SEISMIC RETROFIT OF MASONRY WALLS USING TIMBER STRONG-BACKS

REFERENCE

Develop and validate a simple and cost-effective seismic securing technique for URM solid- and cavity-walls that consider not only costs and performances but also the impact on the building tenants, aesthetics and heritage building fabric.
Full-scale shake-table testing of two cavity and three double-leaf solid clay brick URM walls was undertaken. The vertical timber framing that is typically considered to be a non-structural support for the inner wall lining was used as part of the retrofit solution and was fixed to the wall using mechanical screw-ties in order to form a strong-back.

The critical failure mode for as-built URM walls was one-way bending with crack formation at three quarter-height enabling the wall to act as two separate rocking bodies. In the cavity-walls, bending of the original cavity-ties and subsequent differential movements between masonry leaves was observed.
Standard 90 x 45 mm timber framing can be used as strong-backs, and do not require a specialist construction contractor to install.
Mechanical screw ties provided adequate wall-to-roof diaphragm connection during dynamic loading.
The most effective mitigation strong-backs from wall base to parapet top, which allowed flexural behaviour with a significant reduction in displacement and an increased PGA of three times the as-built condition for both cavity- and solid-walls.
Analytical formulations to predict the response of solid URM walls retrofitted using timber strong-backs and mechanical connections were provided.