Appunti "Building Maintenance Proceedings and Methods"

Building maintenance proceedings and methods

Uni 10604 - 1996

Defines maintenance objective: The maintenance goal is to allow asset usage, maintaining its values and its initial performances during the useful life, supporting technical and regulatory adjustment in case of new performances required by ownership or regulations.

Maintenance

Combination of all technical, administrative, and managerial actions during the life cycle of an item intended to retain the item or to restore the item to a state in which it can perform the required function.

Maintenance plan

Structured and documented set of tasks that include the activities, the procedures, the resources, and the time scale required to carry out maintenance.

Maintenance system

Entity that is able to develop, manage, and control a maintenance plan or schedule.

Maintainability

Ability of an item, under given conditions of use, to be restored to a state in which it can perform the required functions, when maintenance is performed under given conditions and using stated procedures and resources.

Maintenance typology

Routine/planned maintenance

Maintenance carried out to repair, refurbish, and/or replace an item or a building finishing and/or to complete and maintain building systems/plans and machinery.

• Plaster cleaning, partial restoration/recovery
• Window, roof, surface cleaning, refurbishment, and replacement using the same materials, etc.
• Partial refurbishment of external cladding, covering using the same materials, colours, and technologies.

Extraordinary maintenance

Maintenance carried out to refurbish and replace components or parts of a building (structural and non-structural) and to introduce facilities/services (e.g., bathroom, kitchen) or to introduce new building systems/plans/machinery, without an increase in volume, surface, or changing in use.

• Refurbishment of plaster, roof, windows, etc.

Corrective maintenance

Maintenance carried out after a fault recognition, which allows the damaged item/system, etc., to come back to a state in which it can perform its required function.

Preventive maintenance

Maintenance carried out at predetermined intervals of time or according to prescribed criteria intended to reduce the probability of failure or degradation of the function of an item/system, etc.

Predetermined maintenance (ciclica)

Preventive maintenance carried out in accordance with intervals of times or number of units used, without previous condition investigation.

Condition based maintenance (secondo condizione)

Preventive maintenance which includes a combination of condition monitoring and/or inspection and/or testing that ensures the maintenance actions.

Predictive maintenance

Condition based maintenance carried out following a forecast derived from repeated analysis and evaluation or known characteristics of the significant parameters of the item’s degradation.

Degradation and obsolescence

Building’s functional quality is subjected to an entropic transformation during its life cycle. The phenomena which characterized the transformation belong to degradation and obsolescence. Usually, a building or a system, or an item is requested to follow the dynamics that characterize demand. If the asset is no longer able to satisfy the demand requirements, we are going to talk about obsolescence.

Obsolescence

Obsolescence is the inability of an item to be maintained due to the unavailability on the market of the necessary resources that have acceptable technical and/or economic conditions. There are three types of obsolescence:

• Functional obsolescence: Obsolescence process related to changing in demand (usage model, standards, and regulation changing).
• Technological obsolescence: Physical obsolescence process which affects asset’s conditions, related to technological innovations and competitiveness.
• Economic obsolescence: Obsolescence related to market dynamics, consequence of previous obsolescence forms.

Degradation/physical degradation

Detrimental change in physical condition due to time, usage, and/or external causes. Degradation may lead to failure, but also degradation can be caused by failures within the system.

Supposing that the demand’s requirement increases constantly over time, the variance between the performance quality of the building/item/asset and the performance quality requested raises over time. The main objective is to reduce as much as possible the time in which building’s performances do not match the expected performances. So it is necessary to avoid physical degradation by means of restoration actions and facilitate the performance adjustments procedures from an obsolescence point of view. This is possible with two different strategies:

• The degrees of predictability of component’s physical degradation: choose, during the construction phase, the technological elements according to their reliability over time. → Prevention (over substitution or restoration) allows to hinder failures.
• Demand requirements of transformations uncertainty: it consists in facilitating maintenance interventions, reducing the intervention’s time, and leading the system to a higher level of availability and lowering the costs related to the functional adjustment (linked to MUT: mean up time: average time of availability).

Considering that maintainability is the ability of a technological unit to be maintained through restoration or substitution, we can define maintenance as the integration of activities performed to deal with the two forms of transformations that could affect buildings: degradation and obsolescence.

Degradation and obsolescence uncertainty

Degradation and obsolescence both belong to uncertainty. Degradation phenomena are partially predictable, whereas obsolescence phenomena are completely unpredictable. If we observe the relationship between time and probability of degradation or obsolescence transformations occur, we would explain it by the help of a diagram.

