Earthquake of February 6, 2023.
AMw 7.8 earthquake, followed nine hours later by a Mw 7.5 earthquake, struck southern Turkey (Türkiye) on February 6, 2023, leaving tens of thousands of residential buildings, as well as mosques, churches, industrial buildings and silos, collapsed and damaged beyond repair. Bridge structures were severely damaged but did not collapse, with the exception of a simply supported, single-span bridge that fell off its supports at the abutments, with no reported injuries. This provides a stark contrast to the buildings, where the overwhelming majority, if not all, of the more than 50,000 deaths occurred. And this is the official death toll based on the number of bodies that were pulled from the rubble of the collapsed buildings; considering that over 300,000 individual apartments were destroyed, with perhaps several people living in each, the number of people killed in these two back-to-back earthquakes could be many times higher than the official count.
Bridge structures are designed to allow significant damage from a major seismic event, but without collapsing, as should also be the case for buildings. On the older Richter Scale, this 7.8 moment magnitude of the first earthquake translates to about a M 8.1 event, which is of similar size and fault type (right, strike-slip) to the famous 1906 San Francisco earthquake (M 8.2 on the Richter Scale and back-calculated to Mw 7.9 per the United States Geological Survey (USGS). Even the time of day (local time) was similar between the California and Türkiye earthquakes; the 1906 San Francisco earthquake occurred at 5:12 am while the first 2023 Türkiye earthquake happened at 4:17 am, less than an hour apart, and in both cases when most people were still at home, sleeping. The fault rupture length of 180 miles from the Mw 7.8 earthquake in southern Türkiye is equivalent to the distance in California between San Diego and Santa Barbara, with the City of Los Angeles right in the middle.
The structural engineering reconnaissance team (Mountain Goats) included Gulen Ozkula (team leader), Robert K. Dowell (author of this article), Tunc Deniz Uludag, Ayse Hortacsu and Jui-Liang Lin. While the Mountain Goats inspected many different types of structures, the focus of this article is on observed bridge damage. Two significant bridges had severe and unusual damage, the Nurdagi Viaduct and Asi Bridge.
Earthquake Details
Epicenters and fault ruptures of back-to-back Mw 7.8 and Mw 7.5 earthquakes
The epicenter of the Mw 7.8 earthquake was between Nurdagi and Gaziantep at GPS coordinates N 37.225° E 37.021°, while the epicenter of the Mw 7.5 earthquake that happened nine hours later was at GPS coordinates N 38.024° E 37.203°, north of Kahramanmaras and Golbasi. Fault rupture lengths for the Mw 7.8 and Mw 7.5 earthquakes were 180 miles and 99 miles, respectively. The second earthquake is not an aftershock to the first event because it occurred on a different fault, but it was probably triggered by the sudden shaking of the larger Mw 7.8 earthquake, which affected strains and stresses throughout the complex fault system in the region. The maximum horizontal slip from the first event was 30 feet, compared to 20 feet from the 1906 San Francisco earthquake. However, energy release from these two events was similar because the fault rupture length in the famous California earthquake was longer than in Türkiye. For such a large earthquake, the normal (shortest) distance from a structure to the fault rupture line is much more important than the distance between a structure and the epicenter, as the ground shaking intensity contours given above clearly demonstrate.
Measured ground motions
For the Nurdagi Viaduct, the measured ground motions (from Station 2712, the closest strong motion station at 2.00 miles from the bridge) in the EW and NS horizontal directions have Peak Ground Accelerations (PGAs) of 0.607 g and 0.565 g, respectively. Likewise, for the Asi Bridge, the measured ground motions (from Station 3124, the closest strong motion station at 2.14 miles away from the bridge) in the EW and NS horizontal directions have PGAs of 0.659 g and 0.581 g, respectively. In the vertical direction, PGAs were 0.354 g for the Nurdagi Viaduct and 0.589 g at the Asi Bridge. Peak measured horizontal accelerations at the two bridges of 0.607 g and 0.659 g are close to the maximum horizontal acceleration of 0.7 g in California bridge design for a Mw 8 event.
