Loading and deflection limits on Access Platform

[Bartek Sz]

Hi

I have to design an access platform for train depot (access to the roof of the train), cantilevering from the floor on one column as per attached screenshot. What deflection limits would you normally apply to the ends of the cantilever, so people don't feel uncomfortable while walking on it? Also what kind of loads would you consider? Thanks for your input!

 

[BAretired]

Design Live Load 100 psf.
Deflection limit L/720 where L is the length of cantilever.

BA

 

[SlideRuleEra]

Concur with BAretired.

http://www.SlideRuleEra.net

 

[KootK]

What activities will occur on the elevated platform? How many people? Expectations for material and tool storage? For the right application, I'd be willing to take it down to:

- 25 psf based on OSHA light scaffold loadings OR;
- 60 psf based on ASCE7 Walkways and elevated platforms.

I'd personally go with the 60 psf for something permanent like this given that I doubt the extra load will really affect the construction costs all that much.

I like [2 X CANT / 180] for deflection but I think that the important things will be:

1) Deflection estimates that account for column flexure and girder torsion and;

2) Vibration. Maybe try to keep your natural frequency upwards of 8 Hz etc.

 

[TehMightyEngineer]

I'd go for 60 psf as well unless they're putting a lot of equipment and materials up there. 60 psf was used a lot for many of the paper mill work platforms I looked at; for worker access and general equipment storage (welders, few bags of cement, etc.) it was more than adequate.

I also second KootK's vibration and stiffness comments. I've gone on lots of sketchy platforms and the one that I could feel rotate under load, or could feel the bounce from deflection, were instantly suspect. Occasionally I'd run the numbers and find they were fine; but more often than not they weren't adequate. Generally a worker will know what a "safe" steel platform feels like.

Depending on how stiff you need this to be, I'd go for either 2L/360 for high-end design or 2L/180 for a low-end code minimum design.

This is tickling a memory I have about a previous post here on eng-tips. Wasn't there a similar train platform being designed?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[TehMightyEngineer]

If possible; I'd make the center girder a tube for the torsional rigidity. Some knee-braces would probably help add less expensive rigidity of that column to platform connection than trying to create a solid moment connection. The torsion of the entire platform bending the columns/girder is going to be "felt" a lot more than the deflection of the cantilevered beam IMO.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[CANPRO]

Hard to tell from the screenshot, but it looks like your beams start and stop at the column, which would make TME's suggestion to use a tube for the torsion a good idea. I think I'd take the short beams at the column locations and make them continuous over the columns (looks like you're doing this at your end columns but not the intermediate ones) - these continuous beams would support the perimeter steel, which in turn supports the remaining infill beams. Depending on the size of this thing, you might be able to shop assemble "T sections" consisting of your column and continuous beam. I also like TME's idea of using kickers to as a cheaper way to stiffen your connection.

Not familiar with American codes, so can't comment on the code requirements...but regardless of the code requirements, I'd double check with the end-user to make sure they don't plan on putting anything on here that would exceed code minimums.

 

[bob33]

Can you turn the framing around so all beams are connecting to the cantilevers? Less members and connections.

 

[RickyTickyTavi]

Will the deflection create a trip hazard between the platform and the train?

My gut says this is 100 psf platform.

 

[Bartek Sz]

Morning Everyone

Thank you very much for your input. Much appreciated!