Slab on Grade Issues 1

[SteelPE]

I have wrapped up the design of a warehouse recently. The project was a single story warehouse/lt manufacturing facility with an internal mezzanine. The project was tough to get through as the owner hired an independent construction manager and placed him in-between the design team and contractor. The project was designed using IBC 2015.

Unfortunately I have been dealing with slab on grade issues on the project. Getting the client to nail down the specific design criteria has been impossible. The client has varied their design criteria throughout the project and has even gone so far as to change the criteria after the slab was poured (we were told we had final information but then they changed it).

Now the client is requesting an allowable capacity for the slab… an allowable capacity in “psi” for loads placed on the slab. The client wants to take the new load, divide by the allowable “psi” and then come up with a plate size to support that load. I have never heard of such a method. Has anyone else ever given the client this type of number before? How would one go about calculating this.

I am used to designing slabs in accordance with the book “Designing Floor Slabs On Grade” by Ringo and Anderson. This book goes through different types of load (forklifts, isolated columns, racks etc) and give you methods to either come up with slab thickness or calculate capacities. Similar methods can be found in ACI 360R. The problem is, there is no magical number as the methods presented are based upon nomographs and logarithmic equations.

I just don’t know who is being obtuse, myself, or the owners CM. This is now beginning to cause major issues with closing out the project.

 

[bones206]

What would the plate be used for?

 

[SteelPE]

Anything..... there is no specific use for this number.... columns on the slab, rack loading, forklifts..... basically anything they want to put on the slab.

 

[skeletron]

The request from the client is not clear. It appears they want some sort of adjustment range, but they obviously don't have the SE background you have.

You could probably give the client a maximum forklift load based on the design and the equipment available.
As for the baseplates for columns/racks? How about a meet-in-the-middle scenario:
1) Decide on a couple of load ranges and spacing requirements.
2) Provide minimum plate sizes (with a factor of safety)
3) Issue it in a sort of load table.
4) Charge for the time/design. Any deviations will need your input.

 

[JAE]

There is no such thing as an all-encompassing generic “psi” capacity of a floor slab. The psi value will vary depending on the contact area (shear circumference). You could possibly give them a table or a chart reflecting that. But it’s not typically done that way.

 

[dold]

i'd give them "you can have a max load of XXXX lbs over a 4"x4" area". Add in a factor of your choosing based on the fact that, more than likely, they'll be using storage racks with light gauge foot-plates. I.e., the effective loaded area is going to be less than 4x4. Should be fairly easy to come up with a number like this. Bottom line is that they probably don't truly understand what they're asking for, so give them some guidelines. If they mess up their slab, they mess up their slab...

Edit: just realized this is pretty much what skeletron just said.

 

[bones206]

I’m still not fully understanding the plate aspect. Is their plan to lay down loose steel plates to spread out a future hypothetical load if it exceeds the generic load rating you’re supposed to provide?

 

[dik]

I'd start by recording all the particulars the slab design went through and copies of related correspondence.

Having said that, I'd put together a couple of charts showing the range of loads and the areas as well as traffic areas, effect of loads on adjacent point loads, and fatigue requirements. Bill as an extra service.

Dik

 

[Settingsun]

There is no such thing as an all-encompassing generic “psi” capacity of a floor slab.

You can come up with one for the governing case which depends on slab, joint and ground (stiffness) details etc but it will probably be much lower than they're after. Eg strip loads with loaded and unloaded widths set for maximum sagging and hogging moments.

It sounds like they're asking the wrong question. If they have a known load, they should give you that and ask what they need to do to make it work. Perhaps suggest this to them.