Earthquake Spectra
For the Nurdagi Viaduct, the acceleration response spectrum (ARS) plot in the EW direction tends to be larger than the smoothed Caltrans design ARS curve for a Mw 8 earthquake that has a PGA of 0.7 g on rock or stiff soil, and follows a similar trend with increasing structural period, while the NS direction ARS curve is often below the Caltrans design ARS curve. Maximum spectral acceleration values at this bridge are 1.83 g and 1.90 g in the EW and NS horizontal directions, respectively, which are both larger than the peak Caltrans ARS design value of 1.82 g. The EW spectral displacement curve follows a similar trend to the design curve up to a period of about two seconds, then drops below it. In the NS direction, the spectral displacement curve is typically lower than the design curve, matching it only at a period of one second and, again, at four seconds with over two feet of relative displacement. In the vertical direction, the maximum spectral acceleration value was 1.37 g.
The ARS plots for the Asi Bridge also show a higher peak value in the EW direction than from the smoothed Caltrans ARS design curve, but at a significantly longer natural period. Maximum ARS values for the Asi Bridge are 2.15 g and 1.44 g in the EW and NS horizontal directions, respectively, compared to the peak value of 1.82 g from the design curve. Thus, while the maximum acceleration in the EW direction is larger than the peak design value, the largest result in the NS direction is lower than this. A shift to the right for the maximum values of the ARS curves, compared to the design ARS curve, which is based on rock or stiff soil, is a clear indication that soft soil is present at the site of the Asi Bridge, which was later confirmed. Both EW and NS horizontal directions show this shift of the ARS curve to longer periods compared to the smooth design curve. The peak spectral displacement in the NS direction is close to 5 ft at this soft soil site. The Caltrans ARS curve was obtained from an earlier version of the Seismic Design Criteria (SDC), since the current version of the SDC does not have such a graph readily available. The maximum spectral acceleration in the vertical direction was 1.64 g.
Two Significant Bridges with Severe and Unusual Damage
Nurdagi Viaduct
The Nurdagi Viaduct is a large structure in the low mountains of southern Türkiye with cantilever reinforced concrete (RC) columns that have a diameter of 10 feet and a height of about 80 feet. GPS coordinates for this structure are N 37.170960 E 36.699940 and an elevation of 2563 feet. The bridge includes two side-by-side structures with five spans each that are curved and consist of a superstructure combination of precast, prestressed concrete girders and steel girders, with steel used for the longest span. While this viaduct is 18.1 miles from the epicenter of the Mw 7.8 earthquake, it was less than one-tenth of a mile from the fault rupture line; the strongest intensity shaking for an earthquake of this magnitude runs along the region of the fault rupture line, not in concentric circles about the epicenter, as clearly shown in the figure above.
An unexpected plastic hinge formed part-way up one of the large RC cantilever columns, which is unusual since the maximum bending moment is expected at the column base where it interfaces with the fixed footing. The observed damage shows that the plastic hinge formed in the transverse bent direction. Concrete spalled off on both sides of the column, and vertical rebar buckled on one side of the column, with yielded and significantly deformed transverse rebar, as clearly shown in the figures above. For both cases, large moments are required in both transverse bent loading directions. Recent meetings in Ankara, Türkiye, between the author and the bridge engineer who is doing the seismic retrofit design of this viaduct confirmed that there were various bar cutoffs up the column height, which could explain why the plastic hinge formed part-way up the column and not at its base. There was also damage to the abutments, which is expected from such a significant earthquake.
