A special study was made to evaluate the seismic response of the ribs of four reinforced concrete arch aircraft hangars and to assess methods of strengthening the ribs. The study was based on a series of non linear dynamic analyses of the arch ribs, with particular consideration being given to the choice of earthquake records, to the effect of P-delta actions on the arch responses and to the validity of the behaviour predicted by the computer program. The analyses indicated that the arch ribs could become unstable as a result of the P-delta effect acting in combination with reduction in the rib stiffness due to concrete cracking. However, the study also shows that the arches can be stablised very economically by using prestressing strand to provide cross bracing.