Assessing existing structures

Abstract

The focus of this thesis research was on developing an assessment protocol which assists the structural engineer in visualizing the structural possibilities for multi-story industrial heritage constructed between 1910 and 1950. The protocol distinguishes seven assessment steps which can be used to (1) indicate the material and structural quality (2) visualize the structural design possibilities and (3) assess the structural safety. The systematic approach introduced by the protocol stimulates a faster and more efficient determination of the comprehensive capabilities of the concrete load bearing frame. The protocol is developed from the context of integrated designing, giving the structural engineer an independent role in the redevelopment process. The individual assessment steps [the historic assessment, the visual assessment, the technical assessment, evaluation, the preliminary assessment, the [refined] structural safety assessment and the strenghtening assessment] are provided with current knowledge and trends available for the assessment of existing reinforced concrete structures. These steps are subjected to constant change and development. The structural engineer is therefore encouraged to supplement the protocol with experience gained during projects, and with future developments. The engineer is recommended to follow the assessment steps as proposed by the protocol. In cases where this is not possible the engineer should use the protocol to formulate a targeted assessment strategy, which minimizes the influences of the obstructed process. The protocol will provide the information, but requires the motivation and capabilities of the engineer to come to the desired results. QUALITY The first four assessment steps are designed to assist the visualization of the structural and material quality. The steps are focussed in gathering a sufficient amount of information to prevent unexpected situations in the construction or execution phase. The engineer should be aware that in the design of new buildings the information produced on paper will be realized, whereas in the situation of redesigning existing buildings what is realized should be put on paper. The information that is needed to estimate the remaining life span and to perform the structural safety assessment can only be found insitu. In some situations also design information can be found in the original document, but the engineer should be aware that his information always brings a certain level of inaccuracy and uncertainty. The first assessment step is the historic assessment which helps the engineer to collect a sufficient amount of original documentation [architectural design, structural design, reinforcement design, foundation etc.] to visualize the quality and characteristics of the load bearing system. The assessment step is substantiated by a historic database which informs the modern structural engineer about the material and structural characteristics of historic concrete. This concept is introduced to bridge the knowledge gap between past and present and eliminate possible risks occurring from this gap by visualizing the differences in material durability, and provide structural information to clarify the original documentation or when no original information is available to substantiate the making of assessment assumptions. The quality assessment of materials and structures is time consuming, expensive and difficult when performed into detail. The structural engineer is therefore challenged to formulate a targeted and efficient approach without compromising the desired information. To stimulate this process the protocol introduces at the end of the historic assessment a general strategy which helps to formulate the desired and required input and output regarding the visual and technical assessment. The visual assessment stimulates the engineer to visualize the structural and material quality in-situ, by proposing a visual inspection towards deterioration and the optical verification of the original drawings. The assessment V step also provides a description of the non-destructive measuring techniques which can either be used to verify certain forms of deterioration or to verify the test results found with the technical assessment. At the end of the visual assessment the engineer is granted the possibility to take concrete and steel samples for additional research to be performed in the laboratory to indicate, the mechanical properties, the cause of deterioration or to estimate future deterioration. The protocol provides an estimate of the required research pool, and the considerations which have to be taken into account when estimating the sample location. The technical assessment step provides the structural engineer with a general introduction on the different laboratory tests, and their specifications. The quality assessment is completed by introducing the evaluation step, which provides the structural engineer with the general methodology for reinforced concrete repair and protection. This methodology points out the importance for conservation, and a durable and compatible approach. The assessment step also provides a list with the general methods that can be used for repair and protection. DESIGN POSSIBILITIES The identification of the quality and the visualization of the structural design is the basic input of the preliminary assessment stage. This stage is focussed on generating structural redevelopment concepts based on a pragmatic approach, and from the context of integrated designing. This process stimulates the designer’s creativity in visualizing the structural possibilities with respect to functional change. The success of redevelopment seen from a structural perspective depends on the aspects of structural flexibility, the structural adaptability, the relation between building and user, and the standard redevelopment criteria [climate, day light entrance, emergency routing etc.]. To stimulate the successful integration of these design aspects into the redevelopment, the engineer is advised to use a morphological overview. Such an overview gathers the different structural aspects and combines them with possible structural redevelopment alternatives. The overview facilitates the making of combinations, generating redevelopment concepts and to rerun the design steps in a later stadium of the process. As a last note, the preliminary assessment recommends the engineer to generate the structural concepts from the concept of durable and sustainable design. The sustainable perspective requires the engineer to design with an eye on the future, whereas a durable design minimizes the need for functional change in the future. STRUCTURAL SAFETY The safety assessment of existing structures is different than the safety assessment for new structures. The differences can be found in the required aspects of the limit state design, and the considerations towards the structural reliability, the fire safety and the capacity estimation. The [refined] structural safety assessment visualizes the options regarding these different subjects, and clarifies their influences. The assessment stage also gives a general introduction on the methodology influencing the in depth detailing of the calculations and their reflection onto the desired results. In some situations the functional requirements exceed the bearing capacity of the load bearing structure. In such a case strengthening could be an option to improve the chances on redevelopment. Therefore, as last assessments stage the strengthening assessment is introduced. This assessment step visualizes the methodology towards estimating the required strengthening in respect to the ultimate and serviceability limit state design, and gives a general introduction on the strengthening methods available on today’s market.