The Interstate 35W bridge over the Mississippi River in Minneapolis, Minnesota that collapsed yesterday evening was an arched truss bridge; you can see photos and details here. Let’s discuss a little terminology, first, then look at the bridge survey performed in 2001:
The road surface – well, the load or traffic area, to be technical – of a bridge is called the “deck”. On a bridge like this, the part of the box truss that supports the deck can be, obviously enough, called the “deck truss”, though it’s not entirely obvious. The other portions are referred to as the “main truss” and the “floor truss”. Put in layman’s terms, the “main trusses” were the top parts of the span, the floor trusses the bottom of the span, and the deck trusses the parts spanning the main trusses and supporting the deck.
Now, according to the survey (big PDF), the bridge had problems:
Although fatigue cracking has not occurred in the deck truss, it has many poor fatigue details on the main truss and floor truss systems.
But they weren’t too bad:
The detailed fatigue assessment in this report shows that fatigue cracking of the deck truss is not likely. Therefore, replacement of this bridge, and the associated very high cost, may be deferred.
Oh, well, no worries, then. But let’s look at that a little closer:
Although fatigue cracking has not occurred in the deck truss, it has many poor fatigue details on the main truss and the floor truss system.
Wait, the main and floor trusses are in bad shape, but the deck truss is okay, so nothing needs to be done, right? Well, yeah. That’s what it says. However, it also mentions this:
Concern about fatigue cracking in the deck truss is heightened by a lack of redundancy in the main truss system. Only two planes of the main trusses support eight lanes of traffic. The truss is determinate and the joints are theoretically pinned. Therefore, if one member were severed by a fatigue crack, the plane of the main truss would, theoretically collapse.
However, it is possible that collapse may not occur if this happened. Loads may be redistributed and joints may resist rotation and develop bending moments. If the fractured main truss deflected significantly the slab could prevent the complete collapse through catenary action.
Okay – the main trusses had little if any redundancy, and were in poor shape, and the failure of one would – theoretically – lead to collapse. However, joints could resist rotation and “develop bending moments” (not a good thing). As a last wave of defense against catastrophic failure, the strength of the deck slab could have mitigated the deflection of a fractured main truss. Sounds good… except nobody considered what would happen if the deck was compromised by repairs while serving as not only a structural member, but the only thing holding the bridge up.
Part of the bridge appears to have experienced a severe “bending moment”, consistent with the predicted results of a main truss failure. Food for though, no?
Reportedly, the bridge was last inspected in 2004, and no significant problems were discovered. Is it possible, I wonder, that a main truss failed, a result of the problems identified in 2001, between 2004 and now? Is it possible that the deck slab did, indeed, maintain the structural integrity of the bridge? Is it possible, then, that this led to the cracking and other damage that was being repaired this summer? And is it possible that those repairs weakened the deck to the point that it was no longer able to resist the deflection of the main truss, with catastrophic results? I think so. Whether this proves to be the case, only time will tell.
In closing, I’ll leave you with this quote, from the report:
…the bridge could most likely tolerate the loss of a floor truss without collapse, whereas the failure of one of the two main trusses would be more critical.
Update 2 Aug 1340 CDT: Video footage of the collapse suggests the failure began on the south end of the bridge, possibly with the failure of the floor trusses at the support column there. From there, the bridge fails in two more places as the main and deck trusses are unable to handle stresses they were never designed for, ultimately separating cleanly near the middle of the main span; the north end of the span gives way moments later, though it’s largely clouded from view by the splash from the first section to hit the river.