APT - Authenticity vs. Stability
Content
http://www.jstor.org/stable/1504393
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http://www.jstor.org
Authenticity
The
Stability: Conservation Engineer's
vs.
ATTAR
Dilemma
GHASSAN
An engineer discusses authenticity and intervention and suggests a technique to assist engineers and their clients in balancing the demands of historic preservation and engineering.
The conservation engineer frequently is faced with the problem of balancing the structural stability and operating requirements of an historic property with preservation goals. On one hand, strong, stable, easily maintainable, and operational structures are required; on the other hand, the historic design, materials, and construction techniques must be respected. Conservation principles, and the people who enforce them, demand authenticity; building codes, and the people who enforce them, demand safety, strength, and stability. Historians and code officials have had their say on these issues many times and in many places. I want to attempt here to define authenticity and intervention from an engineer's point of view and to describe a simple decision-making process that can be used as a guide in addressing the dilemma of authenticity versus stability. I have also found this to be a useful tool in explaining to non-engineers the resolution of often conflicting demands. This decision-making process can be used for any element identified as contributing to the historical importance of a resource. Authenticity Authenticity is a word often misused in the conservation field. It has been used to describe genuine historic things, but also has been used to describe accurate replicas. Therefore, we must first have a definition that will help us assess the degree of authenticity of a resource before we can determine the effect of any interven-
tion upon it. From an engineering point of view, the authenticity of a structure is a function of three elements: materials, construction systems, and construction processes or methods. These elements affect the appearance of properties, their structural behavior, and the rate of deterioration over time. Authenticity of materials. The materials used in original or period construction - stone, brick, mortar, wood, cast iron, logs, etc. - are the most important factors in determining authenticity because they are the basic elements of the historic property. Authenticity of construction systems. A construction system is the assemblage of materials that comprises one or more building elements and ultimately a complete structure. Examples of systems include postand-beam and balloon frame structural systems, timber and metal trusses, and rubble masonry walls. Authenticity of construction methods. Construction methods are the techniques, tools, and processes used to prepare and assemble materials and systems. Hand hewing of timber, tooling of masonry joints, finishing of stone surfaces, application of paint coatings, and beamcolumn connections are examples of construction methods. Stability From an engineering point of view there are two principal elements of stability to be concerned with
18
AUTHENTICITY VS. STABILITY 19
strength and durability. Strength is the ability of a material or system to carry its own weight plus additional design or imposed loads. Durability is measuered as a function of the useful life of a material or system over time. Both can be affected by weathering, natural decay, external forces, or human intervention. For historic properties, durability is of great interest because one of the goals is the long-term preservation of the resource.
tion below a rubblemasonrywall; structural bracingand reinforcement; cementgroutand epoxy injection; of trussconnections; strengthening of historic and localizeddismantling them elementsand reassembling in their usingthe originalmaterials originallocation,such as rebuilding a sectionof collapsingmasonrywall. restoration Restoration. Structural is designedto recoverthe form,function, detailing,and strengthof a specific periodin the property's history. This work can includethe removal of lateradditionsand/orthe replacementof missingoriginalelements. Structural restoration may involve and consolidationand reconstruction the introduction of submay require stitutematerialsand construction methodsto achievedesiredstrength or operational requirements. reconReconstruction. Structural in structionis the reproduction or whole or in partof a destroyed historic poorlypreserved property.A replicaof a propertyor elementas it at a specificperiodof time. appeared Two types of contemporary struccan also be deturalreconstruction fined: the volumetricreconstruction, which aims at recreating the volume of the historicproperty;and the volumetric expression,which aims to give sufficientinformation of form for the viewerto imaginethe historic property. The ConservationEngineer Beforeattempting to resolvethe conflict betweenauthenticity and stabilof havinga qualifiedconimportance servationengineerinvolvedin the process. The conservation engineer must have the training,experience, and sensitivityto applyestablished conservation principlesand rigorous conventions to the deterengineering minationof appropriate levelsof
ity, it is necessary to understand the
Intervention
