Setting steel house posts in deep concrete footings

Rozz · Jan 04, 2020 3:56 pm

Hi

We are building a house in 89x89x4mm steel posts in deep concrete footings as specified by the engineer. Footings have to be 1.5m deep with the posts floating 800mm off the bottom of the footing (so 700mm) in the concrete. I’m confused how to ‘float’ these posts. The builders sugggest we first pour 800mm concrete in each footing, then once dry we should set the posts and pour the remaining 700mm of concrete. The builders say the two pours of concrete will basically ‘fuse’ together and act like one solid footing. To me it seems a bit unstable. Is his a standard practice? Is this a standard way to float posts in a deep concrete footing? Appreciate any advice you can give.

bashworth · Jan 04, 2020 4:47 pm

The way I have done this in the past is hang the posts of the horizontal bearers which themselves are supported at the finished level.

The holes can then be filled in one go.

MClark · Jan 04, 2020 7:40 pm

A few options I can see.
1. Like Bashworth said you can suspended them with some bearing timbers.
2. If they are simply a steel post with no end plate I would pour the concrete and drop them in to the depth required.
3. Get the engineer to give you an alternative design to have plates so you can do a simple chemset fixing down to the footing.

If the steel critically needs to be plumb and at a exact height option 3 would be best in my opinion if not option 2 has worked for me time and time again.

MC

StructuralBIMGuy · Jan 04, 2020 8:51 pm

Rozz

The builders say the two pours of concrete will basically ‘fuse’ together and act like one solid footing. To me it seems a bit unstable. Is his a standard practice?

That produces a weak Concrete cold joint and should be avoided. hth

Rozz · Jan 04, 2020 9:54 pm

MClark

A few options I can see.
1. Like Bashworth said you can suspended them with some bearing timbers.
2. If they are simply a steel post with no end plate I would pour the concrete and drop them in to the depth required.
3. Get the engineer to give you an alternative design to have plates so you can do a simple chemset fixing down to the footing.

If the steel critically needs to be plumb and at a exact height option 3 would be best in my opinion if not option 2 has worked for me time and time again.

MC

Thanks MC

Could you explain option 3 a little I don’t fully understand?

MClark · Jan 04, 2020 10:12 pm

Option 3 - Speak to the engineer and get them to give you a base plate design for your posts, which you can chemset with all threads into the footing, giving yourself some room for structural grout under the plate so you can shim it accurately.

The end product will look similar to this.

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Mirrmu83 · Jan 04, 2020 10:20 pm

You cant do 2 pours and the concrete will “fuse” together, garbage from your builder (or a carpenter?)

Place a temporary horizontal support over hole which the post can be clamped to. Can set post to required depth and install vertical bracing.

Can be removed at final stages of finishing concrete

StructuralBIMGuy · Jan 04, 2020 10:56 pm

Rozz

MClark

3. Get the engineer to give you an alternative design to have plates so you can do a simple chemset fixing down to the footing.

If the steel critically needs to be plumb and at a exact height option 3 would be best in my opinion if not option 2 has worked for me time and time again.

MC

Thanks MC

Could you explain option 3 a little I don’t fully understand?

Your Engineer has specified a fixed connection with the footings ie Post cast well into Concrete
therefore it will resist Bending & Rotating Moments (M) along with Vertical & Horizontal shear forces (SF) and combination stresses,etc,etc
The example MC has put up is a Pin Connection ( where M=0) and unless your engineer has specified it then it should not be used
I suggest you consult your engineer rather than your builder on structural changes these changes will also require Structural Certifications. Please check with your Council Building dept.
OT Basically Pin joints require Bracing...Are your columns fully Braced? More info required hth

Rozz · Jan 05, 2020 8:10 am

Thanks everyone. Yes I’m not comfortable with two pours. I just need to find a way to suspend the posts on the footing so we can do one pour of concrete. I thin the temporary horizontal support over the hole, clamped to the post, seems the best option.

Rozz · Jan 05, 2020 8:20 am

I have read that the concrete cold joint (two pour method) can be structurally OK if rebar dowels are inserted.

