86 BUILD 126 October/November 2011

RESEARCH

Retrofits of unreinforcedmasonry buildingsResearch into retrofitting unreinforced masonry buildings has found that fibre­reinforced polymer systems can increase their strength and toughness, making them less susceptible to earthquake damage. By Hamid Mahmood, PhD student, and Dr Jason Ingham, Associate Professor, Department of Civil and Environmental Engineering, University of Auckland

Unreinforced masonry was a popular construction material in New Zealand before 1931, but its poor performance in the Hawke’s Bay earthquake reduced its use. In 1965, after the introduction of building bylaw NZS 1900, such construction was restricted to

certain seismic zones. Current practice is to only construct reinforced masonry buildings, but

a significant number of mainly pre­1965 unreinforced masonry buildings still exist. Their partial or complete collapse can result in significant human and economic losses, as seen during the recent Canterbury earthquakes when many were badly damaged.

It is vital to either remove seismically vulnerable unreinforced masonry buildings or upgrade them to be more resistant to earthquakes. Many of these buildings cannot be demolished due to their commercial value or historical significance, so appropriate earthquake strengthening techniques must be developed.

The University of Auckland has been investigating the seismic assessment and retrofit of New Zealand’s unreinforced masonry buildings. One part of the research looked at retrofitting the walls of such buildings using fibre­reinforced polymer materials.

Focus on in-plane loading

The focus of this research was on unreinforced masonry walls that were loaded parallel to their planes (called in­plane walls). In­plane wall damage is frequently observed in earthquakes and often appears as x­shaped cracks (see Figure 1), referred to as diagonal shear cracks.

During earthquakes, in­plane walls may also slide along a horizontal or stepped wall joint, a phenomenon known as sliding shear. Diagonal shear and sliding shear are collectively called shear modes of failure. This research considered seismic retrofit using fibre­reinforced polymer materials on in­plane unreinforced masonry walls likely to suffer a shear mode of failure.

Fibre-reinforced polymers have many benefits

Fibre­reinforced polymer materials are formed by embedding high­strength fibres (for example, glass or carbon fibres) in a polymer resin (for example, epoxy resin) to obtain a composite material suitable for retrofitting structural elements such as beams, walls and columns. Figure 1: Diagonal shear cracks in an in­plane wall.

These materials have emerged in the last 20 years as being particularly suitable for seismic retrofits. They are light, thin (layers of between 0.25 and 5 mm), strong, non­corrosive and easy to install, but limited data is available for their use in retrofitting in­plane unreinforced masonry walls.

Research started with experiments

This research was divided into three parts – experimental work, computer modelling and developing a design guide. The experimental work was further subdivided into three stages.

STAGE A-1 – OLD WALL PANELSStage A­1 of the experimental work was conducted on unreinforced masonry wall panels (see Figure 2a). These panels were constructed using recycled solid bricks obtained from old demolished buildings, with weak mortar being used to simulate decayed mortar.

The wall panels were tested either as built (without fibre­reinforced polymer retrofit) or retrofitted with glass and carbon fibre­reinforced

diagonal shear x­shaped cracks

earthquake motion

in­plane wall

BUILD 126 October/November 2011 87

polymer materials arranged in different orientations (horizontal, horizontal­vertical, inclined, vertical). The panels were loaded along one panel diagonal in an attempt to obtain a shear mode of failure (see Figure 2b).

STAGE A-2 – NEW WALL PANELSStage A­2 of the experimental work involved wall panels constructed using either new or recycled modern solid bricks. Force was applied along both panel diagonals, but not simultaneously, to more realistically simulate the cyclic nature of earthquake loading.

Results from the first two stages were then compared.

STAGE A-3 – RECTANGULAR AND T-SHAPED WALLSRectangular and T­shaped walls with or without window openings were tested in the third stage.

A wall never exists in isolation, and parts of the perpendicular walls act with the in­plane wall to constitute either a T­wall or I­wall. The rectangular walls were included for comparison with international research and to understand how their behaviour differed from that of T­shaped walls.

These walls were constructed using solid bricks recycled from old unreinforced masonry buildings. Force was applied cyclically along the wall top in the horizontal direction until a failure mode was established.

A shear mode of failure was obtained in all three stages of the experiment (see Figure 2b), except for rectangular walls. Fibre­reinforced polymer was applied on one panel or wall face only, in accordance with the current retrofit practice for historic buildings.

Computer models and design guide

Computer models were prepared for verification of the experiments and to provide information for modelling walls that were not tested in this research programme.

Finally, a design guide was developed to fill a previous information gap. It lays out basic mechanical principles for designing seismic retrofits of unreinforced masonry walls that are likely to fail in a shear mode and are to be retrofitted with fibre­reinforced polymer materials bonded to the wall surface. The guide addresses the different orientations of fibre­reinforced polymer – horizontal, inclined, vertical or combined horizontal­vertical.

Findings

Several general conclusions were drawn from this research: ❚ A large increase in shear strength was achieved with fibre­reinforced polymer. It is a viable option for seismic retrofit of reinforced masonry

in­plane walls that are likely to fail in earthquakes in a shear mode. Its use on only one face was effective, but significantly large out­of­plane displacements were observed in wall panels that failed by diagonal shear cracking.

❚ Fibre­reinforced polymer application greatly enhanced the ability of a structure to withstand large deformations, such as those experienced in earthquakes, without losing significant strength. Toughness (a parameter indicating strength and ductility of a structure) was also enhanced, especially of wall panels that failed by diagonal shear cracking.

❚ Horizontal fibre­reinforced polymer used on wall panels with weak mortar does not mitigate the sliding deformation mode of unreinforced masonry.

❚ Vertical or diagonal fibre­reinforced polymer retrofit with or without horizontal application restrains wall panels from sliding. The vertical application was also effective in increasing shear strength.

❚ Insignificant changes in stiffness (i.e. resistance to deformation) were observed with fibre­reinforced polymer.

❚ Glass fibre­reinforced polymer is more effective than its carbon fibre counterpart.

❚ Using more realistic T­shaped walls changed the failure mode to a diagonal shear mode, compared with the failure mode of rectangular walls.This research was partly funded by the Building Research Levy. For more

information, see www.retrofitsolutions.org.nz, then Retrofit Manuals.

Figure 2: Experimental sample. (a) Test set­up for panels. (b) Diagonal shear failure of a panel.