bolehlandconstruction

What if?

2009-08-11T06:55:00.009+08:00

"What would happen if we pour concrete into this?"

Because of asking this stupid question (and take photo in the same time in front of the site superviser), the stupid photographer has earned a 5 minute-long-dirty-words-bombing.

Why the photographer being scolded?

1) Because he is not the supervisor's boss, or paymaster. The supervisor need not to reply in nice way.

2) The supervisor pointed that the reinforcement is not yet done the final part - inserting lapping bars. The act of taking photo of half done work seems "cari pasal" (inviting trouble).

3) Because the photographer was not used to the site language, he interpreted the supervisor's way of communication as "scolding".

From this event, the photographer realized that:

1) The site people knew that the connection is unsafe without additional bars across the main beam.

2) The site people like to do "double lapping" (which consume more steel).

3) Don't comment to the supervisor if you are not his paymaster. And, don't let the supervisor know that you are the one who tell his boss about his fault.

I've been inspired by this event. Since the digital camera or video cam are so popular nowaday, we can fully utilize it in the site monitoring. Before concreting, just ask the engineer to shoot every details and record his comment, like this: "Today is 25 August 09, 3pm. BLC is reporting. Now I am standing near the column at Grid 8/A. You can see the lift core is 2 columns away from here. Now I'm inspecting the beam top bars across this this column. It supposed to be 3T32 at top and 2T25 below, let's see. Oh yes, it is! Lets check the cover, measure from the marking on the formwork... mmm, 30mm... not too bad, still accpetable. This part is ok, let's proceed to the next joint....". Another video camera may be set on the tower crane to make sure that all bars checked by the engineer are there untill the concrete poured are set.

Check every details, record how you check and what did you check. The extra cost is a little bit more disc space which any project management can afford. Then, anyone can check again later on.

Just a wild idea based on limited knowledge. Some other countries have already implementing the similar concept in more efficient way. Anyway, I believe that no matter how advance is it, no system is unbreakable in bolehland.

Yeah, back to the photographer story, someone from the project management told me that the supervisor has put in enough steel bar to lap with this "independent" cage. This joint is ok.

Shrinkage crack or Flexural crack?

2009-07-25T18:41:00.000+08:00

Is it important? Just have a look on the consequences:

Shrinkage crack means that it is not caused by applied load. Just fill cracks with epoxy to protect reinforcement from potential corrosion, then end of story.

Flexural crack means the bridge does not behave like what it's been designed for. It could means underdesign or nonconfirmity to technical drawing and specifications (position of strands, reinforcement, post-tension force applied in every stage, concrete quality...). A lot of people will be questioned, thousands of details are to be checked, hard decission making and then then following by a series of messy and expensive remedial works (or do nothing as well)

As we are only concern about safety of using the bridge, flexural crack may means:

1) The bridge is not perfect, but based on other favorable factors, it is still safe to be used.

2) It is a sign of unfavorable happenings.

All of us need some reliable answer now. How to get it? From the press statement? From this non-sense blogger's unwarranty information? Or from the Consultant's press statement? None of them in fact.

I would like to recall one powerful ancient method to ease your tension. This method has brought human to current level of civilization, even bringing human to the moon and Mars: Basic Scientific Method. Be prepared of squeezing your mind for the rest of discussion.

PART 1 – Observation through Hypothesis
1. Curious Observation
2. Is There a Problem?
3. Goals and Planning
4. Search, Explore, & Gather the Evidence
5. Generate Creative & Logical Alternatives
6. Evaluate the Evidence
7. Make the Educated Guess

Part II – Challenge through Suspend Judgment
8. Challenge the Hypothesis
9. Reach a Conclusion
10. Suspend Judgment

This will be our basis of the following discussion. Anything that is not helping, e.g . emotional word dumping, 14th floor stories etc are strictly prohibited in the comments column. Put your comments here only if you find that is following the above 10 steps. Thank you.

Let me start first:

1. The curious observation is cracks found at Puchong interchange (and now also found at TTDI interchage, which has been painted, check it out).

2. Problem: What are those cracks? (shrinkage crack or flexural crack)

3. Final goal: Recognize the safety level of the bridge; Planning milestone 1: Identify type of crack; Planning milestone 2 to be decided after conclusion from milestone 1 is achieved.

4. Search, explore and gather the evidence.

4a. Hypothesis 1: Those cracks are shrinkage crack

Supporting evidence:

4a1: shrinkage cracks, therefore those cracks could be a shrinkage crack.

4a2: shrinkage crack is normal in concrete.

4a3: Somebody said it is shrinkage crack

Opposing evidence:

4a4: Plastic shrinkage occur within 0 to 2 days of placement, while those observed one occured after years after placement. So they are not plastic shrinkage.

