		2010 NZSEE Conference
	Abstracts

Keynote Address Tribute to Tom Paulay Session 3 Session 4A Session 4B Session 5A Session 5B Session 6 Session 7A Session 7B Session 8A Session 8B Poster Session

Experimental investigation of uplift effect on structures in earthquakes

X. Qin and N. Chouw

In the past, research on structural uplift focused mainly on the rocking of structures which were assumed to be rigid and supported by rigid ground. The contribution of structural flexibility and soil-structure interaction (SSI) were neglected. In this work, shake table tests were conducted to investigate the response of a single-degree-of-freedom (SDOF) system to earthquakes excitations. The SDOF system was a scaled model of a multi-storey RC building. A sand box was used to simulate the effect of subsoil. A comparison of both cases of rigid and sand supports showed that uplift of structures decreased activated bending moments, the maximum horizontal acceleration but increased horizontal displacement. It was also found that frequent uplift may not provide further reduction of structural responses.

Paper P14: [Read]

Dynamic field testing of shallow foundations subject to rocking

T.B. Algie, M.J. Pender, R.P. Orense and L.M. Wotherspoon

Nonlinear soil-foundation-structure interaction (SFSI) of shallow foundations may be beneficial during a strong seismic event. This paper details large scale experiments of the seismic response of shallow foundations, conducted onsite. Prior to the experiments a full geotechnical investigation was carried out to determine soil conditions. A test structure, made from structural steel, was designed and fabricated for this testing. Road construction plates were attached to the structure to create realistic vertical loads and achieve different static factors of safety in bearing. Three series of tests were conducted and are presented in this paper. The static factors of safety of all three tests were 47, 11, and 11 respectively. Nonlinear behaviour of the foundations and underlying soil was evident and comparison between the experimental results is encouraging. Static moment capacity equations are shown to calculate the moment capacity of the tests satisfactorily, however, elastic stiffness values calculated using Gazetas’ equations differed from experimental results. The measured response of the foundations indicates good energy dissipation during the rocking cycles.

Paper P15: [Read]

Simplified Expressions for Modelling Rigid Rocking Structures on Two-spring Foundations

Quincy Ma and John Butterworth

This paper presents a new technique for modelling the dynamic response of uplifting rigid structures subjected to base excitation. The proposed technique exploits the use of a two-spring foundation, and subsequently an equivalent single-degree-of-freedom procedure is established to model the dynamics of the system. A set of simplified closed-form expressions have been developed to estimate the system’s restoring force-displacement characteristics. The simplified expressions only require details of the system geometry and are shown to predict the nonlinear force-displacement characteristics of a rocking structure as closely as those determined from a complicated pushover analysis.

This paper presents two additional numerical examples to demonstrate the use of the proposed technique to simulate the displacement time-histories of a prototype structure under free-vibration-decay or when subjected to earthquake excitations.

Paper P16: [Read]

Soil-Foundation-Structure Interaction Effects on Nonlinear Seismic Demand of Structures

M. Moghaddasi K., M. Cubrinovski, S. Pampanin, A.J. Carr and J.G. Chase

Investigating the seismic demand of structures including soil-foundation-structure interaction (SFSI) effects is a demanding task due to the complexity of the coupled dynamic problem. This task is significantly burdened by uncertainty in the soil and structural parameters and randomness in earthquake characteristics. The objective of this research is to highlight SFSI effects on the seismic demand of nonlinear structures in the presence of these uncertainties and to thus quantify the risk of exceeding response design specifications. This goal is accomplished through a probabilistic Monte Carlo methodology. In the analyses, various soil and structural parameters were combined to generate a large number of realistic models which were subjected to a range of earthquakes with different spectral characteristics. Specifically, 1.36 million nonlinear time-history simulations were run over: (i) models consisting of a SDOF superstructure and a rheological soil-shallow foundation element; and (ii) their corresponding fixed-base models. The demand modification in structural distortion, drift and total displacement due to consideration of SFSI are compared to the results of the corresponding fixed-base systems and are quantified through a comprehensive statistical presentation. The results contradict prevailing views of the beneficial role of SFSI on the structural response and show thus the resulting design guideline assumptions do not hold for all cases. In fact SFSI effects can only be safely ignored with 50% confidence. The rigorous statistical Monte Carlo analysis presented is a significant first step towards reliability-based seismic design procedures incorporating foundation flexibility.

Paper P17: [Read]

Dynamic Field Tests of Single Piles

Norazzlina M.Sa’don, Michael Pender, Rolando Orense, Abdul Razak Abdul Karim and Liam Wotherspoon

This paper presents the results of a full-scale field study of a single free-head piles embedded in Auckland residual clay. Four hollow steel pipe piles, each with an outside diameter of 273 mm and wall thickness of 9.3 mm were installed at a site in Pinehill, Auckland. A series of dynamic tests ranging from low excitation (using an instrumented impact hammer and a low-mass loading of an eccentric mass shaker) to high dynamically-induced force from the eccentric mass shaker was performed during the spring and early summer after the winter wet weather, so that the soil can be assumed to be saturated to the ground surface. Results from low amplitude dynamic tests indicated a reduction in the natural frequency of the system from 9.6 Hz to 8.2 Hz after experiencing a higher level of forcing amplitude. This result indicated the non-linear response of the pile-soil system that was caused by the strain softening of the soil and the formation of a gap between the pile shaft and the surrounding soil.

Paper P18: [Read]

Can tilt tests provide correct insight regarding frictional behaviour of sandstone under seismic excitation?

K.C. Lee, F.S. Jeng, T.S. Huang, Y.M. Hsieh and R.P. Orense

Earthquake induced rock-block sliding is usually analyzed using friction angle measured at the sliding-interface. A tilt test is a convenient test for measuring the required values. However, a tilt test is a test under static condition, and the applicability of measured friction parameters to analyze block sliding under dynamic excitation requires further discussion. This study conducts a static tilt test and a dynamic shaking table test to simulate block sliding with base excitation, compares differences in measured sliding thresholds, and discusses the cause of these differences. Tests on three different materials (aluminium, sandstone, and synthetic sandstone) show that friction coefficients measured by tilt tests are always larger than the ones derived by shaking table tests. Moreover, high frequency tests yield larger friction coefficients, suggesting the sliding threshold is non-constant under excitation. In addition, tests with varying normal stresses of the sliding block show that with increasing contact stresses, sliding thresholds decrease, implying that sliding threshold varies with normal stress. Instantaneous friction coefficients during sliding are also studied in this work. It has been found the frictional behaviour of synthetic sandstone deviates from the idealized Coulomb friction model. The instantaneous friction coefficient varies with relative displacement and relative velocity during sliding. This study preliminarily identifies the limitations of the tilt test when applied to dynamic problems, and concludes that realistic sliding threshold can only be obtained using dynamic tests such as shaking table tests.

Paper P19: [Read]

Keynote Address Tribute to Tom Paulay Session 3 Session 4A Session 4B Session 5A Session 5B Session 6 Session 7A Session 7B Session 8A Session 8B Poster Session