Intervention is the physical work performed on an historic property as part of its conservation program. It can be as simple as routine maintenance or as complex as complete reconstruction. The impact of intervention on the authenticity and stability of a structure can vary from minor to drastic depending on the condition of the property, the scope of work, and the purpose of the work. Structural intervention can be classified into four main categories. In order of increasing intervention they are: stabilization, consolidation, restoration, and reconstruction. Stabilization. Structural stabilization is designed to preserve the existing historic property in its present condition by slowing down the effects of aging and arresting abnormal deterioration. Stabilization work includes: routine maintenance; protection, such as bracing an overloaded member; use of preservation treatments; and minor repairs, such as localized repointing, selective replacement of rotted wood, or partial reshingling to correct roof leaks. Consolidation. Structural consolidation is designed to improve the condition of existing materials and systems. It usually involves the introduction of new materials or construction methods. Examples include the addition of a reinforced concrete founda-
intervention.As yet thereare no for conserprograms degree-granting in NorthAmerica. vationengineering the qualificain evaluating Therefore, for this tions of engineers work, the as followingskills are recommended a minimum: * knowledgeof periodand buildingmaterials, contemporary * knowledgeof periodand buildingsystems, contemporary * knowledgeof traditional methodsof engineering analysis, * knowledgeof techniques of especially physicalinvestigation, non-destructive testingmethods, * abilityto detectand interpret buildingproblems, * ability,and willingness,to take calculatedrisks,and * abilityto work with a diverse team of professionmulti-disciplinary otherenals includingarchitects, historians, archeologists, gineers, exhibit publicadministrators, and the client. the designers, public, The Impactof Interventionon Authenticityand Stability Generally speaking,the amountof in the stabilityand authenticchange of a ity propertywill be leastwith - stabilizathe minimumintervention tion - and will grow as the amount increases to the most of intervention This can be drastic- reconstruction. with a graph(Fig. 1). best illustrated and increasauthenticity Increasing are on the vertical shown ing stability of inand levels (Y) axis, increasing terventionare shown on the horizontal (X) axis. In a theoretical example A and S), we assumemaxi(curves mumauthenticity but minimumstabilityat the point of the conservation initialinvestigation, before engineer's work begins.As the proposedlevel of intervention increases,stability climbsto a maximumbut authenticitydropsto a minimum.The intersectionof the two lines can be called the "optimumsolution"in this
period reconstruction is an accurate
20
APT BULLETIN
MAX.
AUTH.
MAX. STAB.
4_'
~~~~~~I
> MIN. STAB.
S1. MIN. AUTH.
X
Fig. 1. Stability vs. authenticity graph.
smooth. There are many gray areas, subject to interpretation, in the transition between intervention zones. The type of work to be performed, the type of property, the type of structural system and the initial problem(s) to be solved all affect the shape of the two curves and the location of the meeting point. To assist the conservation team in drafting the authenticity and stability curves, the engineer can use an "impact assessment table" as a guide (Fig. 2). On this table the different intervention options are listed. Then, based on the knowledge and experience of the team members, the impact of each option on both authenticity and stability is evaluated, from "nil"
theoretical examplebecauseit results in the most authenticity with the stability. greatest However,in real-world examples the startingpoints arerarelyas extremeand the curvesrarelyas
to "very high." These relative values are then plotted on the stability vs. authenticity graph and the two curves drawn. Depending on the specific case under consideration, a more precise assessment may be required. The stability column in this case can be broken down into different appropriate components, such as durability, compressive strength, resistance to frost action, etc. To illustrate this process, six proposed options have been selected for the hypothetical treatment of a large masonry fortification wall. Option 1: stabilization involving repointing of the existing masonry wall with a traditional lime mortar using traditional methods. There will be almost no change in authenticity or stability resulting from this work. Option 2: stabilization involving repointing with a cement mortar using traditional methods. There will be minor improvement in stability
and a minor loss in authenticity of materials. Option 3: consolidation of the walls using cement grout. This will have some effect on the authenticity of materials and systems and will result in some increased stability. Option 4: consolidation using cement grout with steel reinforcing bars. This will have a higher impact on the authenticity of materials and will considerably increase the stability of the walls. Option 5: reconstruction of the walls using period materials and traditional methods. This will further reduce authenticity but will increase stability, since we can assume that the traditional materials are achieving their maximum strength in their "new" condition. Option 6: reconstruction using modern materials, systems and methods such as reinforced concrete with stone veneer. This will result in the
AUTHENTICITY VS. STABILITY 21
DEGREE OF INTERVENTION
OFIMPACT DEGREE OPTIONS AUTHENTICITY STABILITY
DURABILITY STRENGTH AND/OR METHODS SYSTEMS CONSTRUCTION MATERIALSCONSTRUCTION CONSTRUCTION NIL VL L M H VH NIL VL L M H VH NIL VL L M H VH NIL VL L M H VH
1 STABILIZATION 2
o0 0 0
0 0
0 0
CONSOLIDATION
3
0
0
0
0
4
O00
0
RESTORATION
5
RECONSTRUCTION 6
0
0
0
0
0
0
0
0
Fig. 2. Impact assessment table.
