Additional Footing Detailing 3

[Jandra11]

Hello guys. I need your professional advise regarding this project im working. The project is additional mezzanine and the municipality engr wants to provide additional footing for all the support instead of directly connecting the new columns to the existing subgrade slab(200mm). and this is what what they proposed.

1200x1200x300mm thick foundation.(maximum axial force 300kn)
Question
1) Does top rebars required?
2) Does the new footing should be attached to the exiting slab by shear dowels. and how do you compute for the shear dowel rebars.
3) Is there any code saying that the minimum thickness of the foundation should be 300mm?

Thank you in advance.

 

[Mike Renks]

Hi Jandra11

To answer your questions, please see below:

1) Typically, you'd want to install mesh/rebar top and bottom, as you're moment could be either negative or positive; depending on the types of loading conditions the footing is under.
2) Yes. You need to use dowels to connect two slabs/footings together. I recommend obtaining a technical guide/book called TR34. It's mainly for slabs subjected to wheel loading, however there is excellent background information regarding dowel design.
3) Your thickness is governed by the amount of load you're applying onto the footing and the amount of reinforcement in the footing. Too much reinforcement, and you might have a ductility problem. Too little and your footing may fail or you need a larger/thicker slab to compensate if reinforcement is an issue. There are a lot of factors that affect it.

Hopefully this helps.

 

[dik]

I'd check to see if the 8" slab was adequate. If so, I'd present my findings to the AHJ.

If this is unacceptable with written reason, I'd design the footing... compact a little Gran 'A' using a 'jumping jack' or something similar and cast the concrete. I would not dowel it into the existing slab.

Dik

 

[Jandra11]

thank you for the inputs.
@mike renks even the footing only support axial loading does is still need top reinforcement? and whats the main purpose of the dowel? does it help the additional footing to spread the axial load from the column for soil bearing capacity issue?
@dik yes i checked punching and soil bearing capacity and still they want to proposed additional footing they said the minimum thickness should be 300mm? if im going to propose additional footing can i use the detail in the drawing? or is there any other good detail you can suggest?

 

[P205]

@Jandra11 The footing should be design to carry the load. The dowels would then just be to maintain a level surface across the joint. To me, the dowels, at 150 o.c. seem overkill, and like Dik said, I wouldn't have them.

If the municipality's engineer said to use 300 min, and 300 works, I would just do it and move on. Is there a reason your fighting it, or questioning their requests?

 

[Jandra11]

@P205 thank you.the client wants to just connect it directly to the existing slab to save some money. meaning the dowel doesnt help anything in the design aspect?

 

[Jandra11]

btw is it ok not to have pedestal?

 

[dik]

OP: How large a footing are you looking at? 2' square? or bigger...

If you don't need a pedestal, why construct one?

Dik

 

[dik]

You need to construct something like the attached.

Footing rebar depends on the size and load; it may be possible to use plain concrete, and, not reinforce it.

Footing thickness depends on size and load, and length of anchor rod. If no serious tension on the anchor rod, then can likely get by with 12" long headed bolt, extended about 3 or 4" above the footing I've shown a grout bed under the column base plate, you can get by with flowable grout and about 1/8" or bear it directly on concrete with no grout if load is small. You have to sawcut the slab as required and excavate to the depth plus granular fill. You may only need an inch of so of granular fill, to level the excavation. You may have a little soil 'slough' in from underneath the existing SOG... just make sure you vibrate concrete to fill the void created.

https://files.engineering.com/getfi...-dbbf-472d-8582-395a9e9b8536&file=Footing.pdf

Dik

 

[JAE]

1) Does top rebars required?
[blue] Only if there is significant moment at the base of the column, the base is designed for the moment, or you have any net uplift on the column.[/blue]

2) Does the new footing should be attached to the exiting slab by shear dowels. and how do you compute for the shear dowel rebars.
[blue]As others stated above - the dowels would only be used, in my opinion, to keep the slab/footing top surface level. This would only be needed if you think the footing will settle some amount that would cause problems with the floor, or if your slab needs the footing support under wheel loads from vehicles, forklifts, etc.[/blue]

3) Is there any code saying that the minimum thickness of the foundation should be 300mm?
[blue]I don't know what code you are under but in the US, the ACI code specifies a minimum of 6 inches (150mm) which isn't much. We usually use a minimum of 10" (250mm) and more for column footings.[/blue]

is it ok not to have pedestal?
[blue]Yes we do it all the time.[/blue]



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[SlideRuleEra]

The footing is 1.20 x 1.20 meters and carries the 300 kN column load from a proposed mezzanine.

The client wants... to save some money.

IMHO, this is one of those rare times when the cost effective (i.e. cheap) thing to do is also the best thing to do.

Save money by NOT using dowels (dowels are cheap - time and labor to install them is expensive).
The footing is heavily loaded, 208 kN/m². I assume the floor load is much lower (say, << 20 kN/m²). Depending on soil, a good chance of differential settlement. In fact, I would do the opposite of using dowels - install an isolation joint around the perimeter of the footing to ease any differential settlement.

Save money by NOT constructing a pedestal.
Sounds like the original plan was to put the column directly on the existing slab. So, put the column directly on the footing.

Save money (mostly time and labor) by NOT using a lean concrete working mat.
If the soil is good enough for 208 kN/m² loading, it will likely make a satisfactory working surface without lean concrete.

Unless the client wants only the absolute lowest cost, I would enlarge the footing slightly to 1.50 meters square (cutting ground contact pressure from 208 kN/m² to 133 kN/m²) and increase thickness to 400mm (giving a little more separation between the top and bottom rebar mats... and a top rebar mat may not be needed anyway).

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Jandra11]

Thank you all for sharing your knowledge.
The 1st option is directly connect the new column to the 200mm thick slab. therefore I check the slab for punching (300KN:ultimate load) and soil bearing capacity (120kpa)(see model)

as per this analysis the 200mm thick slab is safe. or do I miss something?

And when I submit the calculation, The municipality engr suggest this kind of this detail (1200x1200x300mm footing with dowel). I was confuse of the footing size 1200x1200 because the maximum axial force is (200kn:service load) which will fail on Soil bearing 140kpa>120kpa and at that time I thought that dowel is to spread the load coming from the column to satisfy the soil bearing capacity.

As of now base on your comments Ill proposed footing which is not connected to the existing slab.
Another question is there any code standard stating the minimum dimension should be equal or greater than 1.2mx1.2m. because im suggesting to minimize the size of the footing at the corner side of the structure but this municipality engr insisting that their minimum standard for mezzanine foundation should be 1200x1200x300mm. with top and bottom reinforcement.

 

[dik]

The dowels do little other than keep the concrete surfaces aligned. They are not really for load transfer. The detail I sent is workable... might consider the 1/2 joint around (ugly).

Dik

 

[Jandra11]

thanks dik. yeah ill propose something like that.