{"id":8432,"date":"2013-07-15T08:00:52","date_gmt":"2013-07-15T12:00:52","guid":{"rendered":"https:\/\/retrofitmagazine.com\/?p=8432"},"modified":"2013-07-10T16:31:40","modified_gmt":"2013-07-10T20:31:40","slug":"the-250-year-old-fort-pitt-block-house-undergoes-an-x-ray-examination-to-determine-the-integrity-of-delicate-timbers","status":"publish","type":"post","link":"https:\/\/retrofitmagazine.com\/the-250-year-old-fort-pitt-block-house-undergoes-an-x-ray-examination-to-determine-the-integrity-of-delicate-timbers\/","title":{"rendered":"The 250-year-old Fort Pitt Block House Undergoes an X-ray Examination"},"content":{"rendered":"

The second half of the 18th century was an exciting time in American history. After declaring its independence and fighting the Revolutionary War, the U.S. became a sovereign nation, free from British rule. A new Constitution was ratified as the law of the land, and George Washington was sworn in as the nation\u2019s first president.<\/p>\n

However, history as we know it may have been very different had it not been for earlier squabbles between the French and British about land where the Allegheny and Monongahela rivers join to form the Ohio River, the site of modern-day Pittsburgh. These disputes led to the French and Indian War, which lasted from 1756-63. During the war, the French Fort Duquesne, which was strategically located at the merge of the two rivers, was completely burned to the ground. The British built their own fort in approximately the same location and named it Fort Pitt in honor of then British Prime Minister William Pitt. They spent the next few years fortifying the fort from natural and manmade threats and, consequently, built three redoubts outside Fort Pitt as the first line of defense against a skirmish.<\/p>\n

\"Phil<\/a>
Phil Martin, radiographer with T\u00dcV Rheinland, is setting up imaging plates for the next exposure during the radiographic inspection of the gun loops at the Fort Pitt Block House. Photo: T\u00dcV Rheinland<\/figcaption><\/figure>\u201cThree sides of this Fort which are not reveted [in brick] having been rendered almost defenceless [sic] by Two successive Floods in 1762, and 1763, I have caused Three Redoubts to be built on the glacis, to cover them. Two are compleated [sic], and the Third going on, as fast as the Weather will permit,\u201d wrote British Colonel Henry Bouquet to his commanding officer General Thomas Gage in 1763.<\/p>\n

Of these three redoubts, one still stands today and is known as the Fort Pitt Block House. The Fort Pitt Society, which owns the structure, has begun a 10-month preservation and restoration project in preparation for the Block House\u2019s 250th anniversary celebration in 2014. The project, which includes masonry restoration and French drain and interior repairs, has begun and should be complete in October 2013. \u201cThis restoration project is a critical first step in our efforts to create a city-wide celebration to recognize this significant milestone,\u201d says Joanne Ostergaard, member of the Fort Pitt Society and coordinator of Block House 250, the anniversary celebration. \u201cThe Block House is quite literally where Pittsburgh began, and it is our goal to ensure it continues to stand as an enduring symbol of American strength and tenacity so future generations can know and appreciate the important role our region played in history.\u201d<\/p>\n

Before restoration work could begin on the Block House, Dirk Taylor, P.E., president and principal of Taylor Structural Engineers<\/a>, Pittsburgh, initiated a series of noninvasive X-ray examinations to determine the integrity of the structure\u2019s wooden gun loops that encircle its perimeter.<\/p>\n

Inside Historic Walls<\/h4>\n

The five-sided, 483-square-foot Block House features much of its original structure. Natural and rough-cut limestone forms the foundation walls. Above the white-oak sill plate lays the first course of horizontal gun loops, also made of oak. The gun loops were designed to protect soldiers inside the Block House while allowing them to see and fire their muskets upon enemies approaching the fort. A 1-foot thick, three wythe brick wall then extends up 8 1\/2 feet before supporting the floor joists of the second story, which includes another course of oak gun loops. A two wythe brick wall then extends to the wood shake hip roof that is supported by oak rafters and a pine deck.
\n
\nSuspecting deterioration in the gun loops during a 2011 conditions survey, a restoration consultant suggested using a small diameter drill to probe around the wood to locate soft spots. \u201cMy feeling was that might be a little too invasive,\u201d Taylor recalls. \u201cI suggested the X-ray strategy. It cost a little bit more money but it wouldn\u2019t damage the wood or leave unsightly holes all over the perimeter.\u201d<\/p>\n

Despite never having used X-ray technology in his 32-year career, Taylor had read about it and understood its validity for unique construction situations, such as locating reinforcing steel in concrete or determining the condition of an old timber structure. Taylor contacted several companies who provide radiographic inspection services. T\u00dcV Rheinland<\/a>, which has locations worldwide, including near Pittsburgh in Aliquippa, Pa., was chosen for the project.<\/p>\n

According to Chris Dugan, radiation safety director for T\u00dcV Rheinland, X-raying a building is very similar to having your bones examined at a hospital. \u201cInstead of using an electromagnetic X-ray machine, like in a hospital, we use actual radioactive materials that pass through the structure and are recorded on a film,\u201d he explains.

