Earthquake-resistant construction in Georgia: houses in Batumi and Adjara

Earthquake-resistant construction in Georgia is settled before the first concrete is poured. Seismic resistance is built into the foundations, the frame and the joints, and adding it later without taking the house apart is almost impossible. Below is what we build into the structural design, and what it makes sense to ask any contractor, even if you end up building with someone else.

Earthquake-resistant construction in Georgia: the code and calculations for your ground

We design the house to the Georgian seismic building code (PN 01.01-09). Under the code, the structural engineer determines what seismic load the house on this particular site has to be designed for, and checks every load-bearing element.

The ground changes the calculation more than you might think: the same tremor acts differently on a house standing on rock and on one sitting on a deep layer of wet sand by the sea. So we don't start the calculation without a ground survey. Taking an “average” soil from the plot next door means either overspending on concrete and reinforcement or underestimating the load. On a slope, the survey team also looks for signs of landslides. Retaining walls there are designed for seismic loads too; there is more on this on the page about houses on steep plots.

What helps a house withstand a tremor

In an earthquake the ground under the house sways, and the force acting on each floor grows with its mass. So the first decision is made at the concept stage: how heavy the house will be and where that mass will sit. A roof terrace with soil and a lawn, a pool on an upper floor, or concrete walls upstairs above a light ground floor all add load exactly where it is most dangerous.

Then we look at the shape. The easiest house to design is one whose stiffness is spread evenly across the plan and whose floors are similar to one another. These are the features the structural engineer deals with separately:

• an open ground floor for parking or shop windows beneath floors with solid walls: the columns below are strengthened, or shear walls are added;
• long cantilevered terraces and bay windows: these are also checked for vertical vibration;
• an L-shaped or U-shaped plan, or parts of different heights: the house is divided by seismic joints into simple blocks, each of which works on its own;
• a staircase or lift shaft in a corner of the house: it shifts the centre of stiffness, and in a tremor the house twists.

Houses like these are built in Batumi, but each of these features shows up in the estimate. That is why the structural engineer reviews the concept together with the architect, while a wall or column can still be moved at almost no cost.

Frame, joints and foundations

We most often build a private house in Batumi on a cast-in-place concrete frame: the columns, beams and slabs are poured on site, and in a tremor they work as a single frame. The frame transfers the load through its joints, so the reinforcement is denser where columns meet beams than in the middle of a span, and bar splices are staggered so that they don't all fall in one section. Member sizes and bar spacing are set by the calculation, and we don't let anyone on site change them “from experience”.

The foundations tie the supports together so that they don't move apart in a tremor. For a concrete or masonry house this is usually a cast-in-place slab or a grid of strip footings, and on weak, wet ground piles tied together with ground beams; we weigh up the choice in our comparison of piles and a slab. A steel frame is lighter and takes less load in a tremor, but it too has joints that need calculating; see our comparison of concrete vs a steel frame.

Masonry and seismic ring beams: how a wall stays in the frame

The walls between the columns don't carry the house, but in an earthquake they work together with the frame, whether we want them to or not. Poorly anchored infill masonry cracks diagonally or falls out of the frame. And masonry built to half the storey height tight against a column, under a ribbon window for example, turns it into a short column, and a short column is the first to fail. So we tie the masonry to the frame:

• every few courses we lay reinforcing mesh or bars in the joints, at the spacing the design specifies;
• we fix the masonry to the columns with starter bars or anchors, and by the sea we use stainless steel anchors;
• we put reinforced concrete lintels over openings and break up long walls with concrete posts;
• we form the joint between the top of the masonry and the slab as the design specifies, so that the slab doesn't bear down on the wall as it deflects.

If the walls are load-bearing, as in a single-storey house, sauna or garage built of blocks, there is no frame, and the masonry itself takes the seismic load. In that case a closed reinforced concrete seismic ring beam runs at floor level and under the roof, and reinforced concrete cores go in at the corners, where walls meet and next to wide openings. We make the ring beam continuous, with the reinforcement lapped at the corners, otherwise it doesn't tie the walls together.

What we check on site

Seismic resistance drawn in the design gets lost on site in small details. Stirrups spaced further apart because “it's quicker”, a bar cut to let a pipe through, spacers forgotten so that the reinforcement lies straight on the formwork. That is why our engineer checks the reinforcement of every joint against the drawing before the pour, and the site foreman photographs it with a tape measure in the shot. The photos go into the handover file, and years later they show what is inside the concrete. If another company is building the shell, you can order this check separately as technical supervision at each stage of the build.

We route the services so that they don't cut through columns or ring beams. Openings for drains and ventilation in beams and slabs are formed with sleeves before the pour, in places agreed with the structural engineer. We don't allow a column to be chased afterwards, “just a little, for a cable”, by our own crews or by subcontracted ones.

A ready-made design, an old house and adding a storey

A design from a catalogue, or one made for another country, was calculated for someone else's ground and someone else's code, so we check its structural section and almost always recalculate it. Our review of a finished design costs from 1,680 ₾, and from its results you know what will have to be redone before you pay for the shell.

With an old house we start with a survey: the engineer opens up the structure in several places, checks the concrete and reinforcement, examines the cracks and states in writing whether the house will carry its existing loads, and new ones if you want to add a storey. A survey with a seismic resistance report costs from 2,820 ₾. Masonry without ring beams is strengthened with mesh, anchors and shotcrete to a calculation, from 235 ₾ per m² of wall; how this fits into the overall plan of work is described on the page about a full renovation of an old house.

How much seismic resistance costs

The structural section of the design with seismic calculations costs from 27 ₾ per m² of the house's floor area. It covers the calculation of the foundations, frame and joints for seismic loads under the Georgian code, plus the reinforcement and ring beam drawings. If we do the whole house design, this section is already included in its price. The strengthening itself is priced in the shell estimate: reinforcement, ring beams, joints and heavier foundations. Based on our estimates, it adds roughly 21–30% to the cost of the shell compared with the same house calculated without seismic loads. That is a guide, not a rule: the exact difference comes from the calculation for your house.

A ground survey on a plot for a private house costs from 3,000 ₾, and it doesn't pay to save on it: without data on the soil the structural engineer builds in a safety margin, and that margin is paid for in concrete and reinforcement. A slope, wet ground by the sea, cantilevers and an open ground floor cost more; a compact house of simple shape costs less. We send a preliminary estimate from photos of the plot and a plan of the house within 24 hours on business days, and the final cost is set on the basis of the design, after a visit to the plot and once the estimate is agreed.