Construction joint in a pad footing?

[psychedomination]

Hi there,

I am a graduate engineer and am currently looking into designing footings for two columns. The loads on the columns are very small at 150kN and 60kN respectively.

Due to the small loads I only needed to use the minimum bending reinforcement for both hogging and sagging moments.

The column taking the 150kN load will be cast against a wall that is to be excavated. The 60kN column is to be cast against the property line. Due to this I designed a combined footing between the 60kN column and the 150kN column.

As such, I needed the 150kN column to have a proper pad footing with no eccentricity.

Before the wall and area behind it is excavated I wanted to put up the columns and beams in place to hold up the floor slab in the event that anything unexpected happened.

I am thinking to pour 2/3 of the footing first on the un-excavated side as shown in the attached drawing. Then excavate the wall and about 3 feet behind it, putting up the required shoring jacks as needed and then pour the other 1/3 of the footing. I am thinking to connect the other 1/3 of the footing with post installed rebars epoxied in and then wetting and roughening the surface vertically before pouring the second 1/3 pour.

What are your thoughts on this seeing as all of the loads are so low? The concrete for the first 2/3 pour would be compacted fine as it will be cast against compacted earth, same with the last 1/3 pour.

I was getting a low bearing pressure of 53kPa in the combined footing.

Any advice would be appreciated.

 

[dik]

I cannot fully understand what you are trying to do. Are you trying to put a 'strap' between two existing footings? If so, then your cranked 'z' bars on the LHS could be straight... cranking them does little or nothing. Your hooked transverse bars could also be straight... If I understand your sketch correctly. Can your 'strap' be widened in the vicinity of the greater column load?

If you can be confident about the epoxy rebar being developed into the existing footing I don't see any problems with your approach. I don't see why you are breaking up the pours.

Detail 1 appears to be a section through the 'strap'; is this correct?

Dik

 

[MotorCity]

These are questions that you should discuss with the contractor. Make sure you understand the contractor's method that he/she plans to use. Often, your idea of sequencing and shoring may be vastly different than theirs. There will undoubtedly be cost implications depending on how it is constructed.

 

[psychedomination]

I cannot fully understand what you are trying to do. Are you trying to put a 'strap' between two existing footings? If so, then your cranked 'z' bars on the LHS could be straight... cranking them does little or nothing. Your hooked transverse bars could also be straight... If I understand your sketch correctly. Can your 'strap' be widened in the vicinity of the greater column load?

If you can be confident about the epoxy rebar being developed into the existing footing I don't see any problems with your approach. I don't see why you are breaking up the pours.

Detail 1 appears to be a section through the 'strap'; is this correct?

Dik

Hi Dik and thanks for the response. I apologize for the original drawing I see that it perhaps is not that clear. I attached another drawing to this post that is better color coded.

Pretty much the situation is that there are two existing footings that are eccentrically loaded with a column. Due to this I wanted to attach the column footings using a combined footing as is generally done for columns along property lines. The right side column has a wall to the back of it that is causing the eccentricity. This initial footing was already poured against the hatched area limit as shown. I am proposing to center the right side column on a normal pad footing. To do this I will need to continue the footing into the area that is to be excavated.

The wall behind the column will be excavated. In place of the excavated wall a beam will be spanning on top of the columns to take the floor load.

Yes that's true, the transverse bars could be straight.

Yes detail 1 is a section through the strap. I have better shown this in the drawing.

These are questions that you should discuss with the contractor. Make sure you understand the contractor's method that he/she plans to use. Often, your idea of sequencing and shoring may be vastly different than theirs. There will undoubtedly be cost implications depending on how it is constructed.

Yes that's true. I was mainly asking from a structural point of view whether a construction joint in a pad footing would be detrimental or not. I know it's not the preferred approach but this is not a standard case. Realistically the footing is perhaps much bigger than it needs to be hence the 53kPa bearing pressure.

 

[dhengr]

Psychedomination:
If you can’t do a drawing and sections which another engineer can understand the engineering intent of, and the construction of, I wonder how you would expect a couple construction rod benders and form builders to understand. Look at every one of your details as if you knew almost nothing about construction or your own intent, and ask, will people understand what I want them to do, when they look at this detail? If it isn’t crystal clear, you need to improve your detail. And, no amount of color coding will fix this. CAD is no smarter than the person using it, and will not fix any misunderstandings.

 

[psychedomination]

Psychedomination:
If you can’t do a drawing and sections which another engineer can understand the engineering intent of, and the construction of, I wonder how you would expect a couple construction rod benders and form builders to understand. Look at every one of your details as if you knew almost nothing about construction or your own intent, and ask, will people understand what I want them to do, when they look at this detail? If it isn’t crystal clear, you need to improve your detail. And, no amount of color coding will fix this. CAD is no smarter than the person using it, and will not fix any misunderstandings.

Thanks for the advice dhengr. I agree, visual communication is one of the most important skill sets of the structural engineer. As a graduate I am still developing that skill set. Mind pointing out what parts of the second uploaded drawing were unclear to you? When I look at the drawing it seems obvious to me what it is showing. The drawing looks to be showing the intent; a strap footing between two existing foundations. Phase 1 (first pour) shown shaded in blue and phase 2 (second pour) shaded in yellow.

 

[BAretired]

The column taking the 150kN load will be cast against a wall that is to be excavated. The 60kN column is to be cast against the property line. Due to this I designed a combined footing between the 60kN column and the 150kN column.

I understand that the two column footings are existing but the columns have not been poured yet. Why not remove the two existing footings and pour a new strap footing in one piece? Column dowels could more easily be placed and footing reinforcement could extend full length. Perhaps the new footing could be made smaller in plan and smaller in thickness.

If necessary a footing can be cast in sections, but it is preferable to cast it in one piece if possible.

Please use Imperial or Metric dimensions but not both. It is bad practice and will likely lead to errors, either by you or by the contractor. If metric, don't use 457 mm for foundation. Footings are not that precise. Round it to 450 mm or if possible, to 300 or 400 mm.

BA