Analyzing the collapse of bridges in Indonesia

Analyzing the collapse of bridges in Indonesia

Sohei Matsuno, Palembang

It was Dec. 5, 1995 when the Air Beliti Bridge along the
Trans-Sumatra Highway collapsed in South Sumatra, some eight and
a-half years before Cipunagara Bridge along the North Coast
Highway of West Java collapsed on July 23.

The two bridges are of the same Callendar-Hamilton (C-H)
design with steel pony trusses. The loads the bridges carried at
the time of collapse (six trucks on the Air Beliti Bridge and
seven trucks on the Cipunagara bridge) are also similar.

The probable reason given for the most recent collapse is also
similar to the one initially given in 1995: An aging bridge
overloaded with heavy trucks.

The C-H type bridge was designed to meet engineering
conditions in Indonesia, and played an important role in the
1970s and 1980s, with hundreds built throughout the country.

Many adjustments to the bridges have also been made since
their initial construction, including efforts to widen the C-H
bridge's applicable span with minimum adjustments in parts and
components.

One of the enduring qualities of the C-H bridge is that it is
well-designed, manufactured and assembled. Hence, it is suppposed
to be stable, workable and bankable. The bridges are designed to
be durable, with a life span of more than 100 years under current
traffic conditions, and all the parts and components of the
bridge are galvanized. The structures are also covered layers of
zinc that prevent them from rusting.

When reviewing the investigation into the bridge collapse,
several important questions should be asked:

If the collapse was caused by aging, why didn't other similar
bridges which are even older also collapse?

If overloading was cited as the cause, why didn't other
bridges the trucks pass along the North Coast Highway also
collapse?

Why was there no warning of the dangers?

If any of the above causes are proven, who should the victims
of the accident claim compensation from?

None of the these answers can exclusively explain the
accident.

If the accident was caused by aging, there must have been
significant warning signs, known in the trade as ominous
deflection, and these could have been detected and a warning
could have been issued.

The investigation into the 1995 collapse focussed on corrosion
and metal fatigue as these are known to be common causes of
bridge collapse.

It was confirmed the bridge had shown insignificant ominous
deflection before the collapse. It was also found the bridge was
fully galvanized. Hence, there was little indication of corrosion
and consequential sectional loss throughout the structure.

After these discoveries, corrosion was deleted from the list
of possible causes. Aging as a cause of the collapse disappeared
at this stage of the investigation.

However, what was observed were apparent beach marks (evidence
of fatigue) on a sheared section of the bridge, which proved a
bridge support, or member, had broken because of fatigue.
However, this kind of fatigue occurrs only when a member is
subjected to alternating deformation -- repetitious bending up
and down, expanding and contracting.

And alternating deformation does not occur as a rule in this
kind of truss design. Therefore it was essential to determine how
this sheared member was subjected to the deformation.

The puzzle was solved when it was discovered that the bridge
was erroneously supported by fixed shoes (hinges) at both ends.
It should have been commonsense to support one end by a hinge and
the other end with a movable shoe (rollers). In this way, changes
of size due to temperature variations could have been
accommodated.

Because of the erroneous shoe arrangement, the upper part of
the hinge (upper shoe) nodded up and down due to the longitudinal
thermal expansion and contraction of the bridge. After a certain
number of cycles, the bridge developed a fatigue crack at a point
where it was connected to the upper shoe. The crack gradually
grew as a result of the alternating deformation until the member
broke. It was subjected to a deformation angle of about plus or
minus 1.15 degrees and took an estimated 30,000 cycles to fail.

The next question was to identify if the erroneous shoe
arrangement was a design flaw or merely an implementation flaw.

In studying the bridge construction manuals, it was found that
instructions in the manual dated Aug. 9, 1977, identified the
need to support one end by a hinge and the other with rollers.
However in bridge drawings dated Jan. 25, 1977, it was indicated
that both ends could use a hinged shoe arrangement.

As it is, it is reasonable to suppose the construction of the
bridge -- which began in 1976 -- used the January drawings.

Future corrections in the scheme of the bridge did not take
effect after it was completed in 1978.

Investigation of some of the 22 other C-H type bridges in the
South Sumatra Province and on the North Coast Highway between
Jakarta and Surabaya found similar errors in construction.

While overloading may have been a factor in triggering the
bridges' collapse, there is a strong likelihood the cause of the
collapse of the Cipunagara Bridge is similar to that of the Air
Beliti Bridge, given its similarity of construction and type.

The writer (ypnt@indo.net.id) was a leader of the ad hoc team
at the Regional Improvements Office II (RBO-II), Bina Marga that
conducted the investigation into the collapse of the Air Beliti
Bridge in December 1995. He is currently a senior lecturer at
Tridinanti University, Palembang.