Integrated Buildings & Structures

​Has anyone perfected a process for specifying what can and cannot be suspended by MEP trades from the underside of a composite slab on deck? 

Over the past few years, with the accessibility and cost effectiveness of "Bang-It" anchors, we've discovered in the field (with increasing frequency) 15" storm lines suspended from a single hanger, trapezes of (24)-6" conduits suspended from two hangers, and other relatively heavy point loads to the underside of composite slab on deck - in some cases, upwards of 7,000 pounds from a single point.  (In some cases, a steel beam flange was located only 2" away, but they "didn't want to mess with fireproofing!") 

Although the MEP hanger specs prohibit anchoring to the underside of composite slab on deck without permission from the structural EOR, it is regularly ignored by subcontractors in their bids.  This is especially true on buildings with intensive MEP systems, such as hospitals and research lab buildings.

We've tried providing the contractor with a maximum psf load that may be anchored to slab on deck (coupled with maximum pipe diameters and individual point loads), but the psf calculation is mystifying to many, and the coordination of such a task is "too cumbersome" that they never complete it.   

While we don't want to be overly stringent (there's no reason a 1" conduit can't be supported by a 5.5" slab on deck), we also want to be part of the conversation to make sure the proposed hanger loads are reasonable. 

We're wondering how others address this issue, and would love to hear your thoughts!

Thanks,
Angie

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Angela Fante P.E., M.ASCE
Philadelphia PA
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The MEP equipment is essentially unknown, even with the specs in hand.  Medical equipment is often changed by the owner at the last minute.

Designing with Vulcraft is available for free: 

http://www.vulcraft.com/catalogs/#Decks%20Catalog

Here you can calculate the concentrated capacity of the composite slab.

 I would check with the deck manufacturer, when the span is short.  I don't expect the capacity to be much greater than the Bang-It anchor.  So the allowable load should be no greater than the capacity of the anchor which would be about 800 lbf when installed in the lower flute.  Subtract the live concentrated load expected on a occupied space (300 lbs) and you are looking at about 500 lbf max.

So the spec would likely read like this:  Allowable load anchored to underside of slab on deck is not to exceed 50 psf (table value - live load) or  a concentrated load of 500 lbf applied to a 3 ft width of deck (calculated effective width).  I would consider adding a sketch to the drawings as well.

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Chad Morrison P.E., M.ASCE
Professional Engineer
Greenville RI
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Hello Angela,

I work for a steel deck manufacturer, Epic Metals. I know discussing proprietary products on this forum is frowned upon, so if you would like to discuss the hangers that we've designed specifically for our deck profiles feel free to contact me at jzuger@...

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Jeremy Zuger EIT, S.M.ASCE
Engineer, Sales Technical Services
Munhall PA
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​

Angie,

I have run into this issue many times in the past with similar frustrations. My experience is there is no catch-all solution. In addition, any solution is going to be interpreted differently from one contractor to the next – or altogether ignored – and you're still dealing with a field issue. Here are my recommendations from my experience.

Coordinate with your MEP designers and get a sense of what they expect to be hung from your structure in the field. Probably the most difficult is fire water piping since that is typically performance specified with the design performed by the contactor (a specialty sub). But the MEP engineers should have a good enough idea to allow you to consider it in your structural design.

For example, a standard 6" diameter water pipe can span about 33 feet which will induce a reaction of about one kip which is of little concern for a typical topping slab. Electrical conduit is typically not a concern either as you point out, and even cable tray supported on B-Line or Unit-Strut trapeze hangers is well within norms. A five-foot wide fully loaded cable tray support for a 20-foot span will induce about the same anchor reaction of one kip. This is assuming a 20 psf design load which is what PIP recommends for cable tray supports. This would cover most typical commercial or industrial buildings.

