12 replies to this topic

[liftmech]

If you hadn't already heard, the northermost span of the Skagit River Bridge on I-5 between Mt Vernon and Burlington, WA, collapsed last night around 1900 PDT. Seems an oversize load may have clipped a beam and caused it to fail, which then compromised the entire span. No fatalities, luckily enough, and only three injuries.

The bridge was built in 1955 and is considered 'functionally obsolete'. The older US 99 bridge immediately to its east was replaced in the late 1990s (?) for the same reason. Having travelled through that corridor many times, I can imagine lengthy delays as traffic is re-routed across other area bridges.

http://www.komonews....lery&c=y&img=22

[Aussierob]

You'd like to think there was some structural redundancy in these things. I have a hard time believing minor contact was enough to separate a member and cause total failure.

[Andoman]

Aussierob, on 26 May 2013 - 07:05 PM, said:

You'd like to think there was some structural redundancy in these things. I have a hard time believing minor contact was enough to separate a member and cause total failure.

There was something else going on there, a buddy of mine is working on the repair, I just met up with him in Tahoe and discussed some of the plans. The minor contact should have never caused this sort of damage, but those old truss bridges are dangerous and should have been rebuilt a long time ago, especially since the maintenance on these structures is general lacking due to the complexity of the designs.

[2milehi]

You can speculate all you want and get further from understanding the truth. Let the experts do their investigation and tell the facts.

This post has been edited by 2milehi: 27 May 2013 - 08:22 PM

[SkiBachelor]

From the latest reports, the load was taller than 15 feet while the bridge at it's lowest point is 14.6 feet. It has been reported that when the load came in contact with the trusses, it caused the bridge to shift and lift off its pylons. Of course we should wait for a full report to be released.

[Andoman]

2milehi, on 27 May 2013 - 08:22 PM, said:

You can speculate all you want and get further from understanding the truth. Let the experts do their investigation and tell the facts.

I wasnt speculating, I was looking over the plans with the contractor's civil engineer doing/designing the repair work.

[2milehi]

Andoman, on 28 May 2013 - 02:16 PM, said:

I wasnt speculating, I was looking over the plans with the contractor's civil engineer doing/designing the repair work.

My bad - you had some first hand information

[Kelly]

Some more background information.
This bridge is made of a series of trusses; the term “series of trusses bridge” is shortened to a more manageable “truss bridge”. The road bed on a truss bridge can be: above, below or “through” the trusses. In this bridge you drive right next to or “through” the trusses.
Generally speaking a truss is a 3 sided structure featuring 2 sides with common force directions and a third side that opposes those forces. The image below shows the two compression components but a truss can also have 2 tension legs and one compression leg (the load direction also changes in this case). The key idea being that most truss style bridges have multiples of smaller triangle shapes that makeup the compression and tension members of the truss building block.
 Pencil-truss-for-web.jpg (88.3K)
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Here is the same concept overlaid on a bridge.
 Concepts.jpg (31.38K)
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With tinker-toys
 tinkertoy-bridge-for-web.jpg (99.85K)
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All members shown
 Members-colored.jpg (98.45K)
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Trusses by the nature of their design are quite unstable and can sometime fail or be damaged while connecting the individual members during construction if care is not taken. Cross-bracing is employed during and after construction to keep the truss stable. The truss below failed numerous times during the taking of this image (see marks on paper) because of no cross bracing to a solid structure - thus confirming this important point.
 Pencil-truss-failed-for-web.jpg (14.14K)
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Here is a top brace connecting 2 trusses adding to its stability, instead of a single beam it’s more economical to build it in the shape of a truss.
 Top-cross-brace.jpg (33.15K)
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Showing the same concepts, the load is outlined in white.
 Top-cross-brace.jpg (33.15K)
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The great thing about a truss is that is it extremely strong for its overall weight. Weight is also proportional to cost, so trusses are also an economical way to span great lengths…and here lies the possibility of many problems with this style of bridge (see below). Let’s go back in time to 1876 for this appropriate statement-

Quote

Bridge building should include design codes, design loads, allowable stresses, minimum strengths, peer design review, construction supervision, and periodic inspection after being built by engineers...Bridge Failure Committee’s suggested recommendations after a prominent 1876 iron truss bridge collapse.

