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Point load on basement slab

BN4537

BN4537

SAWHORSE
Joined
Sep 10, 2020
Messages
898
Location
Kansas
This is something we deal with fairly regularly.

Existing house, basement remodel, the owner wants to remove a bearing wall and replace it with an LVL beam. He gets the beam properly sized, and then he wants to hold one side of the LVL up with a column made of 2 2x4s, or some other effective means. His new column will now be bearing on the regular 4" basement slab, with no footing, that was not originally intended to carry whatever load is on the LVL as a point load.

Clearly, there is a certain amount of weight that the 4" slab can handle, but we don't know if it is really 4" thick, if it is reinforced, or what surface it was poured on.

So, I want to know how other jurisdictions handle this situation? How do you determine if they really need to cut the floor and add a footing, or if they can get away with a small point load on a regular slab?

Best case scenario, I would like to figure out a rule of thumb, like if the point load is only 1,000 lbs. we don't worry about it punching through the slab, more than 1,000 lbs. and they need to put in a footing. Of course, in the real world, it probably isn't that easy...
 
If IRC, 6" think footing minimum. As for sizing footing, take the total reaction, divide by 1500 (or known soil load), then take the square root, then multiple by 12. Value calculated is the minimum width of footing.
 
While not the same situation you might be able to section AH105 section from the 2018 IRC appendix H for some guidance which allows up to a 750 lb load on a 3.5 in slab.
 
If a proscriptive provision does not provide the answer the applicant needs to provide information to justify their proposal. An engineer will be needed to provide that justification

Suggest that the quickest and simplest solution is to locate a spread footing under the new column.
 
These are good answers.

Follow-up question - When do you start requiring an engineer review or a spread footing? Say you had a 3' doorway in the basement in a load bearing wall, and you properly sized a longer header and extended the doorway to be 5' long, with one side bearing on a two 2x4s creating a point load on the slab. Now, you only extended the header two feet, maybe picked up one or two more floor joists, but maybe not a ton of weight added. So, would you require a footing for that? What if you only extended the header by one foot, instead of two?

Clearly, if you had a big span and a lot of weight, you would certainly need PE review or footings added.

So where do you draw the line on an existing house in your jurisdiction? If they only add a little weight to a point load in an existing house would you let it slide, or do you have a no-tolerance approach? If you would allow it, how much weight would you be comfortable with?

I guess I'm really just trying to figure out how much weight you guys would be comfortable with someone putting on a 4x4 area of a 3.5" slab. I mean... I think everyone here would agree that it would hold 100 lbs pretty well... maybe the 750 lbs from Appendix H... might even hold 1,000 if it doesn't have to deal with wind or snow loads... etc...

I'm just trying to get a feel for where I should start getting nervous about the point load on a 3.5" slab. I haven't seen enough failures in similar circumstances to gauge this effectively.
 
If you can confirm it is a 4" slab then maybe you could say the force transfer becomes 12" at the bottom of the slab so you have a 1sqft footing as far as bearing (X soil bearing capacity)....But you would be neglecting any punch through and anything else that makes it engineering...
 
steveray said:
If you can confirm it is a 4" slab then maybe you could say the force transfer becomes 12" at the bottom of the slab so you have a 1sqft footing as far as bearing (X soil bearing capacity)....But you would be neglecting any punch through and anything else that makes it engineering...
Click to expand...
The 12" is the punch shear dim. Assume a 45 deg shear plane through the slab (footing) from the edge of the column. So a 4" square column would have a 12" square effective bearing area. That assumes no reinforcment. It gets much more engineery once you start adding reinforcement and your footing becomes a bending plate with spring supports.

I would be ok with a 1500# point load on a 4" slab by inspection.
 
Core a small hole to see what the slab actual is. Look at table 403.1(1)

or cut out the slab and pour a 24x24 pad, 12 inch thick install 3 laly column the grout in the floor
 
I am in the prescriptive camp. A 4" thick slab (probably 3-1/2") does not meet the requirements as a footer, therefore if you want to use it, you need an engineer to sign off on it. By today's standards, you won't have any problem getting a PE so sign and stamp there name for a price. I would think it would be prudent to just meet the code prescriptively and be done with it. The only thing a 4" slab should support would be partition walls.
 
Applying the prescriptive provisions how did the owner size the LVL without consulting an engineer? For essentially the same price an engineer could size the LVL and address the foundation issue.
 
Here, we have had the floor opened up in every single project at this scope to install code compliant column pads.

You would be amazed how many times I've found no strip footing under a load bearing wall when inspecting these projects. Don't assume anything.
 
Whether the solution is an I-joist or an LVL the manufacturers report is not a proscriptive provision recognized by the code.
 
