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This article is the first part of a three-article series and it deals with full-scale tests of a load-carrying box girder. The two other articles present more details on smaller sub-component levels as well as cap specimens (article 2) and shear webs (article 3). This article also links to the two other articles and brings the main results from them into relevance for a wind turbine blade designer. The investigated failure modes in all three articles relate to the Brazier effect, which is expected to be the key dominating failure mechanism in future wind turbine blade designs. The Brazier effect may also have a significant impact on present wind turbine blades. In this article, a 34 m long load-carrying box girder has been tested in static flap-wise bending, and it has been demonstrated that, for this design, the Brazier effect is a critical phenomenon of great relevance for the ultimate failure strength. The box girder has been evaluated with and without a cap (wire) reinforcement. The cap reinforcement is one out of seven inventions Risø DTU published in 2008, which are all intended to result in a lighter and more reliable blade design. Copyright © 2010 John Wiley & Sons, Ltd.