Slab on Grade for MRI
Slab on Grade for MRI
(OP)
I am designing the slab on grade for MRI, which is about 12kip in weight. To size the slab on grade, I have considered:
1. punching shear from the heavy magnets.
2. make sure the slab is thick enough for the post installed anchor embedment.
3. FRP reinforcing is provided for temp and shrinkage.
I would appreciate if you could point out what other factors have i missed?
Thanks.
1. punching shear from the heavy magnets.
2. make sure the slab is thick enough for the post installed anchor embedment.
3. FRP reinforcing is provided for temp and shrinkage.
I would appreciate if you could point out what other factors have i missed?
Thanks.






RE: Slab on Grade for MRI
An expert is a man who has made all the mistakes which can be made in a very narrow field
RE: Slab on Grade for MRI
RE: Slab on Grade for MRI
vibration isolation is a good point, and yes - isolation joint is provided along ther slab edges
RE: Slab on Grade for MRI
RE: Slab on Grade for MRI
RE: Slab on Grade for MRI
I have always used a thick (12" or thicker) slab with no reinforcing.
DaveAtkins
RE: Slab on Grade for MRI
MRI's are frequently placed on above ground floors. How would they do that if no steel is allowed?
#4's @ 18" O.C. ea way is less than 1 PSF.
But as others have mentioned, other considerations are more important.
RE: Slab on Grade for MRI
all six sides of the room was at least 5 ft thick concrete. there were 4" thick plates suspended in the roof of the vault.
RE: Slab on Grade for MRI
http:
maybe this is the issue:
RE: Slab on Grade for MRI
Have you looked into tuf strand by euclid?
RE: Slab on Grade for MRI
please see below for summary of design guide from advices from my other offices which have done similar projects before, conversation with MRI manufacturer (HITACHI in my case), and study of other structural engineer's drawings:
1. MRI cutsheet will typically specify maximum weight of steel reinforcing in certain perimeter from the magnet. Because ferrous material will impact the accuracy of the process. It is recommended that all reinforcing to be provided by using FRP bars per ACI 440. Make sure you specify correct splice length, strength and proporties for FRP bars. GFRP is what typically called out.
2. The slab on grade thickness is usually governed by:
a. Embedment depth of the post installed anchors, which is usually to be provided by the manufacturer. You dont want to specify a 4"slab, but the manufacturer is putting in 4" embedment anchor for seismic anchorage. it will result in a big change.
b. If vertical vibration joint is not provided around the perimeter of the room (which is strongly recommended by manufacturer), the slab on grade need to be sized to have sufficient mass to resist vibration transmitted from other places.
c. Size for weights of MRI. I was told that most MRIs are not heavy enough to drive the SOG to more than 5" or 6" thick. Certainly, punching shear of the magnets and flexural capacity with subgrade modulus should be checked, but rarely govern in SOG as we know.
3. Make sure you recess the floor for shielding, there is usually a 1/8" to 1/4" of shielding required to be inset on SOG. So, you need to show a recess in the SOG to allow for the shielding. The shielding thickness is NOT by the manufacturer, and usually from the shielding consultant, which is a separate thread hired by the architect. I have confirmed that the steel reinforcing limitation still valid even with shielding. Shielding is for rediation isolation not magnetic isolation. At the same time, if the shielding report specifies a cieling hung shielding, make sure you design the structure above to handle the weight. shielding support framing is by typicsally by UNISTRUT spacing at 4'-0".
this is all I know, just want to share with you all what i found out. thanks.
RE: Slab on Grade for MRI
Metal: These MRI scanners have magnetic fields tuned like Stradivarius instruments. Magnetic metals (iron, nickel & cobalt) will attract the magnetic field of the MRI and throw-off that tune. Our multi-disciplinary approach may mean that the Architectural, Mechanical and Electrical engineers each look at the maximum allowable threshold and say, 'easy, I can stay below that.' But what matters is the cumulative effect of the the metal that they all put into the job. For that reason and in order to protect the function of this $2m piece of equipment, your best bet is to eliminate steel from your slab reinforcing. FRP, carbon fiber, even Austinitic stainless are fine. No steel. This extends below the slab, too, to beams, pipes, conduits, etc...
