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

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

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

(OP)
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.  

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

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

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

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

 

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

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.

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

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

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

(OP)
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.  

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

Good choice.   

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

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

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

BA

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

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.

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

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

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

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.

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

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.

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

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

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

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

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

Thanks for the clarification, Dave.

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

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

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  

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

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

Hir wrong button, sorry.

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

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

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

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

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

The 4 bolt rule does not explicitly require the 4 bolts be non-collinear... :)

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

I wouldn't want to be the theorhetical 300 lb construction worker swinging from the top of a column with 4 anchor bolts in one line.

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

Yep, the last time someone stacked 4 bolts.

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

I thought the 4-bolt requirement was for erecting columns that would be free-standing for some time during erection...like a flag pole.
We use to bolts in crane shafts that are tied to bulding shafts becasue there is never a time when just the baseplate is holding the column.
I agree...use some logic here. Cookbooks are for chefs.  

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

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


I would find it hard to believe that this rule was made by an OSHA "Engineer", as I did not know they had any on staff.

There have been failures during erection of columns with only two bolts that involved injury or loss of life.  Hence the regulation - to protect us from ourselves.

 

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

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

I agree that engineers need to use logic and not just books.  That being said, when there is a legal requirement (whether you believe it to be valide of not) that you choose not to follow, and something bad happens...... you can rest assured that someone will be held to account.  

We as engineers design to the final, constructed state.  Means and Methods are in the Contractor's realm, and since we have no control over how the column is erected, and when/how it is stabilized during erection, it seems prudent (in my opinion) to provde 4 anchor bolts as required by OSHA.  

As noted above, it is based on self-weight (not the load it's carrying), but I believe there is another criteria in there as well (maybe member length?).

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

I couldn't find the column/post distinction on the OSHA site.  But a quick internet search produced this result.  I can't validate it, but it seems official:

A "column" per OSHA is defined as a vertical member that is greater than 300
pounds in weight and/or part of the "primary skeletal framing system".
Anything supporting the main floors, roof and walls of the building and/or
part of the lateral load system is a "column" by OSHA.

A "post" per OSHA is defined as a vertical member that is less than 300 lbs
in weight, axially loaded only, and not part of the primary skeletal framing
system.  Tubular members supporting stairs and mezzanines would be most
likely to qualify as "posts" by OSHA.
 

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

Was there not a thread a few months ago regarding the 4 and 2 bolt situation?

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

I doubt there is a subject which has not already been covered on this site before.  Doesn't stop us from going over them again.

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

The pipe column the OP mentioned is only 100 to 150#, so, based on the definition above, 2 bolts would be OK.

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

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

For OSHA inspectors concern, anybody get injured/killed, a citation will be issued (for vertical permanent member with fewer than 4 bolts) with emphasis on fastening not meeting OSHA recommendation. Consequence? You may prevail through appeal, but first to check with your lawyer & insurance company. Again, it is a practical/practice issue, not strength, and is meant to be fool proof.  

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

The 300# weight limit seems to make sense.  Spandrel columns in single story buildings are usually less than 300#, so the practice in Canada seems to be consistent with the OSHA standard even if it does not have to be.

BA

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

Please read steellion's post again. I don't think OSHA suggests 300# is the cut for fastening concerns. Formal explanation from OSHA is required. Maybe there is already one, but burried in somewhere.

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

Ltwine,
Yup, I think you're right.

BA

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

Just hope no one get misled before OSHA clears its stance on this.

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

Perhaps, a check with OSHA to see if there is a code case?

To minimize cracking at re-entrant corners, I had them core bore the corners and then saw cut from tangent point to tangent point. It was pretty successful.

Michael.
Timing has a lot to do with the outcome of a rain dance.

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