\"This<\/a>
This radiographic image shows the lower-tier gun loops between points K and L, which are about 12 inches apart. Approximately 3 inches to the right of K is a light-density vertical area, which is masonry mortar between wood abutments. About 4 inches to the right of K is a fine, horizontal dark line, about 1 inch
in length. It shows a minor separation in the layers of wood. Directly under the L is a horizontal dark line that runs off the view; this is separation of wood caused by age. The large black area under L is an original opening for the placement of musket barrels. Photo: T\u00dcV Rheinland<\/figcaption><\/figure> <\/p>\n

The shades of gray that register on the film illustrate whether there are discontinuities in the material. \u201cAnything that is dense and solid will not allow all the radiation to pass; some of the radiation will get absorbed or reflected,\u201d Dugan continues. \u201cThe denser the material, the less radiation, which means the film will be a light shade of gray. If there are a bunch of holes or a large hole inside the structure, the radiation basically would pass unimpeded to the film. Any dark shade of gray means the radiation got through a lot faster than it should have.\u201d<\/p>\n

Science of Radiography<\/h4>\n

T\u00dcV Rheinland\u2019s team used Iridium-192 (IR-192), a radioactive isotope, to X-ray the Fort Pitt Block House. IR-192 can produce images for light-density materials, including concrete and up to 3-inch-thick steel. The radioactive material was contained and transported in a camera to the Block House. During an exposure, the camera and an imaging plate are placed on opposite sides of the object to be radiographed. In the Block House\u2019s case, the camera was inside and the imaging plate was placed outside the building. \u201cOnce both items are in place, the IR-192 source is remotely positioned to allow for the narrow beam of intense radiation to be directed toward the wood tier and imaging plates,\u201d Dugan explains.<\/p>\n

During each exposure, the electromagnetic radiation travels similar to light waves: in straight lines at very high speeds. It passes through the structure and interacts with the imaging plate.<\/p>\n

\u201cEach exposure covered approximately 2 feet of tier length with a total of 50-plus exposures required to inspect both wood tiers,\u201d Dugan adds. \u201cEach exposure was probably a couple minutes long, and there\u2019s setup involved, so it was a three-day process onsite.\u201d
\n
\nIndustrial radiography is regulated by the Washington, D.C.-based U.S. Nuclear Regulatory Commission and individual state\u2019s departments of environment. Radiographers are certified by the American Society for Nondestructive Testing, Columbus, Ohio, and must be licensed and complete continuing education to maintain that license. In addition, radiographers, who wear radioactivity monitors on their person, follow strict safety guidelines onsite. \u201cWe completely rope off the area and make sure all personnel who aren\u2019t trained are out of the area,\u201d Dugan notes. \u201cWe do an actual eyes-on walkthrough on every job before each and every exposure to make sure nobody\u2019s inside that building before we make an exposure.\u201d<\/p>\n

\"Aaron<\/a>
Aaron Drake, radiographer with T\u00dcV Rheinland, studies a digital file of a gun loop exposure. Photo: Fort Pitt Society<\/figcaption><\/figure>After its site work was complete, Dugan\u2019s team provided digital images to the engineering firm. Again, similar to a radiologist in a hospital, T\u00dcV Rheinland simply made the exposures available and explained to Taylor and his team what the exposures showed. \u201cT\u00dcV was not the determining party for rejection or acceptance of the images,\u201d Dugan adds. \u201cThe final repair analysis rested with Taylor Structural Engineers.\u201d<\/p>\n

Surprising Results<\/h4>\n

When it comes to construction, radiography most commonly is used on infrastructure\u2014bridges, power plants, water towers and more. Although the Block House was a unique project for T\u00dcV Rheinland\u2019s team, it didn\u2019t create any special challenges. However, the team took the age of the structure into consideration. \u201cWe did everything in slow motion just to make sure we didn\u2019t disturb anything,\u201d Dugan says.<\/p>\n

Despite the building\u2019s age, when the exposures were revealed, Taylor was startled by the results. \u201cI was a little bit surprised there was not as much deterioration as I had anticipated,\u201d he recalls. \u201cI expected to see a lot of rotted wood throughout, but it really wasn\u2019t as bad as I thought it would be.\u201d<\/p>\n

Based on the results of the X-ray, Taylor determined damage was occurring to the wood from the exterior parts of the building. To correct these issues, structural epoxy will be injected into sections of the timbers where internal voids have developed from wood decay; several small sections of the timbers where extensive deterioration occurred will be replaced; sealant will be applied at several open joints where existing sealant is badly deteriorated; and reinforcing steel will be installed to strengthen a failing corner section. The wooden gun loops also will be treated with a borate-based preservative to further protect the area from water and insect damage.<\/p>\n

As the preservation and restoration work continues, the Fort Pitt Society is in the throes of planning the building\u2019s anniversary celebration. Those who worked on the project also are in the celebratory spirit. Dugan, who is from Pittsburgh, was honored to be a part of a project that not only is important to his hometown, but also is a major contributor to the nation\u2019s history. \u201cWorking to help preserve an important national landmark was very rewarding,\u201d he says. \u201cIt\u2019s good to know that modern technology and expert knowledge can save the 18th-century timbers for future generations. I can take my kids and show them this project.\u201d<\/p>\n

Visit the Fort Pitt Block House<\/h3>\n

April 1 \u2013 Oct. 31 <\/strong>
\nWednesday through Sunday, 10:30 a.m. to 4:30 p.m.
\nNov. 1 \u2013 March 31 <\/strong>
\nFriday through Sunday, 10:30 a.m. to 4:30 p.m.
\nThe Block House may be closed occasionally during the restoration for the safety of its visitors. Call ahead before planning a visit.<\/p>\n

Admission:<\/strong> Free, courtesy of the Fort Pitt Society of the Daughters of the American Revolution
\nDirections:<\/strong> www.fortpittblockhouse.com\/directions<\/a>
\nPhone Number:<\/strong> (412) 471-1764<\/p>\n","protected":false},"excerpt":{"rendered":"

The second half of the 18th century was an exciting time in American history. After declaring its independence and fighting the Revolutionary War, the U.S. became a sovereign nation, free from British rule. 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