LVL column loads to basement slab vs adding footers 5

[StructuralGuy68]

Hi all,

I am designing a renovation to my house (built in '69) by removing a load bearing wall (~25.5' span) on the main level of my house. My house is a 1 story rancher with a basement. I used the attic (uninhabitable) loading of 20psf LL and 10psf DL. I am recessing an LVL into the ceiling with rafter ties on joist hangers and carrying the load down to the basement using columns that will be incorporated into walls on each end (one perpendicular an one in-line with the LVL).

The question is when reaching the basement, do I need to install footers? I have original blueprints of my house showing a 4" unreinforced slab. I drilled a test 1/16" hole thru the slab to confirm and it is 4" thick. Using ACI 318-14 section 14.5.5.1 (2.667*λ*sqrt(f'c*b.o*h)) for two way shear I am only getting a 0.71 utilization ratio using the plain concrete 4" slab vs the factored loading that the columns will never see (only way to apply the 20psf LL storage would be to climb over the HVAC in the attic). Cutting into the slab and installing footers is complicated based on one end being basically next to/under the stair stringer (roughly mid height on stairs) to the basement and the other end being next to a masonry laundry shoot which would have to be undermined by the footer. So if possible I would like to bear my columns right on the slab but I would be relying solely on the slab. Thoughts? Things I am not considering?

 

[dik]

It depends largely on tghe soil beneath the slab on how much load you can apply, and on the concrete strength of the slab. A 4" slab will support a light load, but not a large load. It also depends, to a lesser extent, on the size of the load.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[StructuralGuy68]

Hi Dik,

It's tough to know the soil properties and concrete strength (none noted on original blueprints). I assumed 3000psi (pretty conservative) for F'c and 1500psi (IBC min) for bearing but since it's a full basement slab I don't think the bearing would be a factor. Using F'c = 3000psi the plain concrete 2 way shear allowable comes out to 0.71 utilization ratio vs the factored load. Is there something else you are thinking of?

 

[dik]

If the load is light and the soil is reasonable, you should be able to support the post directly on the slab... watch the slab for a few months and if there is any distress, you can add a small footing. Failure, if it occurs will not likely be catastropic...

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape.-John Coates

-Dik

 

[Waseem Raja]

How much load you can apply and the slab's concrete strength mostly depend on the soil underneath the slab. A light weight can be supported by a 4" slab, but not a heavy one. Additionally, the amount of the load plays a minor role.

Understanding soil characteristics and concrete strength is challenging (none noted on original blueprints). Although it's a whole basement slab, I don't think the bearing would be a problem. I made the rather cautious assumptions of 3000 psi for F'c and 1500 psi for bearing. The plain concrete 2 way shear permissible has a utilisation ratio of 0.71 compared to the factored load when F'c = 3000psi is used.

Name: Waseem Raja
Company: The Civil Engineering

 

[EireChch]

Sorry to highjack the thread - but Waseem - I have had a look at your website. Its full of useful information that you are obviously passionate about which is great. But...I would re think the website name. "The Civil Engineering" - it doesnt really make sense to be honest. I dont mean to cause offence but that was just my first thought when I clicked the link.

People typically refer to Civil Engineering as just Civil Engineering, there is no "The" in front. At least to the best of my knowledge...

 

[jayrod12]

I must jump in here and give both Dik (Sorry I respect your opinion a lot, but not this time) and Waseem a bit of a knuckle rap for giving this thread any traction. You've both given the OP essentially a blessing to go forward with this work.

Dik, be completely honest, if a homeowner removed a 25 foot long loadbearing wall and just put posts down on their basement slab, there's no way you're letting that fly. So why now, when there's no risk of your liability are you appearing to support that position?

To the OP, I'm sorry but a 4" thick concrete slab likely will not support the load properly. There's a reason that there are dedicated pads underneath of your teleposts, and a footing (not footer) underneath of your perimeter wall. Because if a 4" slab was sufficient to support those levels of load, then that's what we would put, but it can't, so we don't. Honestly at a rough and dirty number 30 psf of load times 12.25' of opening times what trib width, let's just say 12 feet, you're at a hair over 4000lbs. assuming your post is say 5" square sitting on a 4" slab, you can maybe have a total bearing area of 13"x13" meaning your bearing pressure under that post on the soil is almost 3700 lbs/ft^2. Unless your on the densest of material, essentially bedrock, I can't imagine you're going to get that level of bearing capacity out of the soil.

Bust open your slab and provide a proper foundation below the post. Also, make sure it's an adjustable telepost with the proper capacity to support the loads. You're likely to get some sort of movement of the slab and as such will need the adjustability at some point.

 

[dik]

thanks for the caution jay.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Ron]

If you want to install a footer, use Microsoft Word or Excel. If you want to install a footing, use concrete.

 

[wrantler]

If it is a pain to open up the slab you may want to consider placing a reinforced distribution pad on top of the existing slab. Whether you put it above or inplace of the existing it will still distribute the load across its area in plan.