Determining Appropriate Load Area for Forklift Operations on Concrete Deck 2

[mason2023]

I need advice on a specific load calculation for a project. Our task involves using a forklift (8000 pounds) on a 10-inch concrete deck, designed for a maximum live load of 75 psf. The deck is supported by 24-inch wide by 30-inch deep concrete beams, spaced 18 feet 8 inches apart, and foundation walls about 25 feet apart.

The question is: Should the live load of the forklift be calculated based on the tire contact area (10" x 10" per tire, 4 tires) or the forklift's overall footprint (5' x 3') or something else entirely (does the load spread at all as it travels down through the concrete?)? It is fairly equipment agnostic and a forklift is only one piece of equipment that we are considering.

This decision will impact the safety and feasibility of using a forklift or another piece of equipment on the deck. Your expertise on which area to use for accurate load calculations would be greatly appreciated.

 

[BridgeSmith]

AASHTO has extensive design guidance for concrete decks. do you have access to the LRFD bridge design specifications?

 

[mason2023]

Unfortunately, I do not. I am a contractor who has received contradictory information elsewhere and was hoping that this forum might be able to shed some light on factors that are more or less relevant.

 

[phamENG]

Unfortunately none of us will be able to give you the guidance you want. You'll need to get an engineer with experience in this area on site to review the conditions and determine what kind of wheel loads can be safely supported on your slab.

 

[WesternJeb]

I am going to second phamENG. I began to type out all of the factors that go into it...

The concrete would need to be analyzed both locally (directly under the tires) and then on a larger scale (the load for the entire footprint between each of the concrete beams). How long has it been around? What is the reinforcement?

Those are the most basic items/questions that would need to be addressed and needs to be looked into by a project engineer, as phemENG says above.

 

[JLNJ]

Start with the loaded front wheel loads. The 8000 pounds is presumably the lift capacity and not the total loaded weight. Looking at the contact patch on the loaded front tires will give you a sense of the local punching and beam shear demand-to-capacity ratios.

After that, it's just a matter of moving the loaded wheel loads around to find the worst case for shear and flexure.

Oc course you need to know the slab geometry, concrete strength and reinforcement details.

 

[RattlinBog]

mason2023- Are you the structural engineer analyzing this floor, or are you the contractor concerned with safety of operating equipment on the floor?

Apologies ahead of time if this comes across condescending--that's not my intent. I'm going to make a guess: I'm guessing you're working with an owner on some construction project, and a question came up about vehicle loads on this floor. Someone found an existing design drawing, and there's a note that says live load = 75 psf. If that's the case, I would be cautious about using the 75 psf LL as any kind of metric for vehicle loads. It's usually not that simple (ignoring parking garage design for a minute...)

Here's a little food for thought:
Ford Taurus
Curb weight: 4000 lbs
Wheelbase: 9.5 ft
Track width: 5.5 ft
Pressure: 4000/(9.5x5.5) = 77 psf

Your Forklift
Weight: 8000 lbs (Like JLNJ said, maybe this is just a 4-ton capacity, and you still need to consider the total weight...not to mention unequal axle loads)
Wheelbase: 5 ft
Track width: 3 ft
Pressure: 8000/(5x3) = 533 psf

I feel that trying to calculate the psf live load demand from the forklift and comparing that to the max allowable shown on a drawing isn't the right approach. There's more at stake. I'm making some assumptions about your scenario, but I imagine you get what I mean... Wheel/axle point loads strategically placed to get an envelope of shear, positive bending, and negative bending demands would be a better approach. You would need to check demands on both the slab and the beams. You'd need to calculate the slab design strength for one-way and two-way shear and positive/negative bending. Same with the beams. You need concrete properties, reinforcement size and spacing, span conditions, and other design info to do this justice.

I'll admit that a 10" slab and 24x30 beams could make for a fairly stout floor, but that's largely dependent on the reinforcement design. As others have said, I would recommend hiring a local structural engineer.

