Design of platforms in an industrial building. 1

[mfstructural]

I'm looking into designing a several steel platforms in an industrial building. Since it's not part of the superstructure it should be considered a non-building structure, correct? The platforms support hoppers and have monorails on them to transfer one ton bags of a powder chemical. Chapter 15 of AISC has it's own requirements for non-building structures, so seismic detailing will still be required and satisfied.

 

[Naggud]

You should design them as building structures I would think.

I'm more familiar with the Canadian code but the seismic forces for non-structural components tend to be higher than those derived from the equivalent static shear method (energy dissipation due to ductility etc is not accounted for). You will be designing for higher forces if you assume them to be non structural.

 

[msquared48]

Non-building structure is correct.

Mike McCann, PE, SE (WA)

 

[TLHS]

Non-building structure similar to a buildings.

Depending on what they're connected to, I'd likely just attack this as ordinary frames. A little bit of extra steel in an industrial setting isn't a bad thing, plus usually you'll have a lot of extra lateral capacity in platform structures just from the necessity of hitting slenderness on braces and minimum number of bolts. As long as your connections don't get stupid, it's normally fine.

In your instance, though, this may not be applicable given that you have material storage and a crane. Your seismic load is significantly higher than in the usual industrial access platform. It's worth having a think about the R factors, though, to see if you're actually saving steel or labour with the extra detailing requirements.

Naggud, in ASCE 7 they have separated things into building structures, non-building structures similar to buildings (for non-buildings that are basically framed structures) and non-building structures not similar to buildings (big tanks, stacks, things like that). Then on top of that they have the non-structural component section. They're all reasonably big. The non-building structures sections basically reference the building structures section with a bunch of different assumptions, R factors and other requirements.

That's in comparison to the NBCC, which basically has one section that tells you how to do it for buildings, a couple of clauses that say you should come up with something similar if you've got a structure mostly analogous to a building, and then a couple of clauses about non-structural components.

They've both got their own quirks.

 

[PMR06]

Here's an article from Structure Magazine that talks about Chapter 13 vs 15. The flow chart may help.

http://www.structurearchives.org/article.aspx?articleID=1405

 

[mfstructural]

Thanks all, I'll look into all the resources.

 

[mfstructural]

So I've gotten into the design and have some questions regarding seismic design. I'm trying to determine which R value to use for the non-building structure platform. Initially I'm looking at having the bracing eccentric at the corners of the frame in each frame. I don't see a category for this. Only concentrically braced frames are listed in Table 15.4-1 of ASCE. Would this be categorized as a moment frame since you are essentially creating a moment connection at each corner?

 

[mfstructural]

Another question I'm working through is excitation of the effective self-weight. The top of the platform has a cantilevered monorail that hoists a 2.2k bag of power and moves it over a hatch to dump it in. I'm considering this bag as a LL in the load combinations, but then that means that it doesn't have to be excited seismically. Apparently the bags are held on with 4 ropes and rip on occasion. I imagine that if an EQ occurs and this bag starts swinging it might break free. It's not hazardous chemical so from what I'm told. The question is whether to apply a percentage of it as a horizontal LL? I'm thinking of 30% applied horizontally in combination with the effective DL as a judgment.
Thanks

 

[Bagman2524]

I believe your reference is to ASCE-7-10, not AISC.

The AISC Code of Standard practice section 2.2 list "other steel" that is exempted from being classified as "structural steel". Catwalks, stairs, ladders, etc. are "other steel".

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

 

[TehMightyEngineer]

I designed a very similar platform as you're describing with knee-braces at the corners for a steel frame supporting a large industrial filter (essentially a tank). It was in SDC B in a low seismic area. I designed it under an R = 3 system for braced frames which as far as I know has no restriction on wither the braces have to be eccentric or concentric as long as you design for the resulting forces in the beams, columns, and braces. B.13 of Table 12.2-1 in ASCE 7-05 seems to be the applicable design coefficients and factors.

Maine EIT, Civil/Structural.

 

[CCox]

I work for a wood product company and have designed several steel structures to support air handling equipment. I am just finishing up the design on a tower to self support two large cyclones. Several points to make in regards to your post:
In your case, use chapter 15 for your design. If you have a braced frame system, use R=1 to exempt yourself from using the seismic design manual (AISC 341). Similarly, for a moment frame, use R=1.5 for this exemption. This is similar to the R=3 exemption for normal buildings. The main benefit from this is not having to design the connections for the capacity of the members. With industrial applications, you want everything to be oversized (you can imagine how everything is a target for someone on a forklift). So, you oversize the members, but now you don't have to design a huge, expensive connection to correspond. Talk to any millwright and they will tell you the bigger the better. That is the mill mentality.
The live load to use as a seismic component is up to your judgment. You may find that the member sizes you are specifying are large enough that it won't make much difference. Good luck.

 

[mfstructural]

Thanks for all the input. I initially was going to use R=1 to avoid AISC 341, which was key for me as I didn't want to go that route. Even with R=1, my Cs=0.316. There is one platform that is 8'x8' and 26' in height, so the overturning controls with high tension at the base. The rest are fairly large and shouldn't have issues. I'm going to request them to increase the size to help with this since the sizes they chose were arbitrary.