Experimental evidence for flexibility of a building foundation supported by concrete friction piles

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Abstract

This article explores the possibility to measure deformations of building foundations from measurements of ambient noise and strong motion recordings. The case under study is a seven-storey hotel building in Van Nuys, California. It has been instrumented by strong motion accelerographs, and has recorded several earthquakes, including the 1971 San Fernando (ML=6.6, R=22 km), 1987 Whittier–Narrows (ML=5.9, R=41 km), 1992 Landers (ML=7.5, R=186 km), 1992 Big Bear (ML=6.5, R=149 km), and 1994 Northridge (ML=6.4, R=1.5 km) earthquake and its aftershocks (20 March: ML=5.2, R=1.2 km; 6 December, 1994: ML=4.3, R=11 km). It suffered minor structural damage in 1971 earthquake and extensive damage in 1994. Two detailed ambient vibration tests were performed following the Northridge earthquake, one before and the other one after the 20 March aftershock. These included measurements at a grid of points on the ground floor and in the parking lot surrounding the building, presented and analyzed in this article. The analysis shows that the foundation system, consisting of grade beams on friction piles, does not act as a “rigid body” but deforms during the passage of microtremor and therefore earthquake waves. For this geometrically and by design essentially symmetric building, the center of stiffness of the foundation system appears to have large eccentricity (this is seen both from the microtremor measurements and from the earthquake recordings). This eccentricity may have contributed to strong coupling of transverse and torsional responses, and to larger than expected torsional response, contributing to damage during the 1994 Northridge, earthquake.

Introduction

Earthquake-resistant design of structures must be based on analyses of realistic models of the structure, foundation and soil system, considering wave propagation and all the aspects of nonlinear response. Such analyses require solution of a complicated and difficult to solve system of governing equations and boundary conditions. Hence, it has been necessary to make various simplifications. In doing so, it is important to evaluate the accuracy of the approximations and to define the range of the model parameters for which the approximations are valid. This is best accomplished by careful experimental verification using full scale tests of actual structures.

A common assumption in many models which consider soil–structure interaction effects is that the foundation is rigid. This reduces the number of degrees-of-freedom of the model, and gives good approximations for long wavelengths relative to the foundation dimensions ([7]). For short wavelengths, this assumption can result in non-conservative estimates of the relative deformations in the structure ([23], [25]) and, in general, is expected to result in excessive estimates of scattering of the incident wave energy and in excessive radiation damping ([15], [18], [19]). The extent to which this assumption is valid depends on the stiffness of the foundation system relative to that of the soil, and also on the overall rigidity of the structure ([5], [8]). For a nine-storey reinforced concrete building, extensively tested during the 1970s, the foundation could be represented by a “rigid” slab for NS vibrations (because of stiffening effects of the end shear walls) but not for EW vibrations ([2], [9], [10], [11], [12], [13], [27]). The other extreme is to neglect the stiffness of the foundation system and to assume that the wave energy is transmitted from soil into the building according to the principles of wave propagation ([14], [15], [16], [17], [20]). This approximate approach underestimates the incident wave energy scattered by the foundation and overestimates the energy transmitted into the building. The reality is somewhere between these two approximations, and can be studied in detail only by means of numerical methods.

In this article, an instrumented seven-storey hotel building in Van Nuys, California, is studied. Records of several earthquakes were available for the study, including the 1971 San Fernando (ML=6.6, R=22 km), 1987 Whittier–Narrows (ML=5.9, R=41 km), 1992 Landers (ML=7.5, R=186 km), 1992 Big Bear (ML=6.5, R=149 km), and 1994 Northridge (ML=6.4, R=1.5 km) earthquake and two of its aftershocks (20 March: ML=5.2, R=1.2 km; and 6 December, 1994: ML=4.3, R=11 km). The building is supported by a friction pile foundation. The Northridge earthquake caused severe damage, and the building was declared unsafe. The damage was most severe at the fifth floor, where many columns were damaged, just below the spandrel beam. The specific aspects of the response, which caused this type of failure, have not been deciphered so far. One plausible group of causes can be sought in the large relative deformations of the foundation system (pile caps connected by grade beams; [23]), but the limited number of accelerographs, which recorded the main event, is not sufficient to verify this hypotheses.

In this article, an analysis of ambient noise measurements in the parking lot and on the ground floor of this building is presented. The objective is to describe the deformations of the foundation system during the passage of ambient noise waves (mostly Rayleigh waves caused by surface traffic), and to speculate on how the foundation may have moved during the Northridge earthquake.

Section snippets

Description of the building

The building analyzed in this article is a seven-storey reinforced concrete structure, in the city of Van Nuys (Los Angeles metropolitan area), near the intersection of Roscoe Ave. and the San Diego Freeway (I-405; Fig. 1). It will be referred to as VN7SH for short. It was designed in 1965 ([1]) and served as a hotel until 1994. Its plan dimensions are about 62 by 160 feet (Fig. 2a). The typical framing consists of columns spaced at 20 foot centers in the transverse direction and 19 foot

Earthquake recordings

The first known recorded strong motion in the building is of the February 9, 1971, San Fernando earthquake (Fig. 1). The location of the sensors, three self-contained tri-axial AR-240 accelerographs, is shown in Fig. 3a. During this earthquake, the first strong motion waves started to arrive from N22°E, having originated at depth ∽9 to 13 km below epicenter (Trifunac, 1974). With rupture propagating up towards south at about 2 km/s, the last direct waves were arriving from N 62°E, 9–10 s later.

General overview and objectives

Two ambient vibration experiments were conducted in the building, one on Feb. 4–5 (about two and a half weeks after the Northridge main event) and the other one on April 19–20, 1994 (about three months after the main event and one month after one of the largest aftershocks, of March 20, M=5.2; see Fig. 1). Between the two experiments, the building was temporarily restrained, as it was severely damaged by the main event.

The objective of the first experiment was to measure the dynamic

Discussion and conclusions

One of the more interesting results of this analysis is seen in Fig. 14b, displaying normalized amplitudes of the cross-correlation function of NS velocities for the complete (unfiltered) recorded motions. It shows that during passage of microtremor waves, mainly from west to east, the foundation essentially rotates about a point close to the south-eastern corner of the building (near A9). The EW components of this motion, shown in Fig. 16b, are consistent with this interpretation if one allows

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    1

    Current address: Civil Engineering Department, University of Montenegro, Cetinjski Put BB, Podogorica 8100, Montenegro, Yugoslovia.

    2

    Current address: Geophex Ltd., 605, Mercury Street, Raleigh, NC 27603-2343, USA

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