Structural+Evaluation+of+Historic+Builidngs

Structural Evaluation of Historic Buildings //Scott Molongoski, Student of Architectural Engineering, Penn State, Class of 2013 //

Introduction
====== Historic buildings and structures are found in every state of this country. There are over 2000 buildings and structures recognized as National Historical Landmarks, with landmarks in the states of Pennsylvania, New York, and Massachusetts accounting for over 25% of that number. Figure 1 shows the Old State House, in Boston, MA. one of the many National Historic Landmarks within the city. With so many buildings recognized as historic, methods for the evaluation and rehabilitation of these structures are essential to preserving our past. Many companies around the country specialize in the evaluation and rehabilitation of such structures and they employ a variety of techniques to preserve them. Due to the historic nature of many structures, some of which are over 300 years old, structural evaluations need to be carried out with great care. A number of nondestructive methods using radar and other electromagnetic pulses exist, as well as other in-situ tests such as stress tests and load tests.

There are a number of reasons to perform a structural evaluation of a historic building. Visible structural defects or failures in the structure could prompt an evaluation. The need for proper documentation for a historic building could also necessitate an assessment. Sometimes a client wants to re-purpose a historic building and needs to know the load bearing limitations of the structure, as well as any potential repairs. In rare cases severe events such as earthquakes and fires require a historic building to be evaluated. Yet whatever the reason for performing a structural evaluation, there is a wealth of information that an evaluating engineer must know. Knowledge of historic construction and common causes of deterioration, experience with invasive and non-invasive tests, and the ability to recommend repairs of historic structures are essential parts of conducting a building evaluation. toc

Historic Structural Elements
One of the key challenges in assessing historic structures is understanding the structures themselves. As historic structures, they were likely built in a time when codes were significantly different or perhaps did not exist at all. Methods of construction and materials were also very different than those of the modern era. Knowledge of how loads are transmitted through older structures is essential to proper assessment.

Wood Construction:
The vast forests of North America made wood an ideal choice of building material in the early history of the United States. Wood construction has been used since the dawn of civilization and the construction techniques of Europe were passed to colonial settlers in America. Braced frame construction was the first type of wooden construction used in America. Braced frames featured mortise and tenon joints that connected wood members with a male-female type connection. This method was used in wooden homes, churches, and barns until the late 19th century (Rabun, 2000, 117). The House of the Seven Gables, seen in Figure 2, is an example of colonial wood construction from 1668.

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The balloon frame became predominate in wood construction following innovations in nail technology and in mass production. With balloon frame construction, wood studs were cut square and end-nailed or toe-nailed to other members. A drawback to balloon framing was that studs needed to be the full height of the building, a problem that was corrected in the next framing innovation. By the beginning of the 20th century the platform or western frame became the dominate form of wood construction. This system decreased stud height to just a single floor, allowing each floor to be built as a platform, a benefit that made construction far easier (Rabun, 2000, 119).

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Masonry Construction:
Masonry was another early American building material, most commonly used in important buildings such as cathedrals and government buildings. The clay fired brick was the primary unit of masonry construction in America, but stone was also used. In pre-industrial times bricks were manufactured at a building site and tended to be softer and had less precision. With the advent of mass production and the factory system, bricks became stronger and had a more precise size. No matter how the brick was manufactured however, the same principles governed masonry wall construction. The height of a wall was dependent on the thickness of the wall at the base. A masonry wall must be thick at the base to support the weight of upper floors and of the wall itself. Besides the brick, a masonry wall also needs mortar to bind the bricks together to resist forces in the transverse direction. Lime-sand mortars were used in early American buildings, but they eventually evolved into Portland cement mortars which tended to be much harder (Rabun, 2000, 143). The combination of stronger bricks and mortar led to the increasing height of mass masonry buildings.

