For a flatbed semi-trailer carrying heavy steel pipes that are not blocked by a front header board, how is the minimum number of tie-downs calculated?

1. One tie-down strap for every 20 feet of cargo length
2. One tie-down for cargo 5 feet or less; two tie-downs for cargo 5 to 10 feet; and one additional tie-down for every 10 feet or fraction thereof
3. A flat requirement of 4 straps regardless of pipe length or weight
4. Chains must be placed every 2 feet along the entire length of the trailer bed