Machinery supports vs. structural steel

dgeesaman · May 22, 2025

I work for a machinery supplier. We are sometimes asked to include a custom mounting adapter plate or frame structure to cover an opening in the steelwork that is larger than our machine. This may include some live load requirements since mechanics will need to walk on it to inspect and service the machine. Our bolt designs are handled using mechanical design methods: torqued fasteners, shimming to avoid distortion, etc. Our customers however will blanket specify AISC for "all metal".

For these larger mounting adapters and their bolted connections, what is the technically justifiable line to draw between the structural steel (to AISC codes/practices) and machinery (no AISC codes/practices).

And if the bolted joint between the large adapter plate and steelwork is designed using both systems, is one generally accepted to be more robust?

Thanks, David

human909 · May 22, 2025

I would adopt the 'Duck Principle'

Aka if it looks like a duck, walks like a duck, and quacks like a duck. Then it probably is a duck.

But some things sometimes do look like a Platypus.

If it looks like a structure and acts like a structure then you should probably treat it as a structure. Though I can't comment on how it relates to your code requires as I'm unfamiliar with them.

I deal with this aspect all the time. I am often designing items such as silos, bins, hoppers receivers and other material handling items. The line becomes very blurry at times especially when more and more moving parts, changing pressures are added to the system. Many of these items might look and act like a structure but you buy it from a supplier as a machine or piece of equipment.

dgeesaman · May 27, 2025

The duck test works if you're familiar enough with ducks. I'm a machinery guy and I've never been closely involved with coded structural design.

I know in pressure codes (vessel, piping) a piece of machinery that is attached to the pressurized system does not need to be designed in accordance with the pressure vessel/piping codes IF the stresses in the machinery pressure containing components are dominated by the machines mechanical loads.

It seems that the structural codes offer no such clarity.

It's a shame that this question is for a municipal bid and the bids are handled with great clumsiness: you bid to the entire package or your bid may get rejected. And in my line of equipment I rarely encounter a set of specs that we can simply bid to without exceptions. So you take exception to things that really aren't sensible and if the engineer is ok with those exceptions or all bidders took the same exception, your bid will be considered. Or it will be rejected and you'll never know whether you lost the bid on price or were rejected on the package. You really don't know when to tack on a hefty adder and make your package price look inflated or take exception to an item here or there that a reasonable engineer might realize is a valuable cost savings. So year upon year, the specs get bigger and less tailored to the practicalities of the specific equipment and the bids get wilder and more complicated. And structural code compliance is a new one.

human909 · May 27, 2025

dgeesaman said:
The duck test works if you're familiar enough with ducks. I'm a machinery guy and I've never been closely involved with coded structural design.

I know in pressure codes (vessel, piping) a piece of machinery that is attached to the pressurized system does not need to be designed in accordance with the pressure vessel/piping codes IF the stresses in the machinery pressure containing components are dominated by the machines mechanical loads.

It seems that the structural codes offer no such clarity.

It's a shame that this question is for a municipal bid and the bids are handled with great clumsiness: you bid to the entire package or your bid may get rejected. And in my line of equipment I rarely encounter a set of specs that we can simply bid to without exceptions. So you take exception to things that really aren't sensible and if the engineer is ok with those exceptions or all bidders took the same exception, your bid will be considered. Or it will be rejected and you'll never know whether you lost the bid on price or were rejected on the package. You really don't know when to tack on a hefty adder and make your package price look inflated or take exception to an item here or there that a reasonable engineer might realize is a valuable cost savings. So year upon year, the specs get bigger and less tailored to the practicalities of the specific equipment and the bids get wilder and more complicated. And structural code compliance is a new one.

I hear you. I hear every part of that and I don't even use the same codes as you or work in the same hemisphere.

Structural codes in my experience offer zero clarity for machines/equipment. They assume everything is a "building" or at best something very similar to a "building". Only a small proportion of what I design in my line of work is a "building". The rest is structures around "machines/equipment". Or structures that ARE "machines/equipment".

