Emotional accounts from relatives of the victims and survivors of the partial collapse of a boat dock gangway on Georgia’s Sapelo Island and a newly released video of the frantic rescue efforts paint a dramatic picture of Saturday’s incident.

Before the gangway collapsed, officials said there were upwards of 40 people standing on it. But the Georgia Department of Natural Resources told Channel 2 Action News that its engineering firm calculated that it should have been able to support the weight of 320 people. Here is how the engineers reached the calculation.