gozetas-06-seismic design of found(3)

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SEISMIC DESIGN OF FOUNDATIONS ANDSOILSTRUCTURE INTERACTION

George GAZETAS1

SUMMARY

Two topics of practical significance are discussed : (a) The widely-held perception, reflected in

most seismic codes, that soil-structure interaction (SSI) plays invariably a beneficial role ( and thus

can be ignored in design to be on the safe side) is shown to be the result of a misconception

stemming from the shape of current design response acceleration spectra for soft/deep soil

conditions. Properly normalising the structural period by a dominant period of motion before

statistically processing recorded response spectral values, produces completely different shape and

size of spectral ordinates ; thus an increased period due to SSI may lead to higher response.

(b) Analytical, experimental and field evidence in recent years has started revealing that seismic

design of shallow foundations could overcome the strict limitations of the so-called capacity

design, by allowing significant sliding, uplifting, and even mobilisation of bearingcapacity

failure mechanisms to occur. It is shown that under seismic conditions such dynamic inelastic

nonlinear response may merely lead to acceptable permanent horizontal and rotational

deformations.

1. SSI AND SEISMIC CODE SPECTRA

Seismic codes have an inherent unique peculiarity among civil engineering codes : the actual loading on

structures (arising from earthquake shaking) can not be known in detail even when the intensity of motion is

specified. In fact, the induced seismic loading of a structure depends on the response of the structure itself ---

the consequence of an interplay that is part of soil-structure interaction. The phenomenon increases in

significance as the supporting soil becomes softer. Smooth design acceleration spectra, resulting from statistical

processing of a large number of elastic response spectra of actual recordings, has been universally accepted as

the way to specify earthquake loading in codes. Design acceleration spectra have an essentially constant

acceleration branch and a declining acceleration branch (with the exception of a rather insignificant ascending-

acceleration region of very small periods). The period range of constant-acceleration plateau is larger (up to 1

sec) for softer soils.The increase in natural period (and damping) due to soil deformability, along with the aforementioned

conventional description of acceleration spectra, leads almost invariably to smaller accelerations and stresses in

the structure and its foundation. Thus, the importance of accounting forSSI effects has been often dismissed, to

be on the safe side.

Furthermore, this beneficial role ofSSI has been turned into a dogma. For instance, according to EC85:

For the majority of usual building structures, the effects of SSI tend to be beneficial, since they reduce the

bending moments and shear forces acting in the various members of the superstructure

This statement may indeed hold for a large class of structures and seismic environments. But not always.

There is evidence documented in numerous case histories that the perceived beneficial role ofSSI is an unjust

over-simplification that may lead to unsafe design of both the superstructure and the foundation.

1 Professor, National Technical University, Athens, Greece. Email : [email protected]

First European Conference on Earthquake Engineering and Seismology

(a joint event of the 13th ECEE & 30th General Assembly of the ESC)

Geneva, Switzerland, 3-8 September 2006

Paper Number: Keynote Address K7


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To elucidate this, we first recall that the flat design spectrum of seismic codes does not resemble the spectra

recorded on soft deep soil. For instance, the reader can revisit the spectra of the following records: Brancea

(Bucharest) 1977, Michoacan [Mexico City (SCT)] 1985, Kobe (Fukiai, Takatori) 1995. All of the recorded

spectra of these records attain well defined peaks at periods exceeding 1 second. The large spectral values of

some of these records are undoubtedly the result of the soil deposit resonance with the incoming seismic waves

(most spectacular being in the Mexico City SCT record).

