Grade beam connection to offset piers 1

[SJBombero]

I posted this without the detail in Foundation Engineering but I didn't get much response. I am designing a replacement foundation for an existing residential structure. Originally spread footing. The geotech wants to replace them with P&GB. With the existing structure in the way, what is the the typical method of connecting the piers to the grade beam. The only thing I can think if is a haunch. I have attached my conceptual detail. Is it typical to install the piers straight or inclined?

Detail

 

[KootK]

1) Usually straight piers.

2) The sketch below is what I would expect. It's not so different from yours mechanically but would, most likely, be much easier to form.

3) It would be nice if you had a slab or strap beams to rectify the moment on the piles but I do acknowledge that is not always available.

 

[KootK]

The grade beam could also run through the pile cap with the right detailing if the pile cap would interfere with the existing foundations.

 

[le99]

The connection is basically a lot like a corbel over a column. You shall pay attention to the bending on the pier which may not have accounted for the additional forces.

 

[EcoGen]

If you're not responsible for the pier design itself, I would have the Geotech Engineer somehow sign off on the reactions you determine from the structure and grade beam onto the pier.

EcoGen Consultants LLC
Structural Engineers
ecogenconsultants.com

 

[HTURKAK]

If originally spread footing, probably grade beams designed as tie beams .. Your approach thinking if is a hunch is reasonable as far as grade beams can transfer the superstructure loads ..

I would check the capacity of the grade beams and design the pier with corbel ..

You may prefer to post the foundation plan and pier locations to get better responds..

 

[Brad805]

I don't quite follow the problem yet. Your sketch does not show any existing elements you need to work around. If those are being removed or the existing building needs re-levelling, there are a few other considerations.

It sounds the existing condition is like the revised sketch below. If so, the offset pile cap like Koot shows is what we would do. Simple forming with simple rebar is cost effective even though it requires more concrete. Straight piles are always preferred. Offset piles caps are not complicated to design for a residential application. The loads are small and your pile diameters start at 16" like you show. We typically do not use 16" diameter piles for these applications because once you consider the size of the rebar cage it never feels right to me. It is important to consider what the pile rig must work around. With the typical equipment we use the centerline of our piles are usually at least 36" from any site constraint such as eaves or other framing (see pic).

 

[SJBombero]

Thank you all for your responses. It's good to know my initial sense was not that far off.

We probably won't try to reuse the existing footing at all. It's in a seismic zone and it was built so long ago that the anchorage to the footing is highly inadequate. We would have to re-anchor all the sill plates as well as dowel the new grade beam to the underside.

Regarding the equipment we will be using. There is barely 8' to the adjacent house and quite a few interior piers that will be needed so I expect portable drillers will be used. I didn't realize that was an option but the geotech assured me that it was.

 

[Brad805]

It sounds like you have many steps to detail like one would for underpinning.

I do not think you have space for the type of piles we use. It takes HP to drill those. It sounds like you would at best have space for bobcat sized equipment. I am not too familiar with drilling options for those, but they could be short piles that would be more challenging to design depending on your loads. Keep in mind the amount of tailings that comes out these holes as well. You need space for equipment to remove that as well. I would investigate what equipment will work in the space you have.