Saturday, August 30, 2014

Bored Pile Design : End Bearing Vs Skin Friction

Question : Many engineers in SL simply neglect the skin friction ( not negative SF) of the end bearing piles. Obviously it leads to lager diameter and greater cost but safe design..
Generally, here, the cast in situ piles are considered as end bearing piles and are normally socketed in to bed rock.
However recently I saw a pile design which I thought very risky. These are cast in-situ bored piles with rotary driven type. Pile diameter is 800mm and depth is 19-20m. No rock support in the bottom.the average SPT ( N' 70) is 20. the pile has to bare a design load of 1100 kN.
Is this not a risky and marginal design? how my global friends see it?


As I described in my original post, the local engineers are reluctant to rely on the skin friction of the bored piles owing to the possibility of skin friction loss due to use of bentonite as the stabilization medium. As stated by Tonny Barry, there may be much research which carried out by other countries to investigate the development of skin friction in cast in situ bored piles. However I am not certain how we can incorporate the same results directly to our local conditions which are included different subsurface conditions and construction practice. 
As per the original design, 3/4th of pile capacity is expected to achieve from the skin friction & rest from the end bearing .Obviously this is a floating pile design as mentioned by many. The same FOS of 2.5 was used of both SF and end bearing calculations. I think, this is almost in par with Asrat Workus’ rough calculations provided above. 
Later, two PDA tests have been conducted and as per the results SF is amounted to 2670 KN while only 17 KN of end bearing capacity. It is clear that SF has contributed for the most of the mobilised capacity . As the set measured is only 1mm per blow, the result justifies the fact that shaft capacity of the pile is mobilised at much smaller displacement than the base capacity. 
I think this result contradict the common belief of here, that the bentonite used during the drilling destroys the SF . However I think proper construction methods should used without keeping the bored hole open for long periods. 
Last but not least I have to mention that Pile rest on the firm rock should designed as purely end bearing piles ignoring the SF owing to reasons mentioned by Yassin above. In many cases such as long rock soketed bridge piles, the maximum design load is determined by the stresses in the concrete or pile material and not by the bearing pressure of the rock. 
Once again many thanks for all of you .......

PS Quick Link To full discussion

PDA Test for evaluate Pile Capdity

Saturday, August 23, 2014

What is Redistribution of Moments

The concept of ‘Redistribution of Moments’  which is found in  structural design  is often  not well understood, specifically among undergrads.

Furthermore it is mostly confused with another widely used concept of  ‘Moment Distribution’  which is an essentially a method used to find the BM & SF in structurally indeterminate Beams & Frames.

The Redistribution of Moment is subjected only to moments which derived only by elastic analysis methods.
Moment Redistribution is meant to be that transferring of derived moment from one place to another while not altering the total height of the bending moment diagram. However  it meant to be reduce the BM of one point and in practically that point become plastic and yield which meant there will be a rotation.

Let me clarify this by an example

Let we assume that we got a following Bending Moment envelop from an elastic analyst of the structural element.

source : ALLEN , RF Design to BS 8110

If we do the RF design for this BMD , we may provide the adequate RF for hogging BM of 300kNm and sagging BM of 280 kNm. In that case we neglect that the ability of the element  to withstand the structural effects as an one unit AND also neglect that this envelop was derived  by several load cases. Note that the height of the  BM  is  580

Let assume that this BM envelop was derived by two other BM diagrams.

source ALLEN, RF design to BS 8110

The  height of the each BM diagrams are 400 & 480 respectively.

So we do not want  need necessarily  to design the BM height of 580 that gives by the envelop.

Instead we reduce the maximum BM by some percentage ,let syt 70% and redefined the BM envelop as follows

source ALLEN RF design-to BS 8110

The total BM height is 475 and it is nearly equal to critical elastic BM height of 480.

That is Redistribution of Moments simply explained.