Another phenomenon, however, of seismological rather than geotechnical nature, the forward fault-rupturedirectivity (Somerville 1998), may be an important contributing factor in the large spectral values at T > 0.50 s

in near-fault seismic motions (e.g. in Takatori and Fukiai). An earthquake is a shear dislocation initiating at a

point on a fault and spreading outward along the fault approximately at the prevailing shear wave velocity. The

high velocity of fault rupture propagation toward a site causes most of the seismic rupture energy to arrive in the

form of long-periods pulse of motion, at the beginning of the record (Somerville et al., 1997). The radiation

pattern of the shear dislocation causes such large pulses to be oriented in the direction perpendicular to the fault,

causing the strike-normal peak velocity to be larger than the strike-parallel velocity. The forward rupture

directivity effect appears to increase the spectral values of the horizontal component normal to the fault strike at

periods longer than about 0.5 seconds. Examples of this effect are the Kobe (1995) JMA, Fukiai, Takatori, and

Kobe-University records; the Northridge (1994) Rinaldi, Newhall, Sylmar-Converter, and Sylmar-Olive-View

records; the Aegion (1995) and Lefkada (2003) records ; and many others.

Therefore, it is apparent that as a result of soil or seismological factors, an increase in the fundamentalperiod due to SSI may lead to increased response (despite a possible increase in damping), which contradicts the

expectation incited by the conventional design spectrum. For instance, Mexico earthquake was particularly

destructive to 10- to 12-story buildings founded on soft clay ; their period apparently increased from about 1 sec

(under the fictitious assumption of a fixed base) to nearly 2 seconds in reality, due to SSI. The role ofSSI on the

failure of the 630 m long elevated highway section of Hanshin Expressways Route 3 in Kobe (Fukae section)

has also been detrimental (see Gazetas et al 2005, Mylonakis et al 2005). Evidence of a potentially detrimental

role of SSI on the collapse of buildings in the recent Adana-Ceyhan earthquake was presented by Celebi (1998).

It should be noted that large SSI increases in the natural period of structures (T~

/ T > 1.3) are not uncommon

in relatively tall yet rigid structures founded on soft soil (Tazoh et al 1988; Mylonakis et al 1997; Stewart et al

1999). Therefore, evaluating the consequences ofSSI on the seismic behaviour of such structures may require

careful assessment of both seismic input and soil conditions ; use of conventional design spectra and

generalized/simplified soil profiles in these cases may not reveal the danger of increased seismic demand on the

structure.

To further illustrate the above, results from a statistical study performed by the authors using a large set of

motions recorded on soft soil are presented. The set of motions consists of 24 actual records. The average

acceleration spectrum obtained from these motions is presented in Fig 1, in terms of spectral amplification. In the

horizontal axis, the structural period is presented in three different ways: (i) the actual period T as has been

Figure 1:Average acceleration spectra based on 24 actual motions recorded on soft soil. The periods are

normalized before averaging with: (a) period of peak spectral acceleration (Ta) ; (b) period of peak

spectral velocity (Tg) ; = 5% (After Mylonakis & Gazetas 2000).


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done to derive design response spectra (in the last 30 years) ; (ii) the normalized period T/Tg [where

Tg = effective ground period, defined as the period where 5%-damped velocity response spectrum attains its

maximum (Miranda & Bertero 1994)]; and (iii) the normalized period T/Ta [Ta = period where acceleration

spectrum attains its maximum.]

It is seen that when plotted against the actual period, the resulting average spectrum has a flat shape(analogous to that used in current seismic codes). This shape has no resemblance to an actual spectrum ! This

unrealistic shape is because the spectra of motions recorded on soft soil attain their maxima at different, well

separated periods and, thereby, averaging them eliminates their peaks causing this effect. In contrast, when

plotted against the normalized periods, T/Tg or T/Ta , the average spectrum exhibits a characteristic peak at

values of T/Tg not far from 1, which reproduces the trends observed in actual spectra ! Not withstanding the fact

that determining the characteristic dominant period (i.e., Tg or Ta) for a given site and seismic event is not always

easy, it is clear that current provisions treat seismic demand in soft soils in a non-rational way, and may mislead

the designers with misleading on the significance of SSI. A similar conclusion was reached by Longjun & Lili

(2004), with recorded motions of the Chi-Chi 1999 Earthquake.

