Masonry Magazine April 2009 Page. 36
REHAB AND RESTORATION >>> CASE STUDY
Adjusting Accordingly
IF THERE IS ANYTHING TYPICAL IN RESTORATION, IT IS PERHAPS THAT NOTHING IS TYPICAL. Things rarely turn out to be as they first appear. This challenge is both exciting and satisfying when the project is complete. This is certainly true in the case of One Shell Square, in New Orleans.
One Shell Square was built in 1972 and was, for several years, the tallest structure in Louisiana. The building is a 52-story, double tube system, with a steel core and concrete perimeter. The exterior cladding is in Italian travertine with bronze insulated glass units set in a structural gasket. It was the first southern skyscraper to reach 200 meters in height, and has a total square footage of about 1,732,000 square feet. The restoration project started life in 2004 as a Panel Stabilization/Caulking job, but after Hurricane Katrina in August of 2005, it became a catastrophic, multi-disciplined restoration endeavour.
As is correct procedure, the consultant first tested and verified the suitability of using the most economical solution for panel stabilization, helical wall ties. Travetine from the building was shipped up to the consultant's office in Dallas, Texas, and tests were conducted to establish the optimal tie diameter and drilling technique to maximize the pull-out capacity of the tie in the stone.
Helical wall tiles are the most cost-effective way of retrofitting a connection between the veneer and the structural back-up of a building. The system entails pre-drilling a pilot hole though both components of the wall. Then, using the hammer action of an SDS hammer drill through a special installation tool, the helical tie is driven into place down the pilot hole to make a threaded connection, rather like a self-tapping screw, between the two wall components. No torque is applied during this operation, and the possibility of over-torquing and stripping the thread is, therefore, eliminated. Since the size and form of the tie and the material itself are a given, the only variable is the pilot hole diameter. Varying the pilot hole diameter to optimize the tie pull-out capacity is really the issue when conducting such testing. As a rule of thumb, the softer the material, the smaller the pilot hole required. This, like most engineering, is just common sense. Hard material is more difficult to cut, and the helical tie has the propensity to "ground out," rather than cut a thread form through the material. Similarly, a softer material needs more bearing surface on the thread form to achieve an equivalent pull-out load bearing capacity.
The travertine stone from One Shell Square is considered a relatively soft material, and because of its inherent pourosity and hidden fissures, a 10-mm diameter helical tie was found to consistently produce the best results. After a series of tests in both wet and dry stone, an installation protocol was established for the travertine, which consistently achieved a pull-out load bearing capacity of between 900 and 950 lbf. Setting helical ties in poured concrete almost always generates a pull-out strength in excess of 1,000 lbf, so an ultimate connection strength of 900 lbf, with the travertine being the weakest link, was established as the design parameter. The die was cast, the specification written, and the job was bid.
After the contract was awarded, mock-ups were performed on the building. Unknown to all parties at the time of undertaking the initial series of tests, poured concrete in Louisianna, is the hardest concrete to be found anywhere in North America. A little puff of white dust while setting the helical tie through the travertine and into the poured concrete could easily have been overlooked. But that tell-tail puff of white dust evidenced the destruction of the thread form in the travertine as the the tie vibrated while being set in the hard concrete backup. This required further investigation.
A travertine panel was removed from the building and mounted on 2 X4 boards, so that once a helical tie had been fully installed, the