Abstract
The possibility of foundation rocking of shear wall structures designed to NZS 4203 is discussed. Theory developed by Housner for the free rocking of a rigid block is compared with experimental results from a simple structural model with a number of different foundation conditions. A simple design method for assessing maximum rocking displacements, using equivalent elastic characteristics and a response-spectra approach is proposed, and compared with results from simulated seismic excitation of the model using an electro-hydraulic shake-table. A typical design example is included.
Highlights
Post-mortems on structural response subsequent to seismic attack have revealed unexpected behaviour on some types of structures that can only be attributed to rocking of the entire structure on its foundations
If the structure in Fig. la is designed to NZS 4 2 0 3 f o r Zone A in Reinforced Concrete using Grade 380 steel, aStructural Type factor S = 1.6 and MaterialsFactorM = 1.0 are appropriate
Assuming the wall to be designed on the basis of aflexuralundercapacityfactorof
Summary
Post-mortems on structural response subsequent to seismic attack have revealed unexpected behaviour on some types of structures that can only be attributed to rocking of the entire structure on its foundations. If the structure in Fig. la is designed to NZS 4 2 0 3 f o r Zone A in Reinforced Concrete using Grade 380 steel, aStructural Type factor S = 1.6 and MaterialsFactorM = 1.0 are appropriate. 9 0 , the foundations would need to be designed for an effective S x M value of reinforced concrete: Inspection of the foundations of the apparently unstable tanks revealed clear evidence that the structures had rocked on their foundations. Design to the provisions of NZS 4203: 1976*3) c a n result in situations where rocking of part or all of the structure under seismic attack is probable. Consider the simplified structural system illustrated in Fig. la, consisting of end shear walls providing the required N-S seismic resistance, with vertical load mainly supported by flexible interior columns which do not contribute significantly to lateral resistance. Despite the comparatively squat nature of the structure, the high ratio of tributary floor mass for transverse seismic response to tributary floor mass for vertical loads can result in the base seismic overturning moment exceeding the restoring gravity moment
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