Iron and Steel Construction:
Iron and steel were two new building materials developed in the Industrial Revolution. Initially cast iron and wrought iron were used in combination to construct buildings. Wrought iron was a metal product with very low carbon content while cast iron was iron with higher amounts of carbon and silicon. Cast iron was utilized for compression members while wrought iron was used for the tension members. Iron was a very durable product and could be molded into a variety of shapes such as tubes, squares, H, cross, and channel sections (Rabun, 2000, 163). Similar to current steel construction, iron beams supported gravity loads and then transferred the load through riveted connections into the columns. Eventually iron products were replaced with structural steel, which combined the tension benefits of wrought iron and compression benefits of cast iron. The Empire State Building, shown in Figure 4, is an iconic steel framed building built in Manhattan in 1931. = =

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Historic structures often suffer from a lack of protection against deterioration issues that are easily prevented today. In the past, causes of deterioration were not as well understood and as such buildings do not perform as well against the elements. A common example is the complete lack of brick expansion joints in mass masonry buildings of the past. By understanding the common causes of deterioration in historic structures, it becomes easier to visually assess a structure and determine how best to rehabilitate and conserve it. Masonry and wood are the two materials most susceptible to deterioration in historic structures.

In the United States many historic structures were constructed with brick masonry. There are some possible defects in brick construction. Bricks can be under-fired, creating a greater chance for cracking, spalling, and crushing. Understrength mortar can be used which may lead to crushing of the surrounding masonry. The lack of brick expansion and control joints also restrains the movement of the brick and can result in severe cracking. All of these defects can become a starting point for weathering of a brick wall. Water infiltration can become a serious problem in brick walls. If steel reinforcement was used in the wall, water can and will eventually rust the steel, resulting in a loss of strength. Water trapped in a brick masonry wall is also susceptible to the freeze-thaw cycle, which will accelerate the progressive cracking of the wall (Ratay, 2005, 11). For an investigating engineer it is important to be able to identify types of cracking to determine the underlying problem, whether it be defective bricks, water infiltration, or building settlement over time. Cracks will continue to grow over time until they eventual span an entire wall, as seen in Figure 5.

Many historic buildings from the colonial era were built with wood. The Fairbanks House in Dedham, Massachusetts is a farm house built of timber beams and posts dating from the 1630's, making it the oldest wood structure in the United States (Hunter, 2012). Buildings like the Fairbanks House are still standing today, but they face a variety of deterioration issues. Wood, being an organic material, is very susceptible to deterioration caused by water and temperature changes. Water causes wood to rot and invites fungi, bacteria, and insects like termites to destroy it further. If a wood member deteriorates, it can lose strength and potentially fail when overloaded, as seen in Figure 6. When assessing historic wooden structures, it is important to know what water, bacterial, and insect damage looks like to identify where the root cause of the problem lies.

Prior to a physical inspection of a historic structure as much information as possible about the structure must be gathered. Not only is it important to understand the actual structure of the historic building in question, but also the times in which it was built. Research must be conducted to determine how the structure was built, for what purpose it was used, and what codes and materials were incorporated into the design.

The first order of business in evaluating a historic structure is to locate any construction documents. Having a set of original structural construction documents can be invaluable in determining member sizes, spacing, and other details (Ratay, 2005, 189). Unfortunately the older a building is, the more likely that the records have been lost or destroyed. There are however a number of government agencies that keep records of historic structures. Specifically, the General Services Administration, National Park Service, the Architect of the Capitol, and the Historic American Buildings Survey can be used to locate original construction documents for historic structures.

Determining what codes were in use at the time of a historic building's construction is another important part of pre-evaluation. Knowing what minimum loads a building was built for can give an engineer a good idea of what size and strength members were used. This can decrease the number of invasive tests used when evaluating a building. Knowing the common construction practices of a certain period can be equally important in determining the type of materials used.

In some cases enough research prior to an evaluation can negate the need for the evaluation all together. D. Matthew Stuart's article "Structural evaluations of existing buildings" explains how engineers needed to measure concrete enclosed steel columns in a historic New Jersey building. The only way to measure the steel would have been to perform exploratory demolitions. Luckily the evaluating engineers knew the architect of the building was famous for designing the Woolworth Building in New York City. They then went to the New York Historical Society and found the original construction documents from many buildings designed by that architect, including the building they were assessing (Stuart, 2012). Thus through a little research and digging they eliminated the need for costly and damaging exploratory demolitions.

