Eccentric Pad Footing with point load applied outside of middle third solution 4

[sithlord382]

Hi,

I am in desperate help in solving an onsite issue that has been flagged up by building control. My builders poured a 800mmx800mm pad footing which is taking a point load of 77kN from a column however this load is not applied at the centre of the footing but instead 144mm north of the centre in the y direction. Building control is asking to provide design of this eccentric footing. However i have looked through the calcs the SE provided and there is nothing on how he designed this.

Having looked through the internet I have come to the following calcs:

Eccentricity (e) is 144mm
Point Load is 77kN
e > D/6 therefore the load is being applied outside the middle third of the footing
I am then getting a pressure of 250kN/m which exceeds the allowable bearing capacity of 100kN/m

The house has been completed so how can I justify this to bc without having to rip out the footing and getting a SE to redesign?

Thanks

S

 

[Redtelis]

Even without eccentricity, the pressure will be 77/0.8x0.8=120kN/m2 still greater than allowable bearing capacity 100.

Is this point load 77kN in ultimate design limit state? If yes, this could be reduced to SLS so about 77/1.25=60kN, that gives pressure below 100 (WITHOUT ECCENTRICITY)

In any case, with a footing of 800x800, eccentricity 150mm and point load over 50kN you will never get an applied pressure below 100kN/m2. Your above applied pressure sounds right (250)

 

[phamENG]

You said that an SE designed it - do they show it as eccentric on the plans? If so, they need to provide their justification. If not, then the builder needs to explain why they did it and prove that it's okay.

 

[JedClampett]

Depending on the types of loads (LL?, Wind?, seismic?) that make up the 77kN, maybe the Geotech can give you an increased value, especially for a short term case. A lot of times there's extra capacity there, but they're more comfortable with very conservative values.
You might have to grovel a bit.

 

[sithlord382]

Hi Redtelis,

Many thanks for the quick response.
Yes 77kN is the ultimate limit state.
So if I will never get applied pressure below 100kN/m2, how can what my builders have constructed be justified? ie. what options do I have?

S

 

[sithlord382]

Hi phamENG,

No the SE only gave the dimensions on his structural foundation plan, with no indication (within his calculation pack) how he got to that outcome of 800mmx800mm. They have not mentioned it is eccentric but looking at the plan the column is off centre. Honestly if i had known what and where to look for at the start of the build i would have pulled him up on it.
Builder has built correctly to SE drawings.

 

[sithlord382]

Hi Jed Clampett,

I understand that the safe bearing capacity of 100kN/m2 is conservative, however surely even with grovelling there is no way to justify a pressure of circa 250kN/m? The ground conditions are london clay

 

[phamENG]

sithlord - if the SE put the footing off center on the drawings and the builder built it that way, then your structural engineer needs to justify why he designed it that way.

Your 100kN/m[sup]2[/sup] may be an allowable value. The 77kN load cannot be used to develop a bearing pressure to compare to allowable bearing capacity because it's an ultimate load. They wouldn't be compatible. Here in the States that is common - geotechs give us allowable pressures, so if we design using LRFD (I think you may refer to it as Limit State Design?) we have to convert our reactions to allowable stress values at the soil/structure interface. Without knowing each component of the load in that column, it's hard to do accurately - but as Redtelis indicated there are rough conversions.

Bottom line, get your structural engineer on the phone and find out why he designed it eccentrically and have him explain it to the building official. Where I practice, that's part of the job. If the building official questions my design, I'm obligated to answer - even if it's completely inane and shows a lack of knowledge on their part (I had to give a pro bono 30 minute lecture to a plan reviewer about how wind loads work once).

 

[BAretired]

Are you sure that 77kN is the ultimate limit state? Allowable bearing pressure should be based on service load, not factored load.

800 x 800 is not a very big footing. The easiest remedy is to replace the footing with one that is centred. The column will have to be temporarily supported while the work is carried out.

BA

 

[sithlord382]

Thanks to the both of you. I think I will take your advice and get the SE involved.
It seems like more than just a simple internet calculation is required to justify this. I just wish I understood the structural plans in more detail at teh start of this project.

S

 

[Rabbit12]

Report back here with what you find out.

 

[BAretired]

One of my clients, who was not know as a big spender, had the same problem. His solution was to simply slide the existing footing along until it was centred under the column. He told me that, in his experience, this was standard practice. I was not entirely happy with his solution, but was not overly concerned either, so let it go. The subsoil was typical Alberta clay, which is fairly stiff.

I am not recommending that solution here, however. It has been flagged by building control and they may not go along with that method of repair.

BA

 

[Settingsun]

For checking against bearing capacity, you can consider an effective footing size that is concentric with the load. In this case, 800x512 giving 188 kN/m2 for the 77 kN load.

 

[sithlord382]

Hi Steve,

Just to be clear you are saying that a concentric footing of 800mmx512mm with a point load of 77kn will give a pressure of approx 188kn/m2 exerted by the footing onto the soil?

I'm still failing to understand how this is relevant as the SE has specified in his calcs that London clay has max 100kN/m2 allowable bearing capacity. Could some please explain ths in layman's terms for me.

Thanks in advance.

S

 

[Redtelis]

If what you have built on site is based on SE construction stage drawings, then this footing fail should be addressed with the SE as he must provide calculations that shows that the eccentricity on the footing works.
If the builder didn't follow the SE drawings, then it's the builders responsibility.

 

[Settingsun]

Hi,
The 188 kN/m2 is in comparison to your 250 kN/m2 that you got as the peak of a triangular distribution. It doesn't solve the issue on its own, but it's 25% better off. Then reduce from ULS to working load and it's down to ~150. Still not there but inching closer. You may need to put all the tricks together to get this over the line.