EXPLOITING SSI TO MITIGATE SEISMIC DEMANDS IN BRIDGE PIERS

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1 10NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska EXPLOITING SSI TO MITIGATE SEISMIC DEMANDS IN BRIDGE PIERS S. Farantakis 1, A. Kotsoglou 2 and S. Pantazopoulou 3 ABSTRACT Main objective of the present work is the investigation of the seismic response of Reinforced Concrete (R.C.) bridges with particular emphasis on the interaction effects that occur between foundation soil and the support basis of critical components such as the bent piers. Recognizing that soil-structure interaction (SSI) may affect in many different ways the seismic response of the overall bridge system, a comprehensive parametric study is conducted in order to calibrate analytical estimations of this contribution and to illustrate the possible benefits resulting from exploitation of the SSI effects in improving the overall seismic response. One example is passive mobilization of the foundation soil in shallow or flexible foundations with slender piles during strong ground motions aiming to alleviate the deformation demands in critical subsystems such as the central bent piers in short monolithic bridges. To this end, detailed nonlinear time history analyses were conducted on the finite element models of two typical overpasses from the state of California whose responses obtained from field instrumentation records conclusively demonstrate a strong SSI influence. Variations of the basic bridge structures are considered in order to illustrate how different designs of the overall system may enhance or attenuate the influence of SSI on the bridge dynamic response. From the obtained results it is shown that passive soil mobilization reduces the relative drift demands (damage) imposed on the critical piers by achieving lower intensity, better distributed deformation demand patterns. The results support a recent trend in conceptual design of bridges which postulates that controlled mobilization of the soil should be encouraged and ought to be engineered through proper detailing of the pier and foundation morphology and properties. REFERENCES [1]. A. Kotsoglou, S., Pantazopoulou, (2011). Soil-foundation interaction effects in bridges with monolithic column-superstructure connection, 8th Intnl. Conf. on Struct. Dynamics, Leuven, Belgium, 4-6 July [2]. G. Mylonakis and G. Gazetas, (2000). Seismic Soil-Structure Interaction: Beneficial or Detrimental?, Journal of Earthquake Engineering, Imperial College Press, 4(3): Civil Engineer, Democritus University of Thrace, Dept. of Civil Engineering, V. Sofias 12, 67100, Xanthi, Greece, stylianos_farantakis@yahoo.gr 2 Civil Engineer, MSc, Ph.D, Researcher, Democritus University of Thrace, Dept. of Civil Engineering V.Sofias 12, 67100, Xanthi, Greece,, akotsogl@civil.duth.gr 3 Professor, Department of Civil & Environmental Engineering, University of Cyprus, pantaz@ucy.ac.cy

2 Exploiting SSI to Mitigate Seismic Demands in Bridge Piers S. Farantakis 1, A. Kotsoglou 2 and S. Pantazopoulou 3 ABSTRACT Objective of the paper is the investigation of the seismic response of Reinforced Concrete (R.C.) bridges with particular emphasis on the interaction effects that occur between foundation soil and the support basis of critical components such as the bent piers. Recognizing that soil-structure interaction (SSI) may affect in many different ways the seismic response of the bridge system, a comprehensive parametric study is conducted in order to calibrate analytical estimations of this contribution and to illustrate the possible benefits resulting from exploitation of the SSI effects in improving the overall seismic response. One example is passive mobilization of the foundation soil in shallow or flexible foundations with slender piles during strong ground motions, which effectively contributes in moderating the deformation demands of critical subsystems such as the central bent piers in short monolithic bridges. To this end, detailed nonlinear time history analyses were conducted on the finite element models of two typical overpasses from the state of California whose responses obtained from field instrumentation records conclusively demonstrate a strong SSI influence. Variations of the basic bridge structures are considered in order to illustrate how different designs of the overall system may enhance or attenuate the influence of SSI on the bridge dynamic response. From the obtained results it is shown that passive soil mobilization reduces the relative drift demands (damage) imposed on the critical piers by achieving lower intensity better distributed deformation demand patterns. The results support a recent trend in conceptual design of bridges which postulates that controlled mobilization of the soil should be encouraged and ought to be engineered through proper detailing of the pier and foundation morphology and properties. Introduction A cornerstone of capacity design principles when applied to detailing the pier-foundation connection was to favor a strong foundation weak pier concept, in order to secure that no damage would take place in the less accessible for repair, foundation soil. This idea, through pertinent overstrength factors, led to a general design practice in Europe that favors strong and stiff foundation designs extending to great depths through deep piles so that a perfect fixity condition is not far from reality in most cases. On the other side of the Atlantic, however, foundation designs have always been more lenient, so that the occurrence of SSI effects cannot 1 Civil Engineer, Democritus University of Thrace, Dept. of Civil Engineering, V. Sofias 12, 67100, Xanthi, Greece, stylianos_farantakis@yahoo.gr 2 Civil Engineer, MSc, Ph.D, Researcher, Democritus University of Thrace, Dept. of Civil Engineering V.Sofias 12, 67100, Xanthi, Greece, akotsogl@civil.duth.gr 3 Professor, Department of Civil & Environmental Engineering, University of Cyprus, pantaz@ucy.ac.cy

