I was appalled when I saw the use of strops to hold down an 18-ton vehicle (x5). Back in my SP Howitzer days we even used chains to tie down our 155mm guns to railway cars, let alone an 18 ton vehicle in an aircraft.
An airplane is not a railway car. First, it is made largely of aluminum, not steel. Second, there is a limit to the amount of flex allowed. So, chains are NOT allowed for securing cargo to the airplane. Chains ARE allowed to secure cargo to a pallet in addition to nets and straps; but a combination of the pallet restraints built into the airplane and additional straps as required must secure the pallet and/or cargo to the airplane deck.
The Weight & Balance Manual for the airplane will dictate the number, angles, and distribution of the straps and the number of individual tiedown points that must be used; based on the size, weight, and position of the cargo.