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Concepts of Structural Safety

Uncertainties in an engineering system can be mainly attributed to ambiguity and vagueness indefining the variables and parameters of the system and their relations. This gives rise to

problems in structural reliability analysis which can endanger structural safety.

Structural Designs Are Based On Assumptions

Structural safety and structural reliability are the most

important aspects that have to be taken into considerationwhen any engineer undertakes the design of a structure. The

problem is all the more aggravated in the field of civilengineering because of the uncertain nature of the materials

used in building construction. Materials like sand andaggregates are natural materials which vary widely in their

origin, and hence exhibit widely different physical

parameters of size and strength that are necessary fordetermining the strength of a structure.

Other materials like cement and reinforcing bars are made of materials whose strengths are morecorrectly known and reliable, but in any design the final strength acceptable can come from theweakest parts of the materials used in any combination. This uncertainty makes it all the more

imperative that the structural safety and structural reliability of a structure be correctly assessed.Buildings that use metals for their structural requirements are easier to design for safety and

reliability because of the very determinate nature of the materials that are used. Factors of safetyin such structures are generally much lower as such structures are assessed to be more reliable.

Safety Factors and Reliability

It is necessary that the capacity of a structure to carry loads that are likely to be imposed on it are

determined to a fair degree of accuracy. This will ensure that the structure remains safe during itslifetime and use. While superimposed loads are normally based on acceptable standards that have

been established from observed usage, some structures may have to cater to unexpected loadsdue to misuse, natural conditions like snow, hurricanes, and earthquakes. Design factors do take

into consideration snow, wind, and earthquakes, based on standard observations or statisticscollected in any particular area. But Nature is unpredictable and can cause snowstorms, wind

speeds, or earthquakes that are far in excess of those observed up to a particular date. This doeshowever become a statistic for future designers to contend with, but the present structure then

has to rely on the factor of safety that the designer of the structure has considered.

When the factor of safety is considered as a ratio of the absolute strength or structural capacity to

the load that is actually applied, this will indicate the reliability of that particular design. Mostcountries have imposed their own standards for factor of safety and these are enshrined in laws,

standards, and specifications that are derived by their own institutions. This is however aminimum that has to be included in the design and is no real indication of the reliability, which is

more concerned with absolute capacity to actual load ratios. Designs where the safety or

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reliability factor is the same as the design safety factor,as required by standards or laws laid down, will be

economical designs. Structures where the reliabilityfactor is much higher than the design safety factor would

be referred to as being over engineered and would

probably be structures that are very heavy and costlier.

Using the factor of safety in design to ensure structural

safety and structural reliability does not, however,guarantee the safety of a structure. This safety can only

come about when the standards of supervision ofconstruction and quality and strength of materials used are of the required standards assumed in

the design. The factor of safety can also get compromised when maintenance is lax and thestructure is allowed to deteriorate without the proper protection.

Reliability in Structures

Structural reliability is defined as the ability of a structure to perform its functions as envisagedby the designer. It also indicates the resistance to failure that the structure has and, in case of

failure, its ability to fail without any disastrous consequences. Reliability means that a structureexists without failure, but when design factors are manipulated to very large extents to

completely rule out failure, costs and feasibility can be limiting factors that influence a designer.All reliability factors have to take into consideration that a structure will only be under certain

conditions, and if conditions start existing that have not been considered, the structural reliabilityof a building can be in great danger.

Compared to other engineering disciplines, structural engineering has learnt a lot from mistakes.

It is rather unfortunate that most of these have been caused by failures that have been in thepublic arena and therefore widely publicized. While a number of such failures have been caused

by unforeseen natural or manmade disasters, the fact that they have taken place has led tostructural engineers having to deal with a lot of negative publicity.

The Future Is In Probabilistic Structural Design

Any design of a structure requires that the many uncertainties that are there in the loading,properties of the materials used, and other aspects of the geometry of a structure be taken into

account .This is normally done by assuming conservative values and factors of safety that allowsthe determination of the required design. While such a method of deterministic design will yield

a safe design that will be largely conservative, the results may produce structures that are heavyand costly. Structural engineering has a very heavy reliance on analysis and computation because

of the scale and uniqueness of such works. Allowable stress was the principle for structuraldesign and assumptions that structures behaved elastically.

The new field that tackles reliability in structures is being developed around a concept that the

uncertainties in strengths and loads can be modeled using the theories of probability. The basicnotion is summed up by the premise that structural failure occurs when the structural action is

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greater than the structural resistance. Modeling these two factors of action and resistance asrandom variables and the probability of the limit state that induces failure, can be computed

where the resistance becomes less than the structural action.

The required strength has to be less than the designed strength to ensure structural safety.

Required strength is estimated by performing structural analysis of a structure when it issubjected to the designed loads. The design strength is arrived at by using the principles ofstructural mechanics where the dimensions of the structure and the strengths of the materials

used play a part.

The use of probabilistic structural design has led to structures that are lighter and could therefore be classified as environmentally friendly, as they consume fewer materials and other scarce

resources.

References:

y Probability-Based Load Criteria for Structural Design

y Approximate Probabilistic Optimization Using Exact-Capacity-Approximate-Response-Distribution

By: Jayant R Row