How does a construction site work?

Factual Questions
41

Okay - I did a little research and found OSHA’s report on the collapse, here: https://www.osha.gov/doc/engineering/2008_12_19.html

It turns out that the structural design of the actual bridge was found to be ‘‘generally in compliance with industry standards’’. And while the expected deflections were greater than desirable, this was considered a potential serviceability concern as opposed to a safety issue.

The walkway collapsed during construction due to failure of the temporary shoring, which was a result of several things going wrong, including:

  1. Under-design of the temp shoring beams.

  2. Designing the the temp shoring beams for high-strength steel but not bothering to tell anyone in manufacturing/construction. (The temp shoring would still have been under-designed, but the use of regular strength steel made it that much worse.)

  3. Using 3 beams that were delivered significantly smaller than called for in the temp shoring design - W10x12 instead of W10x19.

  4. Not constructing the temp shoring according to design by increasing the distance between shoring towers, inadequate soil anchor embedment and lack of lateral bracing via diagonal cables between anchors.

In short, I didn’t find any mention that the minimal difference between the weight of wet conctrete and cured concrete contributed to the collapse in any way.

42

I was posting from memory of something that happened in '08. Any inaccuracies are my fault. I should know by now to checks my facts first. I do know that keeping the forest floor as undisturbed as possible was one of the main goals of the design. The skywalk is in a pocket of uncut old growth forest in Midtown Atlanta.

43

Dingdingding!

44

I’m not sure what your response here is supposed to mean, other than you seem to be disagreeing with what I’ve said.

If you think that the concrete being wet was somehow a factor in the collapse, please cite supporting info that:

  1. Wet concrete is significantly heavier than cured concrete - such that an otherwise properly designed and constructed shoring system that neglected only to account for this difference would be likely to fail.

and

  1. OSHA, or anyone else for that matter, deemed the calculated load to be deficient. My understanding from their report is that they took issue not with the calculated load but the required size of beams needed to support it. That is, an accurately calculated load with under-sized beams (my understanding) vs. properly sized beams for an inaccurately calculated load (what I am disputing).

Since the entirety of your response concerns the under-design of the shoring beams, to the exclusion of all the other factors I summarized from the OSHA report, it leads me to suspect that you haven’t given sufficient thought to what actually contributed to the collapse. I remain unconvinced that wet vs. cured concrete had anything to do with this failure.

Given the real world work experience you’ve described in this thread, it would be great if you can shed some light on the two points I’ve outlined above. But your short response to my posts so far is not helpful or instructive.

45

It’s a neat facility to be sure. I’m glad I had this opportunity to look into it as I had never heard of it before. If I’m ever in Atlanta I’d like to check it out.

46

It’s about a mile down the Beltline* on the other side of Piedmont Park* from my place. If your ever around PM me before hand and we can go together. Park Tavern is right there for food and drinks.

*Two other great Atlanta attractions.

47

As I understood the articles I read…
When you thread a rod, you cut into the surface and the thread outside diameter matches the rod, the inside diameter of the thread is somewhat deeper into the rod, a smaller diameter.

The design, as I understood, called for about 10 or 12 feet of smooth rod, a threaded section to hold the first walkway, and then 10 more feet smooth and the end threaded to hold the next walkway. How would you cut threads on this? Put a 20-foot rod on a lathe? Do they make clamp-on thread dies?

The only way around this is to have a section of the rod, that you want to thread, bulge out. (Ie. an intermediate section of say, 1/4 inch bigger diameter?) I’m not an engineer or steelworker, but it seems to me a long rod with a 6-inch stretch of larger diameter for threading in the middle is a pretty specialized (i.e. expensive) piece of material; and the design did not show this.

Logically - the engineer or architect drew fantasy, and the installer did the best he could. He just wasn’t an engineer and did not do it right.

48

I doesn’t matter if the engineer drew flying monkeys holding up the structure.
It is never acceptable for the design to be changed without review by the engineering staff.
Never.
Ever.

“Did the best he could” = people die.

49

Will do. :slight_smile:

I’d like to add the following to my previous post:

After the incident the shoring designer revised their calculated maximum load by decreasing it, as opposed to increasing it. They claimed that an allowable reduction in the estimated construction live load would result in a maximum total load less than what they had specified in the original design.

No mention is made of any increase in the dead load due to wet concrete vs. cured concrete, or anything else.

As I noted previously, OSHA found no problem with originally calculated load and they took no issue with the revised lower load either. They simply noted that even with the reduced load claimed by the shoring designer, the beams were still under-sized.

I really don’t think that the weight of wet concrete had anything to do with the collapse.

50

Sorry, yes, I agree. The guy who made the modifications either (a) did not follow procedure or (b) the procedure was not in place.