3 be suppressed in many examples, particularly in cases where the superstructure stiffness is of comparable magnitude as that of the foundation system. Such is, for example, the case of numerous highway overpasses which are short and monolithic, and therefore excessively stiff. Over the years comparison of the field performance of the two different design concepts has been shifting favorably towards the latter approach, where the soil foundation has some degree of participation in the overall response scheme. What was considered a heresy only a few years ago, namely the idea that there may be less obvious benefits in the undesirable event of soil participation became openly acknowledged and several funded projects openly negotiated the idea that under controlled conditions, SSI effects may prove a very effective means of improving structural performance [1 4]. Thus, today SSI is seen as a potential opportunity, particularly in light of performance based bridge design. Important questions still persist, such as for example: (a) whether SSI can be engineered so that it is mobilized only in controlled conditions (b) How much permanent damage to the foundation is acceptable, at what levels of seismic intensity (i.e., defining acceptable limit states for the foundation soil), (c) How can the design of the bridge superstructure better engage the foundation compliance at the same time limiting the extent of damage to both pier and foundation. These issues are at the core of the present research study, and are discussed in detail in the forthcoming sections through calibration of pertinent analytical models with field records from instrumented bridges and through systematic sensitivity analysis using the nonlinear time history response estimates of various design concepts applied on superstructure models. Mechanics of SSI Participation in Superstructure Response The compliance of the foundation - bent substructure to horizontal force comprises the additive contributions of the two components this is why foundation stiffness in most computer models may be adequately represented by support springs at the base of the pier. Therefore, during lateral sway, which occurs in response to ground excitation, the total drift of the pier-foundation system from the vertical line comprises rotations from flexural deformation and rigid body pier rotation due to bar pullout at the base, as well as the rotation at the top of the pile-cap or footing owing to deformations in the soil, caused primarily by the large overturning moments that are transferred from the fixed end support of the pier. D C Figure 1. Pier-Foundation Substructure (Kotsoglou and Pantazopoulou 2012). Contributions to work-equivalent stiffness for cases with (a) stiff column support, (b) compliant foundation B A

