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Comment by JohnHaugeland | Hacker News Reader

When you go to Pittsburgh for the first time, you will learn that the city has 450 bridges because they're necessary, and you can't just go eliminating a third of them

They can, in fact, maintain the bridges. You might not know this from a brief look at a newspaper article.

What happened is that the bus was too heavy for the bridge. It's PennDOT's fault.

Look up the weight of a bus. Look up the average weight of an American. Look up PennDOT rider numbers. Do the math. They were three tons over on a 26 ton limit.

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"Three‐axle 60‐ft articulated buses are the next most common transit bus in service [in the USA], comprising about 10% of the fleet. The curb weights for these buses currently range between approximately 38,000 and 50,000 pounds, and fully‐loaded weights range from approximately 56,000 to 65,000 pounds." (page iii)[1]

Fully loaded weights for 3-axle US buses from the linked report's executive summary are 28 to 32.5 tons, empty range from 19 to 25 tons. So a not-fully loaded 3-axle bus was very likely over the 26 ton limit.

Three axle buses should not have been on that bridge. Would be interesting to see if that route was changed recently from 2-axle to 3-axle service.

[1] https://onlinepubs.trb.org/onlinepubs/tcrp/docs/TCRPJ-11Task...

From the Post-Gazette article there was a driver and two passengers on the bus, so it might not have been over limit this time but sounds like it would have been at other times on that route.

https://www.post-gazette.com/local/city/2022/01/28/pittsburg...

I used to walk under that bridge quite a bit, hopefully no one was under it when it collapsed (it doesn't sound like they suspect there was, but I guess it could potentially be hard to tell).

oh wow, i hadn't even noticed that it was a three axle

i've been making this argument on skepticism that a two axle bus should have been on that bridge. the standard patransit two axle fleet bus, a cummins isl (which is more than half their fleet) is 2 tons shy of max load when empty, and average american weight breaks load at bus half full.

that's the 61b. when i lived in pittsburgh it was often packed to the gills. i have a hard time imagining that that has changed.

you're right. if it was a 3 axle, that's very significant. hell, an empty three axle (i think they use man sg 220 gawr, which is ~30 tons curb weight) would likely be too much. i'm glad you pointed this out; thank you.

> i have a hard time imagining that that has changed.

Fun trivia, https://www.portauthority.org/system-map/ shows stats on this. It doesn't support deep linking, but I went to the outbound stop just after the bridge, and it says

In FY2021:

Average Weekday Ons: 0.32

Average Weekday Offs: 5.1

Pre-pandemic ridership for CY2019:

Average Weekday Ons: 1.00

Average Weekday Offs: 37.00

It has dropped by quite a bit since the pandemic. The 61A has a daily ridership of 1,643 and the 61B has 1,312.

those are indeed much lower numbers than i had expected

that said, the other person pointing out that it's a three axle bus makes me relatively confident that the fleet contains no domain-relevant busses that had any business going over that bridge in the first place

it's worth noting that when i was growing up, patransit tried to send heavy construction trucks across the larimer bridge. one of the crew members stopped them at the last minute when they saw the bridge and called it in (this is way before pocket internet,) and good thing - basically this would have happened, and in that case, it's a probably five or so story drop onto an active highway. would have been a catastrophe.

What was the limit when the bridge was new?

The limit has been 26 tons for almost 20 years now.

I don't know what the limit was originally, but PATransit should be able to accomodate bridge limits within two decades.

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