It is therefore proposed that in future revisions of seismic codes design spectra for evaluating SSI effects be of

the form plotted in Fig.1 , i.e Sa = Sa (T/Tg)

Such spectra will have to be developed using techniques similar to those used for the development of current

design spectra. There is nothing strange with having different design spectra for assessing SSI --- this type of

spectra will be the critical loading for this mode of response.

2. DESIGN OF SHALLOW FOUNDATIONS WITH UPLIFTING AND SOIL YIELDING

2.1 Introduction

The conventional approach to foundation design introduces factors of safety against sliding and exceedance of

ultimate capacity, in a way similar to the traditional static design. This approach involves two consecutive steps

of structural and foundation analysis :

(a) Dynamic analysis of the structure is performed in which the soil is modeled as an elastic medium,

represented by suitable translational and rotational springs (and, sometimes, with the associated dashpots).

The dynamic forces and moments transmitted onto the foundation are derived from the results of such

analyses along with considerations for inelastic structural response (e.g. by reducing the moments in

columns through the behaviour [ductility] factorq ).

(b) The foundations are then designed in such a way that these transmitted horizontal forces and overturning

moments, increased by overstrength factors, would not induce sliding or bearing capacity failure.

The use of overstrength factors is necessitated by the so-called capacity design principle, under which

plastic hinging is allowed only in the super-structural elements not in the belowground (and thus un-

inspectable) foundation and soil. Therefore, structural yielding of the footing and mobilization of bearing

capacity mechanisms is not allowed. Only a limited amount of sliding deformation and uplifting at the

foundationsoil interface is allowed. However, there is a growing awareness in the profession of the need toconsider soil-foundation inelasticity, in analysis and perhaps even in design [see: Pecker (1998), Paolucci (1997),

Martin & Lam (2000), Allotey & Naggar (2003)]. This need has emerged from :

The large (often huge) acceleration (and velocity) levels recorded in several earthquakes:

- 1994 Northridge (M 6.8) : 0.98 g, 1.40 m/s

- 1995 Kobe (M 7.2) : 0.85g, 1.50 m/s

- 1986 San Salvador (M 5.5) : 0.75 g, 0.84 m/s

which are associated with even larger elastic spectral accelerations (of the order of 2g). Enormous ductility

demands would be imposed to structures by such accelerations if soil and foundation yielding do not

effectively take place to limit the transmitted accelerations.

In seismically retrofitting a building or a bridge, allowing for soil and foundation yielding is the only

rational alternative. Because increasing the structural capacity of some elements would imply that the

forces transmitted onto the foundation be increased, to the point that it would not be technically or

economically feasible to undertake them elastically. Thus, new retrofit design quidelines (FEMA 356)

explicitly permit inelastic deformations in the foundation.


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Even with new structures, it has been recognized that with improved analysis methods we need to better

evaluate performance in terms of levels of damage. For the superstructure, performancebased design

or equivalently displacementbased design have been used for a number of years, with inelastic push-

over analyses becoming almost routine in seismic design practice. It is logical to extend the inelastic

analysis to the supporting foundation and soil.

2.2. Plastic Hinging in Shallow Foundations

Excluding structural yielding in the isolated footing or the foundation beam, three types of nonlinearity can take

place and modify the overall structurefoundation response :

(a) Sliding at the soilfoundation interface. This would happen whenever the transmitted horizontal forceexceeds the frictional resistance. As pointed out by Newmark (1965) , thanks to the oscillatory nature of

earthquake shaking, only short periods of exceedance usually exist in each one direction ; hence, sliding is not

associated with failure, but with permanent irreversible deformations. The designer must only ensure that the

magnitude of such deformations would not be structurally or operationally detrimental. Although this

philosophy has been applied to the design of earth dams and gravity retaining walls, its practical significance for

foundations might be somewhat limited in view of the large values of the coefficient of friction at soilfooting

interface and the passive

type resistance often enjoyed by embedded foundations.(b) Separation and uplifting of the foundation from the soil. This would happen when the seismic

overturning moment tends to produce net tensile stresses at the edges of the foundation. The ensuing rocking

oscillations in which uplifting takes place involve primarily geometric nonlinearities, if the soil is competent

enough. There is no detriment to the vertical load carrying capacity and the consequences in terms of induced

vertical settlements may be minor. Moreover, in many cases, footing uplifting is beneficial for the response of

the superstructure, as it helps reduce the ductility demands on columns. Housner (1963), Pauley & Priestley

