A Skeleton That Sways
The traditional houses in Safranbolu, a UNESCO World Heritage site, appear rigid, but they are engineered to move. Their survival in one of the world's active seismic zones is not accidental. The design relies on a construction method called hımış, a composite system with a flexible timber skeleton and non-structural infill. This technique has been used in Anatolia for centuries. The upper stories of the houses are built with a timber frame made from local pine, fir, or chestnut wood. The builders joined these timbers using mortise-and-tenon connections, which are assembled without nails or glue.
This timber frame acts as the primary load-bearing structure. The spaces between the timbers are filled with materials like sun-dried adobe bricks, rubble stone, or fired bricks, held together with a lime or mud mortar. This infill adds mass and insulation but does not contribute to the structural rigidity. During an earthquake, the timber frame is designed to deform and sway, while the infill may crack or crumble. The design philosophy is to not rigidly resist the seismic forces, and dissipate the energy through controlled, plastic deformation. The houses essentially go with the flow of the ground motion, preventing the catastrophic, brittle failure seen in rigid masonry structures. The upper timber-framed floors often overhang the heavier stone masonry of the ground floor, a feature which also adds to the structural dynamics of the building during a quake.
Tested by Time and Science
The historical evidence for the success of this design is compelling; many of these 18th and 19th-century houses have withstood numerous seismic events. After the major 1999 Kocaeli and Düzce earthquakes in Turkey, observers noted that traditional hımış buildings suffered significantly less damage than modern reinforced concrete structures in the same areas. This empirical success prompted modern scientific investigation.
Engineers at the Middle East Technical University conducted extensive laboratory tests on full-scale replicas of Safranbolu house frames. Using reverse cyclic loading to simulate earthquake forces, they quantified the performance of these traditional structures. The tests confirmed that the mortise-and-tenon joints are essential to the system's resilience. These joints allow for large, non-elastic rotations, enabling the frame to displace laterally by as much as 10 centimeters without failing. This movement absorbs and dissipates a huge amount of seismic energy. The research revealed that the connections are a type of base isolation, a sophisticated engineering concept that decouples the building from the ground's movement. Different infill materials were also tested, with lighter cladding systems like bağdadi (wooden laths nailed to the frame and plastered over) showing excellent performance.