4a5: Autogeneous shrinkage occurs also when the concrete is young. So those cracks are unlikely an autogeneous shrinkage.

4a6: Shrinkage crack does not show significant direction as observed. So, those cracks are not shrinkage crack

4a7: Gradient of thermal fluctuation at the side face of bridge is greater than that of soffit underneath. Thermal shrinkage is more likely to occur at the side face rather than concentrated at the soffit. Therefore, it is not shrinkage crack.

4b. Hypothesis 2: Those crack are flexural.

Supporting evidence:

4b1: Crack lines of a beam/slab under sagging moment are perpendicular to the spanning direction. The crack lines found are perpendicular to the spanning direction, thus they are flexural crack.

4b2: Flexural crack lines of a beam/slab under sagging moment start from the mid span soffit and spreading toward supports. The occurence under observation matches, therefore they are flexural cracks.

Opposing evidence:

4b3: Several crack lines doesn't occur perpendicular to the spanning direction, therefore they are not flexural crack lines.

5. Creative and logical alternative: would it be act of god? (sounds creative, but I can't elaborate more from this)

6. Evaluate the evidences.

>4a1: flexural crack also cracks, so those crack should not only be shrinkage crack

>4a2: flexural crack also normal in concrete, so those crack should not only be shrinkage crack.

>4a3:Somebody could say it as any other thing. The property of cracks is not based on who say it.

I have no objection about the rest of statements.

7. Educated guess (yes, you can question whether I'm educated or not, no problem); it is flexural crack.

8. Challenge the hypothesis. Hypothesis 1 and 2 are both counter-challenging already, I have no Hypothesis 3 to challenge anymore.

9. So, based on my little knowledge, I have no objection that those are flexural crack. (Legal defense notes: I do not have any intention of diverting or influencing anyone to believe in this statement or do any decision based on this statement. Please don't make me liable of anything, this is just purely an academic discussion, or an exercise of basic scientific method)

10. Suspend judgement: The following discussion about the safety level of the bridge can be done based on judgement of this discussion: those are flexural crack.

Buddies, now is your turn. Question those statement, provide more evidence, alternative theories, alternative hypothesis, alternative guess or alternative conclusion. Please be reminded again: no emotional words, no political words etc (ISA applied here). Just purely basic scientific method.

Cause of fatal collapse in Shanghai

2009-07-04T08:58:00.000+08:00

From pop.pcpop.com:

Due to removal of passive soil + 3m height overfill at the active side.

http://video.sina.com.cn/news/c/v/2009-06-28/185139910.shtml

LDP Puchong Jaya Interchange: Major crack line found

2009-06-21T19:50:00.000+08:00

Update on 23 July 09

http://thestar.com.my/metro/story.asp?file=/2009/7/23/central/4371503&sec=central

This blog has been mentioned in the Star newspaper.

JKR, LLM and Litrak has paid great attention on this issue, which deserve a big applause.

One important information provided by Litrak's Engineering Department Senior Manager Mr Dass is that those numbers marked on the soffit are crack length instead of width. After examining all photo in my record, I found that it's true. Therefore, the crack width description and extrapolation posted before should not establish. To avoid misleading those who do not read the comments below, those incorrect words are now formatted with bold red font as caution. No wording has been changed.

I sincerely appologize of providing incorrect approximation of the crack effect.

Anyway, the excessive amount of cracks at flexural-sensitive area are still serious.

I still can not be comfortable with the statement "the cracks were most probably due to shrinkage, and that these were only hairline cracks" especially with the existence of word "probably". Didn't the Consultant advice on the root cause of crack? Are those painting works done in these few days part of Consultant's instruction?

Please don't put me under the spotlight. You know what is the impact of this to a poor layman. Anyway, you do not need to convince me alone. I believe that your Consultant can help you to explain everything professionally to the public.

Older post:

(Click the picture to enlarge)

Structure: Lebuhraya Damansara-Puchong, Puchong Jaya interchange (Exit 1114)

Affected: Puchong residents (335,419 population)

Owner/ Management: Lingkaran Trans Kota Sdn Bhd

Design, supervision, construction and commission: Gamuda

Problem: Crack lines at bridge soffit

Corrective action: To Conduct Condition Assessment on Bridges Located along Lebuhraya Damansara-Puchong by Evenfit Consult.

Photo link from Litrak.com.my

Major crack lines found underneath the bridge!

I bet that not much people take this seriously (except the concessionaire) although thousands were using this bridge everyday either through the top or passing underneath it. The possible reason is that these defects do not causing anything fall down (yet).

The crack locations and widths are summarized in this figure. There are 2 rows of bridges running parallel throughout that supported by abutments at both ends , and 11 rows of piers in between.

