Reshoring deck for forklift load 2

[Dzastr]

Concrete sub wants to reshore garage deck down to mat slab with forklift loading in mind. EOR will not allow forklift to be driven on garage deck without reshores because garage deck was only designed for a 6,000lbs automobile load, forklift weights 8,000lbs and will have a 2,000lbs load.

Forklift Load = 10,000lbs
Forklift Footprint = 20sf

do i need to reshore for a 500psf load, or can I assume the deck spreads the weight out to other shores? keeping in mind that I will be reshoring down to mat slab, meaning there will be no deflection in the garage deck. I have an 8" thick pt deck. what other information about the deck would be usefull in figuring this out?

is there a good place to learn more about shoring and reshoring other than ACI 347 and ACI guide to shoring and reshoring?

 

[msquared48]

How much insurance do you have?

Mike McCann, PE, SE (WA)

 

[Dzastr]

My initial take was to reshore for 500psf. My owner is not excited about that. Looking for a reason to convince myself I am being overly conservative. Here's the deal though, I know this owner has done this numerous times with reshores designed to support only the live load / dead load of concrete placement which is more like 250psf. No issues to the shores but I'm thinking he was probably just dipping into his factor of safety, or the deck is doing a good job of spreading that load to shores outside of the forklift footprint. If there is no deflection in the deck does that distributing of load still occur?

Not trying to rely on insurance. Too earlier in my career for that kinda move. Looking to be legit on this.

 

[dhengr]

Dzastr:
It seem to me that you basically have 5k on each of the front wheels, at whatever their spacing and tread print area are. And, you should move these around on the slab to find max. moments, shears and punching shears for that slab; and then adjust the shoring to make that slab work under those load conditions. I don’t think 500lbs./sq.ft. cuts it. Under max. load on the forklift there will be very little load on the rear tires.

 

[Dzastr]

2000lbs is our max load, the forklifts max capacity is 5000lbs, we just don't have a need to pick anything that heavy up, which is why I spread it over the 4 tires.

ReShore spacing for 500psf based on shore strength alone would be something like 3' both ways or 3'x4'. Pretty tight spacing. Going tighter than 3' on center would not be feasible. Therefore, any issues with punching or shearing would have to be remedied with beams, not increased shores.

 

[jayrod12]

As dhengr stated when loaded very little load will be on the rear wheels and most will be on the front wheels. If it were me I'd be designing the front tires for 40% of the load each and 10% on each rear wheel.

 

[Dzastr]

10-4

How about that deck though? Do I need to assume it offers as much help as a wet paper towel or can I assume it spreads some load out to several shores. What would go into that? I can't wrap my head around that with the fact that I am shoring down to slab on grade and won't be getting any/much deflection from the deck.

 

[jayrod12]

Do you have any reliable info on the deck construction? Thickness, and/or reinforcing? really it won't ever share load in a helpful way beyond the first line of shoring in each direction. AT least in my opinion but others will likely disagree.

 

[Dzastr]

8" pt cable deck. It's a new deck we are building so I can get any info on the deck that I need.

 

[jike]

Dhengr is correct. Majority of load (85 to 95%) is on front tires. Very little is on the back tires. Add impact also.

Can the tires punch thru? What is the bending affect of one concentrated load on the other? Have you checked beam shear?

 

[Dzastr]

my initial concern is not punching through the deck, its the post shores failing. the deck is a parking garage deck and is designed for cars traveling over it with no reshoring. EOR stated reshoring the deck with post shores would alleviate his concerns, he's just worried about the bending caused by the forklift loading over the tributary area of the columns. That said, I'd prefer to check these things myself just to be sure but def going to have to brush up on some of my reinforced concrete design skills first, its been awhile. any recommendations on a paper i could look at to refresh?

the first problem id like to solve though is what i am going to be using for posts. if i space them too far and a fork lift drives over one directly (which will happen) it might not be a very good scene, the ones we own are only rated for about 5500lbs at this height. the spacing required for that situation would be so small it might just be shores practically shoulder to shoulder with eachother. I'd run out of shores quick. Might (likely) need to rent something with a higher rating.

 

[Once20036]

I have a hard time believing that it would spread the load to several shores, given that the bottom of the shores is supported on a mat footing.
In order to get the load distribution that you're talking about, you'd need to experience some deflection at the posts.

Does your slab have a continuous top and bottom mat of reinforcing in addition to the PT? Once you have the posts installed and are running the lifts, I imagine you'll be getting a lot of negative moments where you wouldn't have any with the slab in it's final use.

