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What exactly are "modern design standards" required by bridges? When did the modern era start? Is there really a major moment in bridge design where the stand

by dockd 9y ago

What exactly are "modern design standards" required by bridges? When did the modern era start? Is there really a major moment in bridge design where the standards changed? Is there even broad agreement on what these standards are? (Side note: the bridge pictured in the article is the Bridge of the Gods, just upstream of Bonneville Dam on the Columbia River. There's going to be more than bridge problems if the flow of the river takes out that bridge.)

wiredfool 9y ago
Yes. There are standards, they're the Building Codes. There are several versions of them that have come out over time, and they've been evolving with the advances in the science and engineering. When I was in Engineering school, we were generally taught off of a newer code than was in common use in the wild, but that transition was occurring in the couple of years that I was actually doing bridges. In the case of bridges, you're going to get the basic Steel or Concrete code, maybe some federal guidelines if it's an interstate, definitely state DOT standards. As for modern vs not -- There are definitely classes of bridges that I'd not consider modern. Anything with rivets, or sections that are built up by lots of little pieces (like the old style riveted trusses). Those aren't done anymore for several reasons: onsite labor is more expensive than shop labor, rivets are inconsistent and weak compared to bolts or welds, and the ongoing painting to control corrosion is a nightmare. In places like the West Coast, the older bridges are not built to current seismic standards, which have been evolving as quickly as the rest of the codes (e.g. tiedowns, base isolation, column jacketing). A lot of the older bridges also have details that aren't terribly good for fatigue performance. There was definitely an inflection point, probably in the 80's where precast prestressed concrete started being the go-to design for more than just a plain old girder bridge over the interstate.
WatchDog 9y ago
As a layman, the difference I notice between "modern" and older bridges is, less exposed steel and more concrete.
- kevin_thibedeau 9y ago
  > more concrete That will be heavily spalled 50 years from now. Almost as dumb as building thin flexible skyscrapers with reinforced concrete.
  - ggcdn 9y ago
    I fail to see what's dumb about using concrete for skyscrapers. Tall buildings have been built this way for over 50 years without issue.
    - kevin_thibedeau 9y ago
      It's fine for skyscrapers. The Empire State Bldg isn't going anywhere. It's dumb for thin flexible skyscrapers that will accumulate microfractures in a relatively short time span and whose steel structure is insufficient to support the building in the face of significant concrete degradation.
      - ggcdn 9y ago
        Microfractures =/= global strength loss. Additionally, the concrete itself tends to strengthen over time. I guess I just don't understand what failure mode you are worried about. There are thousands of slender concrete structures that have been in service (thus far) without issue.
- treerock 9y ago
  The Forth crossing (north of Edinburgh) seems to show this progression quite nicely. https://commons.wikimedia.org/wiki/File:Forth_Replacement_Crossing_Artist%27s_impression.jpg https://commons.wikimedia.org/wiki/File:Forth_Replacement_Cr... Three bridges built in 1890, 1964 and 2017 respectively (from right to left in the picture above).
abduhl 9y ago
AASHTO publishes the standard specification for bridges and other road structures. It can be purchased at https://bookstore.transportation.org/item_details.aspx?ID=2211 https://bookstore.transportation.org/item_details.aspx?ID=22... "The AASHTO LRFD Bridge Design Specifications are intended for use in the design, evaluation, and rehabilitation of bridges, and are mandated by the Federal Highway Administration for use on all bridges using federal funding." It is over 2000 pages long without errata. It is updated yearly with major updates less frequently.

by dockd 9y ago

wiredfool 9y ago

Yes. There are standards, they're the Building Codes. There are several versions of them that have come out over time, and they've been evolving with the advances in the science and engineering. When I was in Engineering school, we were generally taught off of a newer code than was in common use in the wild, but that transition was occurring in the couple of years that I was actually doing bridges. In the case of bridges, you're going to get the basic Steel or Concrete code, maybe some federal guidelines if it's an interstate, definitely state DOT standards. As for modern vs not -- There are definitely classes of bridges that I'd not consider modern. Anything with rivets, or sections that are built up by lots of little pieces (like the old style riveted trusses). Those aren't done anymore for several reasons: onsite labor is more expensive than shop labor, rivets are inconsistent and weak compared to bolts or welds, and the ongoing painting to control corrosion is a nightmare. In places like the West Coast, the older bridges are not built to current seismic standards, which have been evolving as quickly as the rest of the codes (e.g. tiedowns, base isolation, column jacketing). A lot of the older bridges also have details that aren't terribly good for fatigue performance. There was definitely an inflection point, probably in the 80's where precast prestressed concrete started being the go-to design for more than just a plain old girder bridge over the interstate.

WatchDog 9y ago

As a layman, the difference I notice between "modern" and older bridges is, less exposed steel and more concrete.

kevin_thibedeau 9y ago
> more concrete That will be heavily spalled 50 years from now. Almost as dumb as building thin flexible skyscrapers with reinforced concrete.
- ggcdn 9y ago
  I fail to see what's dumb about using concrete for skyscrapers. Tall buildings have been built this way for over 50 years without issue.
  - kevin_thibedeau 9y ago
    It's fine for skyscrapers. The Empire State Bldg isn't going anywhere. It's dumb for thin flexible skyscrapers that will accumulate microfractures in a relatively short time span and whose steel structure is insufficient to support the building in the face of significant concrete degradation.
    - ggcdn 9y ago
      Microfractures =/= global strength loss. Additionally, the concrete itself tends to strengthen over time. I guess I just don't understand what failure mode you are worried about. There are thousands of slender concrete structures that have been in service (thus far) without issue.
treerock 9y ago
The Forth crossing (north of Edinburgh) seems to show this progression quite nicely. https://commons.wikimedia.org/wiki/File:Forth_Replacement_Crossing_Artist%27s_impression.jpg https://commons.wikimedia.org/wiki/File:Forth_Replacement_Cr... Three bridges built in 1890, 1964 and 2017 respectively (from right to left in the picture above).

abduhl 9y ago

AASHTO publishes the standard specification for bridges and other road structures. It can be purchased at https://bookstore.transportation.org/item_details.aspx?ID=2211 https://bookstore.transportation.org/item_details.aspx?ID=22... "The AASHTO LRFD Bridge Design Specifications are intended for use in the design, evaluation, and rehabilitation of bridges, and are mandated by the Federal Highway Administration for use on all bridges using federal funding." It is over 2000 pages long without errata. It is updated yearly with major updates less frequently.