or bars of unknown grade should be sent back.
• Ties: stirrups are spaced closer at column ends and through beam-column joints, and the hooks are bent to 135 degrees into the concrete core. A 90-degree hook opens up under repeated loading. It is one of the most common defects we find in older buildings.
• Column size: under TBDY 2018 the short side of a rectangular column cannot be less than 30 cm. The formwork should match the drawings to the centimetre.
• Cover: usually at least 2.5 cm in columns and beams, more in foundation members against soil. Without spacers the bars end up touching the formwork and start rusting within a few years.
• Vibration and curing: concrete placed without a poker vibrator leaves voids. After the pour it should be kept wet for at least a week, a step that often gets skipped in summer.

Even in a two-storey villa, shear walls add a lot of lateral stiffness. Spreading them evenly in plan, preferably close to the corners, cuts down torsion.

How do steel and light gauge steel houses behave?

Earthquake force is roughly proportional to mass. A light steel frame (LSF) house weighs a fraction of a concrete house of the same size, so the sideways force it has to resist is much smaller. That is the system's main seismic advantage.

Light weight alone is not enough, though. In an LSF house, lateral load is carried by diagonal straps and by OSB or cement board screwed to the studs. Screw spacing, strap anchorage and the hold-downs tying walls to the foundation all come from the calculation and have to be installed at exactly that spacing. A board with half its screws missing will not give the stiffness the engineer assumed.

Heavy steel frames are ductile, but their behaviour is governed by the connections, so weld quality and bolt tightening need checking. In both steel systems corrosion protection is part of earthquake safety. A section that has rusted in Şile's salty air is no longer the section in the design. For light steel we use at least Z275 galvanising, meaning 275 g of zinc per square metre. There is more on this on our page on why steel and prefab houses.

Foundations

The geotechnical report decides the foundation. For low-rise houses a raft is the usual choice: it spreads the load and limits differential settlement. Weak ground may need piles, jet grouting or stone columns. On a slope, a stepped foundation and a retaining wall behind the house are normally part of the job, and that wall needs proper drainage. Otherwise water builds up behind it and pushes on both the wall and the house. Where groundwater is high, as in parts of Ağva, the blinding layer and waterproofing under the raft matter even more.

Building inspection and the paperwork to keep

Under Law No. 4708 on Building Inspection, every new permitted building has a licensed inspection firm assigned to it. The inspector checks the rebar before each pour and sends concrete samples to a lab. As the owner you can ask for the geotechnical report, the approved structural drawings, the 7 and 28-day concrete test results and the ready-mix delivery notes. Keep them in a file. When you sell the house, or if anyone ever questions its safety, they are the most concrete evidence you have.

Once the house is finished, compulsory earthquake insurance (DASK) also has to be taken out.

Common questions

Does an earthquake-resistant house cost a lot more?

A code-compliant house uses somewhat more steel and concrete than one that cuts corners, but the difference in total cost is usually modest. What really drives cost up is an irregular design and poor ground. A simple plan keeps the premium small.

Is concrete or steel safer?

Any of these systems, designed and built properly, meets the code's life-safety target. The difference shows in workmanship: concrete quality and tie detailing in RC, connections and corrosion protection in steel.

How can I find out whether my existing house is safe?

Not by looking at it. It takes core samples to measure concrete strength, a rebar survey and a structural performance assessment carried out by a qualified civil engineer.

If you are about to build, spend your first money on the soil investigation and go through the report with the engineer who will design the structure. The foundation type, the number of storeys and often the choice of structural system follow from it. If you would like to compare a reinforced concrete house with a light steel option for your plot, we can come and look at the site.