The bit of metal fell from the lorry as it was heading west on the M1 motorway at Stockman’s Lane in south Belfast on Thursday at about 7pm or 7.15pm.
Police said that a “number of vehicles travelling on the motorway are believed to have sustained damage as a result of striking this piece of metal”.
They added: “Did you witness this incident, or are you the lorry driver?
“You may have information which could help us with our enquiries.
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“If you have any information please call police on the non-emergency number 101, quoting reference 1092 18/05/17.”