World Demolition Summit 2016: Concrete calculations

(Ahmed Amil Khalil, Applied Science International; Photo credit: Rob Kaufman)

Imploding a structure requires a lot of planning. During seventh annual Word Demolition Summit, Oct. 14, at the Marriott Biscayne Bay in Miami, the use of computer models to determine the best approach to implosion was illustrated.

Ahmed Amil Khalil,vice president, international business development, Applied Science International (ASI), Durham, North, Carolina, discussed how his firm was hired to determine the best approach to tearing down Italy’s Pietrastretta Bridge.

The 1-kilometer long (.62-mile) bridge built on a slope in 1970s had many issues, including cracking, rebar erosion and ruptures, in its 21 spans. After many unsuccessful attempts over the years eventually it was determined the bridge had to be replaced.

At first crews from the demolition firm Nitrex employed mechanical demolition to take down the carriageway. “Unfortunately that had to be changed. [They] did not expect one of three beams to separate and started falling with it,” said Khalil. “One of the 3 beams of the span, cut from the other two to be lifted, broke during lifting and in the fall almost drag the launching girder and personnel.”

It was then determined that implosion would to be safer.

Geometrical, mechanical and dynamic characterization of the bridge was done by Italy-based Nitrex.

Nitrex contracted ASI to study stability after weakening of the beams and slab to prepare for blasting as well as stability of the remaining spans against transverse overturning at sudden release of the weight of those spans being blasted.

“With the state of damage, Nitrex went to ASI to do stability analysis,” said Khalil. He said Nitrex had its own demolition technique to weaken bridge prior to implosion and the company “wanted us to check the stability because of damage.”

ASI used extreme loading technology, which was based on the Applied Element Method (AEM).

They studied two different implosion scenarios. Scenario A included imploding three beams separately, while Scenario B involved imploding all the beams at the same time, which while cheaper and quicker was riskier.

Khalil said ASI performed extensive evaluation of different materials. “Reinforcing bars were included in the analysis,” he said. ASI also recorded vibrations and stresses to make sure it would take load without damage.

“A dynamic analysis was performed to study the possibility of overturning of the pylons due to the sudden removal of the bridge sections for both scenarios,” said Khalil. Nitrex decided to go with Scenario B.

He noted, all activities to prepare the blast were foreseen by means of radio controlled equipment with personnel at distance and secured to a safety line fixed at the pylons.

“Demolition went as planned,” said Khalil. “[We compared the demolition] with predictions and they were in agreement.” He added that going with Scenario B resulted in significant savings in cost, time, additional safety and assurance. Crews were able to finish the project within two months.

The World Demolition Summit, organized by U.K.-based KHL Group, publisher of Demolition & Recycling International, in cooperation with the National Demolition Association (NDA), Washington, was Oct. 14 in Miami.