New pedestrian bridge collapses in Miami...

M1911

Moderator
NES Member
Rating - 100%
26   0   0
Joined
Apr 1, 2005
Messages
39,252
Likes
7,152
Location
Near Framingham
Structural engineering


Piles of rocks have significant advantages?
I know you’re being somewhat facetious, but I can’t resist a serious answer.

Stone bridges have lots of disadvantages as well. First, the spans are small and the bridge in question required a large span. Second, they require an enormous amount of labor to construct, along with form work across the span during construction. Third, they are very susceptible to collapse in an earthquake (unreinforced masonry has basically zero tensile strength).
 

MisterHappy

NES Member
Rating - 100%
15   0   0
Joined
Jul 28, 2009
Messages
20,401
Likes
5,130
Location
On the 16 yard line, shootin' for the Lewis!
Structural engineering

I know you’re being somewhat facetious, but I can’t resist a serious answer.

Stone bridges have lots of disadvantages as well. First, the spans are small and the bridge in question required a large span. Second, they require an enormous amount of labor to construct, along with form work across the span during construction. Third, they are very susceptible to collapse in an earthquake (unreinforced masonry has basically zero tensile strength).
Oh, no, I completely understand.

Years ago, I read, "Anyone can build a bridge to support a given load. The trick is to build a bridge that will just barely support a given load."

That's where engineering comes in. It's a balancing act between many goals and constraints.

Still, it reminds me of an instance some time ago, when a village in France (IIRC), that was built on opposite sides of a river valley. 21 bridges connected the two sides. Then there were big floods, and only the Roman bridge was left. They sent out engineers, to figure out why. Probably, because it was a big ol' pile of rocks! [laugh]
 

M1911

Moderator
NES Member
Rating - 100%
26   0   0
Joined
Apr 1, 2005
Messages
39,252
Likes
7,152
Location
Near Framingham
Oh, no, I completely understand.

Years ago, I read, "Anyone can build a bridge to support a given load. The trick is to build a bridge that will just barely support a given load."

That's where engineering comes in. It's a balancing act between many goals and constraints.
I wouldn’t put it that way. When building a highway or pedestrian bridge, the design loads are pretty much specified by design standards, as are factors of safety. You don’t design to “just barely support the load”. The design load is typically a worst possible case. The factor safety is such that you design the structure to handle significantly more (often several times more) than the design load.

The interesting part is more about designing a structure that is inexpensive to build and maintain, while also being durable and attractive.
 

mibro

NES Member
Rating - 100%
13   0   0
Joined
Mar 4, 2013
Messages
12,022
Likes
8,627
Location
Mass.
Still, it reminds me of an instance some time ago, when a village in France (IIRC), that was built on opposite sides of a river valley. 21 bridges connected the two sides. Then there were big floods, and only the Roman bridge was left. They sent out engineers, to figure out why. Probably, because it was a big ol' pile of rocks! [laugh]
Just as you say.

Europe is dotted with 2,000 year old Roman bridges.

A stone bridge with small Roman arches built with Roman concrete will be highly resistant to side loads. Modern bridges are not designed to survive extreme side loads.
 

M1911

Moderator
NES Member
Rating - 100%
26   0   0
Joined
Apr 1, 2005
Messages
39,252
Likes
7,152
Location
Near Framingham
Just as you say.

Europe is dotted with 2,000 year old Roman bridges.

A stone bridge with small Roman arches built with Roman concrete will be highly resistant to side loads. Modern bridges are not built to survice extreme side loads.
All it takes is an earthquake and an unreinforced masonry structure will collapse like a house of cards.
 

mibro

NES Member
Rating - 100%
13   0   0
Joined
Mar 4, 2013
Messages
12,022
Likes
8,627
Location
Mass.
All it takes is an earthquake and an unreinforced masonry structure will collapse like a house of cards.
Maybe so but the Romans designed earthquake-resistant masonry buildings and structures believe it or not.
 

mibro

NES Member
Rating - 100%
13   0   0
Joined
Mar 4, 2013
Messages
12,022
Likes
8,627
Location
Mass.
Jesus H Christ. How could any Civil Engineer see that cracking and not realize that the structure was in immediate peril.
I just watched the video. I agree 100%. Those cracks are at the most critical location in the entire truss. How can you build something like this without having someone on site with sufficient experience to know what those cracks meant?
 

M1911

Moderator
NES Member
Rating - 100%
26   0   0
Joined
Apr 1, 2005
Messages
39,252
Likes
7,152
Location
Near Framingham
Why the designer chose a non-redundant truss structure built out of post-tensioned concrete with multiple cold joints is yet another issue.
 

calsdad

NES Member
Rating - 100%
28   0   0
Joined
Apr 24, 2006
Messages
37,501
Likes
12,183
Location
Chelmsford MA
I just watched the video. I agree 100%. Those cracks are at the most critical location in the entire truss. How can you build something like this without having someone on site with sufficient experience to know what those cracks meant?

Because diversity and inclusion?
 

calsdad

NES Member
Rating - 100%
28   0   0
Joined
Apr 24, 2006
Messages
37,501
Likes
12,183
Location
Chelmsford MA
Thanks for posting that.

The title of the video should be : "Tales of Pervasive Incompetence".

Building a structure like that out of concrete just seems like a really stupid idea to me. I'm no structural engineer - but I have worked with a decent amount of concrete - and the idea that you can have a stressed concrete structure like that with cold joints between the different components - and expect it to all stay together - just seems like abject stupidity.
 

calsdad

NES Member
Rating - 100%
28   0   0
Joined
Apr 24, 2006
Messages
37,501
Likes
12,183
Location
Chelmsford MA
You have no cause to assert that.

I wasn't asserting anything:


ques·tion mark
/ˈkwesCHən ˌmärk/
noun
noun: question mark; plural noun: question marks; noun: questionmark; plural noun: questionmarks
  1. a punctuation mark (?) indicating a question.
    • used to express doubt or uncertainty about something.
      "there's a question mark over his future"


One of the angles that came out about this story early on was that the bridge was designed and managed by a number of female engineers. I haven't read enough of the back story to know if that was true - or not.
 

namedpipes

NES Member
Rating - 100%
3   0   0
Joined
May 7, 2008
Messages
31,357
Likes
13,596
Location
PREM
Jesus H Christ. How could any Civil Engineer see that cracking and not realize that the structure was in immediate peril.
Being psychologically allergic to Boston subways, I have walked across Boston hundreds, perhaps thousands of times. I've seen sights I wish I could unsee.

Somewhere I have some pictures of that fancy bridge they put up next to North Station, taken from underneath. The concrete was cracking and chipping away and you could see countless past repairs.

I don't refuse to drive across that bridge but I do avoid it like the plague.

Another interesting item, an ambitious person could relatively easily ascend from the ground to wide open hatches that lead into control rooms of some sort occupying the, I guess you could call it the "hull" of the roadway. It would be a trick but not at all impossible. Nothing about that bridge screams "robust" or "disaster proof".
 

Keeper

NES Member
Rating - 100%
23   0   0
Joined
Jan 10, 2013
Messages
724
Likes
173
Location
North of Boston
Maybe so but the Romans designed earthquake-resistant masonry buildings and structures believe it or not.
IIRC:
The Romans did, including iron pins in the Colosseum. Part of the Colosseum did collapse in an earthquake over 1000 years ago, but it had already been standing for nearly that long. Also, many of those pins were stolen ages ago, yet the structure stands...for now. Earthquakes continue to take their toll.
 
Top Bottom