Bridge Girder Buckling during Construction in Edmonton, AB
Bridge Girder Buckling during Construction in Edmonton, AB
(OP)
This was the headline story in the Edmonton Journal this morning. Looks like inadequate lateral bracing but I guess we'll find out in time.
http://www.edmontonjournal.com/could+weeks+before+...
BA






RE: Bridge Girder Buckling during Construction in Edmonton, AB
Mike McCann, PE, SE (WA)
RE: Bridge Girder Buckling during Construction in Edmonton, AB
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Bridge Girder Buckling during Construction in Edmonton, AB
RE: Bridge Girder Buckling during Construction in Edmonton, AB
RE: Bridge Girder Buckling during Construction in Edmonton, AB
KootK, I think the abruptness is likely due to the end segments having their permanent diaphragms installed, while it looks like the center segments they were picking just have some temporary erection bracing installed. It's tough to tell, but I think the temporary bracing may have only been lean-on style -- so not providing much actual additional stability against LTB/LFB.
RE: Bridge Girder Buckling during Construction in Edmonton, AB
note the two cranes holding up the two cantilevered sections. if they went a bit too far up or even not completely vertical then you've got a rather wobbly beam and once it gets even a little out of straight, self weight will force it more and more over.
those bracings are puny.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Bridge Girder Buckling during Construction in Edmonton, AB
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Bridge Girder Buckling during Construction in Edmonton, AB
But look also at the lowest picture, furthest right beam going away from the camera. That beam does NOT look like it is "aimed" at its matching beam on the far side of the bridge by at least 6-8 inches. The beams may have been "encouraged" to connect a "bit vigorously" by bending them sideways so they'd connect.
Vertical weight does not seem to be involved. There's no rotation of the webs.
If thermal expansion - how much temperature change on a completely restrained beam - end-to-end of the whole bridge - is needed to force that large a deflection sideways?
RE: Bridge Girder Buckling during Construction in Edmonton, AB
Mike McCann, PE, SE (WA)
RE: Bridge Girder Buckling during Construction in Edmonton, AB
Maine Professional and Structural Engineer. www.fepc.us
(Just passed the 16-hour SE exam, woohoo!)
RE: Bridge Girder Buckling during Construction in Edmonton, AB
https://ca.news.yahoo.com/investigators-review-why...
RE: Bridge Girder Buckling during Construction in Edmonton, AB
Interesting thoughts. Others are wondering if the wind had anything to do with it. Apparently there were fairly strong winds prior to the buckling.
BA
RE: Bridge Girder Buckling during Construction in Edmonton, AB
You have to be very careful interpreting/reading those photos. Telephoto lenses do some really funny things in foreshortening/lengthening lengths in the line of view, which causes lateral distances or vertical distances (perpendicular to the line of view) to appear far more distorted than they actually are. Take a look at videos of highways, railroad tracks, pipelines, etc., it takes the truck hours to get here (long dist. of view and travel) and it’s climbing mountains not just interstate highway grades in getting here. Those photos are deceptive, if you aren’t careful in studying them.
The end thirds of those girders do have some real construction bracing, offering some lateral stability btwn. them as a framing system, and likely some shoring out near their splice ends, at least during erection. I’ll bet the center third of each girder was as flexible as a wet noddle, and they don’t have any decent erection bracing btwn. those portions to stiffen them laterally. I’ll bet they set the end portions which were stiffer, braced them, and felt pretty confident about how stable everything seemed. Then, they assumed they could just hang the center portions off the two end portions, on any single girder line, just like any other hanging beam. They picked those center portions off a truck, from the top flanges, and this would not have given them much hint of how flexible they were. The splices may not even have been made up completely yet, on all the assembled girders. The center portions did hang there until they started removing the shoring under the end of the girder end thirds, and this added significant compression to the top flgs. of the center portions of the whole girder, and all hell broke loose. BA’s original video does show a side view of the bridge prior to the accident, without much/any shoring under the fully assembled girder lines. The point is, the center portions will hang there just fine, but start removing the shoring and you triple or quadruple the bending stresses in the flgs. at mid span.
