Piled foundation with tie beams 4

[mte12]

I haven't been able to find much literature for design of piles and tie beams.
In context, looking at supporting a tall braced steel structure, on soft ground.

(1) What's your opinion for principal reason for tie beams.
(2) For lateral load to be taken to ground, is it more common to have a group of piles at brace node (right sketch) or shed load into other piles via the tie beam (left sketch).
(3) For a tall structure on soft ground, is your opinion that pile foundation with tie beams is appropriate. Or a raft foundation.

Most of the literature seems to be for heavily loaded pilecaps, supporting a single column, using a thick pilecap which distributes load to all piles in the group.
There isn't much information for lateral load transfer using tie beams or for that mater a single thick slab tied into all piles.

 

[kieran1]

(1) Piles should be tied in two directions with beams to accommadate any eccentric bending moments due to pile installation tolerances.

Kieran

 

[HTURKAK]

I will try to respond to your questions based on past experience :


(1) What's your opinion for principal reason for tie beams.

-Tie beams are in general mandatory for seismic regions .In your case , the tie beams should be designed as grid foundation to accomadate eccentricity and provide rigid base to columns . ( In case of the use of single pile with single cap without grid beams , the fixed point would be below the ground ).

(2) For lateral load to be taken to ground, is it more common to have a group of piles at brace node (right sketch) or shed load into other piles via the tie beam (left sketch).

-At braced bays that would be necessary , but i would prefer to design the (grade beams ) or tie beams for the developing effects.

(3) For a tall structure on soft ground, is your opinion that pile foundation with tie beams is appropriate. Or a raft foundation.

-There is no simple answer. If the settlement is a concern , i would prefer piling .If this is a real case , pls share some more info . to get better responds.






He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[mte12]

Thanks kieran1, have heard this before. And to control settlement. Which I agree with both.

Is it also established practice to shed load into other piles, or to take moment via pedestal?

 

[mte12]

Thanks HTURKAK.
I couldn't understand response to (1) but assume it's for eccentricity.
Also with (2) I couldn't quite understand.
This is a real case but it's still in early/study phase. It will be a 30m to 50m tall structure, cladded (storm wind governs), overall footprint is about half the height as per original post, on soft ground.

 

[HTURKAK]

If you want to model the columns fixed at the base, the pile cap for single pile and slender tie beams will not provide the fixed support conditions. You have the following options ,

- If you want to provide fixed support condition, you need to design the tie beams as stiff bending frame elements and justify the fixed end conditions. ( say at least Sj=30EIc/Lc)
- If you want the base nominally pinned base ,( this will be the simplest case) model the columns with pin bases . In this case the single pile cap and slender tie beams will be OK , you dont need rotational stiffness . You may provide anchors for simple conn.
- If you want to use piles as bending elements , you need to add the piles to the model with lateral springs and you need to provide rigid column base and design the piles for BM.

The uplift and lateral load effects are more critical at braced bay column bases.






He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[mte12]

Thanks HTURKAK, the assumption will be for a pinned column base, and also top of pile is pinned, so the moment is supposed to be transferred from pedestal into tie beams.

Only thing is, without the experience I'm wondering why I don't see much mention of this load path, after a reasonably thorough web search. Have not come across any publicly available drawings as evidence, or literature. Only see mention of pile groups with thick pile caps, and thick raft foundations (without piles).

So far what I infer is that ground beams are meant to control settlement, and are beneficial for seismic loads.
For some reason I'm not sure that it's an accepted design to: carry moment from pedestal; and shed load to other piles, for large loads. This is without the experience of a full design.

 

[JLNJ]

You have mentioned "settlement" a couple of times. I doubt that the tie beams will be stiff or strong enough to even out the settlement of the individual drilled pier foundations. If one pile started to settle, your tie beam would need to span two times the column spacing to redistribute the load from the sinking pile. Not likely unless your columns are closely spaced and your tie beam is very deep.

I believe the tie beams are primarily to force the entire structure to act as one unit (which is how we design it). If you need fixed bases, the beams can also be designed for flexure as indicated above.

 

[mte12]

Thanks for response JLNJ.
Can you please explain further on "force the entire structure to act as one unit".