Viaduct Damage Assessment After the 2023 Earthquake in Turkey Part 2

To view the figures and tables associated with this article, please refer to the flipbook above.

Five of the 14 viaducts along the Tarsus-Adana-Gaziantep (TAG) Highway of Southern Turkey were damaged from the 2023 Mw 7.8 earthquake. A companion article in the September issue of STRUCTURE presents the two largest of these viaducts, while this article focuses on the three remaining viaducts, which have similar designs and seismic retrofit strategies as each other. Although these three bridges are smaller than the other two major viaducts, they are still large and impressive structures set in the low mountains of Southern Turkey, and have been in service for just over 25 years. Furthermore, they all had advanced and interesting original seismic designs. However, since the peak ground acceleration (PGA) of 0.607 g, measured at Station 2712, which is the closest strong motion station to all five of the damaged bridges, is about 50% larger than the 0.4 g PGA used in the original designs, these bridges were clearly overloaded, resulting in the observed damage. As shown in Figure 1, the five damaged viaducts are all close to the USGS-defined fault rupture line (in red), while the other nine viaducts on the TAG Highway (O-52 in Fig. 1) are all much further away from this fault rupture.

Seismic retrofit designs for the three damaged bridge structures, the Nurdagi, Sehitler, and Baspinar Viaducts, were done by Cenan Ozkaya, as the engineer of record, working within the Pontem Engineering Co.. Importantly, none of these three damaged bridges collapsed, and all were saved for future use by the on-going seismic retrofitting to larger PGA values than the bridges were originally designed for. The high quality of both (1) the original design details and (2) the construction, are important features that helped save the viaducts during the February 6, Mw 7.8 earthquake and for future use. Retrofitting works are being carried out by the SNH Construction Company, and the owner of the viaducts is the Motorway Division of the General Directorate of State Highways, in the Ministry of Transportation and Infrastructure of the Republic of Turkey.

The Nurdagi Viaduct required emergency retrofitting, prior to the full seismic retrofit, because a plastic hinge that developed part-way up one of its columns was close to complete failure, as discussed here and in an article in the October 2023 bridge issue of STRUCTURE Magazine. Vertical rebar buckled, and transverse rebar yielded and was badly deformed; the emergency retrofit added a steel shell up this one column height, providing horizontal confinement steel and added vertical steel, making up for both the too-small transverse rebar and the potential lost strength of the buckled vertical steel. This viaduct remained open to traffic and, without this emergency work, future aftershocks could have caused a few more cycles of the plastic hinge, resulting in low-cycle fatigue failure of the buckled bars and fracture of the hoop reinforcement, with complete loss of the viaduct.

On February 6, 2023, a Mw 7.8 earthquake struck Southern Turkey, followed by a Mw 7.5 earthquake about six hours later. Both the size of the first earthquake and the right strike-slip fault mechanism are consistent with the anticipated future “big one” in California that will be generated from sudden slip along the San Andreas fault. Hence, because of this, and because bridge design in Turkey closely follows Caltrans methods, there is great interest in California to see how these large bridge structures performed.

The Nurdagi Viaduct is 102 meters (335 feet) from the USGS-defined fault rupture line, has two parallel five-span and six-span bridges on a curve, with single-column-bents, and large, solid, circular reinforced concrete columns of 3 meters (9.84 feet) diameter and maximum height of 24.2 meters (79.4 feet) (Fig. 2). The bridge has a total length of 307 meters (1,007 feet) and is on a steep longitudinal slope of 4%. Each of the parallel bridges is 17.5 meters (57.4 feet) wide, with a space between them.

The reinforced concrete columns act as cantilevers in the transverse and longitudinal directions, with the superstructure spans being either precast concrete girders or steel box-girders, both with reinforced concrete topping slab. Steel was used for the longer spans. Column plastic hinging is the primary damage to this viaduct from the Mw 7.8 earthquake, and of particular concern is that these plastic hinges developed part-way up the column in several instances (Figs. 2-5). Seismic pounding damage (Fig. 6) and local girder buckling (Fig. 7) were observed in the steel superstructure at the expansion joint, which is at the abutment. Reinforced concrete spans are simply supported, with expansion joints at both ends of the viaduct. The maximum span length for the steel superstructure is 80 meters (262 feet).

