driven piles - cut off height?

[Delinquent]

Due to my architect being a complete spaz, I've got to sink my ring beam further into the ground. Though it is annoying I'm not overly worried about digging it out another 4 inches - what does bother me though is the thought of trying to cut down 10 more piles

The piles are 225mm diameter, the rebar cages are 300mm apart - is there any reason I can't just "coerce" the rebar cages down over the piles by 3 inches? By my reckoning it'd only mean spreading 2 of the square formers by 25mm and jobs a goodun - just have 4in more of pile sticking into the beam than intended?!

If I can get away with that I won't have to take all the shuttering out, I'll just scoop below it down to depth and tack on strips of ply beneath the existing shuttering!

 

[Bowness72]

Could you check with your structural engineer who I assume designed the reinforcement for the ground beams ? There is probably a good chance that it will be ok, although it may also depend on the main longitudinal reinforcement bars in the bottom which would also need to be raised the extra difference, which could make a significant difference to the strength (if they extend somewhere near the midpoint of the beam ?).

 

[Delinquent]

I could, but frankly I put him on a similar level to the architect right now, not impressed at all!

If my plan were to be OK, the main longitudinal bars at the bottom would fit down over the piles as well (due to there only being two, and spacing between them being greater than the diameter of the pile), so rather than sitting level with the pile top they would sit 3 to 4 inches below it.

 

[Shytalkz]

The outer bars should slip past the outside of the piles without a problem; however, you've probably got three or four bars in the bottom and the inner ones won't go past. The bottom bars are needed to take the tension in the span and also for anchorage at the support. Generally, 50% of the span steel going 12x bar diameter (if straight bars) past the centreline of the support (the pile) will be ok.

To save you going back to the SE, cut the internal bars 25 or so back from the pile casing and, if there's any loose rebar lying around, put some splice bars over the top of the pile - a T12 about 1200 long will be ok. This will sit at a slightly higher level than the span steel - this is not a problem. These splice bars, together with the external bars carrying on through will give you more than 50% of the span steel area over the pile, so no further checks are required.

 

[Delinquent]

Thanks for the input Shytalks, The not three or four bars in the bottom, just the two! The cages are quite simple T8 300x300mm squares placed at 200mm centres along the four main bars, each being T20, one in each corner of the square!

So in essence, it's just the T8 squares that would have to move, spreading them at the bottom by 25mm or so.

You aren't the first person to suggest there are "normally" 3 or 4 in the bottom though, and going on the shambles this has been so far it does make me question things slightly...

 

[Bowness72]

It isn't necessarily a problem if there are only two bars in the bottom due to the beam being relatively narrow. A ground beam would often be 450 to 600mm wide (although can be wider or narrower) depending what it is supporting. When the beam is wider you need more bars as there is a maximum distance allowed between the bottom bars depending on the forces involved.

 

[Shytalkz]

You aren't the first person to suggest there are "normally" 3 or 4 in the bottom though, and going on the shambles this has been so far it does make me question things slightly...

Perhaps I should have said "generally": each engineer and rc detailer has their own way of doing things, no method is necessarily more right than any other.

As Bow said, there are "rules" as to maximum spacing across the width of the beam between tension rebar, but they wouldn't be exceeded in this instance for two bars. And, even if they were, so what? It's not going to fall down because it doesn't comply with some arcane clause in the code.

The squares are called links and there is absolutely no problem in moving these to miss the pile. They're not required over the pile anyway.

 

[Delinquent]

excellent, many thanks guys Digging commencing!