Obsolescence phenomena may take place since the building has been built (t1). In fact, if we consider the high lead time to build an asset, it is easy to figure that the building could probably not be aligned to the performances required by demand in (t2). Even if this scenario is probable, there is a low level of uncertainty, but after t2 the uncertainty related to the probability that obsolescence occurs increases constantly over time until it reaches in t4 totally unpredictable levels.

Concerning the degradation phenomena, the uncertainty curve presents a different shape. At time t2 already exists a low degree of uncertainty. The potential transformations due to the degradation process let uncertainty grow because of possible mistakes made in design/construction and production phases, which can occur during the Trial stage. Then during the use and management phase, the uncertainty remains fixed to a certain level due to the fact that degradation processes have reached values that don’t increase the probability that a failure is going to take place. Then going close to the end of life cycle, the trend of uncertainty starts to slow down until zero value because the life cycle of a building/component/item is predictable.

Degradation and obsolescence can follow two possible paths:

• Natural ageing: This process occurs when ordinary maintenance is not performed, as any other maintenance type, and the buildings or its items are subjected to their intrinsic degradation over time.
• Constant age: Process that is managed by an ordinary maintenance schedule. In this case, it’s possible to completely restore or partially restore the items or the entire building after the intervention. In the case in which it’s possible to completely renovate the building or its parts, the life cycle of the object is supposed to be infinite because its quality is constantly updated to original levels. On the other hand, a partial restoration leads to a future inadequacy of the building or its components. Most cases are represented by the second situation, i.e., ordinary maintenance that leads to a partial renovation of the item.

Diagram C

The broken line 3 represents the real behavior of a component, in fact after each maintenance intervention (t1, t2, t3,...) the item is brought back to the performance level reported along line 2. During time span “e” performances’ quality decreases according to line 1 slope. Red line “i” is the minimum acceptable performance level, i.e., reliability. This line has a value, arbitrarily defined, included between 0-100%. Reliability threshold suggests that when component’s degradation curve reaches levels under the minimum acceptable value, a substitution is needed.

Diagram D shows how the cyclical maintenance works, after component substitution begins a new component’s life cycle. The new item’s degradation curve, line 5, has a shape equal to the previous one, line 3. About the minimum performance level, a short clarification is needed. Usually, this quality level is flexible, it’s not attributed a precise value to be observed, but an interval of values, as we can see in the last chart (E). This acceptance band allows more flexibility in processes related to substitutions’ scheduling.

Indicators for maintenance planning

• Maintainability: Indicates the capacity of a component to avoid the degradation process.
• Reliability: Indicates the disposal in a component’s working capacity over time.

Maintainability

It is the ability of an item under given conditions of use, to be retained in, or restored to, a state in which it can perform a required function when maintenance is performed under given conditions and using stated procedures and resources. Maintainability is a component/system’s characteristic that integrates both qualitative and quantitative factors.

Qualitative factors of maintainability

• Frequency of maintenance works to maintain the efficiency of the item. This factor is linked to the reliability concept, in the sense that all the maintenance interventions are scheduled according to reliability/durability indexes such as failure rate.
• Organisation and scheduling of maintenance works and logistical support to services.
• Definition of maintenance costs. (Maintenance costs are inversely proportional to maintainability. If an item is highly maintainable, its maintenance costs are lower.)

Quantitative factor of maintainability

• Time required to carry out maintenance work/services.

Component’s maintainability is one of the most representative project requirements talking about designer’s awareness of management problems. Generally, maintenance interventions efficacy mostly depends (70-80%) on decisions made during the design phase. Maintainability is characterized by many other design factors:

• Complexity of the item/system/entity.
• Typological distribution and geometrical features of the building.
• Levels of modularity of building and of its subsystems.
• Accessibility, not only of the building but also of each component.
• Location, size, and spaces’ organization.
• “Portability” (trasportabilità) of items and subsystems.
• Standardisation of components/items according to regulations.
• Reversible nature of components (assembly, disassembly processes).
• Interchangeability of components.
• System flexibility.
• Definition of performance level of items/components.
• Definition of an initial maintenance plan.

In this perspective, it’s necessary to break down the system in all its sub-parts and then provide all the maintenance actions that each part could need according to the potential failures which characterized each item.