For more intensive MEP systems as you describe like hospitals, research labs, etc. I have seen the use of utility support trellises designed for the specific use of the MEP subcontractors to place all their utility lines. To properly design this trellis, you need to have a pretty good idea of what will be supported so you can use an appropriate design load. The advantage of using a trellis is you have control of the connections to the main structure (including seismic restraint if necessary), and you can provide a convenient framing layout of the trellis for the utilities. The disadvantage is, it may require more ceiling space. But in the end, it will probably not be more space than would have been required anyway and it is a known quantity from the beginning that will avoid the possibility of not having enough ceiling space as well as help avoid congestion. Coordination is the key.

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David Chilton P.E., S.E., CP, M.ASCE
Senior Associate Structural Engineer
Burns & McDonnell
Kansas City MO
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I use a big note, such as,
 ''Any unscheduled loading that is applied to the structural components (you may want to list what are components) and not shown in the permit plans and specs shall be submitted to the EOR for review and potential alteration of the structural plans and/or the loading magnitude or pattern prior to the application of the loading.  The same procedure shall apply to unscheduled physical alterations (openings, notches, slices, etc.) in any structural component. The time required the EOR to accommodate the design alteration, if any, for an amendment of the building permit documents will depend on the number, magnitude and extent of the loads and/or physical alterations. The constructor shall not install any unscheduled loads (those not shown by features on the most current version of the permit documents) or any alterations to structural components in anticipation of the EOR fully approving 'as-is' the loading or alteration after it had been completed.''

The structural engineer should never, ever relinquish any control of his or her work product, including the work products of other (specialty) structural engineers in their submissions to the structural EOR.

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Robert Bullard P.E., M.ASCE
President
Ahimsa Technic Inc
Ponce Inlet FL
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First the engineer designing the structure must design the structure so that it can support the future loads.  This means that the engineer designing and specifying the mechanical equipment must provide input as to expected loads and the maximum spacing of supports for the piping, conduit, and ductwork.  While the exact loads may not be know until the design of the system is finalized during construction it should be possible to reasonably estimate the future loads.

It may make sense to provide additional beams where there will be heavy piping loads or equipment loads.  This can be done even though you do not know the exact location of the applied loads. 

The design of the local support members for mechanical, electrical and plumbing equipment needs to be delegated to a registered engineer.  This engineer is often somebody who is hired by the Contractor.  This will be treated as a deferred submittal that will be submitted to the building department. 

The engineer designing the structural frame needs to be provided with the loads to be applied to the structure as well as the details used to connect to the deck or structural frame.  The loads should include both maximum point loads and the overall loads on the frame. 

There are firms that specialize in designing these support systems and providing the trade contractors with the hardware to be used in installing the pipes, ducts, conduits, and equipment.  On some projects the contract documents require that one of  these firms be retained to design these support systems. 

If you have a problem it is likely that either the engineer designing the structure did not think about this issue, the MEP engineers refused to do their job, the contract documents did not delegate responsibility for the design of these support systems, or the contractor did not fulfil his obligation. Basically somebody did not do their job.

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Mark Gilligan P.E., M.ASCE
Berkeley CA
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Original Message:
Sent: 08-28-2017 12:46
From: Robert Bullard
Subject: Hangers from Composite Slabs on Deck

I use a big note, such as,
 ''Any unscheduled loading that is applied to the structural components (you may want to list what are components) and not shown in the permit plans and specs shall be submitted to the EOR for review and potential alteration of the structural plans and/or the loading magnitude or pattern prior to the application of the loading.  The same procedure shall apply to unscheduled physical alterations (openings, notches, slices, etc.) in any structural component. The time required the EOR to accommodate the design alteration, if any, for an amendment of the building permit documents will depend on the number, magnitude and extent of the loads and/or physical alterations. The constructor shall not install any unscheduled loads (those not shown by features on the most current version of the permit documents) or any alterations to structural components in anticipation of the EOR fully approving 'as-is' the loading or alteration after it had been completed.''

The structural engineer should never, ever relinquish any control of his or her work product, including the work products of other (specialty) structural engineers in their submissions to the structural EOR.

