Scissor Man-Lift on Mezzanine

[BSVBD]

We typically design precast mezzanines for 125 PSF (or other).

Garages are designed for 40 psf. Most large pick-up trucks weigh just as much as the typical scissor man-lift. However, the man-lift area is more concentrated with lesser area.

The typical man-lift weighs approximately 4,000 LBS including allowable capacity.

When construction begins, the crew wants to put a 4,000 scissor man-lift on the mezz for various construction purposes. The mezzanine was not designed for this. However, I think that a 4,000 LB lift located anywhere on the precast plan, prior to occupancy, should not have any significant cost increase, if any at all.

We typically have to submit an inquiry to the precast supplier to get approval. We do this several times per year.

How can I provide a blanket statement on the construction documents to prevent this on-going, repeat inquiry from the General Contractor? My thought is to have this note:

"DESIGN PRECAST MEZZANINE FOR A 4,000 LB MOVABLE, SCISSOR MAN-LIFT TO BE LOCATED ANYWHERE ON PRECAST PLAN. STORAGE OR OTHER LIVE LOADS NOT ALLOWED ON SAME PLANK THAT IS TO SUPPORT MOVABLE MAN-LIFT."

Comments? Suggestions?

Thank you!

 

[bearjew]

Your note looks like a good compromise, provided your precast supplier reads it.

We typically design elevated slabs on metal deck over bar joints. Design load is 125psf majority of the time. Reinforcing is welded wire fabric. What we have noticed, is given these conditions, if you run thru the calcs for wheel loads due to scissor lifts the numbers often do not work out for the slab spanning between joists. A 4" slab on deck w/ 6x6-W2.9 WWF on 3' bar joist spans only has a wheel load capacity of around 800lbs.

However A) good luck preventing the contractors from doing it anyways because of the time it saves and access it allows and B) decades of experience tells us that it has been done time and time again with no issues (provided it is a fairly standard sized scissor lift).

 

[jayrod12]

The NBCC requires you to design for a 9kN load over a 30" square area anywhere for floors of offices etc. You're halfway to the full weight of the man-lift at that point. At least that's how I typically justify it.

 

[JLNJ]

You might want to be specific about wheel load and contact area rather than total equipment weight.

 

[KootK]

I'm a precast supplier sometimes. Loads of this scale are a non-issue for most of the common plank products 8" and up. On the order of the shear capacity of a single web perhaps. If the span is short, flexure is a non-issue. If the span is long, you'll get distributed response and the load will vanish in the noise. Specify the note, we'll read it and dismiss it as trivial in most cases, and we'll both spare ourselves the inquiry. The places I could see this causing issues is perhaps a long, narrow bridge or a cantilever. In the case of the cantilever, that might add cost as we try to avoid necessary top strand when required. And there may be back span tie-down issues. I'd expect even that to have a nominal impact though. Chances are the guy doing the quoting wouldn't even register this in the first place.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.