So they are produced to a standard:
R502.1.2 Prefabricated wood I-joists. Structural capacities
and design provisions for prefabricated wood I-joists
shall be established and monitored in accordance with
ASTM D 5055.
R502.1.3 Structural glued laminated timbers. Glued
laminated timbers shall be manufactured and identified as
required in ANSI/AITC A190.1 and ASTM D 3737.
R502.1.4 Structural log members. Structural log members
shall comply with the provisions of ICC-400.
R502.1.5 Structural composite lumber. Structural
capacities for structural composite lumber shall be established
and monitored in accordance with ASTM D 5456.
R502.1.6 Cross-laminated timber. Cross-laminated timber
shall be manufactured and identified as required by
ANSI/APA PRG 320.

That standard is then used to produce manufacturers installation instructions (span charts)..

What's the problem?...Do you require engineering when someone uses joist hangers or do you use the manufacturers installation instructions?

R502.6 Bearing. The ends of each joist, beam or girder shall
have not less than 11/2 inches (38 mm) of bearing on wood or
metal and not less than 3 inches (76 mm) on masonry or concrete
except where supported on a 1-inch by 4-inch (25 mm
by 102 mm) ribbon strip and nailed to the adjacent stud or by
the use of approved joist hangers.

As hangers are not really prescriptive....It is whatever we will "approve"........
 
Mark K said:
Applying the prescriptive provisions how did the owner size the LVL without consulting an engineer? For essentially the same price an engineer could size the LVL and address the foundation issue.
Click to expand...
We accept cut sheets from the manufacturer showing exactly how the lvl will be used. Most of the manufacturers have PE on staff to provide these so we don't need to go to an outside engineer.
 
steveray said:
So they are produced to a standard:
R502.1.2 Prefabricated wood I-joists. Structural capacities
and design provisions for prefabricated wood I-joists
shall be established and monitored in accordance with
ASTM D 5055.
R502.1.3 Structural glued laminated timbers. Glued
laminated timbers shall be manufactured and identified as
required in ANSI/AITC A190.1 and ASTM D 3737.
R502.1.4 Structural log members. Structural log members
shall comply with the provisions of ICC-400.
R502.1.5 Structural composite lumber. Structural
capacities for structural composite lumber shall be established
and monitored in accordance with ASTM D 5456.
R502.1.6 Cross-laminated timber. Cross-laminated timber
shall be manufactured and identified as required by
ANSI/APA PRG 320.

That standard is then used to produce manufacturers installation instructions (span charts)..

What's the problem?...Do you require engineering when someone uses joist hangers or do you use the manufacturers installation instructions?

R502.6 Bearing. The ends of each joist, beam or girder shall
have not less than 11/2 inches (38 mm) of bearing on wood or
metal and not less than 3 inches (76 mm) on masonry or concrete
except where supported on a 1-inch by 4-inch (25 mm
by 102 mm) ribbon strip and nailed to the adjacent stud or by
the use of approved joist hangers.

As hangers are not really prescriptive....It is whatever we will "approve"........
Click to expand...
Boom,.....done and done.
 
Mark K said:
Whether the solution is an I-joist or an LVL the manufacturers report is not a proscriptive provision recognized by the code.
Click to expand...
Not code, but it is an alternative material and method that we accept in our jurisdiction that we deem to be satisfactory and complies with the intent of the provisions of the code.

The whole issue here is that we have a fairly small municipality and we try not to be too rigid if we can tell something will probably work. If we had a larger jurisdiction, there is no way I would take the extra time to study out something like point load on a slab when I could just tell them to put in a footing or get an engineer.

I certainly don't want anything structural to fail, but if it had turned out to be a simple calculation that I was comfortable with I would have been willing to work with people, if I could verify that the structure would be safe. But, the simple answer to make them put in a footing definitely is easier for me...

I appreciate these answers, they were really helpful.
 
Beniah Naylor said:
Not code, but it is an alternative material and method that we accept in our jurisdiction that we deem to be satisfactory and complies with the intent of the provisions of the code.

The whole issue here is that we have a fairly small municipality and we try not to be too rigid if we can tell something will probably work. If we had a larger jurisdiction, there is no way I would take the extra time to study out something like point load on a slab when I could just tell them to put in a footing or get an engineer.

I certainly don't want anything structural to fail, but if it had turned out to be a simple calculation that I was comfortable with I would have been willing to work with people, if I could verify that the structure would be safe. But, the simple answer to make them put in a footing definitely is easier for me...

I appreciate these answers, they were really helpful.
Click to expand...
How do you verify the LVL is properly sized?
Do you require a layout from the manufacturer for I-Joist systems?
 
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