It is tension between two land anchorpoints, transmitted across the top span and hanging down that provides support?
It seems to me you would NOT have to fracture the sturdy verticle girdered supports to bring the bridge down, no:
You would blast at the anchorpoint of one of the two span/land anchorpoints, and like a laundry clothesline snapping at one point, the ghastly rest would gradually, tragically, play out.
I know this cuz the SF Golden Gate Bridge had a similar vulnerability that went unseen for a time, and was later corrected, under high secrecy. That's why the Feds took to guarding those two points so zealously when the T's said they were going to bring it down.
Suspension Bridges are huge and marvelous but they are exquisitely VULNERABLE.
I was in SF in 02 and have pictures of our soldiers guarding the bridge- and posing with overjoyed Chinese tourists. :) The Chinese women were so giddy to have their picture taken with our boys. I thanked the guys for their service. :)
The main cable anchorages of the Mackinac bridge are a long way from either shore. They are huge concrete blocks setting on the bottom. The saboteurs would have to cross several approach spans to get to the anchorage. The anchorages on the Golden Gate bridge are at the waters edge. It would be a major job to place enough explosive to part the main suspension cables of the Mackinac bridge.