4 The effective, or work equivalent (W.E.) lateral stiffness contribution of the overall bent substructure is calculated considering the structure in lateral displacement when it attains a unit displacement at the reference degree of freedom (this is the so-called control node) using virtual work principles: In this regard, the pattern of lateral displacements of the bent, extending from the tip of the piles (point A in Fig. 1b) to the deck (point D) is required; this is the 3-D shape of fundamental mode of vibration, Φ, normalized with respect to the displacement of the point of reference considered as control node CN (Φ CN =1). The displacement at the connection of the pier with the deck is Φ C. Thus the total relative displacement from the tip of the piles to the deck is Φ pier-system =Φ C -Φ A, whereas the relative displacement owing to deformation of the pier is: Φ Φ pier = ΦC Φ B H pier (1) z z= H B where H B is the height of the foundation system with reference to the pile tip. Thus, the equivalent stiffness of the entire pier-foundation system, is, K eq pier syst = K pier Φ 2 + pier K transl. found Φ 2 + found work doneby pier deformation work done by lateral deformation of the foundation soil K rotat found Φ z z= H B work done by rockingof the pile cap &flexuraldef.of the piles (2) In Eq. (2) K pier is the translational stiffness of the pier considered fixed at both ends if monolithically connected to the deck (e.g. pier H 3 transl. K = 12EI / pier ); K found is the translational stiffness of the foundation system below grade (against lateral translation of point B relative to rotat. A), and K found is the rotational stiffness of the pile cap and the system of piles against rotation at point B. Equation (2) illustrates the implications of a non-compliant foundation: Given the translation of the pier-foundation system in the translational shape of the structure, Φ C, it is transl. rotat. evident that a very stiff foundation where K found and K found, will cause the contribution of the last two terms of Eq. (2) to diminish (because the values of Φ found and Φ z z= H B will tend to zero), thereby causing Φ pier = Φ C, i.e. the maximum possible value of pier relative end displacement owing purely to pier deformation alone thereby causing maximum damage to the pier (this is owing to the general principle that the stiffer elements do not participate in the work equivalent stiffness as they do not perform work in the absence of deformation). In the other end of the range of possible deformation patterns, such as in the case of a rocking foundation, the deformation and thus damage, occurring in the pier is substantially reduced, as stated by Fig. (1), with a consequent attenuation of the pier stiffness to the work equivalent stiffness of the overall system and a commensurate increase of the participation of the compliant foundation stiffness. Equation (2) underscores the fact that it is possible to mitigate damage in the pier at the expense of some damage to the foundation soil. Hesitation to accept this as an emerging bridge engineering practice is justified to some extent; in support of the counterargument, the irreversibility of this damage to the soil is called for, as foundation below grade is considered inaccessible for post-earthquake repair. Indeed this type of performance could be considered

5 unacceptable for the usual serviceability earthquake. But at higher intensity events, a marginal rotation at the support alleviates excessive damage to the pier, the functionality of which is essential for the collapse prevention performance objective. For that stage, allowing for controlled amount of a marginal pile cap rotation reduces dramatically the demand in the piers. For this scenario, Kotsoglou and Pantazopoulou [1], proposed the following set of limit states for all the sub-systems that compose the pier system, as illustrated in Table 1, reflecting an increasing tolerance for foundation damage with increasing earthquake intensity. Note that with reference to the resistance curve of the pier-system, compliant foundation, pier-column and support hardware (if bearings have been used at the pier-top to support the deck) all function as springs in series. Thus, the three reference performance levels listed in Table 1, (i.e., A=serviceability for frequent earthquakes, B=repairable damage or even partial replacement for rare events, C=collapse prevention for very rare events) are specified in terms of the acceptable damage to the individual components. To secure minimal soil foundation contribution and damage in frequent seismic events, whereas allowing for some damage at higher intensity rare events, the proposed values of peak soil shear strain are selected so that (a) the limit corresponds to an elastic apparent soil behavior in the former case, and (b) damage to the foundation in the latter case corresponds to a comparable ductility as that used conventionally for design of the piers (for DCM, i.e. q 3.5). Performance Limit: A B C Soil Strain limit, γ Pier Column Relative Drift Ductility Ratio, µ θ System displacement ductility, µ Table 1: Proposed Deformation Limits at performance stages for a well detailed superstructure Investigation of SSI effects in Monolithic Overpasses Using FE Simulation Evidence of the favorable implications of soil compliance on the seismic response of bridges have been recognized several years ago, although there is debate that some catastrophic bridge collapses may be attributed to the modification of the dynamic characteristics of the Soil- Foundation-Superstructure system (e.g. in case of near fault excitations) [4 7]. A characteristic example of the favourable contribution of SSI is a typical highway overpass, known in the literature as Painter Street Overcrossing (PSO) which is an instrumented bridge at Rio Dell in California. It was observed in that bridge that mobilization of the embankments in transverse displacement during earthquakes caused an effective increase in ductility demand of the central bridge columns; yet no damage was observed, with the columns delivered from damage despite their brittle design. The observed improved response of the bridge was attributed by many to partial rotation at the pier foundations, owing to shear deformation of the soil, leading to a commensurate reduction in the relative drift ratios demanded of the pier columns. Based on the above findings a primary objective of the finite element study is to quantify and better understand the effectiveness of SSI in reducing the seismic demands of critical bridge subsystems such as the bent columns. Because the SSI contribution is more prevalent in the case of bridge systems supported on shallow or flexible pile-foundation the study is restricted to typical overpasses that belong to this class. The subject of study encompasses two overpasses located in the USA west coast (PSO-Painter Street Overrossing, MRO-Meloland Road Overcrossing), both instrumented samples of typical 1970 s US design practice (in terms of

