Documenting Movement Over Time

As part of our work investigating buildings and structures, Vertical Access documents existing conditions and collects data about a specific moment in time. In most of our surveys, when we collect information about condition quantities, such as crack width and amount of displacement, it is a snapshot of the current condition. In some cases, it is important to document changes over time. When this is the case, instrumental monitoring can be incorporated into the investigation.

Wire crack gauges installed at plaster ceiling and plaster truss

One type of instrumental monitoring employs electronic gauges that collect and record data in real time. An example of a project where electronic monitoring has been employed is Marble Collegiate Church in New York City. In 2009, a study was conducted of the roof system and plaster ceiling in the sanctuary of the church. The design team for the project was led by Helpern Architects and included Robert Silman Associates and Vertical Access. Following a survey of the ceiling and development of repair documents, RSA and VA worked together to design a monitoring program to record movement at representative areas of the plaster ceiling, masonry walls and wood roof trusses.

Vibra-Tech was engaged to implement the system, which included vibrating wire crack gauges at the ceiling, vibrating wire strain gauges at the roof trusses and temperature and relative humidity sensors. These crack gauges were installed in February 2011, several months before the planned start of the roof repair and plaster conservation work, to measure and report crack displacement of existing cracks in the sanctuary ceiling, strain on the steel tie rods of the existing roof truss system in the church attic, and temperature and relative humidity in both the attic and sanctuary. Data from the gauges is transmitted continuously to a data center in the church. The real time data is available to the project team on a web site. There are also automatic notifications via email or text messages to the project team when movement thresholds are exceeded.

Wire crack gauges installed at plaster ceiling

Another type of monitoring uses crack gauges that must be physically examined to record the data. This more traditional system of monitoring employs crack gauges with sliding plates installed on either side of a crack. One plate has a grid and the other has crosshairs so that any future movement at the crack can be compared to the initial reading. Typically, the crack gauges are affixed to masonry or the other substrate using epoxy. One of the drawbacks of this system of monitoring is that it relies on visually checking each monitor to know whether there has been any movement or not. However, in cases where the purpose of the monitoring is to document any change that may be associated with a known event, this is a reasonable protocol.

NDE on a Rope

by Kelly Streeter, PE

I joined Evan and Keith at the Confederation Building in Newfoundland in May to complete a nondestructive evaluation pilot project designed to challenge our conclusions from our hammer sounding study of the limestone units. While in some cases it may be easy to detect delaminations by hammer sounding alone, the variable installation details at the Confederation Building made it very difficult to confidently predict by ear whether or not a stone “sounded” delaminated.

Delaminated stone used for calibration - on the ground

By using ultrasonics we were able to evaluate the stone’s response to a hammer hit in both the time domain and the freqency domain.  As Evan mentioned in his post, we had the added advantage of easily-accessible, known-delaminated and known-sound units which had already been removed for the renovation of the building’s west wing.    Not only were these units easily accessible, we could clearly see the delaminations in some of these removed units.  We tested those first to calibrate our observations.  It was fairly easy to see the multiple reflections from the delaminated stones, marked with arrows in the plot below, as opposed to the one clear reflection from the sound stone.

We then plotted the power spectral density, or PSD of the recorded hammer hit.  The PSD allows us to convert the power of a signal, as measured over time, into the frequency domain, showing us clearly in numerically and visually (on a graph) what our ear is trying to parse out.  Using this method we can analyze the way the frequency content changes from one stone to another.

The calibration went very smoothly. We found that we could easily identify the differences in both the time and frequency domain plots between the sound and delaminated stone units. We took these observations to the wall to test two different areas on the tower.

And then came the hard part.

Completing Ultrasonic testing - on the wall

It was a challenge (to put it lightly) to manage the rope access equipment, the tablet computer and the hammer and receiving transducer and all of the associated cables.  I found myself wishing I had two more arms.

Another significant challenge was the weather. We had to battle the frequent, low-level mist and rain throughout our visit. I was attempting to get the data but not expose the electronics to excessive moisture.

In the end, it was an excellent and rare opportunity to apply different nondestructive evaluation theories derived for more modern structural materials like concrete and pavements, to dimension stone. We discovered that we could quite easily detect delaminations once we had calibrated the equipment to a known condition.