(1992), and many others have reported that the satisfactory response of some slender structures in strong shaking

can only be attributed to foundation rocking. Deliberately designing a bridge foundation to uplift in rocking has

been proposed as an effective seismic isolation method by Kawashima & coworkers (2005). Moreover, even

with very slender and relatively rigid structures, uplifting would not lead to overturning except in rather extreme

cases of little concern to the engineer (Makris & Rousos 2000, Gerolymos et al 2005).

In soft and moderately-soft soils much of what was said above is still valid, but inelastic action in the soil is

now unavoidable under the supporting edge of the uplifting footing in rocking. At the extreme, inelastic

deformations in the soil take the form of mobilization of failure mechanisms, as discussed below.

(c) Mobilisation of bearing capacity failure mechanisms in the supporting soil. Such inelastic action under

seismic loading would always be accompanied with uplifting of the foundation. In static geotechnical analysis

large factors of safety are introduced to ensure that bearing capacity modes of failure are not even approached.

In conventional seismic analysis, such as in the EC8 Part 5 bearing capacity is avoided thanks to an

overstrength factor of about 1.40. The oscillatory nature of seismic shaking, however, allows the mobilisation

(for a short period of time!) of the maximum soil resistance along a continuous (failure) surface. No collapse

or overturning failure occurs, as the applied (causative) moment quickly reverses, and a similar bearing-

capacity failure mechanism may develop under the other edge of the foundation. The problem again reduces

to computing the inelastic deformations, which in this case meanspermanent rotation. The designer must ensure

that its consequences are not detrimental.

The concept of allowing mobilization of bearing capacity mechanisms in foundation design may represent amajor change in foundation design philosophy (Pecker 1998). However, for analysis of the ultimate response of

a structurefoundation system to extreme earthquake shaking, accounting for such a possibility is necessary.

Martin & Lam (2000) illustrate with an example of a hypothetical structure containing a shear wall connected

with a frame how dramatically different are the results of analyses in which inelastic action in the soil is

considered or is ignored. With inelastic action (including uplifting) the shear wall sheds some of its load onto

the columns of the frame, which must then be properly reinforced ; the opposite is true when linear

soilfoundation behaviour is assumed. Thus, computing the consequences of plastic hinging in shallow

foundation analysis may be a necessity.

The interplay between uplifting and mobilization of bearing capacity mechanisms is governed primarily by

the following factors :

the vertical foundation load N in comparison with the ultimate vertical capacity Nult

the height, h, of the mass center of gravity from the base compared with the foundation dimensions (widthB, length L)


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the intensity, frequency content and sequence of pulses of the seismic excitation.

Under seismic excitation the soilfoundationstructure system can sustain acceleration amplitudes (A) much

higher that the critical acceleration (Ac) required for pseudo-static failure. Even a small dynamic (transient)

factor of safety, i.e. Ac / A 1 sec) and containing longduration

acceleration pulse(s), is far more destructive ; unacceptable deformations start occuring if Ac/A . (Apostolou

& Gazetas 2005, Gerolymos et al 2005).

Moreover, the initiation of uplifting and the mobilization of bearing capacity failure can be quite

beneficial for the superstructure (under certain conditions related with the fundamental period of the structure

and characteristics of ground shaking) (see Gazetas & Coworkers 2003, 2005).

3. REFERENCES AND BIBLIOGRAPHY

Allotey, N., Naggar, M., (2003), Analytical moment-rotation curves for rigid foundations based on a Winkler

model, Soil Dynamics and Earthquake Engineering,23, 367-381.