6 bridge morphology and choice of foundation type), resting on flexible pile-foundation. Furthermore, the behavior of variants of the basic bridge prototypes is studied, in order to examine alternative design scenarios regarding the engineered exploitation of the SSI effects in short monolithic bridges with compliant abutments. Details of the Investigated Bridge Systems The prototypes of the study, known as the PSO and MRO rest on flexible foundation with small diameter piles designed primarily for gravity loads without significant lateral resistance. This design scheme is at odds with modern European design approaches that require heavy foundation with deep caisson piles of a relatively large diameter. The MRO overpass is a regular two-span monolithic structure with abutments supported on approach embankments. The overall bridge length is 63.4m comprising two equal spans of 31.7m. The central bent has a single column of circular cross-section 1.52m in diameter, which is supported on a pile group of twenty five piles of 0.43m diameter (arranged in a 5x5 rectangular pattern, Fig. 2a). The abutment foundations are supported on a series of seven piles arranged along a line and embedded in the embankment soil. A monolithic connection exists between abutments and central bent with the deck superstructure. Similarly, the PSO is a reinforced concrete monolithic bridge with multi-cell box type cross section. It comprises two unequal spans of 44.5m and 36.3m. The bridge is skewed at an angle of 39 ο, and is monolithically connected with the supports at the abutments and central bent. Again the abutments are supported on the end embankments, with the east side resting on fourteen and the west side on sixteen piles arranged in a line series, having a circular dimension of 0.36m and a length of 19m each. The central bent is a two column frame, of 1.56m diameter and a free length of 9.6m, each of them supported on twenty piles arranged in a 4x5 rectangular pattern having a 0.36 m diameter and a length of 11m (Fig. 2b). The variants of the basic bridge prototypes that were included in the study were designed with the intention to increase the available ductility without affecting the load bearing capacity of the bridge; for this reason the number of columns in the bents was increased while their diameter was reduced in order to increase the individual column flexibility to lateral translation. Figure 2. (a) Bridge geometry of the MRO and the MRO variant (MRO Type A), (b) Geometry of the PSO and the PSO variant (PSO Type A)

7 In this framework, apart from the existing MRO bridge the response of an identical bridge variant (MRO Type A in Fig. 2) having a frame bent of two 0.61m diameter columns was studied the circular cross section columns were founded on a separate pile group each, with the piles having identical geometry as those of the MRO. A similar variant of the PSO, denoted in Fig. 2 as PSO Type A had a four-column frame in the central bent, instead of the two in the original structure. Each column has a 0.61m diameter circular cross section and is assumed supported on a separate pile cap. Simulation and Analysis of the Bridge Models Three-dimensional frame models were used for idealization and nonlinear time history analysis of the bridge structures. Those parts of the structure that are expected to remain elastic during the analysis (the prestressed deck, end abutments, cap beam and pile caps) were modeled using linear elastic constitutive laws, whereas the critical subsystems such as the bent columns and piles were modeled using plastic hinges in all possible locations with elastoplastic skeleton curves based on calculated moment-curvature envelopes (obtained from Response 2000). The plastic hinge length, l p, was taken from Eq. (3): l = 0.08 l Φ f (3) p t y where l is the span of the member, Ø t is the typical longitudinal bar diameter of the critical cross section and f y the yield stress of the reinforcement [8]. Apart from the plastic hinge regions in the bridge structure, additional source of considerable nonlinearity in the response is the foundation soil. It is generally known that for cohesive soils increasing shear deformation leads to a commensurate reduction in the effective soil shear constant, G, resulting in significant nonlinearities in the calculated response. It is therefore essential that this effect be explicitly accounted for in the model, with consideration of any accompanying dynamic effects, such as inertial and kinematic soil-structure interactions. In light of the geometric layout of the prototypes, and in particular the small diameter of the piles and the overall compliance of the foundation, kinematic interactions could be neglected without significant error as evidenced by correlation of calculated with measured responses. But the inertial interaction is significant. In the model it is considered through the use of pertinent nonlinear springs of the Bouc-Wen type; constitutive laws were obtained from the G-γ and the equivalent viscous damping (ξ) relationships [9, 10]. Foundation soil was modeled using nonlinear springs spaced at a distance of 1m along the pile lengths, according with soil profile results obtained on site through investigative drilling. Force-displacement (p-y) curves were developed for the springs from the (K-y) soil envelopes, based on the methodology by Reese [11]. At this stage the pile interaction effects (within the pile group) were also considered [12]. Here it is important to note that in order to properly represent the mechanics of the bridge-system, it is equally important to account for the kinematic and inertial interactions that occur between abutment and the supporting embankment soil. In this framework, embankment mobilization was accounted for in all bridge models according with Kotsoglou and Pantazopoulou [5 7] using pertinent lumped mass, stiffness and damping elements in order to account for the mass participation, additional stiffness and damping that are introduced into the system through these interactions.