The Design History
In the 1950s congress passed the Interstate Highway Act that funded an additional 41,000 miles of new highways…along with the growing car industry this boom cycle of road and bridge building occurred for the next 15 years. Also along with that boom cycle came urban sprawl which was the result of all the new roads and spiffy new cars – heck who could blame people, with all the new transportation freedom it was time to literally get out of those hot stinky towns. Paralleling the growth of all the new cars, bridges, and roads came a boom of the steel industry along with the resulting (comparative speaking) high price in steel. So we have low bid Federal bridges and the high cost of steel…tah-daa the truss bridge is the design winner. So at this point we have a bridge designed and built with Federal money and now maintained by the State of Washington.
But wait, is it safe…
This was not a new design and was not noted for any long span or was made of any exotic material. Here is an image of the Forth Bridge, a steel truss bridge (one of the first) that still stands and was built in the late 1880s. This not to say steel truss bridges ever failed, in fact for a short time period they were viewed with suspicion because of frequent failure…all attributable to what the Bridge Failure Committee had recommended in 1876 concerning engineering and inspection.
 Forth-Bridge-for-web.jpg (94.72K)
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Or maybe not safe…
As some of the readers mentioned above, the incident will be investigated by the National Transportation Safety Board (NTSB). They will review: the original engineering, any structural deviations as it exists, WDOT maintenance and operation practices (proper repair of past damaged trusses will be a key issue).
What’s all of this fracture stuff everybody is mentioning?
With any truss it’s “critical” that no component ever break or “fracture” as the truss will instantly fail – this is also known as a “fracture critical” design. A three legged milk stool also has a fracture critical design – there is no redundancy. The bridge was built in 1955 so I am assuming it used a bridge design guide from 1950. In this time period it was very common to build with this type of philosophy. Washington has over 500 fracture critical bridges.
In the early 70s structures concerning public transportation changed to a “redundant design” and the term “fracture critical” is now used to identify that design method.
The term functionally obsolete is applied to structures of this type – they “function” but the design philosophy is “obsolete”.
So were the public or State Transportation Departments ever warned of this unsafe design philosophy?
Yes – a significant failure happened in Minnesota in 2007 and all bridge owners were reminded of this issue that has existed really since those types of bridges were first designed.
When was the last time a bridge component was struck by a truck or large load?
There are at least 10,000 bridges of this type, I suspect they get hit once a day. The image below shows the same design (different State) that survived a truck strike.
 Slater-Road.jpg (93.07K)
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WDOT even has a webpage devoted to a strike on SR 529 (about 40 miles away) that happened December 1:
http://www.flickr.co...57632172072552/
 SR 529.jpg (437.88K)
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…and to a strike on August 2009 to a bridge with a very similar design.
http://www.flickr.co...ith/3878490458/
 Beebe.jpg (146.04K)
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…and yet another in 2006
 Yet-another.jpg (99.62K)
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Investigators had initial fears that the gusset structural connecting plates might be undersized however the pictures indicate they are intact. There are a number of damaged cross-connector trusses from the load impact, these can be seen in the NTSB links below.
 Gussets-all.jpg (99.34K)
Number of downloads: 58  U-5.jpg (97.67K)
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The temporary replacement structure will be a truss span.
Washington Department of Transportation publishes a vertical clearance list for all bridges. The measurements shown are 3” shorter than what is measured (WDOT fudge factor).
It also gives the clearance vs. general lane location and also mentions height distances can vary based on road slope and overhead structural configurations along with an explanatory drawing.
 WA Clearance.jpg (250.19K)
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NTSB gave a number of press briefings, they indicated the load was over 11’ wide, the load struck the bridge in more than one spot and a pilot car (with height pole) preceded the truck before each bridge. They also mentioned there was evidence of past load strikes.
NTSB: http://www.ntsb.gov/..._vernon_wa.html
WDOT bridge clearances: http://www.wsdot.wa....uals/M23-09.htm

[Kelly]

There were quite a few videos from Seattle’s KING 5 news; this one is an audio interview with a ex-trucker that witnessed the collapse in his rearview mirror. He also mentions the pilot car was in front of him and still on the bridge at the time of the failure. It seems the primary cause would be inadequate distance between the pilot car and truck.
http://www.king5.com...-208767601.html
Forgot to post this – arrows point to more damage, lucky the second span did not fall.
 More-damage.jpg (142.24K)
Number of downloads: 57

[Kelly]

The new “temporary” truss has been installed and the bridge is now open. Notice the guard rails only protect the lower truss members. This temporary lightweight truss looks to be heavier than the original span. In the far background you can see ongoing truss repair of the old bridge.
 Skagit-final.jpg (98.7K)
Number of downloads: 57

[SkiLiftsRock]

Here is an article posted today about the permanent fix for the bridge.

http://www.asce.org/...92#.Uiavez_3OVo


(This forum won't let me post the whole article here...)

[Kelly]

NTSB has finished with the preliminary report (+2000 pages) of the incident. It mentions these issues with: the pilot car, load driver and also with the State of Washington’s Transportation Department.
Discrepancy of measuring with all three entities
The pilot car’s driver was using her phone at the crossing
The pilot car’s driver seemed to be using her phone continuously throughout the route
The pilot car’s sounding pole was 1.5 feet taller than the shortest height of the bridge
Other drivers saw the sounding pole strike the bridge numerous times
The load driver strongly relies on the pilot car’s company and driver for bridge height information
The load driver was going over the speed limit
The load driver was too close to the pilot car (would not have been able to stop in time)
The load driver was in the right lane and moved to the far right to make room for a truck passing on the left
The reduced clearances (at shoulder) are published on this arched bridge but not signed at the bridge
Other Washington bridges have seen strikes in the right lane shoulder areas
The bridge had seen numerous strikes before the incident
Other bridges with arched/sloped crossbeam/roadbeds have similar strikes
A few bridge beam gusset plates had slightly reduced strengths from specified levels but these did not contribute to the structural failure

A final report with causes and recommendations is to be released at a later date-

[Andy1962]

thought you guys might want to see what happens when a dump truck driver tries to drive a dump truck across a truss bridge with the dump truck box RAISED.

http://www.huffingto..._n_5639291.html

happened Thursday July 28,2014 location: Burlington Ontario

the usual: One of Southern Ontario's busies roads is closed indefinately one direction (north bound)

a couple of additional pics here. bet we don't get an NTSB type report out of this (like the Skagit river incident) (yes I know different countries and no the Canadian government would not give a damn what happens on a provincial highway) . just fix it and move on.

Attached File(s)

•  n-BURLINGTON-SKYWAY-large570.jpg (32.88K)
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•  skyway_bridge_jpg_size_xxlarge_promo.jpg (59.39K)
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This post has been edited by Andy1962: 01 August 2014 - 02:34 PM