Pile and Groundbeam Design

[carson321]

Hi all. 

 

I feel like im losing the plot going back and forward with building control over my foundation design. 

 

Lot of history to this but right now, I have a screw pile and groundbeam design for my single storey rear extension. Screw piles are in and the rebar in place waiting for the concrete to be poured. BCO has complained about various things first of all thinking that the cage was misplaced, misaligned and all sort of other things. Visit from her supervisor and some setting out by my builder made it clear that this was fine. 

Now for my current predicament, BCO have said they have consulted with their engineers and received a response stating

 

"It’s not entirely clear from the foundation calculations, but it appears that the groundbeam design is to BS8110. If so they do not have sufficient links in their groundbeam as no bar in tension should be more than 150mm from a vertical leg. (clause 3.4.5.5). Bars in tension should be no more than 155mm apart and no more than 77.5mm from the outside face (clause 3.12.11.2)."

 

Piling engineer response was basically  that he believes the spacing is correct but if they wanted extra steel, then stirrups could be placed halfway between the links to reduce the vertical spacing. 

 

BCO further response was then " I have spoken to our consulting structural engineers regarding the design and there are fundamental errors with the calculations and the question that our engineers have posed to your piling contractors has been misunderstood. Therefore, we require a design from a competent structural engineer that provide a coherent justification for the foundation design on site."

 

They never raised any issues with the calculations in the first response, and im not really sure how many different ways their comments on the number of links could be interpreted. 

 

I have attached the original beam design and a photo of the installed rebar. 

 

Any help interpreting this mess or helping me understand if my piling engineer is doing the right thing or not would be greatly appreciated. 

[Oz07]

Sounds like your job has become a bit of a battle for the bco. It's a tough one. Maybe in the first instance copy your piling engineer in on their concerns and get him and them to sort it between them. You want to try to de escalate if you want to get the job moving might need some skillful diplomacy when talking to both sides.  

[ETC]

What centres are your links?

[Gus Potter]

33 minutes ago, carson321 said:

 I have spoken to our consulting structural engineers regarding the design and there are fundamental errors with the calculations and the question that our engineers have posed to your piling contractors has been misunderstood. Therefore, we require a design from a competent structural engineer that provide a coherent justification for the foundation design on site."

That is a bold statement to make!

 

The design of the links can be complex, depends on the level of shear stress, usually in and around the pile heads. I won't comment further as I don't know what loads are on the beams / piles. 

19 minutes ago, Oz07 said:

Maybe in the first instance copy your piling engineer in on their concerns and get him and them to sort it between them.

Plus one to that, good advice. This is what often happens, sometimes a quick phone call between the engineers can sort it all out. Oh, it's ok to design to BS 8110, so long as you are aware of any parts of the more modern Eurocode that superceed the older BS 8110 code. 

[Alan Ambrose]

Stating the obvious - screw piles are fairly novel still. Anything novel gets a bit of extra scrutiny and probably some higher safety factors (explicitly or implicitly) as it's perceived to be higher risk. Nobody wants to make a mistake. Your BCO has called in their engineer because they don't feel able to sign off themselves in the same way they might for a more conventional foundation.

 

Your BCO and their SE may not have dealt with screw piles before (although the BCO's SE is sounding v confident). Your BCO's SE has called your own guy incompetent (or at least that was the BCO's interpretation), which means it's all got a bit out of hand.

 

Suggest your own guy has a call with the BCO's engineer to see whether they can square their differences. It should all be quite simple to agree which is the right design code and determine whether it meets that code. Looking at the situation positively, any extra scrutiny reduces the likelihood of any residual error.

Edited Tuesday at 21:41 by Alan Ambrose

[Nickfromwales]

BCO is just out of their depth / comfort zone, so has gone into the default “ass-saving” mode.

 

@Oz07 is bang on the money, set the two professionals up to communicate directly so you’re 1) not losing things in translation, and 2) are not introducing any emotion into the mix. 
 

Easier said than done, and sometimes we don’t know when emotion compromises any situation.

 

I’ve dealt with one SE lately, and after he stared and pondered and scratched his head, I just said get your lead SE to say yes as this is all perfectly fine, please. 
 

He left, did as I asked, lead SE confirmed all my adaptations were spot on and the lot was signed off without further hesitation.

 

Been doing ‘construction’ for north of 30 years, and not had a proposal fail to date.

 

They just need to tweak their antennae’s to align, and the problem will disappear quickly. 

[saveasteading]

23 hours ago, Alan Ambrose said:

Your BCO and their SE may not have dealt with screw piles before

I'd be cautious too, because I've heard a lot of guff from mini-pile sales-people, thus start from a position of some suspicion.

ie I have been given amazing claims that felt exaggerated to me. It was at an exhibition aimed at the public, not the industry.

(To be clear, I've used big piles many times, but never these little ones, which may supply a less distributed resistance and be more liable to ground movement, especially in clay.

3.7m span between screws is quite a long way.

 

Piling engineer response was basically  that he believes the spacing is correct but if they wanted extra steel, then stirrups could be placed .  Says who? That isn't good enough. How do we know the accuracy of the conversation and result?

It needs to be on a drawing or in writing from their SE., and if it has already been assessed then that is an easy thing to confirm and will be approved by the bco.

 

That seems to be plenty of reinforcement.. that isn't the issue really,  so  it should be resolvable.

 

I don't like the generic look of the drawing btw.. 'piles to suits conditions' is not site specific.