I would suggest you not to do what you intend to do. Because the supporting structure is wood, wood is not a good support structure when the top is glass. Usually glass they use metal structure. More solid and sturdy, plus lasting compared to wood. If you really need to, better support the cross section of the wooden beams first, then only cross. And do not step in the middle. Step where the cross wooden beams are at. If its not you who installed it, we dont know if the glass is tempered or not. Dont risk it.