Catwalk access around circular tanks 3

[apartux]

Hello!

I've done some structural work before, but nothing really unconventional. I've got a (slightly) oddball situation, and I'm curious about how it's normally handled. It's a catwalk for maintenance access to two FRP tanks. Due to space contraints and access requirements, they'd like the grating to be cut around a portion of the perimeter of the tanks. I'm putting together a preliminary stab at how I think it ought to look, but before I present the concept I was curious if anyone would be willing to sanity-check it.

Attached is a plan-view sketch (catwalk elevation will be ~9' above finish-floor.) The concept is braced frames for column-lines A, B, and C, a moment-frame for column-line 2. I've got beams running as sort-of "chords" around the perimeter of the tanks, ultimately stability-supported by the plan-south moment-frame. The grating will be cantilevered off of the "chords," supported by clips or angle along the main plan-south beams. Stability for the columns along column-line 1 will be provided through the long chord-beams to the moment-frame.

Does the basic idea as presented make sense? Even though it's a small, lightly loaded structure, I'm a little nervous about using a single moment-frame for E-W stability; not for any reason that I can justify, it just seems unconventional. But apart from providing base-fixity for small HSS columns (and any associated added cost with that) I'm not sure how to arrange it, apart from abandoning the concept. Would anyone with a bit more structural experience take a glance at this? Thanks a lot!

 

[topsisu]

Some possible issue to be concerned with:
1. How would the guardrail on the edge of the platform be supported at the cantilevered grating edge?
2. Does the tanks have a perimeter guardrail if you are walking on the top of the tank?
3. Do you have a fall protection system for 100 % tie off if there is no guardrails?

I would frame a platform around the entire outside with 2 holes for the tanks (octagon framed opening) and have guardrail around the outside perimeter.

 

[BAretired]

Why not cantilever the catwalk from the wall of each tank? That would avoid any additional foundations and columns. If necessary, diagonal braces could be added to support the catwalk.

BA

 

[apartux]

topsisu: for the guardrail, I was toying with the idea of raising the HSS column height to the top-of-rail elevation, and cantilevering it from that. As the cantilever length is short (~2.5') I can't imagine it would be more cost-prohibitive than setting new columns.

BAretired: The tanks will be fiber-reinforced plastic, so I'm avoiding making it load-bearing, especially with regard to the out-of-plane forces that would result from a cantilever from the tank wall.

 

[BAretired]

Have you considered fiber reinforced plastic members and grating for the catwalk? Again, I would suggest a cantilevered arrangement but would reinforce the tank wall with suitable vertical members.

BA

 

[SlideRuleEra]

apartux - Assume you are planning on using steel grating, not FRP. Suggest not cantilevering the grating. The tie-down bolts / clips that are normally used with steel grating are small. Loading on the cantilever portion could create significant uplift on these connectors that they are not really designed for. Welding the grating to its supports may not be wise. In an industrial application it is common to remove gating sections from time to time for maintenance an other (sometimes unexpected) reasons. If the tie-downs are damaged or are not replaced exactly right you could get failure the next time the cantilever portion is loaded.

Suggest that you use only simple span or continuous span supported grating. Keep the framing basic so that the edges of the grating form cords around the tank exterior. Of course, there will be some gaps between the grating edges (the cords) and the circular tank. In most operational situations these gaps are of no concern. Have the edges of the grating banded. Use grating tie downs, but only to keep the grating from shifting, not for structural reasons.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[paddingtongreen]

If not part of the tanks, I would put those main diagonals down just below the longitudinal support member, I would run a parallel member along between and at the sides of the tanks, supported by perpendicular members sitting across the diagonals. I would have bent angle support/toeboard along the edge around the tanks, supported with another cantilever on the centerlines of the tanks similar to what you show. If this is in a code country, there will be design requirements for the handrail that you cannot get from cantilevered grating.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[Bagman2524]

Agree with Slide Rule. Cantilevering the grating is not a good idea. Structural members should be put as close the perimeters of the tanks as possible to produce simple spans.

Faith is taking the first step even when you can't see the whole staircase. -MLK

 

[BAretired]

Nobody has suggested cantilevering the grating so it is unclear why three respondents have mentioned it. It would seem to make good sense, however to cantilever the structure supporting the grating since the width of catwalk is only 2.5'.

The cantilevered frames would be tied to the tank wall and should be FRP material which offers excellent corrosion resistance. I would visualize the FRP grating spanning 2.5' between FRP beams supported on the cantilevered FRP frames. The advantage of tying the catwalk to the tank is that it derives its lateral stability from the tank itself, eliminating the need for wind resisting frames.

FRP structural shapes are available from several sources and come in similar shapes as structural steel including HSS, WF, angles, channels and others. See the following link:

http://www.strongwell.com/products/structural-shapes-and-plate/

BA

 

[jayrod12]

BA, his sketch seems to show cantilevered grating so I believe that is where the idea is coming from.

 

[msquared48]

The guy is a space cadet. Maybe you could recommend the use of Balsa wood for the platform if he persists.

Mike McCann, PE, SE (WA)

 

[msquared48]

If you also spec the mandatory use of anti-gravity boots, he will love you.

Mike McCann, PE, SE (WA)

 

[BAretired]

Okay Mike, thanks.

BA