RE: Bridge Girder Buckling during Construction in Edmonton, AB
It does look like only the center portion has buckled. And looking at the very beginning of the video in the article that BA linked to, you can see there are 6 girders in place. Only the 3 shown in the pictures above have buckled. Maybe faulty bracing between the 3rd and 4th girder?
http://climate.weather.gc.ca/climateData/dailydata...
RE: Bridge Girder Buckling during Construction in Edmonton, AB
40 tonnes is only 500-600lbs./ft., the girders are about 135' long aren’t they, maybe not so outlandish, 1.2 cu.ft./(ft. of length). I didn’t exactly have the term ‘elastic buckling’ on the tip of my tongue, but I’ll buy that. I don’t see any sharp kinks anyplace or local bucking in the top flanges, so I was certainly thinking, not much real yielding, localized yielding, had taken place. The top flanges have nice clean curvatures in their rolled over distortions. The top flgs. of the middle portions also seem narrower than the flgs. on the end third portions, thus less stiff laterally. The webs and stiffeners look pretty good and straight, and the bottom flg. rolled right with the webs. The bottom flg. saw almost no yielding or high stresses, so they’re good, but I’ll bet they bent the hell out of parts of the splice pls., and maybe a little bit of the girders right at the splices. If the City, County, Province will give them a bit of a break, I’ll bet those center portions can be heat straightened with no loss in ultimate strength of the girders or final bridge. The top flgs. will req’r. the most work, localized yielding and residual stresses from the heating, but they’ll finally be encased in the deck haunches/conc. Furthermore, as you load the girders, in the final bridge, you essentially just stress that local region back up to its previous max. on a slightly shifted stress/strain curve (elastic portion of the curve) before it continues up the original stress/strain curve, but the conc. is there to help with that too.
These plate girders/bridges are at their most dangerous during erection. Nobody wants to spend too much money on temp. bracing, it’s only going to be there for a short time. There won’t be any strong winds during that short time period. Do we really need all that bracing, flip a coin. And, the worst of the worst is the first two girders, until you get some real solid bracing btwn. them, then the rest can be braced back to the first two while a crane hold them in place for a short time.
RE: Bridge Girder Buckling during Construction in Edmonton, AB
Would the bearings have been fixed against sliding during erection?
RE: Bridge Girder Buckling during Construction in Edmonton, AB
In this case it looks like there was a conversion from bending to torsion as some minor deviation in the straightness or loading got amplified by the self-weight of the girders. Looks exactly like the failure one gets from a simply supported vertical strip of cardboard with a bending load.
Is it uncommon to place the cross bracing before the crane releases the weight to the supporting structure?
RE: Bridge Girder Buckling during Construction in Edmonton, AB
The permanent cross-frames I design for straight plate girders are usually K braces made of at least a L4x4x3/8. If the girder spacing to height ratio becomes to low I will switch to X braces. I also specify hardwood 4x4 struts at 4' maximum spacing for the bottom flanges and 1/2" tension ties at 4' maximum spacing for the top flanges. This helps to keep everything braced when the deck is being placed. After seeing this I don't think any of it is overkill.
RE: Bridge Girder Buckling during Construction in Edmonton, AB
It's most common to have a reduced number of braces required before the crane releases the girder (enough to provide stability with a minimum lateral load during the work period), and then more braces that need to be installed before the end of the shift or before winds exceeding X mph occur.
Of course, that requires the contractor plan in advance to know how long a weather window they need to get a girder and bracing up, compare that to their weather forecast, and make the right go/no-go call.
It's very possible that pair of braces at the center were the release bracing, that they were planning to install additional bracing prior to the end of the shift, and couldn't because the wind speeds exceeded their crane's specs. Either poor planning or just unlucky.
Although that doesn't explain why the buckling didn't occur until monday morning.
RE: Bridge Girder Buckling during Construction in Edmonton, AB
RE: Bridge Girder Buckling during Construction in Edmonton, AB
RE: Bridge Girder Buckling during Construction in Edmonton, AB
RE: Bridge Girder Buckling during Construction in Edmonton, AB
"If you're not part of the solution, you're part of the problem" - John McClane,Die hard
RE: Bridge Girder Buckling during Construction in Edmonton, AB
"We shape our buildings, thereafter they shape us." -WSC
RE: Bridge Girder Buckling during Construction in Edmonton, AB
That's not what happened. From the articles posted:
"Crews were working on the girders Monday morning when some of them buckled without warning, he said.
