Pittsburg Convention Center failure 5

[JAE]

This article on ENR is about a failed section of floor at the Pittsburg DLL Convention Center. I note the following statement from the article:

[blue]The structural-steel slip connection of a girder at the X-9 expansion joint that runs in the short direction of the David L. Lawrence Convention Center in Pittsburgh is a detail that at least some structural engineers don’t favor, a source told ENR.[/blue]

http://www.enr.com/news/buildings/archives/070222.asp

I guess I’m surprised that anyone would design a thermal slip connection in framing where the slip occurs via bolts sliding in horizontal slots. What they describe happened was that the bolts eventually gouged out the hole and reduced the ability of the connection to slip.

I’ve for years never used this detail (not that I’m all that smart) but because I was taught that you use either double support beam/column arrangements or Teflon seats (which apparently is the resulting repair).

Is this expansion joint via slotted hole detail used that much? I’m just surprised it was used on a project of this magnitude.

 

[UcfSE]

JAE, would you mind elaborating on the alternatives you use?

 

[JAE]

Well, as I said, I have tended in recent years to use double columns with separate beams to create an expansion joint. At this facility they apparently just slotted the holes on one side.

I've also used a seat (similar to what the article suggests was the repair concept) where the beam sits atop a steel seat with a teflon pad connected to the beam which rests upon another pad (either teflon or stainless) which is bonded to a steel backer plate. The whole teflon arrangement usually has some slotted holes to allow bolts to restrict any ultimate sliding-off that might occur. The bolts are vertical and connect the seat to the bottom flanges of the beam, but only loosely torqued to still allow the slip. Many times the lower pad is bonded to a neoprene or other elastomeric pad to allow slight rotation at the beam end without gouging the teflon.

Here's a couple of links:

http://www.aaatech.com/TRISLIDE Brochure 03.pdf

http://www.sepbearings.com/fluorogold.pdf

 

[vincentpa]

I have a pdf showing animation of the cause. I can't post it though. I don't know where to uplink it to. Any ideas?

 

[dik]

Thanks JAE for the excellent links... I've used the latter often if a teflon slider is required; also used elastomeric pads often depending on the movement anticipated.

Dik

 

[DaveAtkins]

Wow--without the benefit of hindsight, I don't see anything wrong with the connection detailed. Why did the bolts gouge down into the outstanding legs of the connection angles? Were the angle legs too thin, causing a bearing failure?

DaveAtkins

 

[sundale]

Excellent post (as usual) JAE. This is kinda scary because the failed connection really does not "look" that bad to me. I promptly looked at some expansion joint details that I had drawn on a project a year or so ago (I did it OK, whew!)

I usually do the double framing out of a simplistic and lazy desire to not detail a connection able to accomodate thermal movement. This is by far the best way to go for the engineer and contractor. I have seen both seated connections with a teflon bearing or elostmeric pads and I have also seen horizontal long slotted holes in the various firms I have been with.

Another thing that I did not consider obvious until recently is that a teflon slide bearing is NOT that frictionless. The coefficient of static friction for a teflon bearing is on the order of 0.15 to 0.18 or so. These things have low kinetic friction, but it has to first move to mobilize it. On the cryongenic vessel I was supporting, the static frictional force imposed a larger shear at the slide bearing than the seismic shear imposed at the restrained end. This was a big surprise to me at the time.

 

[jt12]

So would've the connection been ok if threads were excluded from the slots? Someone will probably be writing a paper on this in the future. It sounds like in modifying the failed girder a few years back, they constrained these slots on this girder from fully being able to slide as well. The slotted connection used is perhaps a bit questionable for heavily gravity loaded members, but I would likely still be willing to use something like it for lightly gravity loaded beams that I need horizontal slop on.

 

[JEmH]

jt12,
I thought that they only "fixed" the single failed girder connection back in 05. I'm not sure if the threads were included in the slots or not. However, I wouldn't think that it would have mattered. In my messing around with old bridges, I find that every steel on steel movable connection is frozen. I think that the biggest problem with this connection in any use is the hassle of inspecting it. During construction, the temptation the torque down the bolts is overwhelming.

vincentpa, could you post your pdf to mytempdir.com?

 

[JAE]

JemH - your point about steel-on-steel movable connections freezing up is what I was originally thinking about when I questioned this type of connection in an expansion joint setting.

The original article stated that "some engineers" question its use and I was wondering how many engineers out there actually use it. I was taught that it was a poor connection design.

 

[Galambos]

a good number of stadiums around the country use this connection. i think i remember seeing this connection at heinz field in pittsburgh just across the river.

 

[jt12]

JemH:

The article wasn't real clear, but I read that by fixing that adjacent girder, they may have limited this girder from sliding, - but then the article went on to blame the gauging for preventing the sliding too. I'm sure we'll hear more research on it with time. The slide connection pictured isn't great, and definitely isn't appropriate for nasty bridge environments, but I would still be willing to use it in certain circumstances.

 

[vincentpa]

This is the link to a portion of the report with a little animation.

http://www.mytempdir.com/1233866

 

[JAE]

vincentpa - I couldn't get through to your link.

 

[JLNJ]

I was able to view the link. It's a 21 page PDF with a very good visual on what happened. It is fortunate the damages weren't more serious.

I'm afraid that the inner workings of a "simple" end connection are not all that "simple" and we need to be careful when we piggy-back a new twist on to a tried and true detail.

 

[gtaw]

I followed the link. It worked without any problems. It is a great link!



Best regards - Al

 

[JAE]

OK - I got through (my own thick head)...

The connection is what I thought it was.

I absolutely don't think that simple slotted holes can serve as an adequate expansion joint in a beam to beam connection.

The "fix" that is shown on the last slide is what I think ought to have been there from the start.

 

[hokie66]

I agree completely with JAE on all counts. Would never use this type connection, especially for a major member like this beam. Just to add another argument against, this type connection is very noisy. May not be an issue in a convention center, but spent a night in a hotel once detecting where the "rifle shots" were coming from. Found slip of a similar joint was occurring in fits and starts, and each release was accompanied by a noise which disturbed hotel guests in the vicinity of the joint. Much better to have double line of support, but if architect can't be convinced, carefully designed guided sliding joints will work in many cases. Resist using halving joints, as that restricts access if maintenance is required.

 

[SteelMover]

I agree with the above statements that the connection design was poor. Tolerances in a connection like that would never guarantee that you have more than a few bolts bearing until you actually start gouging surfaces, and by then you've already created sticking points in the connection. My heart would have been in my throat trying to justify design loads on those 2L's.

A hardened bolt sliding on soft steel will gaul the surfaces, and in bridges that will lead to rust and seize the connection. For me, I do stadium steel work quite a bit and I get the luxury of using large linear journal bearings or mechanical hinged linkages at bearing seats to allow constant quite movement. Though our travel demand is often (+/-) 12" or more.

 

[doct9960]

Updates from ENR...

http://enr.ecnext.com/free-scripts/comsite2.pl?page=enr_document&article=nebuar070305