The effect of diagonal bracing on the earthquake performance of a steel frame building

Diagonal bracing, intended to increase the lateral strength of two steel frame buildings at the University of Concepcion, was broken by the Chile earthquake of May 21, 1960. The unbraced structures then resisted the earthquake of May 22 with no further structural damage, thus raising the question whether the bracing was either necessary or beneficial. To examine this question, a digital computer study was made of the strengthening effect of the diagonal bracing when the buildings were subjected to ground motions recorded at three different earthquakes. The results indicate that for some earthquakes, the bracing may be beneficial. However, for other cases, the bracing induces increased forces in the frame which exceed the strengthening effect of the bracing. Thus, the braced structure actually may be less earthquake resistant than if it were unbraced.