At the Edge of a Continent

Imagine what the Norse sailors who first settled the site now known as L’Anse aux Meadows in Newfoundland must have experienced traveling across the Atlantic Ocean 1,000 years ago; or what the English explorer John Cabot felt when he purportedly landed on Newfoundland at the end of the 15th century. These sailors, venturing west from northern Europe, found the edge of a new continent. While a four-hour delay on the outbound flight and a cancelled return flight are hardly hardships compared to what these early explorers suffered, they do make one realize that St. John’s, only three hours from New York City by airplane, is still an insular and isolated place.

Cape Spear Lighthouse, photo by Keith Luscinski

Following Cabot, explorers from other European countries landed on the shores of Newfoundland, and in 1583 Sir Humphrey Gilbert declared the island to be a colony of England. Although France was granted land rights to parts of Newfoundland in 1713, the island was effectively a British colony until 1907 when it acquired dominion status. Finally, a tightly contested referendum held in 1948 determined that Newfoundland, together with Labrador to the north, would become a province of Canada.

Quidi Vidi Lake, photograph by Keith Luscinski

The Confederation Building, constructed in 1959-60, was built to house the Newfoundland and Labrador House of Assembly as well as other government offices for the newly established province. The building consists of an eleven-story tower with three to five story wings on three sides of the tower. The exterior is mainly clad in brick, with St. Marc (Quebec) limestone used at the parapets and window surrounds.

Confederation Building with west wing under scaffold for renovation and Kelly at top of tower

Exterior renovations began in the fall of 2009 to address the deterioration of the window systems and masonry cladding. With work at the west wing ongoing, Vertical Access was retained by Erik Jokinen to assist with the investigation of the limestone on the tower. During three days of field work this week, Vertical Access technicians Keith Luscinski and Evan Kopelson performed a hands-on inspection of the upper areas of the tower, documenting masonry conditions and hammer sounding over 1,000 limestone units. Kelly Streeter joined Keith and Evan for two days on site to perform a pilot study on the use of acoustic emission as part of the investigation.

Evan with St. John's and Atlantic Ocean in the background

A key question in the study was how to identify blind delaminations that may not be visible on the exterior surface of the stone and are difficult to detect by traditional hammer sounding. As part of the pilot study, acoustic emission, a nondestructive techniques based on ultrasound, was applied to stone units previously removed from the west wing of the building. Both sound units and units with visible delaminations were tested in the pilot project setup. Following calibration of the testing procedures to the specific stone used on the Confederation Building and type of delamination encountered, Kelly performed ultrasonic testing at two areas of limestone at the top of the tower. Initial results are promising in being able to identify blind delaminations based on the frequency pattern of the acoustic emission. Also evident from the pilot study is the need for careful calibration to the specific conditions likely to be encountered.

Kelly performing ultrasound testing on south facade

Vertical Access’ Top 10 of 2010: Project 2 – University of Buffalo Alumni Arena

The second project Vertical Access completed in 2010 and would like to highlight is the exterior investigation of Alumni Arena on the north campus of the University of Buffalo. VA performed the work for DiDonato Associates in April, focusing on the exterior masonry walls.

Alumni Arena, also called the Health, Physical Education & Recreation Building, was designed and constructed in two phases. The main field house on the south side of the structure was constructed as part of Phase I in 1982. This portion of the building was designed in 1978 by a project team including architect Robert Traynham Coles and structural engineer Sargent Webster Crenshaw & Folley. Phase II was designed by Robert Traynham Coles with consulting engineer Ammann & Whitney in 1982 and constructed in 1985. The Phase II project comprises the north half of the building and includes pool facilities, ball courts, locker rooms and other physical education spaces. Overall, Alumni Arena is over 500 feet long in the north/south direction and 426 feet wide in the east/west direction, with the highest exterior walls reaching nearly 80 feet at the field house. A distinctive feature of the building is the space frame truss employed in the field house. The general wall construction at both the Phase I and Phase II portions of Alumni Arena consists of concrete masonry unit (CMU) back-up separated from the face-brick by an air cavity.

Vertical Access used a combination of aerial platforms and industrial rope access to perform the hands-on investigation of Alumni Arena. As part of the investigation, VA used a wall tie locator to map out the location of wall ties at representative areas. VA documented existing conditions using TPAS. The annotated drawings produced with TPAS helped to identify fault patterns. Quantities collected during the survey with TPAS were used to produce repair budget costs. After analysis of the initial survey data, VA performed additional investigative work using borescopes. The purpose of the borescope investigation was to confirm the presence of wall ties and connections between relieving angles to the back-up masonry at representative areas. See video footage from two borescope probes here.

 

Read about Project 1: Union Theological Seminary Brown Tower.