In one case, the plastic hinge formed about half-way up the column height and caused spalling of cover concrete on both sides of the section, indicating large curvatures in both transverse directions at this same column location, buckling of the primary vertical rebar, as well as yielding and severe deformation of the transverse hoop reinforcement (Figs. 2-3). This Indicates a couple more cycles could have caused failure of the plastic hinge and complete collapse of this viaduct, stemming from this location.

Column plastic hinges are designed to occur at the column/footing interface, where earthquake moment demand and capacity values intersect. However, primary vertical rebar cutoffs up the columns were designed for this structure, resulting in a sudden loss to the moment capacity, causing the intersection of moment demand and capacity curves to move up, and away, from the bottom of the column, as shown in Figure 3. Modular expansion joints completely failed at both ends of the bridge, and concrete was severely damaged at these locations due to severe impact forces (Fig. 6). A steel superstructure beam struck the edge beam wall, causing buckling of the steel beam (Fig. 7). At the damaged beam, the longitudinal sliding pot bearings were also damaged and lost functionality.

The seismic retrofit design consisted of adding full-height steel shells to most of the columns (Figs. 8 and 9), which provided added vertical and transverse steel, as well as making up for the buckled column rebar at Bent 5 (Fig. 3). In fact, the steel shell was added to this Bent 5 column as an emergency contract since traffic was still flowing over the bridge, and any aftershocks could have failed the plastic hinge, resulting in the complete loss of the viaduct. Footings were increased in size in all three dimensions, with added rebar, as shown in Figure 8. Nonlinear Time-History Analysis (NTHA) was used to assess the behavior of this bridge, to ensure that the retrofitted structure satisfies the Turkish National Earthquake Design Code.

This viaduct has side-by-side superstructures with eight spans each, single-column-bents, and solid reinforced concrete columns of 4 meters (13.1 feet) diameter (Fig. 10). It has a total bridge length of 310 meters (1,017 feet), and the tallest column is 54.8 meters (180 feet) high. The superstructure consists of precast, prestressed concrete girders, with reinforced concrete topping slab. Spans are simply supported, with expansion joints at both ends of the viaduct. As with the Nurdagi Viaduct, the primary damage concern for this bridge is the onset of plastic hinging in the columns away from the column/footing interface. This is caused by primary vertical rebar cutoffs up the columns, reducing the moment capacity at that section. Figures 10 and 11 show the cover concrete spalling off at various column locations, as well as significant horizontal cracks opening up, where vertical rebars were terminated. In addition, settlement occurred at the approaches to the bridge and damage was made to the expansion joints, earthquake blocks, connection slabs, elastomeric bearings, barriers and facade elements. This viaduct is 212 meters (696 feet) from the USGS-defined fault rupture line.

Columns were retrofitted with partial-height steel shells to make sure, in a future earthquake, that plastic hinges don’t form up the column height, above the column/footing interface (Fig. 12). Multiple tie-downs were also added to the footings for increased overturning moment capacity, as shown in Figure 12. The tie-downs go through the footing and deep into the soil. For column sections of secondary importance, far above the column/footing interface, bidirectional single or double-layer composite fiber reinforced polymer (FRP) wraps were added to increase column shear capacity and confining pressure (Fig. 12).

Baspinar Viaduct is very similar to Sehitler Viaduct in layout, column lengths, and damage. It has 8-span parallel bridges with single-column-bents, solid reinforced concrete columns of 4 meters (13.1 feet) diameter, and a precast, prestressed girder superstructure, with reinforced concrete topping slab (Fig. 13). The total bridge length is 311 meters (1,020 feet), with the tallest column of 58.3 meters (191 feet) height. Spans are simply supported, with expansion joints at both ends of the bridge. This viaduct is 148 meters (486 feet) from the USGS-defined fault rupture line.
Damage includes onset of column plastic hinges above the column/footing interface due to primary vertical rebar cutoff, settlement at the approaches, and failure of expansion joints and concrete from impact loads. The same seismic retrofit strategy used for Sehitler Viaduct was used for this viaduct, adding column partial-height steel shells and strengthening the footings with multiple added tie-downs that go through the footings and deep into the soil. FRP wrap was also added to the top parts of tall columns to increase their shear strength and to provide additional confining pressure.

