M1 bridge collapse: How it might have happened

An independent structural engineer who submitted a report to the Department of Labour’s inquiry into the M1 bridge collapse, suspects that bolts were missing at more locations on the structure than were mentioned at the inquiry’s first hearing on Tuesday.

Gregory Harington, a structural engineer with many years’ experience, submitted a report to the department on February 12, highlighting three basic issues that might have contributed to the tragic event.

The incident took place on October 14 2015, near the Grayston off-ramp in Sandton, killing two people and injuring 19 as the temporary structure collapsed onto vehicles travelling on the busy highway.

Commissioner Lennie Samuel, who leads the inquiry, confirmed receipt of the report to Moneyweb. He said when someone with specialised knowledge such as Harington offers information that may add value to the investigation, such a submission may be considered.

Moneyweb also asked another highly experienced engineer to read Harington’s report. He agreed that the propositions are quite plausible.

The three issues Harington pointed out are (refer to his sketch and corresponding numbers below):

1 – Vertical supports comprising Formscaff Super-Shores (proprietary steel element designed for vertical support.)

2 – Vertical supports at median comprising a double set of Super-Shores

3 – Lateral supports comprising Form-Scaff Kwik-Stage scaffolding (versatile, robust support work)

4 – Lattice girder comprising +_ 6m-long x 2.3 m-high modular units by Form-Scaff

5 – Position of joint to be made on site between Western and Eastern sides of continuous lattice girder

6 – Positions of connections to be made on site during assembly

• Missing bolts (5) that should have joined two pre-assembled halves of the lattice girder (4). The lattice girder is the temporary horizontal structure, on which the concrete bridge deck spanning the highway would have been constructed later;
• Missing bolts (6) securing the median support structure to the concrete base at the bottom and lattice girder above;
• Inadequate lateral support (3) at the Eastern and Western ends of the structure to accommodate lateral forces.

Statements made by interested parties at the first day of the public inquiry were consistent with two of his suggestions.

The Johannesburg Development Agency (JDA), who contracted Murray & Roberts (M&R) to construct the pedestrian bridge, stated that the missing bolts at the joint between the two pre-assemblies of the lattice girder, was mentioned at a meeting five days after the fatal incident. On that occasion a representative of M&R said the missing bolts “did not affect the structure”.

The JDA questioned this statement.

Harington, who visited the site after the incident and studied the large amount of visual material published in the wake of it, says the way the components fell issignificant.

The Eastern half and the Western half of the lattice girder assembly separated in the middle with the Western half falling flat onto the road surface, while the Eastern half rotated and landed on its side with the top surface facing southwards. He says if joint correctly, the structure would have functioned as one continuous beam.

Harington stated in his report: “The apparent discontinuity at the median zone appeared unusual, as one would have expected a correctly-constructed modular girder to act as a continuous unit, which would not easily permit the relative rotation and vertical shift (which had occurred).”

With regard to the inadequate support at the ends of the bridge (abutments), Harington states in his report: “A conclusion easily reached is that the 48 metre-long deck would not have moved 8 metres in the Westerly direction and collapsed if the abutments had been adequate.”

He continues: “To the experienced eye the scaffolding structure as-built appeared flimsy as though it had been improvised on site.”

This submission also seems to be supported by statements made before the inquiry. On Tuesday representatives of Formscaff, who supplied the detailed drawings, said M&R (which constructed the scaffolding support) deviated from the drawings and only part of the scaffolding, called Kwik-Stage, was completed at the time of the incident.

Harington bases his third submission on the median support that was not properly secured to either the concrete base or the lattice girder, on the lack of distortion of the steel following the collapse. This support consisted of Formscaff’s Super-Shores, a proprietary steel element designed for vertical support.

“Photos show a lack of distortion at positions where Super-Shores should have been bolted to foundations and lattice girders above, this is indicative of structural failure without any resistance being offered at available connection,” he states in his report.

No mention was made of this during the first day of the hearing.

Moneyweb sent the report to M&R for comment. Spokesperson Ed Jardim responded: “We were not aware of the report and it has not been tabled in the commission and therefore we have not considered it as yet.

“Our experts have conducted an investigation of the remnants on site (having had actual access to the site and associated evidence) and are considering all elements of the collapse.

“These expert reports will be tabled in the commission in due course. We are therefore unable to comment on outside parties’ views and observations outside of the commission itself.”

Moneyweb asked Harington, who was retrenched from M&R last year, whether his interest stemmed from sour grapes. He denied this, saying the industry has been quiet in the wake of the incident and he wanted to make a contribution in order to prevent anything like this from happening again. “Engineering facts are engineering facts,” he said.

The public hearing continues on April 19.

Read Gregory Harington’s full report here.

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