8 Meloland Street Overcrossing MRO Depth in m MRO Soil Profile [14] Painter Street Overcrossing PSO Depth in m PSO Soil Profile [15] from 0-0.3m from m from m from m from m Soft Sandy Clay Compact Fine Silty Sand Interbedded with stiff Silty Clay Slightly Compact Fine Silty Sand Interbedded with Stiff Silty Clay (Sand 50% Clay 50%) Stiff Clay Slightly Compact Silt to Fine Silty Sand Interbedded with Thin Stiff Clay Seams From 0-1.0m Soft Brown clayey silt with sand From m Slightly compact brown silty medium to fine sand with clay binder From m Slightly compact olivebrown silty fine sand with some clay binder From m Compact olive sandy silt with clay binder From m Very stiff brown clayey fine sand from Compact Fine Silty Sand From m Compact light brown silt from From m Compact to slightly Stiff Clay Interbedded with compact light brown silty Slightly Compact Fine Sand fine sand Table 2. Soil profiles for the MRO and the PSO case studies below grade (level 0 corresponds to the underside of the pile cap). For a complete investigation of the problem each separate bridge type that was investigated in the study was modeled considering all possible alternative support conditions in the central bent foundation in order to illustrate the implications of foundation compliance: this includes the variants of (a) fixity conditions at the base, (b) detailed modeling of the foundation components using Bouc-Wen springs. (a) (b) Figure 3. Implemented F.E. models (a) MRO and (b) PSO Analysis Results Ground acceleration records from three significant earthquake events characterizing the seismicity of the greater region of the bridge locations were used in the time-history simulations.

9 These are, (a) the Petrolia, Rio Dell (1992) earthquake, having an epicenter located 29km away from the instrumented PSO overpass in the Cape Mendocino area, California, (b) the Imperial Valley (1979) earthquake with an epicenter at just 0.5km away from the instrumented MRO bridge and (c) the Northridge earthquake which occurred in the greater area (Los Angeles, 1994) (Fig. 4). Figure 4. Ground acceleration records considered in the investigation (a) Petrolia earthquake (1992) (b) Imperial Valley (1979) (c) Northridge (1994). Based on detailed nonlinear dynamic analysis conducted for the selection of ground acceleration records, the displacement ductility demand µ of the central bent columns was estimated (Fig. 5) for the bridges, considering different foundation conditions (e.g. stiff or flexible foundation). PSO (as built) PSO (as built) PSO (modified) PSO (as built) Figure 5. PSO Dynamic Nonlinear Analysis Results: Column Displacement Ductility µ