 

[mason2023]

Appreciate the insights so far. I'm a contractor considering a bid for this project. The bid documents specify a 75 psf Live Load (LL) limit for the existing slab, but lack detailed information about its condition. Given that the slab is scheduled for removal, it's likely not in prime shape. My immediate task involves removing stones before slab replacement, yet the General Contractors (GCs) haven't confirmed whether using heavy equipment on the slab is feasible. The example comparing the Taurus to the forklift is exactly why I am asking these questions.

Hiring an engineer for a detailed assessment during the bidding phase isn't feasible due to time and budget constraints. It's a tough spot to be in! Certainly, if we win the bid, we'll engage engineers to formulate a safe and practical plan. For now, I'm trying to gauge the feasibility of using a forklift or a similar vehicle for the job. I need to understand the best, worst, and most likely scenarios for stone removal, considering the uncertainty about the slab's capacity to support heavy equipment. This would help in planning whether a straightforward approach is possible or if we'll need to devise a more complex solution.

 

[RattlinBog]

That's a tough place to be. Unfortunately, I don't have enough experience in navigating that to give you much advice. Could the GC ask the owner to clarify how the 75 psf LL limit was determined (by a PE hopefully)?

A slab can certainly distribute load to a decent-sized area, but that can be challenging to determine, especially without any reinforcement info. Maybe the 77 psf Tauras vs. 533 psf forklift isn't a fair view of real-life distribution, but I know I wouldn't feel great sticking my neck out if I didn't have all the facts. I've checked things like this before using wheel loads placed to get the largest demands.

Not a lot else I can add that would be useful...good luck

 

[phamENG]

Staging demolished material and driving machinery on an elevated slab will require some considerable effort from a construction/demolition engineer. Have you worked with one in the past? If so, you may want to lean on that relationship and get a fee and some preliminary thoughts from them. They can probably give you a high level idea so you can estimate costs, and then provide you with a detailed plan for staging, paths of travel, etc. once you have the job and have signed their engagement letter.

 

[SlideRuleEra]

mason2023- There are ways to get an approximate answer, but you need to clarify one item:

Does the forklift weigh 8000 lb. with no load? If so, what will be weight of the load?
or
Is 8000 lb. the total weight of the forklift and the load?

 

[mason2023]

I wouldn't be responsible for the demolition of the slab. I just need to remove some stones, and then get them out of the area. The GC has its own headaches to worry about with the removal of the slab.

What I am hearing is that I need to proceed cautiously, probably carry a fairly significant amount of money for engineering, and lean towards worst-case scenarios while hoping for the best.

 

[EZBuilding]

Are you going to pile up all of the stones in the middle of one of the beams while the forklift is being used to move material around? Is the GC going to have pallets of titanium plating on the slab while the forklift is being used? Do you have existing structural drawings which identify reinforcement, or just dimensions of the elements. Is the stone mud set directly on the slab, or is there a topping slab above the structural slab?

It becomes very challenging - and risky - as an engineer to get involved in these discussions as we do not have a complete understanding of the means and methods and sequencing of construction. This question is more complicated than how you have prompted it.

For your original question - in a vacuum - I would look to find the forklifts product data sheet to see how it identifies load distribution between weights. I would consider both the weight in the contact area and as a distributed load in checking the floor system capacity.

 

[mason2023]

@Sliderule - I have located a forklift that weighs 8000 pounds and can lift up to 5000 pounds which would cover an area of about 16.5 sf. Alternatively, I have also located a small telehandler that weighs 11,000 pounds and occupies 44 SF. So it's heavier but the weight is spread out more.

 

[Enable]

I'm a contractor who uses heavy equipment on existing slabs for demolition. Usually we shore for anything greater than the weight of a small skidsteer.

Why not qualify your bid assuming the acceptability of your equipment to be used and exclude any shoring or strengthening and design therefore if it ends up being required?

EDIT - Also your last post seems to suggest you are thinking about your equipment as producing a UDL. It does not. It produces discrete point loads at the tires, which vary significantly from front to back when you have a full payload. You really should just qualify your bid if this is not something you have significant experience with.

