The four main horizontal beams are indeed steel I beams. They are quite heavy as I am sure you can imagine, but worth the efforts. They combined with the other steel materials conveys a sturdy yet lofty space. An end result that always looks like a sound investment, certainly one that will surely be around for a very long time to come. We appreciate your question,comments and any reviews you may have, never hesitate, we are always happy to help. Many thanks, LJDH