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.
------------------------------
William Moorhead P.E., S.E., M.ASCE
Alpharetta GA
------------------------------
Original Message:
Sent: 08-25-2017 14:26
From: Angela Fante
Subject: Hangers from Composite Slabs on Deck
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
------------------------------
Angela Fante P.E., M.ASCE
Philadelphia PA
------------------------------