It seems I'm on the other side of the fence to you. Over 90% of my work is for mechanical engineering firms who are now facing similar "ENTIRE PACKAGE" requests. For me it has worked out well as I'm the lone wolf in the manufacturing world. For you to make things work you either need to shift gears into the structural world yourself, or try to gain a willing partner/employee who works in the structural space.

Just don't be like some idiot mechanical engineers that I saw attempt to design a complex moment frame structure with extremely high loads. It was an absolutely disaster that I had to pick up at the 90% design stage. (Basic steel design I'm sure mechanical engineers can do just fine, just like I can do basic mechanical design.) Beyond that we should stick with what we know.

JStephen · May 27, 2025

We do tank work. In the AISC Code of Standard Practice, they define "Structural Steel" and that definition specifically excludes "tanks".
Generally, as a contractual issue, we'll qualify that anything we furnish that is attached to the tank is defined as part of the tank and designed and welded via tank codes rather than structural codes. (The big issue being whether weld procedures and all are per AWS D1.1 or ASME B&PV Code Section IX).
Note that definition gets fuzzier as you move into silos, etc.

dgeesaman · May 27, 2025

human909 said:
It seems I'm on the other side of the fence to you. Over 90% of my work is for mechanical engineering firms who are now facing similar "ENTIRE PACKAGE" requests. For me it has worked out well as I'm the lone wolf in the manufacturing world. For you to make things work you either need to shift gears into the structural world yourself, or try to gain a willing partner/employee who works in the structural space.

I am perfectly happy to call in consulting engineers for this kind of work.

My feathers ruffle when I sense it's gross overkill (like an adapter plate that covers a larger opening and extends a couple feet past the natural boundary of my machine), and they won't give clarity on what is and isn't structural and I have to guess whether to include structural design code work on it (higher cost if competitive bidders exclude it) or don't (not to spec).

I'd like to say that the RFI process helps here but 90% of the time we miss that or simply can't dig deeply enough into the spec package at that time to root out this stuff. I think in my latest example the added PE stamps and such were late additions.

I think when there is an actual vessel or piping involved this question goes away. The above issue comes up when the machine is mounted to concrete via an oversized adapter plate / bridge with seismic calculations and anchoring designs. We can do the calcs and such to IBC but we don't have structurals on staff.

Thanks,

David

human909 · May 27, 2025

JStephen said:
We do tank work. In the AISC Code of Standard Practice, they define "Structural Steel" and that definition specifically excludes "tanks".
Generally, as a contractual issue, we'll qualify that anything we furnish that is attached to the tank is defined as part of the tank and designed and welded via tank codes rather than structural codes. (The big issue being whether weld procedures and all are per AWS D1.1 or ASME B&PV Code Section IX).
Note that definition gets fuzzier as you move into silos, etc.

Just today I was asked to supply seismic loads for liquid tanks. These aren't large vessels. 2-3m diameter, 2-3m high. Our seismic code specifically excludes liquid tanks. So yeah...

But no biggie... I'll stand by my calculations these aren't big vessels. And the aspect ratio means that the more complicated aspects of liquid tanks don't come into play as much.

JStephen said:
Note that definition gets fuzzier as you move into silos, etc.

Agreed.... But in AS and EU code silos pretty much fall under structures. Oh, except now AS has discontinued their silo code and we are now cast adrift!

So for silos I'm comfortable. But this is the world we play in. A good engineer learns that there is life beyond codes.

JStephen · May 27, 2025

The last time I checked, a "structure" was defined as "that which is built", so it includes houses, cars, televisions, and kazoos, so not an especially useful definition.
In ASCE 7, tanks fall under "non-building structures" (as do silos, pressure vessels, etc) but are still not considered as "structural steel".
The workers are categorized as boilermakers rather than ironworkers also.

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Machinery supports vs. structural steel

dgeesaman

Mechanical

human909

Structural

dgeesaman

Mechanical

human909

Structural

JStephen

Mechanical

dgeesaman

Mechanical

human909

Structural

JStephen

Mechanical

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