"All of a sudden four of them bent simultaneously."
and "It was around 2:15 a.m. Monday and crews were installing the sixth of seven girders at the 102nd Avenue Bridge when the steel beams spanning across Groat Road began to bend before their eyes. "
The first article has a pretty neat before/after shot. Click on the photos tab at the top.
RE: Bridge Girder Buckling during Construction in Edmonton, AB
Is it me or in the photo before it bends when its all straight is the crane holding up the nearest spar not vertical? It wouldn't take much to induce a bit of twist in the girder and then when it relaxes a bit feeds into a bend which may not have been seen and then bang. If they were installing them one after the other and constantly moving position it wouldn't take much to get the lift non vertical and induce an ever so slight sideways force.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Bridge Girder Buckling during Construction in Edmonton, AB
I'm betting the girders will not have to be replaced.
BA
RE: Bridge Girder Buckling during Construction in Edmonton, AB
http://i.embed.ly/1/display/resize?key=1e6a1a1efdb...
BA
RE: Bridge Girder Buckling during Construction in Edmonton, AB
Those beams would have done their bending behind the crew's backs as well. I guess a watched beam still bends.
The real question - did the engineering firm do a stability analysis for each piece as it was placed to see that the constraints were sufficient or did the construction firm blow off required reinforcement as too time consuming? Or both?
In a former life it was often a question for me about the analysis of imperfect parts for stress concentrations, deviations to load paths, assembly loads, and the like. Stress analysis seemed to always be centered on nominal parts with no consideration for variations and a lot of reliance on safety factors.
RE: Bridge Girder Buckling during Construction in Edmonton, AB
Thermal loads causing the buckling are unlikely IMO. Standard practice in Alberta is that the contractor is not allowed to pour the grout pad beneath the bearings prior to all of the steel being erected and free spanning. In that case the bearings would be resting on steel pintles and the only restraint would be through friction. As noted in the news articles, the girders were fine at 2:10am and buckled by 2:15am. First guess is one girder went as the tension was being let out of the crane lines and the others followed.
The "lean on bracing" installed does not look to be erection bracing. It is likely the top and bottom chord of the permanent diaphragms. As others have noted, it is common to only install the bare minimum amount of bracing required to provide adequate bracing to the girders for supporting their own self weight during erection. Once the girders are up the rest of the permanent diaphragms are installed.
It's close to a 99% probability that there was engineering done for the erection of the girders. This was not a small contractor and regulations in Alberta are strict. If Alberta Transport is responsible for the bridge, they would not allow erection of any of the steel without sealed erection plans that include everything up to how the load is to be relieved on the shoring towers after all of the steel is erected. If it is City of Edmonton responsibility, the requirements may be a bit less.
RE: Bridge Girder Buckling during Construction in Edmonton, AB
In regards to the bearings, it is possible this bridge was designed with integral abutments, in which case the girders would have likely been bearing on elastomeric bearing pads with no longitudinal restraint.
RE: Bridge Girder Buckling during Construction in Edmonton, AB
This is all of course just an assumption at this point however it seems reasonable.
RE: Bridge Girder Buckling during Construction in Edmonton, AB
I can only see this being caused by buckling caused by temperature effects, but what we can't guess is what was done to leave the structure vulnerable.
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Bridge Girder Buckling during Construction in Edmonton, AB
On Monday, officials said a mistake made by the subcontractor hired to install the girders led to the twisted metal.
“The investigation indicated the buckling was a result of the subcontractor responsible for installing the girders misinterpreting the precise bracing details and requirements required,” Barry Belcourt with the City of Edmonton said.
At the time, there were concerns that high winds played a role in the incident – Belcourt confirmed the wind was not a factor.
Belcourt said the subcontractor, Supreme Steel, fabricated the girders and designed the plan on how to install them – but the plan wasn’t followed.