Performance of burnt clay hollow block slab under seismic action in Uganda

Abstract

A storeyed building has different structural elements each working to complement the other in resisting seismic actions. Since most of the building mass is present at floor levels, earthquake induced inertia forces primarily develop at the floor levels due to ground vibrations from which they travel horizontally through the slabs and beams to columns and wa11s, and then to the foundations where they are dispersed to the ground. Ninety five percent (95%) of storeyed buildings in Kampala the capital city of Uganda have their slabs constructed with burnt clay ho11ow block reinforced concrete slabs. Slabs containing hollow blocks are constructed with the hollow side of blocks end to end running in one direction, with a rib between rows of blocks in which steel reinforcement is laid. This constrains the slab to act as one-way spanning. For a structure subjected to seismic loading, the requirement for a diaphragm is that the slab should have equal strength in both horizontal directions. This is because horizontal seismic action is described by two orthogonal components considered as independent and represented by the same response spectrum. According to EN 1998, (Euro Code 8), one of the guiding principles governing conceptual design against seismic hazard is the bi-directional resistance and stiffness at storey level. Seismic load is applied to the slab as a compressive load in the plane of the slab. This study determined the load capacity of hollow blocks parallel and perpendicular to the boles in the plane of the slab. Hollow block slabs measuring 750 x 800mm were produced. These were cured for 28days and loaded parallel and perpendicular to the direction of hollow blocks and ribs. Also, the hollow blocks were loaded parallel and perpendicular to the direction of holes. It was found that the load capacity of burnt clay hollow blocks when loaded parallel to holes varied from I 03.35kN - l 23.93kN giving a compressive stress of 2.3 /mm2 - 2.8N/mm2 , whereas the load capacity, perpendicular to the direction of holes ranged from 45.6kN - 47.4kN giving a compressive strength of I .ON/mm2 - l.05N/rnm2• For the slab, the failure load varied from 3 I 5kN - 375kN when loaded parallel to the rib, giving a stress of l .97N/mm2 - 2.34N/mm2 and the failure stress perpendicular to ribs varied from l.04N/nun2 - l.2N/mm2.