Vehicle Lift Seismic Design
Vehicle Lift Seismic Design
The bldg structure was originally design-built in the early 80s.
Bldg has 3 main areas. Plan view imagine a rectangle, split into thirds, center third is steel braced frame structure with tilt wall facade; left and right side are steel moment frames, with Conc Tilt wall supporting the roof. One end has an additional steel moment frame supporting an overhead Maint Shop rolling crane. The other end just has the concrete tiltwall supporting the roof and interior steel moment frame supporting internal equipment and sundry gear.
Focusing now on this later end, without the crane. This is where somebody wants to install 12- 26,000 lb vehicle lifts.
(They saw an add, wanted to install equipment off the shelf with a rotorhammer and electricians tape,...already installed 12- 15k to 16k lifts, which are now failing (equipt age), and want to replace them yesterday with 12-26000lb lifts.
Steel frame is supported with concrete footings with perimeter strip footings and bond beams about 40ft oc forming several square bays (plan view). Strip footings are no more than about 18" wide by 14.5"deep, while the bond beams are 16" wide by 24" deep.
Looks like the roof is supported by tilt wall structure which sets on the perimeter strip footing, with steel columns CL offset about 14" parallel to the tilt wall structure, about 7" gap between them.
The entire floor is a 6" thk rein concrete slab with only #3 rebar 16" oc ew. FF is 12" above the bond beam, so except at the columns I don't know what's between the bond beams and the 6" slab.
Above is just to give a feel of the structure.
Now for the Vehicle Lifts.
I'm in SDC D using say SD1 of 0.68 and SDS of 1.14g.
Manufacturer says he can install his lifts, provided the slab is 12" thick 4000 psi with #6 60k rebar 10" oc. Only problem is I'm at 6" with #3 rebar and after 30 years, I wouldn't be surprised if it isn't corroded through.
Manufacturer's typ seismic design for 15k lifts some 15 years ago altered his design to 14" thick slab and 2 course of worst case #8- @ 6"oc and 2nd course #4 @ 6"oc ew.
Since mine is heavier lifts, it's probably a more stout cross section. Since the seismic will need to be designed both directions, the slab replacement will end up selectively demo'g each bay slab (which has POL containment) at about 36' sq per bay.
Manufacturer's installation calls for about 14-1"x10" epoxied post install anchor bolts. For seismic (2 way lat forces), this probably will need to be modified to about 25 bolts, I suspect.
Here's my real question.
Even if I get the slab properly designed for the lift loads,
1) What seismic provisions would you design for? Nonstruct Equip?
Elevator? I'd probably error on the side of safety and go with heavier demand, but there may be some more eloquant treatments.
2) Construction engineering,..selective demo of slab. Do I risk the slab is separate from the bond beams, or did they place the concrete in one lift? I could core, but I don't want to core the bond beam.
3) Construction Engineering,..during construction, I suspect the slab wt is being used in the original design for overturning moment. I don't have original calcs. I can go back and redesign to quantify, but if so, I end up spending twice the design effort needed.
4) If I just replace the slabs and equipt, total project cost is likely to come in about 15% of the total bldg cost. Of course I could tear down 1/3 the bldg and rebuild from scratch, but it'll take more time.
5) Additional risk factor just to make this project more fun: Entire bldg is surrounded by about a 40acre tarmac of 8" unrein concrete on a 2% grade over native 4% grade, which when originally placed, didn't receive subsurface drainage design. 20 years after construction, cavernous voids formed after rainstorms, about 3 ft deep x 12' wide x 80ft long, later filled by others with grout, about 200ft downstream from the bldg. Source of the erosion water never has been verified.
I'm leary to open up Pandora's Box
There is a good chance if the project doesn't go with the slab replacement, they won't get funded for another 10-20 years. Existing lifts show no signs of slab failure, so slab replacement will likely work.
Other than a more dedicated structural investigation, any recommendations/ideas/considerations you might put in a scope of work for a DB project?
I'm thinking this through on the fly, but am trying to step back with a little more wherewithal at this point.
The existing lifts were installed over the past 20 years without documentation, nor any professional review. Tenant maintains vehicle which have steadily increased in weight. I wouldn't be surprised if the reason for lift failure hasn't been to overloading the lifts with too heavy of vehicles, but the foundation shows no sign of damage or cracking.
Somebody ran a ground penetrating radar over some of the slab and reported it was quite a bit thicker, but that report might have been from either a poorly calibrated machine or running it over a bond beam. Just thinking out loud....
Project funds dry up in a week and it is wanted now.
Prudently speaking I tend to follow some rules:
1) Don't throw good money after bad.
2) Don't rush the design.
3) Render a professional service.
4) Even if it isn't fully designed, there is something to be said for working results.
FWI, I don't see gauge length in the manufacturer's installation recommendations for his concrete anchors to meet seismic codes.
Tenant wants vehicle lifts and hates pits.
Curious if anybody else had had similar issues with vehicle lifts and the seismic codes????