 

[oldestguy]

Ok, then give them a chart with plate dimensions and total load. As the load gets larger decrease the psf used. Then also add on the chart a plot for plate thickness (cannot be made up by more than one plate - requires thick plates to be on hand at larger loads). This is due to not ex ceding the yield psi of the plate steel. Practically speaking this will soon tire them of their odd choice. Also give a top limit for the load-plate size. This way you don't overload the slab. For guidance see if pavement design charts might somehow be the governing sizes and loads

 

[GC_Hopi]

At least I am not the only one with "particular" clients. Buy them a structural book and send them on their way. Lol J/K

 

[SteelPE]

Yes, I agree it sounds like they are asking the wrong question. At one point during this 5 month ordeal they referenced placing an internal mezzanine on the slab. To prove my point, I generated a capacity of the slab based upon various base plate sizes 8x8, 10x10, 12x12, all the way to 18x18 etc.... I then back calculated the capacity of the slab in "psi" for the client. The capacity varied from 250 psi to 100 psi. A 100 psi load, if they used this value over the entire slab, would equate to 14,400 psf which would violate the bearing capacity of the soil under the slab. I told them this as well.

In a conversation with the CM, who said he wasn't trying to be argumentative, said if I can calculate the thickness requirement for a give rack load then I should be able to calculate this "psi" number. At that point I gave up and said "While I would like to help you solve this problem, it is something that I do not know how to calculate. Therefore, I can not give you the all encompassing allowable number you are looking for".

I have considered giving them the allowable bearing pressure/144, but I don't think they would like this.

This issue has completely destroyed the project. Over 5 months of changing requirements, research in trying to give them an answer, phone conversations etc I have about 40 hours tied into this issue that I can not be reimbursed for.... on an item we give very little thought in when preparing a proposal (might be allotted 1 hour of design time for the project).

 

[Settingsun]

What's the slab thickness, concrete strength, and CBR/subgrade modulus?

 

[Aesur]

Since they don't specify the baseplate sizes, weights, spacing, locations, etc. and all you have to go by is really the soil bearing capacity, do a calc of soil bearing pressure / 144 sq-in, ie: 1500 psf / 144 sq-in/sq-ft = 10.4 psi. Then explain to them without constraints (spacing, loads, sizes, etc..) this is all they are allowed as you cannot guarantee they won't overload the slab soil bearing capacity.

 

[dold]

Jeez guys, seriously? Can we not cut out a 3'x3' square and put a footing in? Or whatever it needs? Seriously, the amount of time it has taken to develop load tables for putting bigass baseplates on slabs or whatever vs. cutting a hole in the slab on a cost basis seems trivial to me? $120/hr vs $15/hr? You must have spent many hours coming up with those tables. Unless this mezzanine is 10k sq.ft. they could have already have had this done. No?

Unless this is some sort of production line where they absolutely cannot cut the slab?

 

[Rabbit12]

dold, did you read the thread? They aren't asking only about a mezzanine. They want an all-encompassing number they can use for anything they might dream up over the life of the structure.

FWIW, I really don't think it's possible. Too many variables including the thickness of the steel plate. That's going to a critical element.

 

[dold]

Ah. I read it but it seems to have gone in one eye and out the other. Apologies.

 

[bones206]

Just reading between the lines... I suspect the owner is trying to avoid having to pay for engineering later when the facility usage changes or they want to add a mezzanine. So they are trying to get their floor rating "certified" ahead of time so they have a blank check to do whatever they want without hiring an engineer. At first I thought that they just needed some education/expectation management, but now I think they might know exactly what they are doing and just trying to take advantage of their structural engineer.

 

[WinelandV]

Honestly, the floor was designed for the requirements given. I fail to see how "load rating for the slab on grade" could possible be covered under your contract. This is definitely outside the scope of normal practice - I'd push back hard on doing this under the original contract.

 

[SteelPE]

bones206

How difficult is it for them to give the slab information to someone in the future and then have them check the slab to see if footings are required? I have gone into my building plenty of times to find "prefabricated" mezzanines on the floor. We just exclude them when we issue our final sign off on the building.

I have see some amazing stuff placed directly on slabs with no additional reinforcing. I have one client who put a crane system on his 5" thick slab w/o additional footings. I have no idea how it's not an issue, but 5 years later not a crack.

I don't know what this client is up to. I just know it's causing a headache for everyone involved. I just need to make sure I am approaching the solution in a similar manner to others in the profession.