Spiral stairs for storage tank 2

[Borimer07]

Customer has requested this instead of the usual ladder/cage, never prepared drawings for one before.
Anyone have experience with these and care to share some insight:

1. What program was used to model it? Trying to do it in Bricscad but it's 3D capability is not up to snuff.
Main issue is modeling the channels to form around a helix.
2. Where can one find any standards? Only PIP specs. I've found are for linear type stairs.

The basic design we were going to go by:

Vessel: 204" OD
Height: from to grade to top of grating/platform = 235"
Span: roughly 120 degrees. Too steep?
Stair Risers: channel 12x20.7#/ft
handrails: 2.5x 2.5x 1/4 angle
treads - 12-1/8 x 36 galv. grating

Any help or insight would be greatly appreciated!

 

[MJCronin]

Looks like another case of the client shooting himself in the foot ...

The client's tank engineer is probably very inexperienced ... (Is this his first storage tank ?)

Your tank has a diameter that is somewhat small (17 feet) but still suitable for a spiral staircase. Although it can be done, spiral staircases on tiny tanks end up looking like something that should be in an amusement park for kiddies.

On the tanks I have specified, I recall a general "rule of thumb" minimum of 20 feet diameter

The UL 142 tank code refers to an OSHA standard for spiral stairs and dimensions:

Sub-paragraph 1910.23(d)(2) of the Occupational Safety and Health Standards, Title 29 of the Code of Federal Regulations

The OSHA standard also has many dimensional requirements for tread width, rise and top gate and fall protection

Spiral stairs are prone to long term expense because of the need to paint and replace when corroded. Ladders and cages are cheaper when they are simply ripped off and replaced ....

You have to ask the client; Why do you need to go to the top of the tank so often ? Why is it that a caged ladder is unsuitable ?

If he has no answer, tell him that the caged ladder is FAR LESS EXPENSIVE than spiral stairs ....



MJCronin
Sr. Process Engineer

 

[racookpe1978]

You will find it is extremely (did I mention extremely?) difficult to 3D roll (bend and twist) a spiral for rectangular HSS, for flat bar, or even for small (1-14, 1-1/2 inch or 1 inch) tubing and shapes. And those are shapes that are symetrical about the vertical and horizontal axis so they fit in the bender slot uniformly

It is even harder for WF, Tee, or channel shapes which are unsymetrical going through the rollers.

Round shapes (for handrails, for the structural frame itself) are not easy either, but at least you do not have to twist the rolled shapes axially every foot that the handrail or stair web goes up and rolls around the tank.

I've built sewpt spiral stairs. They are very hard, very time-consuming.

A "standard spiral" stair for large diameter tanks that is cantilevered out from the tank wall can be made from near-straight short tangent sections (approximating the final spiral) between each stanchion. But a 17 foot diameter is a bit small a radius for that practice without looking ugly and tacky.

 

[IFRs]

I would also advocate a vertical ladder or even straight stairway going from grade to top of tank.

But, if they insist, it's not rocket science. A quick look at my current hard drive shows dozens of spiral stairways going back to 2006. Mostly on API 650 and AWWA D-100 tanks. Smallest was 12', a few in the 20 to 30 foot diameter range, most over 50'. Some were no stringer, some were single stringer, others double stringer. All had 45 degrees as one of the primary angles - either at the tank shell or at the inner stringer. All spiral stairs had flat bar stringers (if there were stringers) , most had bolt-on serrated grating treads, all were open risers. If you use 45 degrees at the tank shell your spiral will go 235" around to go 235" up. That's about 132 degrees. OSHA has rules about tread depth, rail height (mid- hand- and top-rails all needed). Certainly round rails are going to roll easier than angle. Treads 12" wide may not be deep enough. The simplest design is to have treads welded to the tank shell, square bar posts at each tread with hand- and top-rails welded to the posts.

I would not get too worked up about modeling it in 3D but there are shortcuts if you really have to. If you really need to extrude the stringer around a helical path. I would strongly suggest not using channel for the stringer since the complications are not worth it and you don't need the strength assuming you can connect to the tank at regular intervals.

Some non OSHA compliant images at that otherwise illustrate the basic concepts:

https://thumbs.dreamstime.com/z/spiral-stairs-fuel-tank-farm-14049608.jpg

http://ameristruc.com/dev/wp-content/uploads/2012/06/spiral-staircase.jpg

https://s3-media3.fl.yelpcdn.com/bphoto/9t0BqyO6D03vfVNn-2JlnQ/o.jpg

https://i1.wp.com/

http://www.wwtank.com/wp-content/uploads/2015/03/stair_walk.jpg?resize=493,500

 

[JStephen]

Check the current OSHA rules for rise/run, handrail/stairrail requirements, required strength, etc. Currently, a separate guardrail and handrail are required. Ideally, rise/run should match other area stairs, but the newer rule prohibit some of the common combinations in former use.
Simplest stair is formed checker plate welded to the shell.
If a double stringer design is desired, generally the stringers will be flat bar, not channels. For a larger radius, they'll ship flat and be pulled to a curve as they are bolted up. It's possible, but may or may not be practical, to run them through a plate roll on an angle. Handrails can be rolled to a circular arc of somewhat greater radius than the projected horizontal radius, and fit up by twisting.
The analysis is not rocket science, and in general, it will be cheaper to add additional supports than to do enough analysis to show they aren't required.
Note that AISC provisions for straight members are not applicable to helical members, so you're on your own there. Which actually simplifies things.

 

[MJCronin]

This entire discussion should be placed into the "Storage Tank Forum" ...

MJCronin
Sr. Process Engineer

 

[Borimer07]

Thanks fellas for the responses, as I've gotten very few on other forums. Those of you that have done stairs before I'm curious as
to what CAD flavor you used to do the modeling? I'm very tempted to look into Solidworks but perhaps there are others worth consideration?

Currently I'm using Bricscad which has 3D on a par with Acad plus sheet metal utilities but the workflow/interface is not the best.