Car lift over framed/steel decking 2

[COEngineeer]

The owner wants a couple of 4 posts car lifts in his garage. I think I am going to use steel decking and 4" of concrete. I will put steel beam right underneat the lift posts. How do you think I should attach the post? Shall I just anchor them to the elevated slab? Or shall I somehow connect the post straight to the steel beams? Comments?

 

[grandcasita]

As the owner, I'd prefer to have it mounted on the concrete, so I could move it, but as the structural, I'd like to lock the location.

Depending on maximum lift capacity, can the slab handle the post load if it off the beam by "x" inches"?
It is unlikely that the loads will be much more than 2 to 3 k per post, and the right base plate might give flexibility in locating them.

If you can design for a little flexibility, I'd try do that.

Otherwise, you could stub up from the beam with a WF or HSS and cap it with a plate which would be at finished floor level, allowing the lift baseplates to be welded in place.

 

[COEngineeer]

Grandcasita, I think I will use HSS with a plate at the finish floor. I think I will tell them to weld all thread rod on the plate so I can bolt down the posts (so they can remove it if they have to). Do you think I should put expansion material arround the HSS?

 

[twinnell]

Make sure you leave a little room for deadload deflection. If you set the base plate at finished floor elevation, then place the deck....the beams will deflect and lower the base plate below the finished floor elevation. No need for expansion joint material around the post. It isn't put around columns going through floors, so I don't see why here.

 

[COEngineeer]

Ok.. last question. So the capacity of this lift is 9000 lb. So I just go ahead and divide that number to 4 (assume the owner will load it up to max capacity). So I have this live load, plus the dead load of the lifts. Then I design the floor for 50 psf LL OR 3000 point load. Do you think L/480 LL and L/240 DL deflection limit is enough? Do I need to look at vibration when they run the lift (I have never done this calc. before). Thanks for your help!

 

[grandcasita]

tw,
I don't see a way for this to happen, unless slab and beam do not move together.

The cap plate would generally be larger than the tube, so the plate would effectively connect the slab to the beam at that location. Which brings up the question about this apparently being non-composite, and the vertical tube becoming a shear connector...another topic for another thread. At any rate, you probably don't want a path for spilled motor oil and antifreeze to run from under the lift to the area below, so I'd say "no" to expansion material.

As to loading, do not assume equal load on each post - pickup trucks (esp. with diesel engines) are heavier in front. Design for some variation... then factor it. On the other hand, the lift may be designed for 9k/4 on each post, so that might be your live load.

50 psf LL over the lift area plus the lift load is probably overdesign. It is likely that you'd design the slab for the 50 (or whatever), but the structure below would be designed for the greater of either the psf LL or the vehicle weight/lift post load. The weight of the empty lift can probably be called dead load, since it will be "permanently installed equipment."

ASCE 7-02:
Section 4.7.2 might also apply.
Also check note 1 on Table 4-1.
Note that 50 LL changed to 40 LL (in 2002) but the allowable reduction also changed.

IBC Table 1604.3: L/360 LL & L/240 L+D for floors. YMMV

 

[COEngineeer]

Wow.. thanks grand. I hardly ever look at ASCE. I just look at IBC most of the time unless it refers something to ASCE. I will double the load for the lift to 18k.

 

[UcfSE]

I wouldn't assume the load goes equally to all four posts. That's like saying the cog of the load is in the same location as your center of supports. I would also increase the load for dynamic impact. Contact the manufacturer of the lift and ask if they have a special deflection criterion they require or recommend. You should consider using the maximum load of the crane and not just the maximum load rating to help ensure that an overload will fail the lift and not your structure.

 

[csd72]

I dont see why you are making it so complicated. Why dont you just drill straight through the slab and through the top of the beam. Chemical anchor around the beam for shear.

These types of things are much easier if they can be installed after the main members are in place, it is much easier to locate them more accurately on site with the slab already in place.

I would ask the client to get loads from the manufacturer as I doubt that the motors e.t.c. will all be located centrally. for initial design divide by 4 then add 20% to each point load, then confirm these when you get the details from the manufacturer.

I would also note the design load on the drawings in case they change manufacturer.

These things move fairly slow, I doubt there will be any significant vibration issues.

 

[twinnell]

grand,

I was stating that during concrete placement, the beams will deflect under the weight of the wet concrete. The hss and plate will deflect with the beam since it is attached. But the top of concrete will remain at the desired finished floor elevation, we hope. if you set the plate height at finished floor elevation, then the finished plate elevation, after concrete placement, will be beam deflection below the finished floor elevation. I don't know the beam spans or what the deflection is, but we had similar situations where we had 1" dead load deflections that we had to account for.

 

[COEngineeer]

csd, thanks for the reply. So you suggest to only increase 20% instead of 100% that asce suggested? If you want to drill trough the slab and and trough the beam, how do you make sure you are lined up? I think the column base plate will be wider than my steel beam width. Now they even want me to wood frame the floor instead of steel decking.

 

[grandcasita]

So it will be a flat concrete slab, with wood forms, on steel structure, no?

As to lining up the anchors, that was my point in the first reply... keep the location of the lift flexible, designing for the lift to rest on the concrete slab. The loading will be on the order of 80-100 psi under the post baseplates, so concrete compressive capacity isn't an issue.

I do not see a need to anchor to the structure, just KwikBolt or adhesive anchors to the concrete.

 

[COEngineeer]

Ok, I just didnt feel comfortable putting the post on the slab without a beam underneath it. I will use 1 1/8 plywood w/ 4-5" of concrete with LVL joists and steel beam.

 

[csd72]

Sorry COEngineer,

I should have read your post more closely (my day jobs distracts me from my eng-tips posts-not the other way around).

Anyway, you should design it as an elevator and double the loads, then (maybe)add 20% as a preliminary measure to allow for non-uniform loads.

It is essential that you get exact loadings confirmed by the manufacturer because you may actually find the car loads are only on 2 posts!

 

[COEngineeer]

I am just going to follow my gut, I am doubling the lift load, use steel decking, use steel beam underneath the posts, and just use chemical anchors to the slab. I will also design it for L/600. Thanks everyone!