greatest loss of authenticity but probably will achieve the highest stability. By plotting these points (Al and S1, A2 and S2, A3 and S3, etc....) on the graph and drawing the two curves (Fig. 1), the conservation engineer can then identify the ideal theoretical solution represented by the intersection of the two curves. Does this mean this is the solution that will be implemented? No, not necessarily. One important factor not represented on the graph is cost (in general, the greater the degree of intervention, the higher the cost). However, this graph and the table will provide a tool for discussion with the owner or client of the consequences of a variety of possible
decisions. For example, if authenticity is considered most important, then everyone involved in the decision must know that a lower level of stability can be expected. If that is not acceptable (to code enforcement officials, for example), the graph can indicate how much authenticity might be affected by the required improvement in stability. The graph and table are not substitutes for qualified professional judgment. When dealing with historic properties, the selection of the optimum intervention is very complicated and cannot be simplified to a formula or a point on a graph. The conservation engineer and the entire conservation team need a considerable
amountof knowledgeand experience in orderto identifyand evaluateall possibleoptions. The process described hereprovidesconservation with a workingtool professionals whichwill assistin the decisionmakingprocessand which can help themarriveat an appropriate balance betweenhistoricauthenticity and structural stability.
out of a coursehe presented to Canadian officialsto developtheir government
conservation requirements.
GHASSAN ATTARis an engineer and Chief, Period Engineering Section of the Canadian Parks Service. This paper grew sensitivity to historical values and
Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=aptech. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected].
Association for Preservation Technology International (APT) is collaborating with JSTOR to digitize, preserve and extend access to APT Bulletin.
http://www.jstor.org
Authenticity
The
Stability: Conservation Engineer's
vs.
ATTAR
Dilemma
GHASSAN
An engineer discusses authenticity and intervention and suggests a technique to assist engineers and their clients in balancing the demands of historic preservation and engineering.
The conservation engineer frequently is faced with the problem of balancing the structural stability and operating requirements of an historic property with preservation goals. On one hand, strong, stable, easily maintainable, and operational structures are required; on the other hand, the historic design, materials, and construction techniques must be respected. Conservation principles, and the people who enforce them, demand authenticity; building codes, and the people who enforce them, demand safety, strength, and stability. Historians and code officials have had their say on these issues many times and in many places. I want to attempt here to define authenticity and intervention from an engineer's point of view and to describe a simple decision-making process that can be used as a guide in addressing the dilemma of authenticity versus stability. I have also found this to be a useful tool in explaining to non-engineers the resolution of often conflicting demands. This decision-making process can be used for any element identified as contributing to the historical importance of a resource. Authenticity Authenticity is a word often misused in the conservation field. It has been used to describe genuine historic things, but also has been used to describe accurate replicas. Therefore, we must first have a definition that will help us assess the degree of authenticity of a resource before we can determine the effect of any interven-
tion upon it. From an engineering point of view, the authenticity of a structure is a function of three elements: materials, construction systems, and construction processes or methods. These elements affect the appearance of properties, their structural behavior, and the rate of deterioration over time. Authenticity of materials. The materials used in original or period construction - stone, brick, mortar, wood, cast iron, logs, etc. - are the most important factors in determining authenticity because they are the basic elements of the historic property. Authenticity of construction systems. A construction system is the assemblage of materials that comprises one or more building elements and ultimately a complete structure. Examples of systems include postand-beam and balloon frame structural systems, timber and metal trusses, and rubble masonry walls. Authenticity of construction methods. Construction methods are the techniques, tools, and processes used to prepare and assemble materials and systems. Hand hewing of timber, tooling of masonry joints, finishing of stone surfaces, application of paint coatings, and beamcolumn connections are examples of construction methods. Stability From an engineering point of view there are two principal elements of stability to be concerned with
18
AUTHENTICITY VS. STABILITY 19
strength and durability. Strength is the ability of a material or system to carry its own weight plus additional design or imposed loads. Durability is measuered as a function of the useful life of a material or system over time. Both can be affected by weathering, natural decay, external forces, or human intervention. For historic properties, durability is of great interest because one of the goals is the long-term preservation of the resource.
