if i get a core sample of concrete in a newly built existing structure and found out based on laboratory test that the compresive strength of the sample is below 3000psi, does it conclude that the structure is unsafe or structurally unsound.
Are there other factors needed to be considered in determining structural soundness of the structure?
Answer ( Eng. Yasela)
Well , whether structure is unsafe or not depends on several factors.
first issue to be resolved , what is the design strength of the concrete that used in design of the structure. Presuming it is 3000 psi, as you have mentioned, below the par values of 3000 psi required further inquire . Further it is important to evaluate by what margin it get failed.
innitially, you can carry out a non destructive test such as hammer bound test though the particular tests are not widely accepted . then you can carry out core drilling and test the sample. by means of these tests, you can evaluate the actual strength of the concrete of the structure.
with this figure you can check the design calculations. many distinguish concrete practice worldwide use factor of safety for the material and strength. if the actual concrete strength figures are higher than these values which is after the applying of factor of safety, the strength of the structure should not be in question
however durability aspects of the structure may compromised due to serviceability conditions such as width of the crack, fatigue etc. these issues are to be examined by a experienced structural engineer.
In bored pile construction , there are few widely spread methods in stabilizing the bored hole. This post discusses these methods in summary.
The most common and economical solution is to use bentonite
and the main idea behind using bentonite is to stabilize the bored hole. Owing to
the high density, it prevents water inflow to the hole. However, if it is a floating
pile or depend significantly on skin friction to resist load then the use of
bentonite is controversial.
If bentonite is not the solution then the next economical
solution is to use temporary casing. Steel
casings are widely used in the industry. However extraction of the casing
during the construction is challenging explicitly in deeper depths.
Inserting a temporary steel casing- checking for versatility of the steel casing using the buble
In that case you can decide for permanent casing and it has
to be ensure that you have considered
the cost allocation for that. This can be decided with the specialized pile
contractor.
The mix design of the
concrete using for piles also play a pivotal role and there are many
specification on this subject. BS 6235,ACI 357,Dnv are few of them…
Generally cement water ratio is around 0.4 for the submerged and minimum cement content
would be 360 kN/m3. Minimum 28 days characteristic strength would be 35 N/mm2. Of
course these are general specification and designer has the authority to adopt
tailor made solution. The Portland cement is preferable and low content of tri calcium aluminate is preferable to reduce
the risk of sulfates attacks, if it is offshore construction explicitly.
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 .......
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.