The Nurdagi, Sehitler, and Baspinar Viaducts are significant bridges, and all were damaged during the 2023 Mw 7.8 earthquake that occurred in Southern Turkey. These three viaducts are similar to each other, as was their damage, with large, solid, circular reinforced concrete columns that have diameters of either 3 meters (9.84 feet) or 4 meters (13.1 feet). They all have single-column-bents, so that in the transverse direction the columns act as cantilevers. Because the superstructure is simply supported at the bents, the columns also act as cantilevers in the longitudinal direction. Normally, a plastic hinge is designed, and expected, to occur at the column/footing interface, where the earthquake moment demand and capacity envelopes intersect, but these three bridges showed plastic hinges developing above this expected location.

The Nurdagi Viaduct had one plastic hinge that formed about half-way up the column, resulting in spalling of cover concrete and buckling of the primary vertical rebar, as well as yielding and severe distortion of the transverse hoop reinforcement. This plastic hinge was close to failure, which could have brought down the entire viaduct. Onset of other column plastic hinges above the base were also seen, at this viaduct and the other two viaducts discussed in this article, but not at such high levels of plastic curvature and rotation. This unusual re-location of column plastic hinges was due to primary vertical rebar cutoffs up the column height, that all three of the damaged viaducts discussed herein had, which shifts the intersection point of column moment demand and capacity envelops to above the column/footing interface.

The seismic retrofit for these three viaducts consisted of placing steel shells around most of the columns, either full-height or partial-height, and increasing the footing dimensions, with added rebar and steel micropiles. At less important parts of the longer columns, far from potential plastic hinge positions, FRP strengthening was applied to the columns, with fibers provided in both directions.

While these three significant bridges of the 14 viaducts along the important TAG Highway were damaged, none of them collapsed and all were saved for future use by the on-going seismic retrofitting, with no closure to vehicles. It is expected that under a future large earthquake of similar size to the 2023 earthquake, these retrofitted bridge structures would not be critically damaged. However, it is unlikely that an earthquake of this magnitude would hit these structures again within their remaining service life. ■

About the Authors

Cenan Ozkaya got his B.S., M.S. and Ph.D degrees from the Civil Engineering Department of Middle East Technical University-Turkey. He is working as Technical Manager in the PONTEM Engineering Company.

Robert K. Dowell received his B.S. degree in Civil Engineering from San Diego State Univeristy (SDSU), and his M.S. and Ph.D degrees in Structural Engineering from the University of California at San Diego (UCSD). He is a licensed Civil Engineer (PE) and a Professor of Structural Engineering at SDSU.

Faruk Yildiz got his B.S. degree from the Civil Engineering Department of Yildiz Technical University-Turkey, and is working at the Motorway Division of the General Directorate of State Highways-Turkey.

References

[1] Dowell, R. K. (2023).  Reconnaissance Report of Observed Structural Bridge Damage: M_w 7.8 Turkiye (Turkey) Earthquake of 2023.  SDSU Structural Engineering Research Project, Report No. SERP – 23/03, March 2023.

[2] Dowell, R. K. (2023).  Observations of the Bridge Damage Caused by the M_w 7.8 Türkiye (Turkey) Earthquake of February 6, 2023, STRUCTURE Magazine, October (Yearly Bridge Issue).

[3] 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

[4] Kale, O., Sandikkaya, M.,A. (2023) Seismic Hazard Report to be Used in Seismic Retrofit Works of TAG Motorway Viaducts, TED University.

[5] Dowell, R. K. (2004).  Time-History Analyses versus Measured Seismic Response of the 5/14 Connector Bridge, Report No. DH-04-02, submitted to the California Strong Motion Instrumentation Program (CSMIP), Dowell-Holombo Engineering, February.