StructuralBIMGuy · Jan 05, 2020 8:44 am

Rozz

I have read that the concrete cold joint (two pour method) can be structurally OK if rebar dowels are inserted.

That is if they remember to insert the reinforcement and yet another issue?
Here make sure structural concrete elements are properly inspected and certified
Inspections structural stages. hth
BTW photos do help others and your engineering plans-> Details should have notes to follow

Rozz · Jan 05, 2020 9:10 am

StructuralBIMGuy

Rozz

I have read that the concrete cold joint (two pour method) can be structurally OK if rebar dowels are inserted.

That is if they remember to insert the reinforcement and yet another issue?
Here make sure structural concrete elements are properly inspected and certified
Inspections structural stages. hth
BTW photos do help others

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Here’s an image if it helps. Steel base plates on posts in deep footing. Carpenter wants to do two pours so we can sit the posts on the first pour. I don’t trust two pours but am told rebar will solve the problem.

StructuralBIMGuy · Jan 05, 2020 9:42 am

Yes that may be correct, but the certified engineering details has changed and will require re certification, amendments and an engineers signature/stamp? eg, certified Engineering Plan and details
Hence my above reference to checking with the Councils Building Department Surveyors
Carpenters or builders can not alter structural elements because it's convenient for them to erect or they didn't allow for that engineers detail in the quote. These problems always comes back and bite the OB on the rear.
And with Council Building depts now taking a harder line on structural non compliance..
I recommend you do it correctly and avoid structural repairs or worse pulling it down
OT Details should always be read in conjunction with Architectural drawings
Otherwise everyone replying is guessing , eg, how high are the columns,etc
"The better the information given the better the reply"
my2c
1. Start a post on your build
2.Put up the architectural Drawings ie Floor plans ,Elevations,etc
3.Tag in the experts, there are many that give generously
4.It becomes a permanent record in the event of future issues and helps other
5.6.7
Cheers
Chris

MClark · Jan 05, 2020 11:45 am

Rozz

StructuralBIMGuy

Rozz

I have read that the concrete cold joint (two pour method) can be structurally OK if rebar dowels are inserted.

That is if they remember to insert the reinforcement and yet another issue?
Here make sure structural concrete elements are properly inspected and certified
Inspections structural stages. hth
BTW photos do help others

Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅

Here’s an image if it helps. Steel base plates on posts in deep footing. Carpenter wants to do two pours so we can sit the posts on the first pour. I don’t trust two pours but am told rebar will solve the problem.

Two pours is out the question unless you are given a design by the engineer to do so.

Always get the engineer to provide a design that is certified. Here in VIC we now need to send these designs to the building surveyor for approval and update the stamped drawings. So be sure you comply to what is required, your builder should know.

MC

The_Billy · Jan 28, 2020 3:12 pm

As said above contact the engineer and get them to re specify this as a chem set fixing as the method he has highlighted is the strongest but not the most practical, even if you suspend it well, the pouring of concrete may stuff up your set out marginally throwing out structure. Chem set fixing will involve pouring your footing to the required height, and just bolting the post down at the top. Generally engineers don't like to specify chem set fixings as there may be concerns around uplift forces, but I am yet to see this being an issue on projects I've done.

esky · May 15, 2020 9:27 pm

Pour a footing and chemset, you wont get them in the right spot otherwise, cast-in plate is another option.

Structural.Review · May 27, 2020 1:22 pm

- Excavate to 1700mm
- Pour 200mm concrete
- Use 20mm non shrink grout
- Bolt down base plate and post to your structural engineers spec
- Pour the rest of the footing

Two pour to 700mm might work as well but you need to confirm if cold joint is allowed with your structural Engineer.

alexp79 · May 27, 2020 1:41 pm

What is your foundation design?

How those posts are supposed to be linked together? With timber or with concrete?

I would be second to saying that you would need to create your bearings first, prop them, attach posts to the bearings at the right height and then pour the footings with the posts being already inside them.