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Robert Bullard P.E., M.ASCE
President
Ahimsa Technic Inc
Ponce Inlet FL
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Angie

You have expressed the age old problem of how to get the contractors to read the contract documents.  My solution is a combination of the advice given prior.  I would request both the Architect and MEP designer to have requirements for review of hangers included in the contract specifications,  Prior to bidding I review the mechanical drawings and try to catch any special conditions that I need to address on the structural drawings.  Prior to construction at a startup meeting I remind the contractor to review the specifications and the drawings.  Then you usually hear "We didn't account for that,  do you really need it".  However, doing those things usually catch the mistakes before the slab is cast.

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David Thompson P.E., M.ASCE
Principal
KTA Structural Engineers Ltd.
Calgary AB
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Load bearing capacity of concrete anchors installed
into the bottom of concrete filled metal deck floors
can be found in the evaluation reports pertaining to
specific anchor types from different manufacturers.
One can go to ICC Evaluation Service, LLC (ICC-ES) and enter 'anchor' in the product category. This will bring up the approved evaluation reports from different manufactures. Load capacity of the anchors can be found therein. In addition to verifying the capacity of individual anchors one must also verify the capacity of the concrete filled metal deck as a structural members spanning between the supporting beams.

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Sharadkumar Pandya P.E., SE, M.ASCE
Structural Engineer
Hercules CA
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​Angie: If you haven't already done so (and I am getting from your note that your dilemma has some history), you may want to consider what's contained in the publication " C-2011 Standard for Composite Steel Floor Deck-Slabs" (SDI ANSI C 2011), along with the tables in SDI's "Floor Deck Design, 1st edition" publication (the manual). 

Beginning on page 13 of SDI ANSI C 2011, there is a discussion and a protocol presented to evaluate concentrated load effects.  There is a method of considering an equivalent uniformly distributed load spread over an effective width (be), from a concentrated load.  This method is conceived for a load presented on top of the slab, by a plate, with dimensions (b1 and b2), which obviously does not correspond neatly to an anchor from underneath - someone will have to use further judgment to get around that (or decide that he/she can not, I suppose.) 

I had occasion about a year ago to slog through the calculations, for an evaluation I was doing, and I created a spreadsheet to keep track of things -- there are moment, direct shear and punching shear checks to be performed, plus deflection.  One thing that helped a lot, in addition to the C-2011 standard and the SDI manual, was an example problem found in the Canam deck catalog, on page 33.  The example is from a design standpoint, and you are doing evaluations, but it is still the best starting point, along with the publications. 

It is a lot of figuring, and if you have many of these loads bunched together, then it may be close to impossible to get comfortable with how to work a solution, but  for cases with a more or less isolated single concentrated load, I like that once the equivalent load is determined, it all goes pretty rationally, using more or less standard concrete engineering, and the table data from the manual.  The table data from the manual gives the the phi-My, phi-Vn and moment of inertia.  If you use the quantities in the tables, you don't have to go back to rock bottom first principles, as presented in the appendices of C-2011.  I used tables 3, 4, and 8 from the manual in my calculations.  If your slab has no steel mesh, then you will not have much capacity in the direction opposite the ribs, and that will put you at a disadvantage.  In my evaluation, the slab I was working with had 6x6 - W2.1X2.1.  The C-2011 is a free download, but the Floor Deck Design, 1st edition publication has a cost. Good luck to you on this.

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William Moorhead P.E., S.E., M.ASCE
Alpharetta GA
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Angie,
We are very familiar with the issue you raise and have not perfected a solution, but we are making progress.  
Our approach is a combination of many of the items from other respondents.

We
- provide details in the structural documents indicating load limits and spacing criteria
- make sure the structural specs are clear 
- do not want to have to answer the question from the MEP contractor asking whether "X" is acceptable or not.  We want the coordination and design responsibility to reside with the contractors as they know their systems 
- work with the MEP consultant and review their details and specs.
- make sure that the MEP specs don't refer the contractor to the structural engineer (us) to verify their approach
- mark up MEP specs so that the contractor needs to determine hanging loads and review limits on structural documents.  If they exceed limits, hire a delegated design engineer to design the approach including any attachments to slab

Best of luck

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Bradley Hughes P.E., M.ASCE
Senior Structural Engineer
Flad Architects
Madison WI
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