[[image:failures/Crack_monitor.jpg align="right" caption="Figure 7, Crack monitoring device. Photo Credit National Park Service"]]
There are a variety of different methods to evaluate the condition of a historic structure. Evaluation tests fall into one of two categories: invasive and non-invasive (also known as non-destructive or NDE). Invasive methods of evaluation are physical tests of the structural system and materials to determine aspects such as flexural and shear capacity. Opposite of that are non-invasive tests, which use alternative methods such as radar to avoid physically damaging or altering the structure.

The first form of structural evaluation should always be a visual assessment. This phase of investigation includes inspecting all parts of a historic structure for potential deficiencies such as brick and masonry cracking, water damage, or excessive wall deflection. All problem areas of a structure should be documented with photographs and cataloged in some form of database. Any cracks should be measured as well and fixed with a crack monitoring device such as the one shown in Figure 4, to monitor the development of the crack over time. A visual assessment of a structure can also include some basic preliminary tests. One basic test is sounding a floor slab or masonry wall with a hammer to listen to the reaction. If the hammer does not rebound then the masonry is in poor condition, while if it rings sharply and rebounds then the masonry is in decent shape (Feilden, 2003, 211).

A preliminary visual assessment can identify the causes of a number of deficiencies. But in some cases a structural problem may not be apparent just by looking at a wall, beam, or column. For instance it is impossible to determine the load capacity of a centuries old wooden beam just by taking a photograph. In these cases invasive tests can be employed to determine capacities and stresses of members.
 * **Stress Measurement:** A tool called a flatjack can be used to determine a walls compressive stress and deformability. These tests require the removal of masonry mortar between bricks, followed by the placement of a pressurized flatjack in the empty joint. The pressure of the flatjack can be adjusted to fit the original mortar-bed joint geometry, which is then measured by a gage to determine the vertical compressive stress at a point in the wall (Schuller, 1995, 54).
 * **In Place Shear and Flexural Tests:** For shear strength, one brick and a head joint are removed from the opposite sides of a chosen test unit. In the place of the removed brick a hydraulic ram is used to push on the test brick, while a gage is placed over the gap created by removing the adjacent head joint. The ram applies a force and the gage records the amount of displacement. The results are then used to determine the wall's resistance to shear forces. Flexural strength tests also require the removal of bricks to subject the wall to flexural loads.
 * **Large Scale Load Test:** Vertical and horizontal load tests can be performed. A vertical load test involves placing dead weight on a certain floor area. This dead weight can be sand bags, bricks, or metal slugs. In other cases tubs are constructed and then filled with water to determine the load bearing capacity of a gravity system. In any case the deflections of structural members subjected to load tests are monitored very closely so that if sudden large deflections occur, the load can be removed. Secondary shoring should always be used to prevent collapse in the event that an area is overloaded. Horizontal load tests can be constructed using hydraulic equipment to simulate wind and seismic forces on lateral members (Schuller, 1995, 56).

While invasive load tests can be extremely useful in determining the capacity of structural systems, they can be impractical for use on sensitive historic buildings. Many historic buildings still have the original building materials which lend to the general authenticity of the structure. Invasive tests would likely require the destruction of original building material so they are generally avoided. There are a number of non-invasive tests that can be employed on sensitive buildings to determine the capacities and deficiencies of structural members.
 * **Infrared Thermography:** This test requires the use of infrared cameras to detect surface temperature distributions within a structural member. Instrumentation can pick up radiation emitted by a wall or slab and render a thermal map, which can be used to locate and monitor moisture.
 * **Impact Echo:** Applies a physical impact and then measures the displacements near the impact point. An impact generates several types of waves that can be used to locate discontinuities, holes, and other defects in a wall or floor that would be otherwise undetectable without breaking into the wall. Waves types included shear waves, surface waves, and longitudinal waves. An equation dividing the time between arrival of longitudinal waves at a receiver by the wave speed equals the depth of an anomaly. Impact-echo tests are most applicable to heterogeneous materials such as masonry and concrete (Cecire, 2003, 21).
 * **Ground Penetrating Radar:** Electromagnetic waves are sent into a structural component such as a wall or slab to locate anomalies. The waves are used to find embedded steel, utilities, or holes in the structure, though the results will be less detailed with GPR than other tests like impact-echo. However, ground penetrating radar has the advantage of being easy to operate and quick to perform (Cecire, 2003, 25).
 * **Acoustic Emission:** This test is used to continuously monitor a structure to detect an impending failure. It uses a microphone to listen to sounds generated by rapid energy releases caused by sudden crack growth or plastic deformation. Though acoustic emission is capable of detecting the onset of failures, it is very expensive to run and requires expert knowledge to interpret the results (Pielert, 1996).

