Helical screw pile / grade beam foundation

[matty54]

Hi there,
I'm new to Eng-tips, but I have found lots of useful information from the threads in the past so I decided to join.
My question regards a pile foundation for a pair of large metal building systems. buildings will house cranes and sawmilling equipment. I plan on using grade beams throughout to help pick up moments and lateral forces from the rigid frames. some of the larger equipment will require their own slabs supported by piles separate from the building foundation. foundations for the equipment will be poured after the machinery. Does anyone see any issues with pouring the equipment slabs directly on top and around the grade beams? Essentially enclosing the grade beams in the equipment slab.

 

[thoughtofthis]

Generally, equipment foundations are separate for a reason like vibration, settlement, etc. Connecting them may be problematic. Secondly, equipment gets replaced and new foundations will be poured. If the grade beam is a part of your lateral force resisting system, that will now include the equipment foundation.

Regarding the building, if grade is constant around the building, it looks like the LFRS of the PEMB is supported on top of a retaining wall. I think this presents a few code issues. Also, pile foundations under walls need to be staggered for stability.

 

[matty54]

Thanks thoutofthis, I did not consider the future replacement of those slabs. I think I am going to try and split the slabs up between the grade beams instead. As for the building, it will indeed be a metal building system with a varying height retaining wall all the way around it. I thought maybe I would be able to design the wall and pedestals strong enough to handle the torsion from the column feet and soil pressure. Other than that maybe adding tiebacks?

 

[dik]

Welcome, and enjoy your stay...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

Not sure what you mean... if normal grade beams, the passive soil pressure around the grade beam provides lateral restrait and I often use it to take the lateral loads off the piles. If a retaining type of structure, the retaining wall is often designed as that with the lateral PEMB loads taken out into the floor supporting slab.


Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[matty54]

Hi dik, yes that is what I am going for: using the grade beams to provide the lateral restraint, but because the PEMB loads are 8' above floor slab and grade beam, I thought there might be an issue with that.

 

[HTURKAK]

Hi Matty,

IMO, when we say the perimeter grade beams does not define the situation .. In this case ,it is better to say Perimeter Grade Walls moreover retaining wall also..

-Apparently the span and size of tie beams are not suitable for the use of moment resisting frame element and is not a good idea in this case. If the size of the tie beams are selected for moment resisting element, still will not be feasible and the tip deflection at the top of perimeter walls will be magnified..

-The use of single screw piles under each pillar will not be sufficient for the moment due to wind and active soil load. It is better to use spread ftg under the pillars with two piles .

-Regarding the interior columns , the use of single pile is questionable when the spans and size of the tie beams considered..

- I will suggest you to ask supervision of an experienced structural eng.
I attached the revised sketch for your info.

 

[dik]

If 8' above, then you have some serious moments to design for; you might want to use counterforts or pilasters in the retaining wall.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[thoughtofthis]

Not sure what you mean... if normal grade beams, the passive soil pressure around the grade beam provides lateral restrait and I often use it to take the lateral loads off the piles. If a retaining type of structure, the retaining wall is often designed as that with the lateral PEMB loads taken out into the floor supporting slab.

I was referring to the piles beneath the retaining walls between the columns. IBC 1810.2.2 "Deep foundation elements supporting walls shall be placed alternately in lines spaced not less than 1 foot (305 mm) apart and located symmetrically under the center of gravity of the wall load carried, unless effective measures are taken to provide for eccentricity and lateral forces, or the foundation elements are adequately braced to provide for lateral stability."

Regarding the exterior wall height and the column bearing on the top of the wall: A perimeter wall is not unusual for a PEMB, but it usually is just to pick up the bottom of the siding and to provide some frost protection. Occasionally, you pick up a jamb. Heights are generally less than 4 ft. What is shown here is totally different. It's a retaining wall supporting a PEMB. By supporting the column on the wall, you have inserted a new LFRS element between the moment resisting frame R=3 and the foundation. There are several issues with this configuration: 1. Which LFRS do you have below the column? Cantilever concrete column? Possibly, but this has implications for the design of the building (R=1?) 2. Should you/Can you switch LFRS without lateral support at the joint? I don't think you can use the seismic coefficients from ASCE 7 - maybe it's possible if you created and tested a new LRFS per ASCE 7 12.2.1.1. 3. Your overturning moments will be huge. Overall, I would not recommend this configuration.

If it were me, I would separate the the wall from the building by some inches to allow for drift. Bring the steel down to the top of foundation. Run the wall behind the column. Use passive pressure and sliding to resist lateral from the building if possible (you may be able to include passive from the retaining wall on top of the footing). If piles are required for whatever reason, you'll need to rely on the lateral capacity of the piles. Helical anchors and micropiles are not really suited for this. You'll hear that they have some lateral capacity or that you can batter them if you want more. In my experience, steer clear. I would be looking at auger cast, drilled piers or H-piles. Grade beams may be required based on your SDC.

 

[matty54]

Thanks for all the responses. thoughtofthis the wall itself isn't supporting the columns or any other significant vertical loads, it only acts as a retaining wall. Columns all have their own reinforced pedestals, so I am looking at using cantilevered concrete column along with the lateral pile capacity to support the lateral load. Having the walls behind and bringing the columns down lower would be great but unfortunately the building isn't going to be able to change.