Methodology for the assessment of face-loaded unreinforced masonry walls under seismic loading

Abstract

In the early 1980's US researchers (ABK, 1982) subjected full-scale specimens representing a face loaded wall element spanning between two adjacent floor diaphragms, to earthquake motions. They found that a single horizontal crack tended to form near mid-height of the test specimens and another crack formed at the test bed floor, and that the walls were able to sustain large displacements, comparable with the wall thickness. This ability to withstand large displacements without collapse resulted in the walls having a significant post cracking seismic resistance. The term "dynamic stability" was used to distinguish this type of behaviour from the behaviour that might have been expected from static force calculations.

Subsequently this concept was used to develop the current New Zealand National Society of Earthquake Engineering (NZNSEE) Guidelines for the assessment of face loaded walls. These Guidelines are based on the equal energy method and the initial elastic stiffness of the wall (Priestley, 1985). However, in this study the current NZNSEE Guidelines were shown to be an unreliable, and often very unconservative, method of predicting the seismic resistance of URM walls.

In this study a more reliable method of assessing the seismic behaviour of face loaded URM walls, originally proposed in a previous EQC Research Foundation funded project (Blaikie, Spurr, 1992), has been developed to a stage that is suitable for design office use. A computer model was used to test and refine the methodology, and full size walls sample were laboratory tested to calibrate the computer model. The influence of "near fault" earthquake motions on the wall response was included as this may become an important consideration for locations near to active faults such as Wellington.

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Technical Abstract

An assessment methodology that can be used to predict the seismic stability of a cracked faceloaded URM wall was refined and developed in of this study. The methodology makes use of both the acceleration and displacement response spectra for an earthquake motion. The acceleration spectrum is used to predict the earthquake intensity that will just open the joint cracks in the wall. The displacement spectrum is used to predict the earthquake intensity that will generate mid-storey wall displacements equal to the displacement at which the wall becomes unstable. Modification factors are applied to allow for the effect of storey boundary conditions and to allow for amplification of the earthquake motion due to the overall building response and any diaphragm flexibility.

It was found that a relatively simple formula can be used to calculate the period of the motion of a cracked face-loaded URM wall when the peak mid-storey displacement is 60% of the displacement at which the wall becomes unstable. The formula is not dependant on the wall slenderness, overburden load, wall thickness or presense of top flexural fixity. The period calculated using the formula can be used in conjunction with a displacement spectra as part of the methodology used to predict the stability of face-loaded URM walls.

Design charts have been prepared to enable rapid design office assessment of a face-loaded wall in terms of the NZS4203 Loading Standard design EQ spectra. Similar design charts could be prepared for other earthquake records or code design spectral intensities using the proposed methodology.

A three-storey, dynamic inelastic computer model was used to examine the effect of a number of parameters on the seismic stability of a cracked face-loaded URM wall. Parameters examined included interaction between the face-loaded walls segments in adjacent storeys, effect of building flexibility and effect of diaphragm flexibility and/or yielding. The analyses indicated that the earthquake intensity required to collapse a face-loaded wall, as indicated by the computer modelling, is conservatively predicted by the proposed methodology. However, the current New Zealand National Society of Earthquake Engineering (NZNSEE) Guidelines were shown to be an unreliable, and often very unconservative, method of predicting the seismic resistance of URM walls.

The free vibration responses of 2 test specimens were modelled using the computer model. The tests demonstrated that the computer model can be used to model the dynamic displacement response, and hence the seismic stability, of a URM face-loaded wall.

The response of the 3-storey wall model was evaluated using an earthquake motion recorded in the near-fault zone during the Kobe earthquake. The near-fault motion had a quite different frequency content and spectral shape from that used as the basis of traditional code design spectra. The analyses showed that the formulae proposed for calculating the seismic resistance of face-loaded URM walls, when the building and diaphragms are rigid, can be used for a wide range of different earthquake motions including near fault earthquakes.

However under the near-fault earthquake motion, amplification factors included in the assessment methodology to allow for building and diaphragm flexibility, must also increase
with increasing building and/or diaphragm flexibility. Poor performance of face-loaded URM
walls in near-fault earthquake zones is predicted.