Thursday, July 23, 2009

Major Crack Line found on LDP Puchong Jaya Interchange



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

Add Image
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.


Originally published in : BOLEHLANDCONSTRUCTION


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