Anastasopoulos, I. (1999). Analysis of the failure of two bridges in the 1995 Kobe earthquake, and the role of

soil, Diploma Thesis, National Technical University, Athens, Greece

Apostolou, M, Gazetas, G., (2005), Rocking of Foundations under Strong Shaking : Mobilisation of Bearing

Capacity and Displacement Demands. Proceedings of 1st Greece-Japan Workshop : Seismic Design,

Observation, and Retrofit of Foundations, (editors : G. Gazetas, Y. Goto, T. Tazoh), 131-140.

Calvi, G.M. and Kingsley G.R. [1995], Displacement-Based Seismic Design of Multi-Degree-of Freedom

Bridge Structures,Earthquake Engineering & Structural Dynamics, Vol 24, pp. 1247-1266

Celebi, M. [1998]. Turkish Earthquakes: Two Reports. Lessons from the Adana-Ceyhan Quake and the Dinar

Aftershock,EERI Newsletter, Vol. 32, No. 9, 8 pages

Cremer, C., Pecker, A., Davenne, L. (2002), Modeling of Nonlinear Dynamic Behaviour of a Shallow Strip

Foundation with Macro-Element.Journal of Earthquake Engineering, 6(2).

FEMA 356 (2000) : Prestandard and Commentary for the Seismic Rehabilitation of Buildings.

Gazetas, G. and Mylonakis, G. (1998), Seismic soil-structure interaction: New evidence and emerging issues,

Geotechnical Earthquake Engineering and Soil Dynamics III, ASCE, P. Dakoulas, M.K. Yegian, & R.D.

Holtz (editors), Vol. II, pp. 1119-1174

Gazetas, G, Apostolou, M., & Anastasopoulos, I. (2003), Seismic Uplifting of Foundations on Soft Soil, with

Examples from Adapazari.Foundations: Innovations, Observations, Design and Practice, British

Geotechnical Association, Thomas Telford, 3750.

Gazetas. G. (2005), Seismic Analysis of Shallow Foundations : Beyond EC8, ATTI, Conferenze di

Geotechnica di Torino, xx Ciclo , Geotechnical Design with Eurocodes, Politecnico di Torino, Italy.

Gazetas, G., Anastasopoulos, I., Gerolymos N., Mylonakis, G., Syngros, C., (2005), The Collapse of the

Hanshin Expressway (Fukae) Bridge, Kobe 1995 : SoilFoundationStructure Interaction,

Reconstruction, Seismic Isolation, Frank Rackwitz (Hrsg.) Entwicklungen in der Bodenmechanik,Bodendynamik und Geotechnik, Festschrift zum 60. Geburtstag von Univ. Professor Dr.-Ing.habil.,

Stavros A. Savidis, (), pp. 93-120.

Gerolymos, N., Apostolou, M., & Gazetas, G. (2005), Neural Network Analysis of the Overturning Response

Under Near-Fault Type Excitation.Earthquake Engineering and Engineering Vibration, 4(2).

Housner, G., (1963). The Behavior of Inverted Pendulum Structures During Earthquakes. Bulletin of the

Seismological Society of America. 53(2), 404-417.

Kawashima, K., Hosoiri, K. (2005), Rocking Isolation of Bridge Columns on Direct Foundations. Journal of

Earthquake Engineering, 6(2).

Longjun Xu and Lili Xie (2004), Bi-normalized response spectral characteristics of the 1999 Chi-Chi

earthquake, Earthquake Engineering and Engineering Vibration, Vol. 3, No.2, pp. 147-155.

Luco, E. (1982). Linear soil-structure interaction: A review, Earthquake Ground Motion and Effects on

Structures, ASME, AMD, Vol. 53, pp. 41-57

Makris, N., and Roussos Y.S. (2000), Rocking Response of Rigid Blocks under Near-Source Ground Motions.

Geotechnique, 50(3), 243-262.


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