tion below a rubblemasonrywall; structural bracingand reinforcement; cementgroutand epoxy injection; of trussconnections; strengthening of historic and localizeddismantling them elementsand reassembling in their usingthe originalmaterials originallocation,such as rebuilding a sectionof collapsingmasonrywall. restoration Restoration. Structural is designedto recoverthe form,function, detailing,and strengthof a specific periodin the property's history. This work can includethe removal of lateradditionsand/orthe replacementof missingoriginalelements. Structural restoration may involve and consolidationand reconstruction the introduction of submay require stitutematerialsand construction methodsto achievedesiredstrength or operational requirements. reconReconstruction. Structural in structionis the reproduction or whole or in partof a destroyed historic poorlypreserved property.A replicaof a propertyor elementas it at a specificperiodof time. appeared Two types of contemporary struccan also be deturalreconstruction fined: the volumetricreconstruction, which aims at recreating the volume of the historicproperty;and the volumetric expression,which aims to give sufficientinformation of form for the viewerto imaginethe historic property. The ConservationEngineer Beforeattempting to resolvethe conflict betweenauthenticity and stabilof havinga qualifiedconimportance servationengineerinvolvedin the process. The conservation engineer must have the training,experience, and sensitivityto applyestablished conservation principlesand rigorous conventions to the deterengineering minationof appropriate levelsof
ity, it is necessary to understand the
Intervention
Intervention is the physical work performed on an historic property as part of its conservation program. It can be as simple as routine maintenance or as complex as complete reconstruction. The impact of intervention on the authenticity and stability of a structure can vary from minor to drastic depending on the condition of the property, the scope of work, and the purpose of the work. Structural intervention can be classified into four main categories. In order of increasing intervention they are: stabilization, consolidation, restoration, and reconstruction. Stabilization. Structural stabilization is designed to preserve the existing historic property in its present condition by slowing down the effects of aging and arresting abnormal deterioration. Stabilization work includes: routine maintenance; protection, such as bracing an overloaded member; use of preservation treatments; and minor repairs, such as localized repointing, selective replacement of rotted wood, or partial reshingling to correct roof leaks. Consolidation. Structural consolidation is designed to improve the condition of existing materials and systems. It usually involves the introduction of new materials or construction methods. Examples include the addition of a reinforced concrete founda-
intervention.As yet thereare no for conserprograms degree-granting in NorthAmerica. vationengineering the qualificain evaluating Therefore, for this tions of engineers work, the as followingskills are recommended a minimum: * knowledgeof periodand buildingmaterials, contemporary * knowledgeof periodand buildingsystems, contemporary * knowledgeof traditional methodsof engineering analysis, * knowledgeof techniques of especially physicalinvestigation, non-destructive testingmethods, * abilityto detectand interpret buildingproblems, * ability,and willingness,to take calculatedrisks,and * abilityto work with a diverse team of professionmulti-disciplinary otherenals includingarchitects, historians, archeologists, gineers, exhibit publicadministrators, and the client. the designers, public, The Impactof Interventionon Authenticityand Stability Generally speaking,the amountof in the stabilityand authenticchange of a ity propertywill be leastwith - stabilizathe minimumintervention tion - and will grow as the amount increases to the most of intervention This can be drastic- reconstruction. with a graph(Fig. 1). best illustrated and increasauthenticity Increasing are on the vertical shown ing stability of inand levels (Y) axis, increasing terventionare shown on the horizontal (X) axis. In a theoretical example A and S), we assumemaxi(curves mumauthenticity but minimumstabilityat the point of the conservation initialinvestigation, before engineer's work begins.As the proposedlevel of intervention increases,stability climbsto a maximumbut authenticitydropsto a minimum.The intersectionof the two lines can be called the "optimumsolution"in this
period reconstruction is an accurate
20
APT BULLETIN
MAX.
