Boardwalk Live loads

[struct_eeyore]

Hello all,

I cannot find anything in ASCE 7 on live loads for a boardwalks. What I have is a narrow, 5' wide boardwalk that runs through some swamp - the whole thing sits about 1' above the ground. This is not a pedestrian bridge, just a nature trail. I have at the moment designed it for 60psf, which really seems like an overkill. Can anyone suggest a source to verify my load? (Would like to reduce it ideally) Thanks!

 

[Ron247]

I know it may not make sense but I would use no less than 70 psf and maybe as high as 100 psf. Here is why. Restaurant, Bars and Churches are designed for 100 psf. It is not that the entire area is packed with people, but a lot of people can congregate in a tight area. That area may be the middle of a structural section, the end of it and later move to another structural section.

Point is, let the rare "double-faced people looker" come out in the open for 20 minutes and that structural area may be packed with people for 10 minutes. You only have to exceed collapse load for 3 seconds or less. Remember when all the people were on the Golden Gate bridge?

 

[dauwerda]

It sounds like you chose 60 psf based on it being a walkway/elevated platform per table 4-1 in ASCE 7. I think this is the correct (edit, minimum) value to use unless this is for some private residence not open to the public. I have seen plenty of tours or field trips on "just nature trails" where you have a large group of people closely packed together which makes 60 psf sound pretty reasonable to me.

 

[StrucPEng]

If you use any live loads from ASCE 7 as a guide I would say 60 psf is light. Given that any situations where there is a corridor, walkway, aisle, assembly, exit, or any other manner of publicly accessible commercial walk 80 psf is the minimum. With the exception of maintenance catwalks. I personally would use the 100 psf live load for design but the situation might be up to your judgement given you know the project and the usage.

 

[Ceinostuv]

Hey Structee,

I've noticed that you've made quite a few posts regarding these kinds of pseudo-coastal structures. Someday I'd like to hear what you've learned

I've only done a few of these, so there may be someone more experienced who can offer an 'out' to the following - from my research and conversations with some of our local jurisdictions, I would say you should design for either 60 psf (per IBC, designed as an elevated platform) or 90 psf (per AASHTO, as a pedestrian bridge). I would personally use AASHTO loads based on the 'boardwalk' verbiage, unless I could justify it's use as an observation platform or something...

Judgement-In-Training

 

[SlideRuleEra]

...boardwalk that runs through some swamp...

Totally agree with the others, 100 PSF. Live load rating is for a "brand new" boardwalk.
Outdoors, in a swamp, probably little if any maintenance, it won't be long (even with treated lumber and galvanized fasteners) before live load rating is <100 PSF... and continuing to drop as time passes.

http://www.SlideRuleEra.net

 

[Ron247]

it won't be long (even with treated lumber and galvanized fasteners) before live load rating is <100 PSF

Good point, I totally did not think of longevity. Making a 10% to 20% error in "design judgement" is not the end of the world. But designing for 60 psf when it should be 80 psf would be 25% before you then add longevity.

 

[struct_eeyore]

Point taken - I will, secretly, up my loading to 80. I'm afraid 100 might get me laughed out of town.

@Ceinostuv - FEMA P55 has been my go-to whenever coastal comes up. In addition, I've had to deal with the "Construction Guidelines in FL for Minor Piling-Supported Structures Constructed over Submerged Aquatic Vegetation" - ACOE (this one caused some specific issues for the boardwalk in fact). Lastly, drift has been an issue. Had to do some forensics on a job that, by all standards, was designed correctly - steel portal at core. However, recurrent, non-design storm events, caused drift oscillations that resulted in flashing separation- whole building began to rot - 2 mil to restore, lawyers probably made as much - engineer was never charged in the wrong - my takeaway was that drift limits should be much tighter.

 

[JStephen]

https://www.youtube.com/watch?v=_2Y9ruxqmOY

 

[SlideRuleEra]

Thank you, Ron247. I've lived and worked a career in coastal South Carolina (plenty of work in swamps and salt marsh).
Wooden walkaways, porches, docks, etc. collapse on a regular basis here. Recent example (with video of the aftermath) of a local dock failure that dumped 20 people in a tidal creek just two weeks ago.
Two years ago, I got to experience a partial collapse myself - an ocean-front elevated deck suddenly dropped about 6" before wedging itself into place when the wedding party gathered for a photo.

structee - Eighty (80) PSF is probably ok for fresh water (assumed since you did not mention salt or brackish tidal water level variation). Suggest oversizing supporting posts, and having them pressure treated for fresh water use (0.80 lb/ft³, CCA, or equal).

http://www.SlideRuleEra.net

 

[Canuck65]

Any public walkways we have designed have been 100 psf, including other engineering firms in our area. Typically in public areas where crowds of people can gather for fireworks displays, boat shows, etc... Done a few elevated platforms in marshy areas, and also designed them for 100 psf. I would be relaxed with the deflection criteria at 100 psf for a boardwalk, but not with the strength criteria. I really do not think the overall construction cost will increase that much between 80psf and 100 psf. There was a set of public finger docks in my area that were designed for 50 psf and started to fail during a boat show with a crowd of people on them.

 

[Ron247]

Anytime I get complaints from Owners about heavy live loads in situations like this, I tell them to get out their high school year books and look at the Beta Club, Glee Club etc all packed on a Spiral Staircase or a Balcony for their school photo. Then they understand.

 

[Settingsun]

Australian Standard AS 2156 is for walking trails if you're interested in a directly-relevant but foreign standard. Loading is given according to a classification system.

Classes 1 & 2 = 5 kPa (100psf). Frequent encounters with others and moderate/high level of facilities (a popular and developed track essentially).

Classes 3 & 4 = 4 kPa for viewing areas and 3 kPa for the access ways. Occasional encounters with others, infrequent facilities, moderate fitness but no experience in bushwalking necessary.

Class 5 = 3 kPa viewing area and 2 kPa access ways. Somewhat remote, you won't come across many other people, should have bushwalking and navigating experience.

No reduction for large supported area in any case.

Think carefully before applying Class 5 in USA 48 states. Australia is the same size but <10% population so we have a lot more cases where this would apply.

 

[MIStructE_IRE]

100psf (5kPa) for me! Gut feeling and old requirement of a now superceded code I still refer to!

 

[Agent666]

To add to the aussie numbers posted above, in NZ we also have a similar standard for outdoor structures with similar numbers. Like ~25 years ago we had a serious collapse of a public walking track viewing platform the had no engineering into a ravine, killing I think 14 people when they fell ~40m. It lead to this standard being developed to apply universal minimum design criteria to these types of structures.

 

[KootK]

I forgotten about this until somebody mentioned the Australian standard: Link. it's pretty handy even if you need to tweak the lumber products abit for North American use. I've had a couple of instances where I've been able to get in early on this kind of project and convince the design team to just do it like the manual shows. Pretty dreamy from a few perspective. Of course, folks often have their own ideas that take things in a different direction.

 

[bridgebuster]

The USDA has guides for trail bridges; they use 85 psf. I’ll post the drawing whenI get home. AASHTO requires 100 but under certain conditions 85 psf is permitted. I would go with 100 psf;60 is too light IMHO for a public walkway.

 

[human909]

If there is some flexibility in your codes and interpretation then another approach is to look at the consequences. If the failure could be progressive or not be particularly dangerou then that is one thing. If the failure could result in collapse and serious injury/death for those on the platform then that is another.

Pedestrian structures spend most of their life being over engineered. But it is the 0.01% case that you need to considered. And that 0.01% case could be very serious as Agent666 mentioned.