Building Post supported on Slab-On-Grade

[RWW0002]

There have been several posts in the past about slab supported post loads, but I am having a hard time finding code justification for supporting lightly loaded post loads on thin slab-on grade.

I have a situation where I have many interior posts that the building owner would like to bear on a 4" slab on grade. There is a maximum of 2k per post and they are spaced such that the slab carries the load no problem (both by pca method and by Ringo and Anderson equation for slab-supported post loads)

My issue is not with capacity, but on code justification for supporting building loads on a slab. (I am under 2006 IBC) ACI 318 specifically excludes SOG unless "they transmit vertical loads or lateral forces from other parts of the structure to the soil" And if the slab is considered a foundation or plane concrete, I am forced to use 8" minimum slab thickness (for plane conc. 6" above bottom reinf. for "foundation")

Supporting loads (sometimes substantial loads) on 6-7" slabs is very common in industrial facilities and I have seen many light metal buildings with load-bearing interior walls with no foundation elements other than an exterior turndown. Is anyone aware of an out in the building code or ACI-318 that allows this type of constructon?

 

[amec2004]

The best two references would be

ACI 360R-10 Guide to Design of Slabs-on-Ground
PCA Slab Thickness Design for Industrial Concrete Floors on Grade

ACI 360R-10 Guide to Design of Slabs-on-Ground

Search the key word "concentrated load" and you will get all the info. There are different design methods/charts for design of slab-on-grade.


PCA Slab Thickness Design for Industrial Concrete Floors on Grade

You can also check the "POST LOADS" in PCA Slab Thickness Design for Industrial Concrete Floors on Grade and there is step by step design example for POST LOAD on page 12

anchor bolt design per ACI 318-11 crane beam design

http://www.civilbay.com

 

[RWW0002]

amec,
Thank you for the references. I am not having trouble caclulating the adequacy of the slab to support loads. I am looking for code justification (specifically IBC) that allow a building to be supported on a 4" slab.

Thanks,

 

[ztengguy]

Can you thicken the slab at the posts to justify?

 

[RWW0002]

Yes, thickening the slab is the alternative if 4" slab cannot be otherwise justified. I am hoping to avoid that since it would result in 20% of the floor being thickened due to the number of posts.

This is another one of those cases where the contractor/owner states that " We have done it like this on ALL of our other buildings, why do I have to do it different now"

It is one thing if I can tell him that it is needed for strength/serviceability reasons, but much harder if it is (seemingly) arbitrarily code mandated.

 

[MiketheEngineer]

I have done it - seems to work OK. Use treated timbers or steel plates Maybe you could justify by using a 6'' x 6'' ot 8'' x 8'' steel plate???

 

[msquared48]

If you look at the available capacity direct to bearing (45 degree prism here), the soil would have to support slightly better than 2000 psf with an unreinforced concrete slab. The current minimum in the IBC is 1500 psf for most situations without a geotechnical report.

One option to get around this woiuld be to mount the 4X4 posts on a 8" square steel bearing plates, or go to R/C 8X8 posts, then the numbers would work without increasing the slab thickness, if the jurisdiction would accept that reasoning.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[RWW0002]

@ Mikethe Engineer
When you say that you've done it,do you mean bear columns on a 4" slab or that you have thickened slabs directly under posts in this type of situation?

@ Msquared
The posts are light light-gauge steel with a bearing plate now (although the bearing plate is smaller than 8x8).

The project has gone away due to unrealated issues, but I am still wondering how other engineers are dealing with this type of structure. The building in quesion is a single story "self-storage" metal building. Aparently with these type buildings all of the unit partition walls are load-bearing metal stud walls with studs (or posts) spaced at 5-6' o.c. with a bearing angle at the base. when you talk to the metal building guys, they all seem to say that the walls almost always bear dirrectly on the slab. Outside of communicating the design intent directly with the jurisdiction code officials, I am not sure how to meet the letter of the building code.

 

[msquared48]

Are these flange to flange studs joined to form a box section?

Are the studs in a wall at a door jamb, or isolated?

If there a steel stud track inviolved, another idea would be to fill in the steel stud track with concrete and a couple of #3 horizontal rebar to try to spread out the load. Would have to add some additional tweek to this to get it to work, but it may be possible.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[MiketheEngineer]

Done it both ways depending on the load. Prefer to use 6x6 or 8x8 plates. Solves two problem - timber to concrete contact and spreads load.

 

[RWW0002]

Thanks for all the thoughtfull responses.

@ msquared
The studs are single CEE members with no track.

I agree that with a little thought and proper detailing the slab can carry the loads. I just don't see any way to get away with bearing building loads on concrete less than 8" thick under IBC/ACI-318. I am all for engineering judgement when it comes to code interpretation and engineering decisions, but this case seems pretty explicit.

 

[DaveAtkins]

Ultimately, you must make the final decision. Personally, in this case I would be comfortable with meeting the intent of the codes (safety and serviceability) while not meeting the letter of the codes.

DaveAtkins

 

[msquared48]

If I remember correctly, there is also a provision in the IBC code for the use of engineering principles to furnish a design that works, but is not really within the guidelines of the code.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[XR250]

Don't forget, if there is 4" of gravel under the slab, that will spread the load as well.
I would not worry about it. I have yet to see a column punch thru a 4" slab in my 20 years of doing this.

 

[woodman88]

Not sure if I would do it but...

Have you look it 2006 IBC Section 2304.11.2.7 Posts or Columns. "Posts or columns supporting permanent structures and supported by a concrete or masonry slab or footing that is in direct contact with the earth shall be of naturally durable or preservative-treated wood...."

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.