Following the on-site investigation of a historic structure, an engineer should prepare recommendations for the repair and rehabilitation of the structure. The engineer that does the structural evaluation of a building should also be familiar with the various methods for rehabilitating that structure. Recommendations should be made to determine how a repair should be done, what materials should be used, and what sequence to carry out repairs. Special attention should be payed to the aesthetics of new repairs so that new materials look like the original materials.

Historic buildings and structures must be constantly monitored and maintained through the passage of time. The older the structure, the more important it is to maintain it's appearance and structural health as it is more susceptible to deterioration. Since the creation of the National Historic Preservation Act of 1966, there has been a concentrated effort toward preserving this country's past. Many important historic buildings have been subject to various deterioration and structural problems. The following examples highlight the structural assessment and preservation efforts of some of those historic landmarks.

Fallingwater, one of the most enduring examples of Frank Lloyd Wright's work and indeed, of American architecture, had a serious structural issue dating back to the original construction in the 1930's. The first floor concrete cantilever experienced large deflections the moment its formwork was removed, deflections that only increased with time. By 1995, the deflection had reached an astounding 7" and it was decided that something had to be done in order to preserve Wright's masterpiece. Several firms and agencies, such as Robert Silman Associates and the Pennsylvania State University were hired to assess the structural condition of Fallingwater (Meek, 2010). Both invasive and non-invasive test were utilized. The primary cause of the excessive deflections was inadequate steel reinforcement in the cantilever. A structural analysis model using SAP 90 accurately determined the proper reinforcement needed in the cantilever. The investigation also led to the removal of the floor system where a large amount of damage to the system was found. In the end, external post-tensioning was used to correct the excessive deflections of the cantilever.

The United States Capitol Building in Washington D.C. is one of the proudest symbols of our country as seen in Figure 5. The dome of the building is a cast iron structure that was completed in 1863. Since its completion, there have been a number of structural assessments of the dome; in 1956, 1994, 1998, 1999, and 2000. These investigations looked at a number of issue concerning the dome, including the cast iron members, water drainage system, lead abatement, and cast iron plates. While there have been some issue with thermal expansion that have led to cracking, the Capitol Dome is in otherwise excellent shape (Ariosto, 2010). The many and constant structural assessments, as well as routine maintenance and repairs, are necessary work that must be done in order to keep our historic structures alive and well.

The H. Lawrence & Sons Rope Works is a complex of factories built in the 1850's in Brooklyn, NY. The building was constructed with brick masonry walls, cast iron columns, and timber roof and floor joist systems. Dominick R. Pilla Associates was hired to evaluate the structural condition of the factories and recommend the appropriate repairs and restorations to modernize the buildings. The investigation found a number of issues including; large settlement cracks due to improper backfilling, deformed roof purlins due to multiple layers of roof finish, rotted and fire damaged floor areas, severe column corrosion, and deteriorated brick walls. All of the damaged material was fixed with modern building materials and the entire complex was brought into compliance with New York City building code. As a result of the assessment and rehabilitation, the Rope Works won the Historic Preservation Award from the Brooklyn Chamber of Commerce and it has been placed on the National Register of Historic Places (Pilla, 2010).