10 MRO(as built) MRO (modified) Figure 6. MRO Dynamic Nonlinear Analysis Results: Column Displacement Ductility µ From the analysis results it is evident that displacement demands for bridges with flexible foundation are significantly decreased as a result of foundation rotation. On the other hand the computed rotation of the foundation of the modified bridges with flexible columns was negligible. Figure 7. Pile Cap rotation in the variants of (a) the MRO and (b) the PSO bridge. Conclusions The favorable effects of foundation compliance in moderating the seismic ductility demands of the columns was confirmed in the present study; however, it was found that the magnitude of the effect depends on the compliance of the bent columns to lateral translation as compared to the compliance of the foundation. Stiff piers (such as those used in European bridge designs) render the soil very vulnerable causing larger rotations and damage (plastic hinges) in the upper end of the piles near the connection with the stiff pile caps, whereas flexible column designs deliver the foundation from damage by undertaking larger share of the lateral drift demand without significant damage. The complexity of the analysis of the bridge system is excessive: the model includes the deck, piers / abutments, foundation substructure including the interacting soil, as

11 well as the mobilized embankment inertial and kinematic interactions. The large number of parameters and associated uncertainties as to the proper values dictate the need for conducting extensive parametric investigations of this problem in order to yield the most favorable combination of column stiffness and foundation arrangement. This application is particularly relevant to short (stiff) bridges where the embankment participation to the dynamic response of the bridge prevails, driving the bridge deck to lateral rigid body translation, thereby imposing significant displacement ductility demands in intermediate piers (e.g. bent columns). An open issue is the rotation limits for the pile-groups at different levels of performance- proposed values in this paper need to be further correlated with field investigations before they may be generally adopted in practical design. References 1. Kotsoglou, A, Pantazopoulou, S. (2013), Exploitation of Foundation Flexibility on the Seismic Performance of Bridge Piers, SEI-Structural Engineering International; IABSE, Volume 23, Issue 2, Pages , Antonellis, G. and Panagiotou, M. (2013), "Seismic Response of Bridges with Rocking Foundations Compared to that of Fixed-base Bridges at a Near-fault Site." J. Bridge Eng., /(ASCE)BE (Oct. 10, 2013). 3. DARE: Soil Foundation Structure Systems Beyond Conventional Seismic Failure Thresholds Application to New and Existing Structures and Monuments Chopra A., Goel R. (1999), Capacity-demand-diagram methods based on inelastic design spectrum. EERI Earthquake Spectra; Volume 15, Issue 4, Pages , Kotsoglou, A., Pantazopoulou, S. (2007), Bridge-Embankment Interaction Under Transverse Ground Excitation. Earthquake Engineering & Structural Dynamics; Volume 36, Issue 12, Pages , Kotsoglou, A., Pantazopoulou, S. (2009), Assessment and modeling of embankment participation in the seismic response of integral abutment bridge. Bulletin of Earthquake Engineering; Springer, Volume 7, Issue 2, Pages , Kotsoglou, A., Pantazopoulou, S. (2010), Response Simulation and Seismic Assessment of Highway Overcrossings, Earthquake Engineering & Structural Dynamics; Volume 39, Issue 9, Pages , Priestley M.J.N., F. Seible., and G.M. Calvi, 1996, Seismic Design and Retrofit of Bridges, John Wiley & Sons, New York. 9. Drosos, V., Gerolymos, N., Gazetas, G. (2012), Constitutive model for soil amplification of ground shaking: Parameter calibration, comparisons, validation, Soil Dynamics and Earthquake Engineering, Elsevier, Volume 42, Pages , Y. K. Wen. (1976), "Method for Random Vibration of Hysteretic Systems," Journal of the Engineering Mechanics Division. ASCE, Vol No. EM Brown, D., Morrison, C., and Reese, L. (1988). Lateral Load Behavior of Pile Group in Sand. J. Geotech. Engrg., 114(11), Comodromos, E and Pitilakis K. (2005), "Response Evaluation of Horizontally Loaded Fixed-Head Pile Groups using 3-D Nonlinear Analysis", International Journal for Numerical and Analytical Methods in Geomechanics, Vol. 29, No. 6, pp Maragakis, E., Douglas, B. And Abdel-Ghaffar, S. (1994), An Equivalent Linear Finite Element Approach For The Estimation Of Pile Foundation Stiffnesses, Earthquake Engineering And Structural Dynamics, Volume 23, Pages , (1994). 14. Heuze F.E. and Swift R. P. (1991), Seismic refraction studies at the Painter Street Bridge site, Rio Dell, California. Report UCRL-ID , Lawrence Livermore National Laboratory, Oak Ridge, TN, 1991.