Do I need a separate footing if a post is added on slab on grade 2

[dcceecy]

The existing basement has 5" slab on grade reinforced with WWF.
Now I have a new post with a load of 5.72 kips.

If I assume the soil capacity is 2000 psf, it seems I need a 3 ft^2 of footing.
I am going to use 3 1/2" standard pipe.

if I can hide this post in a parition wall and put a steel plate below the pipe, and anchor the plate into slab on grade.

Do I need the separate footing? because if I need one, I have to cut the existing slab.

if I do not put separate footing, is that load going to make the slab on grade around the post crack?

any suggestions? Thank.

 

[COEngineeer]

Are you removing a load bearing wall? Because if you do, there is probably a footing already. What are you doing exactly?

Never, but never question engineer's judgment

 

[JAE]

ACI limits footings to a minimum thickness of 6" (see ACI 318, section 15.7).

 

[concretemasonry]

If it is a residential basement slab, there is no guarantee it is actually a minimum of 5" thick. If you are removing a basement partition wall the load is distributed over a large area. Many residential basement slabs are placed with little control over the compaction and there could even be voids under the slab.

I would saw the slab to remove it and create a separate footing of proper dimensions and thickness. Then you know what have and are responsible for.

A concentrated load on an unknown slab is not reliable especially if there are any early signs of cracking.

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[msquared48]

Just do it right and cut a 2X2X8" deep hole through the slab with a couple @4 each way. Don't forget that you need four anchor bolts here (OSHA).

You are correct that the slab may crack if it is only bearing on the slab with no existing or new footing.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[dcceecy]

The client wants to cut an opening through the floor (concrete planks) and the post is from the opening framing.

Actually the client is remodeling an office building.

I was trying to save cost. if i do not put a footing and cracks show up, it'll cause trouble for us.

I think I am going to cut the existing slab and put a 2'x2'x12" deep footing for the post.

Thanks.

 

[msquared48]

Good choice.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

A separate footing is a good choice. I think 2'-6 x 2'-6 x 12" would be a better choice.

BA

 

[apsix]

I agree that using the slab is inappropriate for the loading, however requiring four anchor bolts, citing OSHA, surely is not necessary for a 3 1/2" pipe x 10'? long column.

 

[msquared48]

Read it and weep. It's for erection safety reasons. Nothing else.

http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_id=12746&p_table=STANDARDS

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

apsix,
Aren't you glad we don't have to put up with that nonsense?

 

[CTW]

if i do not put a footing and cracks show up, it'll cause trouble for us.

Why would it cause trouble for you? When the square cut is made in the slab for the footing, the slab will eventually crack anyway due to the re-entrant corners resulting from the cuts.

 

[steellion]

Msquared, what about stair posts for steel pan stairs? I see them with anchored with 2 bolts all the time. I know columns require 4, but it seems stair posts are exempt for some reason.

 

[DaveAtkins]

OSHA allows posts to have just two anchor bolts. Posts are supposed to be non-load bearing; however, OSHA defines what a post is based on the self-weight of the post, not what load it carries.

DaveAtkins

 

[JrStructuralEng]

4 anchor bolts for a tiny post. Engineers need to use logic, not just books.

 

[steellion]

Thanks for the clarification, Dave.

Jr, you can't just ignore codes that you don't like.

 

[Ltwine]

It is my understanding that OSHA does not concern strength, but focus on construction/erection stability to prevent undesirable incidents. Should anything could fail and bear serious

 

[Ltwine]

....consequence, use 4 bolts (min).

Hir wrong button, sorry.

 

[BAretired]

Two bolts per column are frequently used in Canada when placing a column on a narrow grade beam. Four bolts would require a pedestal which adds expense to the project for no valid reason.

BA

 

[Ltwine]

I guess 4 bolts requirement is pure US-OSHA business, which was meant to be fool proof at any cost.