Asi Bridge over the Asi River
The Asi Bridge consists of two side-by-side, six-span bridges of precast, prestressed girders and a RC topping slab with Asphalt Concrete (AC) overlay for the driving surface of the superstructure. GPS coordinates for this bridge are N 36.255050 E 36.204300 and elevation of 315 feet. The columns and footings are cast-in-place RC. At the time of the inspections the bridge was still being used by vehicular traffic. Recent meetings between the author and the bridge designer confirmed that it is no longer used by vehicular traffic, and it will be replaced with a new bridge. The Asi Bridge was 80.8 miles from the epicenter of the Mw 7.8 earthquake, but only 2.5 miles from the fault rupture line, with the damage and measured intensity of shaking clearly showing the significance of the distance to the fault rupture line rather than to the epicenter of a large earthquake, as discussed previously.
Unusual and severe damage occurred at the ends of the simply-supported precast girders, with cracking and spalling of web concrete over long distances from the support at the abutments. This occurred on both interior and exterior girders. With vertical spectral accelerations of above 1.6 g, as given above, the girder ends lifted off their supports, one side and then the other, often losing the rubber bearing pads that had supported them. Thus, the girder ends were impacting in the vertical direction, concrete-to-concrete, as well as in the longitudinal and transverse directions, resulting in severe multi-directional cracking and spalling. This forced the prestressing strands to slip forward to maintain a transfer length, which further exasperated the stresses and spalling. Also, vertical web rebar buckling occurred, pushing the concrete out in both directions, increasing the length of spalling and damage. In some cases, there was no web concrete remaining, with only the rebar cage toward the girder ends supporting the shear loads at the abutments. Prestressing was lost, including whole strands coming out of the girder. In addition, lateral loading caused exterior and interior shear keys to fail, while longitudinal loading resulted in column plastic hinges at their base, as expected. Significant settlement occurred at both approaches to the bridge.
Summary
The Mw 7.8 earthquake and the Mw 7.5 earthquake that occurred nine hours later, both on February 6, 2023, caused extensive damage to southern Türkiye, with tens of thousands of buildings collapsing, resulting in more than 50,000 people killed and leaving millions without homes. Industrial facilities, silos, churches, and mosques also collapsed. Of particular interest is that while bridges were heavily damaged, only one collapsed; a simply supported, single-span bridge that slipped off its supports at the abutments, with no injuries reported. There is an important distinction here; buildings are privately funded and operated, with little or no oversight to their design and construction, whereas bridges are state-owned, designed and built to a higher engineering standard. This good record of bridges in Türkiye demonstrates that it is possible to design and build high-quality structures to withstand a major earthquake without collapse. Buildings had low concrete strength and poor rebar detailing and were often not built to current seismic specifications, resulting in the extreme number of buildings that collapsed. Measured ground shaking intensity maps and observed bridge damage levels showed that the normal (closest) distance from a structure to the fault rupture line was much more important than the distance to the epicenter of the earthquake for a large seismic event.■
References
Ozkula, G., Baser, T., Dowell, R. K., Hortacsu, A., Huang, C. W., Ilhan, O., Lin, J. L., Numanoglu, O. A., Olgun, G. C., & Uludag, T. D. (2023). Earthquake Reconnaissance Team Report: Türkiye Earthquake Sequence on February 6, 2023. Learning From Earthquakes. https://learningfromearthquakes.org/2023-02-06-nurdagi-turkey/index.php?option=com_content&view=article&id=80
Dowell, R. K. (2023). Reconnaissance Report of Observed Structural Bridge Damage: Mw 7.8 Turkiye (Turkey) Earthquake of 2023. SDSU Structural Engineering Research Project, Report No. SERP – 23/03, March 2023.
Ozkula, G., Dowell, R.K., Baser, T., Lin, J.L, Numanoglu, O.A., Ilhan, O., Olgun, C.G., Huang, C.W., Uludag, T.D. (2023) Field reconnaissance and observations from the February 6, 2023, Turkey earthquake sequence, Natural Hazards (2023) 119:663-700. https://doi.org/10.1007/s11069-023-06143-2