Can you define a narrow aisle where the forklift is allowed to travel? Or does it need access to the entire deck?

I think renting stronger post shores is a good place to start.

 

[RHTPE]

What is the slab span? With an 8" P/T slab, I'd assume the slab spans 24-27' or so.
What is the out-to-out of the forklift's front wheels?
What kind of shores are being used for reshores? Steel, aluminum or wood?
The design superimposed live load for the 8" slab would likely be 50 PSF, with some minimum wheel load.

This method of moving beam forms and slab deck tables is done often in multi-story parking structures with no adverse effect, though I can't be certain about the typical gross weight of the forklift + load.

Assuming a forklift with a 3' tread width + 2' contributing slab width each side, there would be at least a 7' slab strip supporting the weight of the forklift + load. With an assumed slab clearspan of 22', the service load strength for this 7' wide strip has to be near to 3.85K in shear + 14 ft-K in bending.

The moment from the forklift on an unreshored slab would max out at about 27.5 ft-K, so obviously reshoring is required.

As far as the reshores go, most steel or aluminum post shores at parking garage shore height have a SWL close to the total load imposed by the forklift + load (10K). Single post shores should have a factor of safety of 3:1.

If reshored down to an immoveable surface (probably no such thing, other than a massive mat slab), shear will be a concern. If reshored to an equally stiff 8" elevated slab below, then load sharing occurs, each slab bearing approximately 1/2 of the applied forklift load. Maximum slab shear force gets cut in half as well. Add another level of reshores and the applied load gets distributed (more or less) equally to 3 floors, which gets you pretty close to the service load capacity.

I typically work with service loads when designing reshoring. It is possible to justify a smaller load factor when dealing with construction loads as they can usually be more reliably determined. If you go this route, you could use a construction load factor of say 1.2 or 1.3, which could give you a bump of 23-33% in slab strength. I find that juggling the load factors makes the analysis more confusing for those who have to follow the analysis.

The critical situation is when the slabs are reshored to the ground, as ACI tends to treat the ground as an immoveable surface, and the reshores as infinitely stiff.

Of course, the reality is different. The gound supports will behave more like springs as will the reshores. The imposed construction load distribution to completed floors tends to put more load in the uppermost completed floors because of this.

JMHO.


Ralph
Structures Consulting
Northeast USA

 

[Dzastr]

Ralph,

i have spent the last few days trying to figure it out myself in order to avoid having to ask you stupid questions, but i haven't been successful.

What calc are you doing to yield they 3.85 in shear and 14 ft-k in bending? Also having trouble backing in to your 27.5 number,

Also, we are shoring down to an 18" mat slab, safe to say we wont get much spring out of that, but the re-shores will provide a little. The reshores we are using at steel, and at this height they will get about 5.5kips

 

[RHTPE]

Dzastr,

Obviously this requires a more in-depth discussion than I would care to type in a forum like this.

My contact info is on my web site. I'm not clear on how much we are allowed to share in a post in this forum, so I'll leave to your ingenuity. It's a dot com.

Ralph

Ralph
Structures Consulting
Northeast USA

 

[RHTPE]

Dzastr - Did you ever come to a satisfactory conclusion regarding the reshoring for the forklift?



Ralph
Structures Consulting
Northeast USA

 

[precast78]

I have not read all of the responses but please be careful with forklifts! They often use solid tires or foam filled tires and the deflection of the tire is very minimal to distribute the load. Your load will almost like a line load.

 

[Archie264]

Dzastr,

M = (7')(.050 ksf)(22'^2)/(12) = 14'k

V = (7')(.050 ksf)(22')/(2) = 3.85k

That said, call Ralph. Take him up on his offer; don't go this one alone.

 

[Dzastr]

So far we are actually have fairly good luck using pallet jacks. It is not ideal, but might get us through this job, providing us enough time to get this figured out prior to our next project. hasn't hurt production much either.

I will keep you all posted.

 

[Dzastr]

Just to touch on that Moment calc though, referencing Lindenburg's Civil Engineering Reference Manual, APpendix 47.A, rather than using


(eqn #5) w*l*L^2/12

shouldnt we be using

(eqn #22) (wL^2/12)(1-6(a/L)^2+4(a/L)^3) where a=distance from support to distributed load?

Seems like the way eqn 5 is being used, really what your doing is spacing a 7*0.05=0.35k load over the entire beam.