sandbox open-air fireplace

I wonder how heavy that sand will be, particularly when it gets wet. Should you call in a sructural engineer?

As a penthouse owner, I believe I have had this discussion previously with architects, and I seem to remember that fireboxes/firepits will not pass Manhattan building code regardless of how far they are installed from the building's wall, unlike gas grills which are allowed if they have a dedicated gas line. I suggest your architect talk to their expediter, and have the expediter talk to the city for code requirements before you consider installing this. In addition, your building's BoD may not be thrilled to have an open firebox/firepit on their terraces or roof, and this may be immediately disallowed under your house rules - so you'll also need to take a look at the offering plan, house rules, and amendments, and get clearance from the BoD of your building as well.