Preserving our collective past through historic structures is a noble cause. We have a responsibility to keep alive the places and symbols that have added a thread to the fabric of history. Imagine a nation without the enduring symbols of the Statue of Liberty, Capitol Building, or Fallingwater. By preserving historic structures, not only can we learn how they were built, but also how the people that built them lived and worked. Through the preservation of historic structures we can preserve the very character and pride of our nation.

Bibliography:
This wiki page has information about the structural assessment and repair of the United States Capitol Dome in Washington D.C.
 * "United States Capitol Dome Restoration and Rehabilitation Project" Ariosto, Tim. Failures Wiki. 2010.**

This thesis paper goes into great detail about the various nondestructive evaluation (NDE) methods for historical structures. Techniques described include impact-echo, infrared thermography, and ground penetrating radar.
 * Cecire, Ashley Skye. (June 2003) "Nondestructive Evaluation of Historic Structures." Massachusetts Institute of Technology Libraries. (October 2, 2012).**

This book has a general overview of the various methods used to preserve historic buildings. Part of the book describes the ways that historic buildings are evaluated.
 * Feilden, Bernard M. (2003). "Conservation of Historic Buildings.//"// Architectural, Boston, MA.**

An internet article giving the age and history of the Fairbanks House in Dedham, MA. This house is the oldest wooden structure in the United States.
 * "A Grand House in 17th Century New England" Hunter, Elizabeth I.V. Fairbanks House Historical Site.**
 * [] (November 13, 2012)**

This wiki page has information relevant to the structural assessment of Frank Lloyd Wright's Fallingwater, an exceptionally historic building.
 * "Fallingwater: Restoration and Structural Reinforcement" Meek, Tyler. Failures Wiki. 2010.**

This portion of standards by the ASCE Committee on Structural Condition Assessment and Rehabilitation of Buildings describes several tests performed with their inherent advantages and disadvantages. This is only a select few pages of the ASCE 11 standard.
 * Pielert, J., Baumer, C., and Green, M. "ASCE Standards on Structural Condition Assessment and Rehabilitation of Buildings." Standards for Preservation and Rehabilitation, ASTM STP 1258, S.J. Kelley. Ed., American Society for Testing and Materials. (1996). pp126-136**

This article explains some of the evaluation procedures for a 150 plus year old factory in Brooklyn, NY. The article goes into condition assessment, code compliance, as well as the roof, floor, and masonry wall evaluations.
 * Pilla, Dominick R., and Tong, Xiaoli. Structure Mag. (September 2010).****"Evaluating Historic Structures for Adaptive Re-Use."**
 * [] (October 2, 2012).**

This book goes into depth on assessment methodology, historic American building systems and historic materials assessment among other topics.
 * Rabun, J. Stanley (2000). "Structural Analysis of Historic Buildings: Restoration, Preservation, and Adaptive Re-Use Applications for Architects and Engineers.//"//**
 * Wiley, New York, NY.**

This book describes how buildings in general, not just historic ones, are assessed.
 * Ratay, Robert T. 2005. "Structural Condition Assessment."****John Wiley,** **Hoboken, NJ.**

This publication describes NDE and In-Situ tests for masonry construction. NDE tests include survey tests, wave transmission tests, ultrasound pulse velocity tests, and impact echo tests. In-Situ tests include stress measurement, deformability measurement, shear tests, flexural tests, and large-scale load tests.
 * Schuller, M. P., Atkinson, R. H., and Noland, J. L. "Structural Evaluation of Historic Masonry Buildings." Association for Preservation Technology International. (1995) pp51-61.**

This internet article goes into some detail about evaluating a structure with both demolished openings and X-rays just to name a few. There is also an example of structural evaluation on an historic building in New Jersey.
 * Stuart, D. Matthew.** **"Structural evaluations of existing buildings" (April 2012) [] (October 2, 2012).**

Additional Resources:


 * ASCE Guideline for Structural Condition Assessment of Existing Buildings.**

http://www.nps.gov/tps/standards.htm
 * National Park Service: Technical Preservation Services.**