You can easily find information on the web concerning the supposedly miraculous spiral staircase at the Loretto Chapel in New Mexico via a quick Google search. Alternatively, start with the Wiki Encyclopaedia account (Wikipedia: Loretto Chapel (https://en.wikipedia.org/wiki/Loretto_Chapel)). This site has links to other sites, such as that of the Sceptical Inquirer, that debunk the claims that there is anything miraculous either in the provenance or construction of the staircase. The basic argument is that the carpenter is most likely known (and is not St. Joseph) and that the artefact is indeed physically possible and doesn’t require a continuous miracle to hold it in place.

In support of the latter claim,

1. Firstly, the inner edge of the screw effectively acts as a supporting pole, and
2. Secondly, lateral movement is prevented by an innocuous-looking bracket that links the staircase to an adjoining pillar.