Recycled Pedestrian Bridge 1

[phamENG]

I'm analyzing a maintenance catwalk (designed and fabricated by never installed) for use as a small pedestrian bridge. Span will be about 30' to cross a drainage ditch between the parking lot and industrial facility. I've run the analysis and it doesn't quite work considering the 90psf load from the AASHTO pedestrian bridge specs. This thing is only 3 feet wide - no passing. It's also not in an area that would be subject to crowd loading. 90psf seems excessive to me in this instance. If you assume 250# workers walking in a single file line on the bridge with 3ft per person, you have 250#/9ft2 = 27.778psf. I wouldn't drop it this low, but given the relative predictability of the usage here what are the general thoughts about reducing to 40psf?

Vibrations is another issue. In this case, I'm inclined to give a warning that it is likely to vibrate - either spend the money to fabricate a new structure or put up with vibrations. The girders are W12x14's. They are thoroughly braced and strength is no issue, but it's flexible. A quick rundown of the DG11 checks for approximate natural frequency and acceleration puts my minimum Ix at about 1110in4 to be within the suggested limits for an outdoor pedestrian bridge.

Thanks.

 

[BAretired]

I agree that 90 psf is high for such a narrow bridge. Maybe 60 psf would be a reasonable compromise (no particularly good reason for selecting that number).

Alternatively, instead of specifying a design load, perhaps it would make sense to check it out and report to your client a safe live load. Let them decide whether or not it is okay.

BA

 

[cdill21]

My general thought would be to use the load required by the local code. Absolutely nothing is stopping people from gathering on this bridge, however unlikely. ASCE 7 would recommend MINIMUM 60psf LIVE LOAD (Table 4-1).

EDIT: BAretired. I didn't see your recommendation until after I posted. Nice work.

Chris

 

[Ron247]

I would not drop that low (40 psf). I may consider 60 psf at the least. I am going to challenge some of your statements.
[ul]
[li]You can pass at 3' wide. People do it every day on stairs.[/li]
[li]Over the years, you can get a crowd on the bridge. Since it is a drainage ditch, let something get trapped in the water or need rescuing (a puppy) and see how many people line up on the rail to look down the channel. Let a heavy elderly person fall and see how many people line up to help, watch or wait to get past. It will be a line and it may be 2 people wide.[/li]
[li]People stop and talk without regard for others getting past them. It happens on pedestrian bridges every day.[/li]
[/ul]

Edit: I posted my 60 psf min without seeing the other responses with the same load. I had the same thought.

 

[phamENG]

Thanks for your opinions. I'll re-run the analysis at 60psf - it should work well enough.

 

[phamENG]

Thanks, Ron. To answer your challenges:

- I'll grant they can pass, but one of the purposes of this bridge is provide two routes (two way traffic) so people don't need to pass.
- When I say ditch, I mean more of a deep swale. About 3' from underside of structure to the invert of the ditch. Being trapped in the water is unlikely. The presence of a puppy is also very unlikely. In an emergency, someone would just wade in. This will not be used by the elderly. It leads from a parking lot to a heavy industrial facility. There may be a few "old timers" in their mid to late 60s, but these are 65 year olds that can bench press a small bus...
- A worthwhile consideration on a public use bridge, but I don't think it applies as much on this. Chance meetings don't occur on this bridge - you know the person because you're working with them and have had the last 12 hours to talk to them. You also know everyone else on site, and I haven't met anyone too shy to ask people to get out of the way...

I understand your concerns and I would be agreeing with you if this were in a public park or otherwise used by a larger and more diverse population. In this case, though, the remoteness and restricted access to the site mitigates many of the concerns I would usually have with long term variability and unforeseen conditions.

 

[SlideRuleEra]

I've run the analysis and it doesn't quite work considering the 90psf load...

What value did you get?
Agree with BAretired, instead of arbitrarily picking a loading value, suggest starting with the results of your analysis and working backwards to see what loading value you can justify... with, as suggested by others, 60 PSF being an absolute minimum for industrial use.

http://www.SlideRuleEra.net

 

[cdill21]

PhamENG, I am glad you decided to use 60psf as your baseline, but it is important to recognize WHY. Please do not confuse unlikely with unreasonable. Yes, it is unlikely that a group of people will gather on this bridge, but it is not unreasonable.

It WOULD be unreasonable to assume that a bunch of full grown silverback gorillas will form themselves into a pyramid 30' wide and cross this bridge . (Please note that there are not current accepted LRFD specifications available for gorillas, you will need to use ASD)

Chris

 

[phamENG]

SRE - I was getting live load deflections around L/290. Strength utilization around 0.8, but I wasn't comfortable with the deflection. Reducing to 60psf will certainly fix the problem.

I always forget about those last 3 lines in the table, alone on their own page with the footnotes...

