Is a professional engineer required to certify this design?

My guess is that a P.E. would *not* be required, since failure of the dolly is not a danger to the general public. The insurance company for the people making the dolly and using it *may* want it tested, or certified to meet some particular standards. Some places would require a 'calculation' with signatures from at least two of 'preparer', 'reviewer', and approver. The approver would most likely, but not necessarily be, a P.E. Alternatively, there might be a drawing or set of drawings to describe the dolly and the metals and welds to make it. The drawings might have similar signatures. Trade people may feel that this regulation is not necessary, or that simple testing after fabrication is adequate. Engineers may be more likely to do what *they* have been trained to do.