CHAPTER 2: LITERATURE REVIEW
2.16. Assessment of existing structures
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system. The envelope analysis strategy allows for the determination of maximum and minimum moment and shear conditions.
According to Gartner (2008), the following structural checks are relevant to green roof systems and should be taken into account, when applicable:
1. Verification of irregularities in seismic mass for conditions of conventional roof and fully saturated roof conditions.
2. Structural elements supporting green roofs i.e. gravity beams, seismic drags/collectors and any other connections, must be evaluated for conditions of high bending in combination with axial loads.
3. Careful evaluation is needed for punching shear in concrete slabs.
4. Careful evaluation of plastic hinges that are expected in lateral systems.
5. Consideration to the sequencing of construction of the shear wall and bracing, in an attempt to prevent dead loading
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structures utilising the principle of structural reliability and considering specific problems on existing structures. In addition, the ISO 13822 code explains why current standards with regard to structural design are not sufficient for a reliability assessment of existing structures and for the design of the applicable repairs and upgrading (Holicky, 2010).
Furthermore, Holicky (2010), states that present design codes do not provide procedures for the assessment of the current state and resistance of materials of existing structures.
The ISO 13822 code further states that structures that are designed and constructed utilising earlier codes or those that have been designed and constructed utilising “good construction practice” with no codes applied may be considered safe to resist actions and may be considered serviceable for future use.
Figure 2.16.1: Various structural assessment levels (after Hille, et.al., 2006) To conduct an assessment of an existing structure two main objectives are established i.e.
the minimisation of costs and the assurance of structural safety.
2.16.1 Structural safety and serviceability:
The main aim of the structural safety and serviceability assessment is to ensure that the structure or parts thereof do not fail under loading. This form of assessment is carried out for ultimate limit states and are inclusive of (Hille, et al., 2006):
• Equilibrium loss of a structure or parts thereof
• The achievement of a structures resistance capacity
• The transformation of a structure or part thereof into a mechanism
• The instability of a structure of part thereof
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A limitation of structural use may arise through a reduction of serviceability. It is based on this fact that serviceability assessments may prove to be necessary. Serviceability limit states are inclusive of (Hille, et al., 2006):
• A reduction in the working life of a structure through localised damaged
• Deformations that affect the efficient use of a structure
• Discomfort caused to people through excessive vibrations 2.16.2 Cost minimization:
Systems to manage single structures were developed in an attempt to minimise the overall cost through the optimisation of inspections, maintenance and repairs. The primary task of such processes is to assess the structural conditions in order to determine its current state and evaluate the future performance of a structure (Hille, et al., 2006).
2.16.3 Methods of structural analysis 2.16.3.1 Simple analysis methods
Simple structural analysis involves using basic conservative methods to calculate load effects through simple structural models. Typical simple analysis methods are inclusive of space frame analysis under a simple load distribution and linear elastic material behaviour, resulting in lower bound equilibrium solutions (Hille, et al., 2006).
2.16.3.2 Complex analysis methods
Complex analysis methods are utilised in instances where simple analysis methods fail.
Complex methods are refined and include methods such as finite element analysis and non-linear methods such as yield line analysis, in an attempt to result in higher capacities.
Complex methods model material behaviour such as shrinkage and creep of reinforced concrete structures and take into account interactions between components such as bondage and tension stiffening in reinforced concrete structure. This is done in an attempt to uncover hidden reserve capacities and reduce conservatism. In addition, to conduct a complete probability safety verification, stochastic finite elements can be utilised to model a structure. Stochastic finite elements take into account the spatial correlation of the applicable random variables (Hille, et al., 2006).
2.16.3.3 Adaptive models
Adaptive modelling automatically updates structural parameters utilising measured data such as changes in displacements, strains or damage values (e.g. crack width) to make
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new information about the structural behaviour during assessment available (e.g. long- term monitoring) (Hille, et al., 2006).
2.16.4 Reliability verification
Although structural analysis is utilised to obtain information with regard to the structural state, reliability verification is done to assess the actual evaluation of the safety and serviceability margin of an existing structure. Essentially, reliability verification describes the distance between the actual real state and the limit state of the structure.
Approaches to reliability verification is graphically illustrated in Figure 2.16.4.1 (Hille, et al., 2006).
Figure 2.16.2: Reliability verification approaches (after Hille, et. al., 2006) 2.16.4.1 Deterministic verification with global safety factors:
The deterministic verification method is commonly referred to as the traditional way of defining safety. The method is based completely on experience and the safety measures are empirical. The most common deterministic safety measure is the global factor of safety. The global factor of safety is defined as the ratio between the resistance and the load effect. A typical method of deterministic verification is the concept of permissible stress. Another concept is that of the ‘load factor’. The load factor concept represents the ratio of the ultimate strength of a member to the working load. Deterministic methods of verification that contain only one global factor of safety contain considerable amounts of uncertainty and is therefore recommended to be utilised exceptionally within the assessment of existing structures (Hille, et al., 2006).
2.16.4.2 Partial safety factors:
The semi-probabilistic approach is based on the principle of limit state. The primary concern of the approach is to ensure that failure does not arise within a component of the existing structure of the structure itself, commonly described as Ultimate Limit State
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(ULS). In addition, it is important to analyse the structural effects of applied loading that may arise in a serviceability failure, described as the Serviceability Limit State (SLS). As a result, partial safety factors are established as a safety measure to guard against variations in the design parameters that may arise on the resistance or load side (Hille, et al., 2006).