 

[phamENG]

Silverback gorillas, no, but I bet I could wrangle some black bears for a pretty impressive picture...

 

[dhengr]

PhamENG:
But, keep in mind…, given its location btwn. the plant bldgs. and the parking lot, and the fact that the whole second shift clocks out at the same time, there does tend to be a one directional crowd at intervals. This tends to cause unison walking to keep from stepping on the heels of the guy ahead. And, while 60lbs./sq.ft. seems reasonable, the single direction of traffic could tend to overload one side of the bridge a bit.

 

[cdill21]

For the record, I would insist upon calling your local code officer and finding out what standards you need to follow. You don't want to have this thing installed, only to find out later that you were required to use the 90 psf load from AASHTO.

 

[SlideRuleEra]

phamENG - IMHO, the deflection criteria can be relaxed for a short (30') single span, but that is certainly your call. You have determined that 30 PSF is a reasonable "typical use" loading. Don't be too concerned that if the walkway is "packed" as shown in cdill21's photo, the walkway will (safely) deflect L/290, or so. I could argue that (safely) deflecting under extreme loading is a good thing... tends to keep users from "packing" even more people on the walkway.

On another matter, anytime an older item is reused, keep in mind just how old it really is. In this case, I'll bet you analyzed the beams as A992 steel, which is probably is... unless the walkway is over, say, 20 years old which makes A36 steel more likely.

http://www.SlideRuleEra.net

 

[phamENG]

dhengr - fortunately the facility doesn't do a mass changeover. Each employee has to do an individual turnover within a general timeframe, which results in a sporadic release of workers.

cdill21- thanks. I'm familiar with those images from the AASHTO pedestrian bridge spec commentary.

SRE - thanks for your thoughts on deflection. That's where I started, but felt determining a maximum load that would meet minimum performance was better than justifying the performance of a particular load - if that makes sense. That seems to be the general consensus, even if I started by throwing out a load that was entirely too low. And you do bring up a good point about age. This is A992 - it was fabricated last year but either didn't fit or a better solution was found (the fabricator couldn't remember which).

 

[SlideRuleEra]

...felt determining a maximum load that would meet minimum performance was better...

One more suggestion, then I'll be quiet. If the only concern is excessive deflection (L/290 at 90 PSF), and, say, L/360 is acceptable then:

You can quickly calculate that load of 72.5 PSF gives a deflection of L/360... say 70 PSF. The reason I'm being stubborn is to get the load rating as high as safely practical. Industrial structures have a hard service life... but at the same time, don't want to arbitrarily put artificially low restrictions on there use.

At one of our plants, if 60 PSF loading had been proposed only because it was a (minimum) value in a code and the structure would easily work at that loading... I would be (temporarily) speechless.

http://www.SlideRuleEra.net

 

[JLNJ]

If something were to happen and I were the attorney for the harmed party, I'd ask why you checked it with such a tiny uniform load when the code calls for 80/100 psf for corridors.

"I guessed at the number of people and how much they might weigh..." is not going to play too well to a jury.

 

[XR250]

Seems like a bridge that narrow and long would potentially have some stability issues from unbalanced side to side loading.

 

[Ron247]

phamEng: While I feel 60 psf is sufficient, it will depend on what a code dictates. Choose a reasonable code and check it for that. While AASHTO is a code, is it the one you should use in this situation or is there a reasonable code that requires less than AASHTO?

As far as using site/business specific data to design to, that can be something to steer clear of. It is way too hard to determine everything the future holds.
[li]When they need to paint the 2 bridges. I bet they paint one first and steer all traffic to the 2nd one. So, for a few days, you have bi-directional traffic on your bridge.[/li]
[li]If they do not paint at all and one bridge eventually collapses from poor maintenance, when they investigate they will use the shotgun approach. In checking the design, they determine a more heavily built bridge would have collapsed 7 months later from poor maintenance but would not have on that particular day.[/li]
[li]They sell the building to someone who does clock in and out in shifts.[/li]
[li]They change their own method of shift change.[/li]
[li]They sell the building to a Santa Claus training academy. Now you have old fat guys that move slow across the bridge.[/li]

If the drainage ditch is so narrow under all amounts of rain, why not shorten the bridge a little?

Curious of what everyone thinks of posting the bridge for like no more than 5 people as an example. Do you think that would increase or decrease liability?

 

[JoelTXCive]

PhamEng - What are the weather conditions for this bridge? Do you have to worry about picking up a snow or ice load in addition to your live load?

 

[BAretired]

It's an existing bridge; you didn't design it. You simply report to the owners and the AHJ how much load it can carry in the absence of further remedial action. You do not hide the fact that it does not meet AASHTO requirements for loading. You do it all in writing and if the powers that be say "leave it as is", you leave it as is. Otherwise, you take steps to beef it up to meet the code.

Simple, and you avoid liability.

BA