Vibration: These machines generate their images by triangulating-in on molecule-level signals. That's remarkably challenging by itself... now imagine trying to do this while the patient is wiggling. The two primary sources for vibration are Earth-borne and structure-borne. Earth-borne vibration is a function of what vibration sources you have (highways, rail-lines, construction sites) and your soil conditions for transmitting these vibrations. Structure-borne vibration can come from pumps, air handlers, motors, fans, even foot or rolling-cart traffic. An MRI's vibrational sensitivity has spectral 'hot spots' where the amplitude and frequencies are particularly sensitive. Vibration analysis is always recommended, but requires a fully operational building around where the MRI will be at the time of study to be accurate. SoG is typically best when you know you don't have unreasonable Earth-borne vibration, and floating the slab by separating its perimeter from the building's structure helps eliminating structure-borne vibration transmission. On elevated floors, the rigidity of the entire frame must be considered relative to vibration sources and paths to the MRI.
Shield: All MRI's require radio frequency (RF) shields in a six-sided box. The load of these is negligible, but they do nothing, zero, to contain the magnetic energy of the MRI system (which is harmless to people's bodies, but can be very disruptive to medical devices like pacemakers). Magnetic shielding may be designed for the suite, and is made up of plate steel, the loads of which are not inconsequential. Remember (above) how I said to try to eliminate all steel in the slab construction? How does this square with the idea of adding large plates of steel? When steel shielding is added, it must be designed in such a way that the attractive 'pull' that it exerts on the balanced magnetic field of the MRI is equaled out. The MRI has some internal tuning ability, to correct for this, but often if there's enough steel shield material on the floor it exceeds the scanner's 'shim threshold' and requires that compensating steel be placed in the ceiling to 'balance out' the field. Because the plenum above the ceiling is much further away from the center of the magnet than is the floor, the amount of compensating steel above my be 2 - 3 times what is in the floor. Obviously this could get structurally significant very quickly. Also, the engineering and installation costs for steel magnetic shielding are very significant. Often it is less costly to increase the size of the area around the MRI than it is to attempt to shoehorn it in and correct for conflicts with steel shielding. Lastly, depending on your shield vendor and what sort of shielding you ultimately have, the slab recess should probably be 1-1/2" to 2". The shield vendor will probably want to put on their own top-seal, then install their shield, then install a protective layer over it.
Path: From a structural standpoint, the other thing to consider is the path of travel. Unlike nearly every other piece of imaging equipment, MRI's can't be 'broken down' to anything much smaller than the final product for delivery. This means that doorways often must be torn out along the delivery path. To get the MRI magnet from point-a to point-b, imagine that you will need a path that is 8-feet tall by 8-feet wide (verify the particulars with your MRI vendor's requirements). And while 12 - 24 kips isn't an impossible load to design the slab for, remember that this load is going to have to roll across the floor. On elevated slabs, this may require temporary shoring (another reason to have these on grade).
Lastly, most professional errors and omissions coverage assumes that the value of the designer's work is the building, and most of the time that's true. For MRI suites, however, the value of the professional designer's work is also in the operation of the MRI, a piece of equipment worth 2 - 4 times the value of the suite into which it's placed.
If you have any other questions about MRI suite design (issues like equipment interference, exclusion zones, screening protocols, shield vendors, design standards), I encourage you to look up www.MRI-Planning.com
RE: Slab on Grade for MRI
I'd appreciate some elaboration on this point.
RE: Slab on Grade for MRI
RE: Slab on Grade for MRI
I believe that designing to the minimum acceptable standards for today's MRI equipment dramatically increases the likelihood that the next MRI, or the one after that, will necessitate significant changes to the suite that would otherwise be unnecessary had the suite been designed in anticipation of more stringent requirements of future equipment.
As for the site I suggested, it is completely different than it was just a couple weeks ago! It is quite promotional. If you Google it, perhaps you can view a few older cached pages.
RE: Slab on Grade for MRI
Do the requirements tend to get more stringent with subsequent generations of MRI?
RE: Slab on Grade for MRI
In short, there's the potential that both of the major environmental siting challenges (vibration and metal) will become more stringent over time as a result of both clinical and technical changes to the MRI systems.
RE: Slab on Grade for MRI
You can find frp reinforcing by searching plastic lumber; they use it all the time.
RE: Slab on Grade for MRI
I suggest you talk with the equipment vendor about your concerns. They will give you feedback of their many, many previous installs, and let you know if they think something might be a problem.