For clarity, each space between supports (span) are labelled and colored. The U-turns and roads underneath the bridge are shown as well. From here, you can see how lucky was the owner that all defected parts are just nicely not above the road. That's why this issue were never brought up to newspaper.

Let's see some pictures taken on 21 June 09.

At 12th span, the whole stretch of soffit cracks. The crack lines have been filled by something by bridge repair company that makes them so obvious.

12th span: You can see the crack width marked by the bridge repair company. One of them is 1.5mm

11th span: The crack lines in 11th span are spreading out from its mid span and they are not as widespreaded as the end (12th) span.

11th span: The bridge on left hand side

10th span: Crack width 2.1mm

10th span: Except these funny marks that doesn't looks like structural crack, I couldn't see any crack line marking here. I also can not find any crack line by nake eye through 4m height distance. But, please help me to remember that this span does not crack on 21 June 09, this information could be useful one day in the future.

5th to 9th span: Nothing special found

4th span: Some more crack lines found and they are "still young" compared with others.

4th span: Crack width 1.25mm

Before showing you some even more suprising pictures, please allow me to share some information on how may a civil engineer look into the cracks.

The easiest way to assess the seriousness of this problem is to compare the crack width with the allowable one in the code of practice --- it is 0.3mm

So now you see:

0.3 vs 1.5 (12th span), 2.1 (10th span) ---- 5 to 7 times the allowable crack width.

What does it means?

The reinforcement steels are now exposed to corrosion. After sometime, when the reinforcements cross sections are largely rusted to the extend that the remains are incapacle to withstand the tension force anymore, they will yield, or just snap off.

Not serious enough?

Let's go further down. This may need some effort to understand.

Steel elongation limit: There's a requirement on the steel bar elongation limit. For high yield deformed bar (don't bother with this jargon, just treat it as steel bar), the first limit is 0.2% and the fatal limit is around 12%.

The 0.2% elongation indicates a point where the strain is permanent. For example:

-You pull a 1m long bar to 1.001m (the elongation is 1mm, the strain is 0.1%) then release, the bar shrink back to 1.000m. The strain is not permanent.

-You pull the same bar to 1.005 (elongation = 5mm, strain is 0.5% which is exceeding 0.2%) then release. The bar does not shrink anymore or just shrink a little, e.g. 1.004mm or 1.0045mm, which is permanent. Once you apply the same force to it again, it will go further.

If you keep pulling the 1m bar to 1.12m (12%), the necking will appear and prepare to listen to a big bang sound --- the steel snapped.

Where are we now?

We have gone through some critical requirements. So, what is the steel elongation of the bridge? Is it less than 0.2%, between 0.2% and 12% or has already exceeded 12%? Please allow me to do some pluck-from-the-sky since I can't count exactly how many crack is there:

-Assume the 12th span is 30m (believe me, the owner prefer me to assume it as long as possible for the following calculation)

-Assuming there's 300 cracks across the span, each of them are 0.5mm (compared with 1.5 shown in the figure, I've discounted alot)

Then, elongation of the bottom bars is now 300*0.5 = 150mm

Strain = 150/30000 = 0.005 or 0.5%, Oops, these bars are now on the way to the hell (>0.2%). Every overloaded truck passing on top of it will induce additional elongation to it, no return.

Let's see more photos:

The 1st span: before you proceed to the next photo, please guest the crack width...

The 1st span: DALANG!!!! It's 8.65mm!

Say there's 100 cracks with average width of 3mm, and the span is 30m, what is the strain now? (10%)

How about assuming 4mm crack width? (13.33%, fatal)

How about the span is 25m instead of 30m? ...

How about....

So, why this bridge is still standing?

Because, it is not solely rely on the reinforcement. Supposingly, this is a post-tensioned bridge. The tendons may still playing their role on sustaining the bridge from breaking down, but they are now unable to sustain the bridge from crack badly.

By assuming it as a post tensioned bridge, the following may be the reasons causing these crack:

1) Underdesign

2) Underprovided prestressing force

3) Inappropriate input of design parameters e.g. material properties, creep coefficient.

4) Out of tolerance in construction

5) Act of god (most of politicians like this one)

Some other defects:

The 1st span: The exposed bottom reinforcement to be patched

The 1st span: Some more exposed bottom reinforcement to be patched

So now comes to the final question: is it safe?

It depends on who say it.

For me, I will try to avoid using it for this moment.

For the owner, it is subject to the appraisal report of the bridge repairing company.

For the bridge repair company... I can feel that they are now like a sardin between their paymaster and the public. I hope that they are able to convince me to use the bridge again.

For the politicians... I don't know. But I recalled the MRR2 series in my mind now.