MY objection was to “The original design was fine. The problem was not the original design.” If the design was fine, there would most likely have been no changes to the design. The trigger to the whole event was a design that was fantasy. The actions of the guy modifying the hanger design, and substitutes to box beam specs, contributed.

Plus, the problem should have been obvious the moment the steelworker boss looked at the design; or if he lacked spatial imagination and Blueprint 101, as soon as the rods were delivered. There was plenty of time to ask the engineer(s).

There is still the risk from the improperly-constructed box beams, but since the top of the second hanger failed first, that was obviously less of a risk than the 2-hanger design.

IMHO the majority of spectacular failures start with poor design (As exampled by Nava’s post about all the utilities being run in the same space) . Quality Control issues often just compound the problem. working around design flaws is an exercise in itself. If the GC or the owner have made change procedures paperwork onerous and time-consuming (the proverbial 20 sign-offs for every change, no mattr how trivial) then installers will be temped to gloss over any changes rather than wait for approval; and their judgement of trivial vs. important is not the same perspective as a structural engineer’s.

51

In one of the Maryland suburbs of DC, the county and the transit agency have been trying to build a big subway-bus transit center for years and years. It’s finally almost done, and it’s become riddled with major cracks arising from structural defects. Accusations are flying among the ocunty, the designers, and the contractor. A long and tortured history lies behind it.

Someone else was supposed to handle the paragraphing, but he’s running late due to delays on another thread.

52

[quote=“Mister_Rik, post:40, topic:665849”]

…every single person I knew who worked in construction (at the labor level) was unemployed half the year./QUOTE]Of course, some people consider that a feature, not a bug. I knew one construction worker who would work half the year (they worked on projects that often ended up with lots of overtime at critical points, so at union scale, they racked up a decent salary over half a year), and take the winter to travel and take care of family.

53

Yes, this is correct. As previous posters said, it was inadequate shoring. Once concrete cures, it is, itself, a structural member. It helps hold itself up. While it’s wet, it needs shoring to hold it up, because it’s contributing nothing to the structure until it cures.

54

Back to the question in the OP, projects like this are a measure of controlled (to a degree from not to reasonably) chaos.

I worked for a while as a vendor which would supply equipment to subcontractors.

In our industry, there would be a specific consultant, who would hired the design the system and then work to manage the subcontractor.

Sometimes the consultant would work under the general contractor and other times they would be hired directly by the future tenant of the building or the part of the building.

One problem is that the buildings are necessarily being designed several years out, but technology is changing rapidly. As new products are announced, the clients want the latest and greatest, which cause a cascade of changes and the changes don’t get communicated everywhere.

And then vendors will make promises which should never, ever be made, including expected ship dates for new products in addition to issues which are out of their hands, such as shortages of parts.

The scope of the projects would be in constant flux as various parties on the client side would be involved. Inevitably, the clients would have the wrong department in charge of a project and getting spec clarified is a real life telephone game where one person whispers what she heard to the next and so on.

A lot of the expertise does come from the vendors. However, the architects, consultant and contractors have the responsibility to substantiate the claims.

One project I was involved with had an accident that could have been a disaster has things been a little worse and if the ball room had been occupied.

They had 30 some-odd projectors and sound equipment on a platform suspended from the ceiling and which would be raised and lowered. Anyone can guess the likely outcome, which did, in fact. Occur. Fortunately, a safety cable prevented the platform from crashing completely to the floor when the hoist failed.

I want to say that the right sized was spec’ed and the wrong one purchased and installed, but it may have been the wrong one spec’ed in. The hotel was not opened yet, so it didn’t make the news and the people who knew were very tight lipped.

We made money because they had to replace our equipment as well. Total cost for everything was in the several hundred thousand dollar which was relatively minor compared to what it could have been.

On a personal level, knowing what could happen, I hired a second architect / inspector when I had a house build in Tokyo. That was fortunately because he discovered that a main supporting pillar had been cut by the carpenter because the plans were incorrect and it hadn’t been caught.

55

Not at all. My previous post on the matter spoke to this exact subject: concrete forms need to be adequately supported during a pour. I was in agreement with you.

56

I’m claiming liquidated damages!

57

It’s my brother-in-law; what can you do?

58

Ah - I had thought your reply to me was implying that the under-design of the shoring was due to ignoring the ‘‘extra’’ weight from the concrete being wet. Apologies for my misunderstanding.

59

Can’t tell you how many times I’ve seen forms blow out because somebody forgot a few snap ties. This, of course, was much more serious.

60

Or several rows of whalers that end in a vertical line.
Or not enough T shores.
When they were building I210 near JPL they had a huge bridge collapse due to not enough shoring under the forms. IIRC about 12 workmen were killed.
I was working in the neighborhood and saw and heard the collapse.