Reliability

Usually, the choice of a component depends on the quality/cost rate, where quality represents the adequacy of the item to future usage. In this perspective, quality definition lays on two main factors:

• How the component fits the standard level.
• How is the component reliability (ability to perform the required function over a period of time).

The first quantitative definition of reliability is the “probability that an item will carry out its required function for a predetermined period of time, under stated procedures.” This leads us to a definition with a probabilistic nature (obtained with calculations) assigned to the reliability concept. So the time in which the component is requested to perform its function affects its reliability value. Reliability is inversely proportional to using time, and depends on:

• Performances’ level required
• Time
• Context and usage model
• Costs → to be evaluated for the entire component’s life cycle.

To obtain the reliability curve and calculate components’ life duration, we have to define some functions:

• Probability density
• Cumulative distribution
• Reliability function
• Failure rate

The Failure Rate function is the most important one due to the fact that most components concerning reliability are classified according to their failure rate. It is essential to calculate the MEAN TIME TO FAILURE (MTTF) variable which identifies life duration of a component without undertaking maintenance activities. It provides an evaluation of item’s useful life, defining a time frame in which the failure rate is quite constant.

FAILURE RATE function represents the probability that an operative unit will fail in the period of time (t+dt) considering that it has been working until time t. This function could be considered as the failures number in the unit of time (the speed of failures occurrence).

"Bath tub" graph common to most technical elements describes three main phases:

• Trial stage: Phase during which failures, related to materials’ defects, errors, or production mistakes, occur. Over time the failure rate continuously decreases due to the substitutions of the components which have already failed. This leads to a stable value of the curve.
• Useful life: During this phase, characterized by constant values of failure rate, we can have random failures caused by unpredictable events.
• Ageing (and quick degradation): The final phase is characterized by random failures and failures caused by the natural ageing of components, which are going to reach the end of their useful life.

If we include the reliability function inside the failure rate curve, we obtain the graph above.

Looking at the diagram, it is possible to underline some preventive maintenance strategies to be adopted to hinder failures:

• Each intervention carried out during the trial stage would be negative because it would bring back the failure rate value to its higher initial value, and by so doing, a new trial stage would start.
• For the Useful life phase, the maintenance strategy to be adopted would be corrective maintenance. Interventions would be executed after failure occurs.
• In the last stages of useful life phase, a preventive maintenance intervention would lead to a much longer time frame in which failure rate results to be constant and would also bring reliability values back to a higher level.
• During the Ageing phase, preventive maintenance execution would be positive because it would bring failure rate values to lower levels considered constant over time.

FAILURE RATE = 1/MTBF

MTBF = T/R = TOTAL TIME/NUMBER OF FAILURES

MTTF = T/N = TOTAL TIME/NUMBER OF UNITS UNDER TEST (estimated for each operative unit)

Failure's classification

Classification based on failures’ "speed"

• Gradual failure: Event that occurs after a progressive change in element characteristics. This type of failure is easily predictable through a preventive analysis of component’s functioning.
• Unexpected failure: Unpredictable failure which occurs suddenly.

Classification based on failure events consistence

• Partial failure: Failure triggered by component characteristics changes. This type of failure does not compromise the component operating level required.
• Total failure: Failure triggered by component characteristics changes which lead to a complete loss of functioning requirements.

Classification based on the combination of failure’s speed and consistence

• Catastrophic failure: Failure event that results to be sudden and total.
• Degradation failure: Failure which results to be both gradual and partial.

Classification based on life cycle (failure rate function)

• Failure during initial phase: Failures which occur when failure rate is decreasing.
• Failure during useful life phase: Failures which occur when failure rate is constant. These failures usually belong to random/catastrophic types of failures.
• Failure during quickened degradation phase: Failure which occurs when failure rate is increasing.

Classification based on failures’ consequences

It is necessary to evaluate the potential failures’ effects on the system, to safeguard the reliability of the system.

• Lesser failures: Failures which cause functional degradation of the components without influencing the system or the safety of the building’s users.
• Significant failures: Failures which are not so dangerous for the system or for people’s safety.
• Critical failures: This type of failures may cause the loss of the primary function of the system. Their consequences are significant for the system and the environment. These effects generate a discrete risk for people safety.
• Catastrophic failures: Failures which cause negative effects on the system, on the environment. These failures represent a threat to people’s safety.

Classification based on failures’ causes

The failures which belong to this kind of classification are those which are generated from design/production/use mistakes. They can be classified as follows: