Question

1. Provide a simple definition of structural design.

2. Give a description of both the LRFD and ASD design approaches. What is the fundamental

   difference between the methods?

3. Identify three sources of variation in the strength of a structure and its components.

4. Provide two examples of serviceability limit states.

Answer 1

Structural Design -

Structural design is defined as methodical investigation of the stability, strength and rigidity of the structures. In other words, Structural design is methodical investigation to get the economical specification of all the strcuctural members constituting structure to carry out the predicted load safely.

The main purpose of the structural design is to produce a structure capable of resisting all the applied loads without failure during the whole intended life of the structure.

Allowable Stress Design (ASD) - It is also known as Working Stress Design (WSD) method which is based on the principle that stresses developed in the structural members should not exceed a certain fraction of elastic limit.

This is old method of design which only considers elastic strength of material, so it limits the allowable stresses to a fraction of this limit (e.g. 40-50%). All loads are considered as service loads and no factor is applied to increase these services loads.

This method does not take into account the Plastic and Strain Hardening stages of material, therefore, it becomes overly conservative in certain situations. It produces unsafe results in other situations due to considering loads at service load values only.

Further serviceability limits are also not considered in ASD method, which may result in structures which though safe but do not fulfill their intended purpose.

Load and Resistance Factor Design (LRFD) method This method is based on the principle that strength i.e resistance of various materials is scaled down by some factors while the applied loads are scaled up by some factors thereby the structural elements are designed using reduced strength and increased loads.

The strength of materials considered for design is the ultimate strength, which results in utilization of elastic, plastic and strain hardening stages of material thereby giving economical and safe design consistently.

The factors by which strength is reduced depends on the confidence of predictability of strength of the material. Also load factors are more for those loads which are highly unpredictable than loads which can be more accurately predicted. Thus load factor for dead load is less that that for live load or wind load as dead load will not vary as much as live or wind loads.

LRFD method also considers serviceability limits like maximum allowable deflection, cracking etc. in addition to the strength design.

Thus we can say that LRFD method has a more rational approach as compared to ASD method and that is the reason why LRFD method has largely replaced ASD method in design of RCC and Steel Structures.

The fundamental difference between ASD and LFRD method is that Allowable Stress Design compares actual and allowable stresses while LRFD method compares required strength to actual strengths.

LRFD has higher available strength when directly compared to the ASD available strength. LRFD will result in stronger structures for more highly dynamic loads and ASD will result in stronger structures for less variable (more predicable) loads.

Sources of variation in the strength of a structure and its components

The three major sources of variation in strength of a structure and its components are Material, Manufacturing and Testing. There are many possible reasons for strength variability under each of these three categories of sources.

Variation due to Material source -

There is variation in strength of structure due to Variations in raw material variations i.e characteristics of cement, fine aggregate (silt, grading), coarse aggregate (dust/bond), admixtures, water content etc.

Variation due to Manufacturing source -

There is variation in strength of structure due to variations concrete manufacturing process, batch to batch variations i.e ingredient weights (water, cementitious, admixture), mixing, transporting, delivery time, temperature, workability, air content etc.

Variation due to Testing source -

here is variation in strength of structure due to test variations resulted from Sampling, specimen preparation, initial/fi nal curing of specimens, transporting, test procedures and equipment etc.

Examples of serviceability limit states -

Serviceability limit state design of structures consists of various factors such as durability, overall stability, fire resistance, deflection, cracking and excessive vibration.

Serviceability refers to the conditions under which a building is still considered useful. Should these limit states be exceeded, a structure that may still be structurally sound would nevertheless be considered unfit.

For example, a skyscraper could sway severely and cause the occupants to be sick, yet be perfectly sound structurally. This building is in no danger of collapsing, yet since it is obviously no longer fit for human occupation, it is considered to have exceeded its serviceability limit state.

Another example of this is deflection. A loaded cantilever beam will have a deflection at the free end (actual behavior) that must be kept less that an allowable deflection (allowable behavior).

Serviceability limit states tend to be less rigid requirements than strength based limit states since the safety of the structure is not in question. Serviceability limit states generally neither tend to put people's lives at risk nor do they risk property damage.