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?


Answer: 

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.









Sunday, June 29, 2014

Flooding to the Correction

In the last post, I assured that I will write on the measures which we took to protect the bridges and embankments owing to damages caused by scour. After a devastating rainy season which happened during the project implementation, we were convinced on the additional requirements of the protection measures which were not included in the original designs.

The following photo shows the scale of the damages owing to the flood event. Even though all bridge structures were not submerged by the rising flood level, the approaches were badly damaged.



In an another location, the temporary baily bridge, at the bypass road,  which was constructed to cater the traffic during the construction was totally submerged while the new  bridge structure stands firmly owing to correct choice of HFL during the design.

Note the  red colour  bailey bridge.


The temporary bailey bridge is submerged while new bridge unaffected.



The following bridge is located at a lagoon. Similar to other locations, the bridge approaches have been inundated.




The next photo depicts the scale of the damages in the downstream of the approach roads near to bridge structures.




After witnessing a such a devastating flood , we took concrete  remedial measures  in order to withstand the future flood events.



I keep those for next post as  I feel that this post is lengthy enough for a day :)

Sunday, June 22, 2014

The bridge on a river where no single drop of water to be found.

Even in a dry season , very few water paths are  get dry as following one  where not single drop of water can be found.

This bridge is located in  Mankulam - Mulathive road, where I worked as the Consulting Bridge Engineer for the project in 2012/13

A Bridge on a river where you can not find a single drop of water.


While structure was under construction,   few visitors to the site raised their eyebrows on the requirement of a bridge at particular  location. However,  the Hydraulic studies of the area gave us a very different figures which are drastically differ from the depiction of the picture.

I was fortunate to witness the theory meets reality.  During the time of project execution, bridge met its highest flood which it was designed and stood against it.

Same bridge but with many drops of water

The bridge was designed with a 600mm of minimum  freebord height for the high flood level and it is adequately designed as seen in the picture.

However,  at the time of  flood , the bridge was  not in its completed form. The approach roads and the embankment protection measures were not completed. As a result, the embankment was damaged and later it was rectified.

Looking forward to write more in my next post on the subject of  bridge protection  measures  that we took after the rainy season.





Thursday, May 15, 2014

The importance of Hydraulic Jump

I caught this video  from FB post..........



video


This video shows the importance of the  correctly design HYDRAULIC JUMP at the  spill.

I can remember the effort of my Hydraulic Professor who took great effort to teach us the phenomena during my undergraduate studies.

The hydraulic jump is designed to dissipate the energy of the water flow, so that it flows in safe speeds inthe downstream.





Wednesday, May 7, 2014

Cracks in Prestress concrete Beams


I came across this problem last year. Although the issue has been solved, still I think we can share our views on this.

The problem occurred on 19m long pre tensioned PSC beam.  The cracks, in the range of 0.2 mm – 0.5 mm,  were noticed on the top flanges of the 19 m beams and  near the two lifting positions of the each side ends of the beams as shown in the following figure.


cracks positions of the beam

closed view of the cracks

The crack positions, sizes and the propagation are almost identical in every beams at the site and I had my  uncertainties on the structural integrity of the defected beams.

The cracks have appeared on the places where maximum hogging moment occurs during the lifting of beams. Hence these are considered as flexural cracks ( Not shrinkage or early thermal cracks) and possibly owing to lifting of the beams before the concrete achieve its characteristic strength in Transfer Stage.  

I would highly welcome your opinion on what next to do?




Friday, February 28, 2014

Monolithic Buried Concrete Box Culvert Design: Load Cases and Applicability of Hydrostaic

Three cell box culvert which I designed  for  NRCP
Last few days, I spent most of my work hours on reviewing the designs of concrete box culverts.

As local practice is based on British codes, the most relevant  document is identified as part 2 & part 4 of Bs 5400. The additional guidelines are  provided by BD 31/01.

Appendix A of BD 31/01 shows many diagrams on  load cases which are to be considered. It includes following combinations.

1.) Max. Horizontal loads with Max. vertical loads ( No Traction)
2.) Minimum vertical load with max. horizontal Loads (No Traction)
3.)Maximum vertical load with minimum  horizontal loads ( No traction)
4.)Traction with max. vertical Loads
5.) Traction with min. vertical loads

The checks on stability  is conducted  as follows;

6.)  Sliding : min. vertical loads with buoyancy on foundation slab. ( primary live loads are included with min. factors as it is necessary include traction forces)

7.) Bearing : max vertical loads with buoyancy on  foundation. All loads are nominal.


In all above conditions , it is necessary to concern whether to  include live load surcharge on to  both abutment walls or not.

In the first five of loading cases, BD 31 is silent on applicability of  Hydro static Pressure on walls.

However elsewhere , in cl 3.1.4 it says, /// when appropriate , the effects of hydro-static pressure and buoyancy shall be taken in to account. The increase in pressure on the back of the walls  due to hydro static  pressure at a depth  Z meters below water level shall be taken as 10Z(1-K)   kN/m2///

Please see the following figure which i took form my note book and captured from my mobile camera. it  explains how this 10Z (1-k) is derived.


However BD 31 does not well explain   that how hydrostatic forces do   applicable in the first five load cases.


Contrary , the american LRFD documents which are basically based on Marston's studies on the subject , provide more details on this.

The following picture that referred from a design guidelines of Kansas Department of Highways is self explanatory.


The Department of Transport of Massachusetts  LRFD guidelines provides following recommendations on the load cases;

1.) Maximum  vertical load in deck slab and Max. outward load on the wall ( DL max+ Earth vertical max+earth horizontal minimum+hydrostatic max)

2.) Minimum vertical load and maximum load on inward wall of the abutment ( DL Min + Earth verticle Min+ earth horizontal max)

3.)  Max vertical load  and max load in inward wall of the abutment ( Dl max+Earth verticel max+Earth horizontal max+ live load  max)

It is clear that there  is not  general agreement  between distinguish  design guidelines on the load cases and more specifically on the  hydrostatice loads and the  uplift /buoyancy.

I am of the view that hydrostatic pressures become critical when a sudden discharge  flows through the barrel without no time allowing Ground Water level to  rise. Such a phenomena could happen in the cases of upstream flood or owing to spill of upstream reservoir.

In the same line of argument , water level can be differ in two sides owing to rise of ground water level but stream flow is at regular levels. I am in doubt whether is this could happen in real?

In other cases hydrostaic forces are in balance or have little impact on the total effects.


construction of the 3 cell