Our question is what size and alloy aluminum tube can be used to replace steel tube in a vertical load application. Our current spec is 1.5” OD STD steel tubing 7 feet long. We will use 6 poles attached to steel footings to support vertical stacking of containers that each weigh 1500 lbs. the bottom container will see 4500 lbs. from vertical stacking. the containers are 45” wide and 87” long. There is one pole in each corner and 2 in the center on each side. Six tubes of steel 1.5” x 7’ steel is to heavy. Can we use aluminum tube round hex or square and what alloy ?