Denny's Sign Failure 8

[dik]

https://www.ctvnews.ca/world/toweri...gh-wind-killing-1-injuring-2-others-1.6239328

Was it removed before an investigation... seems a little hasty.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[spsalso]

The plate showing on the bottom of the fallen sign is the underside-of-the-sign plate.

This pictures shows the top-of-the-post plate still in place:

I do wonder if someone decided to weld the two plates together. And, in doing so, weakened the metal around the bolt holes.



I know next to nothing about wind loads on a tall sign. But. By scrounging ballpark numbers, I came up with 5000 pounds at 100mph wind. I also did the same for two 3/4" bolts, and got 40,000 pounds. 40,000 is a lot bigger than 5000.

Seismic? I doubt anyone bothered when this sign went up. A lot matters on the weight of the sign, way up there. I doubt it weighs/weighed much--500-1000 pounds max(?).



spsalso

 

[IFRs]

The link in the original post said the sign weight was 1.3 tons.

 

[FacEngrPE]

The pix above the plate on the bottom of the sign seems to still be attached.
The plate from the top of the post appears to have been torn loose from the post. There seem to be a combination of bolt failure, weld fatigue and a portion that is torn plate - likely occurred last.
A second look - the plate welded to the sign is missing all 4 corner bolts. The paint remaining is deceptive, but may indicate the plate at the top of the post has an opening the diameter of the monopoly. Loose bolts are more likely to fatigue from alternating loads than tight bolts, this is a location where I would be surprised if bolt tightness is ever checked.

It is interesting to see what the ASCE 7 2010 wind design requirement is. Tool Used is [URL unfurl="true"]https://asce7hazardtool.online/[/url].
[ul]
[li]Risk Catagory 1, ASCE 7 2010[/li]
[li]Wind Speed 105 Vmph[/li]
[li]10-year MRI76 Vmph[/li]
[li]25-year MRI84 Vmph[/li]
[li]50-year MRI90 Vmph[/li]
[li]100-year MRI96 Vmph[/li]
[/ul]Value provided is 3-second gust wind speeds at 33 ft above ground for Exposure C Category,

Question - What ASCE 7 risk category seems (is) appropriate for a sign in a parking lot when the sign is say 100 ft above the parking lot? Does the ASCE wind value include a factor for the dynamics of buffeting (the wind and the sign interacting with each other).

 

[spsalso]

"A second look - the plate welded to the sign is missing all 4 corner bolts."

No, it's not. As I mentioned earlier, you can see two remaining bolts in the picture I posted above. You can also see them in this one:

The above still was captured from the video below:

https://www.youtube.com/watch?v=bTRJ1_qJ6SQ

spsalso

 

[MintJulep]

If the mast's top plate remains in place, as shown by spsalso's picture, then what accounts for the circular outline on the bottom of the sign's plate?

 

[spsalso]

I'm not seeing a circular outline. I see rust encroaching from the outer edges, moving towards the center in a pretty randomy way.

My guess is the plate was painted white when it was new. And what ain't white is rust.



spsalso

 

[spsalso]

Ya know what would be fun? Fly a photo-drone up to the top of the big tower and get a really high quality picture of what it looks like--things like the condition of the bolt holes and extent of rust and such.


spsalso

 

[IRstuff]

spalso, you can clearly portions of a circular edge in both your and my photos, particularly the upper right quadrant showing an obvious circular boundary , while the other 3 quadrants show a circular outline in the rust. It seems obvious that the white paint degraded from the outer edges. The sign itself sits on a metal column that's presumably welded to the square plate. The weight of the sign would manifest itself through the metal column, so that protects the white paint more than the remaining portions of the flanges.

It could be my imagination, but I'm seeing almost 3/4 of an obvious circle, only the lower left quadrant is a bit random

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[IFRs]

Those bolts look pretty small compared to the plate thickness and too few to develop the strength of the plate or for any redundancy. Unless they were tightened fully, I can see wind buffetting eventualy loosening them, allowing water in, then corrosion, then failure.

 

[spsalso]

Yeah, I see the curves now.

But is it our "imagination", or is it real? There'd have to be a physical "thing" to cause it--maybe some half-assed gasket material?



spsalso

 

[spsalso]

The bolts DO look awfully small and sparse. But note that the two remaining bolts don't appear to have failed, as in snapped. It looks like they pulled through. One of them doesn't even look bent.


spsalso

 

[TugboatEng]

The circle is where it was welded to the column. Welding causes distortion that bends the plate towards the weld. This leaves inside the circle high and outside low. The contact was inside the circle. Outside had low contact pressure which allowed for corrosion.

 

[MintJulep]

I think Tug has explained the circle.

 

[TugboatEng]

Lots of experience with poorly constructed equipment in highly corrosive environments.

 

[IFRs]

Yes but - doesn't one picture show a plate remaining on top of the column?

 

[Alistair_Heaton]

There won't be just a ring of poor pen weld with fatigue faces from slag

 

[Brian Malone]

Pretty surprising that something so massive would be mounted using four relatively small bolts. By being elevated high on the pole makes regular inspection impossible. If the design has been used nationwide, Dennys has major liability hiding at every restaurant that has an elevated sign. I am going to make sure I stay away from these signs!

 

[spsalso]

"The circle is where it was welded to the column."

But it wasn't welded to the column. It was bolted. To the other steel plate that probably WAS welded to the column.

There are two square steel plates. One is at the bottom of the sign. The other is at the top of the pole. The sign is placed on top of the pole with a crane, and the bolts are inserted and tightened.

The plate that "was welded to the column" is still on the top of the pole.

The one we are examining is attached to the remainder of the sign by passing a square section tube through a matching square section hole. THAT was most likely welded, but it wouldn't make the described circular marking out on the edge.



The "artifact" might be the result of welding done on the top of the plate. There might be additional pieces between the square steel plate and the yellow part of the sign, to spread the load. I would be very wary of using only the (nominal) 6" square tube inserted through the hole in the (nominal) 2" thick plate to transfer loads.

In the picture below, you can see a sort of spacer between the plate and the yellow part of the sign. But it DOES appear to be square, not round. Additionally, you can see one of the bolts still in place:

spsalso

 

[Alistair_Heaton]

I don;t think there is anything left on the pole.

The circular butt weld from pole to steel plate has gone.

And more than likely the white paint your seeing was welded through when they put the plate onto the top of the pole. Quite likely welded in the pissing rain.

 

[TugboatEng]

I'm not suggesting the weld that caused this artifact is on the contact faces even on this plate. The lower plate is welded to the circular column. The contact pressure at the faces of the two plates is the same. Therefore, the effects caused by warping of the lower plate during welding could translate into the bottom face of the upper flange.