 

[mason2023]

@EZBuilding - No matter what process is used, each stone will be completely removed from the deck upon removal. There will not be any stacking on beams or anywhere else on the deck. At most we would have one machine and perhaps 6 people working on this deck (200' x 25') simultaneously. This is more like a surgical extraction and not a demolition operation.

The watertable that we are removing is not sitting on this concrete slab at all. The deck was poured up against the granite, burying the bottom several inches and locking it into place. I already know how to deal with that part of this mess.

 

[mason2023]

@enable - oh, there are definitely going to be many, many detailed qualifications on this one! Your comment about UDL vs discrete point loads gets to the heart of my questions. I know that when we lift something with a forklift almost all of the load is on those front tires. It becomes a question of how far does that load spread within the concrete.

In this case shoring below the deck is impossible. Either we can use a small machine without shoring, or we have figure out a way to move 4000# stones by hand. Luckily I do have a workable plan B.

 

[XR250]

Based on some of the youtube videos I have watched, you might consider the full weight of the forklift and payload on one front wheel!

 

[BridgeSmith]

The load / footprint analysis could be very misleading. Our bridge decks, designed for 32,000 lb axle loads, are only 8" thick. Granted, they don't typically span 18' between girders. However, we also have slab bridges that span up to 30', that are only 14" thick. This is possible because the distribution of load through concrete spreads the load well beyond the footprint of the vehicle wheels.

I'm not saying the slab will support the forklift. I'm just saying that it very well could, if the reinforcing is adequate, and in decent condition. The concrete itself would have to be in very poor condition to affect the capacity significantly.

Also, the design live load on the interior beams presumably would be 1350 lbs/ft (75 psf * 18'). Depending on the span of the beams, they may adequate to support the concentrated load of the forklift. If it's feasible to route the forklift along the beams, you could probably use it. For reference, if the beam span is more than 12', 1350 lb/ft generates a larger moment on the beam than an 8000 lb point load.

That said, I would recommend communicating your dilemma to the GC. Let them know that without additional information about the as-is capacity of the slab for equipment loads, you will have to assume completion of the work without the equipment, and your bid will be necessarily be much higher as a result. They may or may not deem it worthwhile to do the leg work to provide the information and get better bids. If they don't care to provide it, I suggest you bid it assuming that using the forklift is not an option. You may lose the job to someone else who assumes the slab will support the equipment, but then they're taking on that risk.

OTOH, there's a very wide difference between the loading that will damage a reinforced concrete slab, and the load that would produce catastrophic failure, with numerous warning signs in between.

 

[Enable]

As a contractor I appreciate your diligence in thinking about these issues. Bidding against hooligans that have no clue what a UDL is gets tiring, so your efforts are indeed appreciated.

That said, I agree with my colleagues above that it's impossible to tell you from the armchair. It usually takes a site visit for these kinds of things and reliance on what has worked in past projects in the same locale (given the local nature of building practices, and construction issues). I say that as both a contractor and an engineer.

Here is my contractor advice (not engineering advice)

1. Ask the GC how they plan to remove the slab. It's hard to imagine they are doing so without any shoring, though sometimes that is possible. If they have a system to support the slab (perhaps even from above) then maybe you can use that for your work item and qualify as such.

2. If item 1 goes nowhere then I would consider qualifying the bid as I mentioned in my previous post

3. If I didn't want to qualify for whatever reason (e.g. to keep my GCs happy and reduce problems for them) I would bid using very light weight equipment (so no forklifts, probably just buggies for movement) and consider how to break the stones into smaller, hand-movable components. My first thoughts on that front are expansive grout - drilling and let the grout break it for you - or hand chipping. But you seem like a smart contractor so I am sure you have better ideas how to do this given your familiarity with the project.

4. Consider how my competition is going to do this and ensure that my questions in item (1) are taken seriously such that the GCs are aware of these issues and don't just take a low bid from a silly mason not considering the problems at hand. Then reevaluate my bid and add a good percentage on top for (A) unexpected problems and (B) a premium for my services now that the GCs know I am the guy who will cause them less issues.

5. Submit bid and drink a few beers.