how is the damage explained?
It was damaged in a major fire in 1633, but in the fire of 1666 this damage acted as a fire break and prevented the flames from further damaging the bridge and crossing to the south side of the Thames.
London Bridge Is Falling Down
It was damaged in a major fire in 1633, but in the fire of 1666 this damage acted as a fire break and prevented the flames from further damaging the bridge and crossing to the south side of the Thames. 
story behind the nursery rhyme london bridge is falling down UNANSWERABLE what is this rhyme's origin? The oldest extant version could be that recalled by a correspondent to the "Gentleman's Magazine" in 1823, which he claimed to have heard from a woman who was a child in the reign of Charles II .
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London Bridge Is Falling Down [SEP] Age and damage explanation
Until the mid-eighteenth century the Old London Bridge was the only crossing on the Thames in London. It was damaged in a major fire in 1633, but in the fire of 1666 this damage acted as a fire break and prevented the flames from further damaging the bridge and crossing to the south side of the Thames. With its 19 narrow arches, it impeded river traffic and flow. Central piers were removed to create a wider navigational span. Widening and the removal of its houses was completed in 1763, but it remained relatively narrow and needed continual and expensive repairs.