RE: Slab on Grade for MRI
Another important factor is that MRI scanners are not appliances, not in the conventional sense. The magnetic fields from the MRI interact with the building, and the ferrous content of the building reshapes the magnetic field that the MRI uses for imaging. The less the building's design messes with the MRI, the better images it is capable of producing. This means that the MRI equipment is clinically / financially viable to the owner for a longer period, extending their equipment replacement cycle. Given both the costs of the equipment and lost revenue during replacements, this is not insignificant.
And speaking of revenue, today's insurance company reimbursement rates provide razor-thin margins for most MRI providers (here I'm speaking about the US). Given that gross revenue and operating expenses for these devices are very, very high, interruptions to service can have grave consequences to the provider's bottom line. Anything that can help assure trouble-free operation of the MRI scanner and minimize servicing time is very important to the owner.
And lastly (and specifically on the issue of structural design of floors), I'm right now reviewing a design of an MRI suite where the structural designers must have looked at the allowable steel values for this particular MRI scanner...
Scanner specs for steel
B.F.F. PSF
0" 0
3" 2
5" 3
10" 8
13" 20
The structural designers' floor structure (elevated) is...
B.F.F. PSF
3" 1.8 (#4 rebar, 18" oc / ew)
4.5" 2.5 (centroid of 2" metal deck under 3.5" conc)
10.5" 12 (PLF centroid weight of framing stiffeners under magnet)
12.5" 43 (PLF centroid wight of beam)
Yes, 1.8 PSF is under the 2 PSF threshold. And yes, the 2.5 PSF of the metal deck is under the best-fit curve of the tolerance whose next point is 3 PSF at 5". The problem is that these aren't independent tolerances... they're cumulative. If you have 1.8 PSF of rebar at 3", you've used 90% of your allowable threshold. That means that you only have 10% left at any other distance! That's 10% of 3 PSF at 5", or 10% of 8 PSF at 10", or 10% of 20 PSF at 13".
Will the magnet still scan? Probably, because the design criteria are a bit conservative. But certain types of scans may not work as well, or be as clear, or take longer to acquire. The client will yell at the MRI manufacturer, who will provide the latest software upgrade, scratch their heads, and say 'what else would you like us to do?'
RE: Slab on Grade for MRI
I don't know about the experience of everyone else, but I'm usually given "the manufacturer's criteria" to design around. Those are usually quite difficult. I've never tried going back to the arch and attempted to sell the idea of designing around more extreme criteria for the possibility of the future being more severe. I suspect that they'd shoot back something like: "Well, how do you know that the next generation of MRI won't be more robust and have easier criteria?" From a common sense standpoint, I would've assumed it'd be just as likely to go that direction.
I don't see how to approach this from the strl engineer's standpoint. We're given "the criteria" that will make the MRI "work." It sounds like you're proposing that SEs should go to the arch with an argument that sounds something like "Well, I don't believe these criteria that GE's physicists and engineers came up with are stringent enough. I think we should spend even more of the owner's money and more of our time to try and design to something more stringent for [fill in the blanks]." Maybe my experience is odd, but I don't think that would sell, easily anyway.
Do you have monetary numbers for this? Option A vs Option b. If so, then I can see how one could put together a lunchtime presentation for the arch's office and explain to them why one should try to significantly exceed the mfr's criteria. Otherwise, it sounds like an assumption only and is too vague to weather a serious argument.
Again, do you have some service time numbers, Option 1 vs Option B, that one could show to an architect? If not, then it sounds vague and I'm anticipating it being a pretty hard sell.
One issue I have with argument such as these is that MRI have been installed on many, many floors since the mid 80s. The firms I worked for have put many of them on cip concrete and steel-framed floors and I don't think they've ever heard of a problem with any of them. We just follow whatever mfr criteria are given. I'm sure there are examples of problems, but we've never had anybody alert us to any of them.
RE: Slab on Grade for MRI
RE: Slab on Grade for MRI
If we can design for the loads, and simply substitute FRP or carbon fiber for the reinforcing for a CIP slab, both design and cost differentials are negligible.
I would hope that the architect would have enough respect / fear of botching the install of this multi-million dollar piece of equipment that they'd be receptive, particularly if you accept my prior assertion that, at least for CIP, the burdens are negligible. Yes, structural steel buildings and elevated floors are more difficult, but not deal-killers. And I am saying that I don't believe that the GE criteria are stringent enough... not for the life-cycle of the building. Apart from the space requirements, vibration sensitivities, metal tolerances, dead loads, have all slowly ratcheted up. Designing to the limits of the tolerances, today, may mean tearing out major portions of a suite for the next generation of equipment.