AUTH.
MAX. STAB.
4_'
~~~~~~I
> MIN. STAB.
S1. MIN. AUTH.
X
Fig. 1. Stability vs. authenticity graph.
smooth. There are many gray areas, subject to interpretation, in the transition between intervention zones. The type of work to be performed, the type of property, the type of structural system and the initial problem(s) to be solved all affect the shape of the two curves and the location of the meeting point. To assist the conservation team in drafting the authenticity and stability curves, the engineer can use an "impact assessment table" as a guide (Fig. 2). On this table the different intervention options are listed. Then, based on the knowledge and experience of the team members, the impact of each option on both authenticity and stability is evaluated, from "nil"
theoretical examplebecauseit results in the most authenticity with the stability. greatest However,in real-world examples the startingpoints arerarelyas extremeand the curvesrarelyas
to "very high." These relative values are then plotted on the stability vs. authenticity graph and the two curves drawn. Depending on the specific case under consideration, a more precise assessment may be required. The stability column in this case can be broken down into different appropriate components, such as durability, compressive strength, resistance to frost action, etc. To illustrate this process, six proposed options have been selected for the hypothetical treatment of a large masonry fortification wall. Option 1: stabilization involving repointing of the existing masonry wall with a traditional lime mortar using traditional methods. There will be almost no change in authenticity or stability resulting from this work. Option 2: stabilization involving repointing with a cement mortar using traditional methods. There will be minor improvement in stability
and a minor loss in authenticity of materials. Option 3: consolidation of the walls using cement grout. This will have some effect on the authenticity of materials and systems and will result in some increased stability. Option 4: consolidation using cement grout with steel reinforcing bars. This will have a higher impact on the authenticity of materials and will considerably increase the stability of the walls. Option 5: reconstruction of the walls using period materials and traditional methods. This will further reduce authenticity but will increase stability, since we can assume that the traditional materials are achieving their maximum strength in their "new" condition. Option 6: reconstruction using modern materials, systems and methods such as reinforced concrete with stone veneer. This will result in the
AUTHENTICITY VS. STABILITY 21
DEGREE OF INTERVENTION
OFIMPACT DEGREE OPTIONS AUTHENTICITY STABILITY
DURABILITY STRENGTH AND/OR METHODS SYSTEMS CONSTRUCTION MATERIALSCONSTRUCTION CONSTRUCTION NIL VL L M H VH NIL VL L M H VH NIL VL L M H VH NIL VL L M H VH
1 STABILIZATION 2
o0 0 0
0 0
0 0
CONSOLIDATION
3
0
0
0
0
4
O00
0
RESTORATION
5
RECONSTRUCTION 6
0
0
0
0
0
0
0
0
Fig. 2. Impact assessment table.
greatest loss of authenticity but probably will achieve the highest stability. By plotting these points (Al and S1, A2 and S2, A3 and S3, etc....) on the graph and drawing the two curves (Fig. 1), the conservation engineer can then identify the ideal theoretical solution represented by the intersection of the two curves. Does this mean this is the solution that will be implemented? No, not necessarily. One important factor not represented on the graph is cost (in general, the greater the degree of intervention, the higher the cost). However, this graph and the table will provide a tool for discussion with the owner or client of the consequences of a variety of possible
decisions. For example, if authenticity is considered most important, then everyone involved in the decision must know that a lower level of stability can be expected. If that is not acceptable (to code enforcement officials, for example), the graph can indicate how much authenticity might be affected by the required improvement in stability. The graph and table are not substitutes for qualified professional judgment. When dealing with historic properties, the selection of the optimum intervention is very complicated and cannot be simplified to a formula or a point on a graph. The conservation engineer and the entire conservation team need a considerable
amountof knowledgeand experience in orderto identifyand evaluateall possibleoptions. The process described hereprovidesconservation with a workingtool professionals whichwill assistin the decisionmakingprocessand which can help themarriveat an appropriate balance betweenhistoricauthenticity and structural stability.
out of a coursehe presented to Canadian officialsto developtheir government
conservation requirements.
GHASSAN ATTARis an engineer and Chief, Period Engineering Section of the Canadian Parks Service. This paper grew sensitivity to historical values and
