Earthquake-Resistant Construction with Porotherm Clay Block

Wienerberger, manufacturer of Porotherm clay blocks, was founded in 1819 and is today the largest producer of clay products. They are also the leading global innovator in the development of structural clay components with the 'fabric first' approach being a trendsetting world leader in the building industry.

In order to meet higher seismic requirements, a new block was developed by Wienerberger to be used in very high seismic zones in Europe: the patented Porotherm-S block. This block uses extra conventional mortar distributed through internal pockets within the blocks, thereby minimising thermal leakage through the joints. It is well understood that mortar joints become cold bridges, decreasing the thermal properties, so Wienerberger developed a particular clay mixture to retain thermal conductivity as well as maintaining high mechanical strength. Cyclic shear tests have been performed on this Unreinforced Masonry System (URM — no steel reinforcing), where vertical compression was kept at a level of 0.60 N/mm² in order to simulate the load of three upper storeys. The results from these tests showed that the structure still hadn't reached its Ultimate Limit State (ULS) at a Peak Ground Acceleration (PGA) of .75g. By comparison, New Zealand's PGA zones range from 0.13g in the upper North Island to Wellington at .45g, and our severest at Westport being 0.50g. The performance level of Porotherm clay blocks can well be appreciated for construction of New Zealand homes using a reinforced Confined Masonry Construction system.

The basis of the design is Confined Masonry Construction (CMC) in accordance with the Eurocodes*. The three forms of masonry construction are Unreinforced Masonry Construction (UMC) — no longer used here, Fully Reinforced Masonry (FRM) — all our concrete masonry design here, and Confined Masonry Construction (CMC) — the masonry elements are confined within.

Each standard block has 60% porosity. In addition to the standard blocks, the system has blocks which contain a 175mm diameter void into which reinforcing bars are placed and solid poured to form reinforced concrete columns, with an effective cross-sectional dimension of 150 x 150mm to link with top and bottom reinforced concrete ring beams. These columns are positioned either side of openings exceeding 1.5m² in area at all corners and structural wall junctions at 5.0m centres max.

The design philosophy for Stellaria's clay block projects to date is a system consisting of aerated fired clay blocks, core range designation R25Th+, nominally 500mm long x 250mm thick x 249mm high, stretcher bonded using a 1mm thin-bed mortar joint proprietary system in accordance with DIN 1053-1, Z17.1-628, EN 998-2.

Average compressive strength = 9.0 N/mm². Base value (unfactored or allowable compressive strength) = 1.5 N/mm². Density = 6.8 kN/m² (including voids).

As with all masonry design, the structure relies heavily on diaphragm restraint at ceiling level being achieved with either construction ply in the lower seismic zones or pre-stressed concrete in the higher seismic areas.