I wish I had the means to collect this information! I've heard it enough times, anecdotally, that I believe it (or I've swallowed my own Kool-Aid). As an example, a few years ago there was a brain imaging center for a state university. The project was a stone's throw from the hospital, and the brain imaging center and the hospital got very similar magnets from the same vendor at about the same time. There should be no real difference between them, right? Except one was shoe-horned into an existing hospital suite with conventionally-reinforced floors, and all other sorts of potential interferences (within tolerances, all), and the other was installed in a purpose-built suite with minimized ferromagnetic materials in construction, had a floating SOG with FRP. The MRI vendor service engineer remarked that the magnet in the purpose-built suite for the brain imaging project produced better images and required less periodic 'tune ups' than the comparable scanner in the hospital.
The difficulty in quantifying this is that the problems manifest as imaging equipment troubles, and not building system troubles. Even if there were grave problems, they show up in the output of the MRI scanner. Unless the MR manufacturer engineers go back and assess the construction of the suite, it could be that nobody would ever connect those dots (this is purely a illustrative and not meant to cast aspersions on your firms' designs).
If structure is 10% of the cost of a building, and MRI technical spaces make up 10% of the floor area, and structural changes to accommodate special requirements of MRI represent a 10% construction cost premium, this contributes 0.1% to the cost of the building. Contrast that with the $1.5 - $2.2 million that one MRI scanner costs. Even if I'm 100% wrong, that seems a very modest 'insurance premium'.
RE: Slab on Grade for MRI
I very much appreciate your perspective on this subject. I'm very interested in it. So far, I've only been involved as a designer, but now am doing research on the subject.
RE: Slab on Grade for MRI
If there is enough ferromagnetic metal (iron, nickel and cobalt) nearby to the MRI scanner, it throws off the 'tune' of the precisely-balanced magnetic field. Depending on your MRI machine, there may be a couple of different ways that this can be corrected.
The 'old school' way is to use shims, pieces of ferromagnetic material with well-defined magnetic properties. These are, quite literally, stuck to the body of the cylindrical magnet in masses and configurations that correct for the distortions of structural steel nearby. This is a laborious process, but can be accomplished during installation. As long as the building steel isn't changed, the shims should maintain a balanced magnetic field.
The 'new fangled' way (which is used in concert with metal shims) is active shimming, achieved by an array of electromagnetic coils built into the MRI scanner itself. These are tuned to produce counterbalancing magnetic field effects to the building steel.
One of the challenges to the old school metal shim method is that there is a direct correlation between the thermal properties of the shim metal, and its magnetic properties. If the temperature swings out of specification by even a couple of degrees, F, the magnetic balance for the MRI scanner gets thrown off.
The chunks of shim metal are also right next to the high-energy radio frequency magnetic field coils, which are typically actively-cooled because they generate so much heat. If the active-cooling system isn't up to snuff, or if the HVAC system sees the room temperature specs as 'suggestions' instead of 'requirements', the image quality from the MRI can start diminishing during the day as the heat from the coils builds.
All of this goes to show the interconnectedness of the elements in an MRI installation. 'Conventional' quantities of steel in the structure require that the MR equipment manufacturer adds metal shims to the magnet, which reduce the operational tolerances of HVAC and equipment cooling systems. The HVAC then becomes a linchpin (through this long chain of causation, begun with the structural design) to image quality.
As I said in an earlier post, the inter-relationship between the MRI equipment and the building in which it's sited is unlike any other piece of equipment you can think of.
RE: Slab on Grade for MRI
RE: Slab on Grade for MRI
RE: Slab on Grade for MRI
@structuresguy The 'short' and 'wide' bore MRI systems are making the physical body of the MRI smaller, but by making the 'imaging space' larger, this actually puts greater demands on magnetic field balance. And as imaging capabilities go up, one of the ways that these improvements are leveraged is by imaging smaller and smaller structures. Think of it as adding a new lens to your microscope, going from 10x to 25x. Beyond magnifying the image, you also magnify distortions introduced by vibration.
Now, having said all of that, the MRI equipment vendors go to great pains to 'build in' corrective measures into their MRI scanners, in an effort to not ratchet-up the design burdens. These corrective measures, however, don't always coincide with the sensitivity advances. There are 'lags' in the protection, where a number of scanners, perhaps the next one your client buys, don't have the protections to compensate.