Working Stress Method And Limit State Method 6z731s

For working stress approach, service loads are used in the whole design and the strength of material is not utilized in the full extent. In this method of design, stresses acting on structural are calculated based on elastic method and they are designed not to exceed certain allowable values. In fact, the whole structure during the lifespan may only experience loading stresses far below the ultimate state and that is the reason why this method is called working stress approach. Under such scenario, the most economical design can hardly be obtained by using working stress approach which is now commonly used in the design of temporary works. For limit state approach, for each material and load, a partial safety factor is assigned individually depending on the material properties and load properties. Therefore, each element of load and material properties is accurately assessed resulting in a more refined and accurate analysis of the structure. In this connection, the material strength can be utilized to its maximum value during its lifespan and loads can be assessed with reasonable probability of occurrence. Limit state approach is commonly used for the majority of reinforced concrete design because it ensures the utilization of material strength with the lowest construction cost input. The design of reinforced concrete structural involves the knowledge of loads, material properties and factor of safety. The working stress method is refered to as deterministic because it is presumed that loads, permissible stresses and factor of safety are known accurately. In physical situations, however, these parameters are not always known and the analysis must be based on predictions. The prediction of such parameters is not always known and the analysis must be based on predictions. The prediction of such parameters is normally based on experience or on field data. The design of a structural member depends on how closely an actual situation is predicted. Parameters that involve the element of prediction are refered to as non deterministic and there is no guarantee that they will actually occur. This forms the basis of limit state design. In the limit state design method these parameters are deterministic based on observations taken over a period of time. These parameters will thus be influenced by chance or random effect not fast at a single instant but throughout the entire period of time or the sequence of time that is being considered. Such a process is refered to as a stochastic process. In a rough sense a stochastic process is an phenomenon that varies to some degree unpredictably as time goes on. It is also refered to as a Random process. In the limit state design procedure stresses in an element are obtained from the design loads and compared with design strengths. In the working stress design method the stresses. The main distinction between the two methods lie in the fact that in the former, a member is considered in its limit state, whereas in the later, in its working state. The required more stringent conditions that loading conditions leading to collapse as well as partial safety factors be determined very carefully and accurately in the limit state design.

Working Stress Method 

The Stresses in an element is obtained from the working loads and compared with permissible stresses.

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The method follows linear stress-strain behaviour of both the materials.

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Modular ratio can be used to determine allowable stresses.

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Material capabilities are under estimated to large extent. Factor of safety are used in working stress method.

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The member is considered as working stress.

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Ultimate load carrying capacity cannot be predicted accurately.

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The main drawback of this method is that it results in an uneconomical section.

Limit State Method 

The stresses are obtained from design loads and compared with design strength.

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In this method, it follows linear strain relationship but not linear stress relationship (one of the major difference between the two methods of design).

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The ultimate stresses of materials itself are used as allowable stresses.

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The material capabilities are not under estimated as much as they are in working stress method. Partial safety factors are used in limit state method.

How is Working Stress Method (ASD) different from Limit state method (LRFD or LFD)? Assumptions, Advantages and Comparisons Limit state method (LRFD or LFD) “A limit state is a condition beyond which a structural system or a structural component ceases to fulfill the function for which it is designed”

Various limit states are Strength limit states: With respect to strength in shear, flexure, torsion, fatigue, bearing, settlement, bond or combined effects. Serviceability limit states: With respect to deflection & cracking. The appearance, durability and performance of the structure must not be affected by deflection & cracking, buckling, stability Special limit states: Damage or collapse in extreme earthquakes. Structural effects of fire, explosions, or vehicular collisions.

Design Assumptions and advantages of Limit state method 1.

Partial safety factor for material (γm) for yield and ultimate stress.

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Working loads are factored (increased) as per partial safely factor (γ f) causing Limit State of strength.

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The design strength is calculated dividing the characteristic strength further by the partial safety factor for the material (γm), where γm depends on the material and the limit state being considered.

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Post buckling and post yielding plays important role in estimating capacity of structural elements at Limit State.

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Deformations are evaluated at working loads.

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This is based on the behavior of structure at different limit states ensuring adequate safety against each limitation.

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Tensile strength carried by of concrete is zero.

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The stress block represents in a more realistic manner when the structure is at the collapsing stage (limit state of collapse) subjected to design loads. Does not obey Hook`s law

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It consider the variability not only in resistance but also in the effects of load. Concept of separate partial safety factors of loads of different combinations in the two limit state methods.

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Concept of separate partial safety factors of materials depending on their quality control during preparation. Thus, γ mfor concrete is 1.5 and the same for steel is 1.15. This is more logical than one arbitrary value in the name of safety factor.

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A structure designed by employing limit state method of collapse and checked for other limit states will ensure the strength and stability requirements at the collapse under the design loads and also deflection and cracking at the limit state of serviceability. This will help to achieve the structure with acceptable probabilities that the structure will not become unfit for the use for which it is intended.

Working stress method 1.

Since the specifications set limit on the stresses, it became working stress method

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Factor of safely for yield stress, allowable stresses are less than ‘f y’.

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Pure elastic approach for analysis of structures under working loads.

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Yielding or buckling never occurs at working loads

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This method is based on the condition that the stresses caused by service loads without load factors are not to exceed the allowable stresses which are taken as a fraction of the ultimate stresses of the materials, fc’ for concrete and fy for steel.

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It deals only with elastic behavior of member perfectly elastic at all stages of loading; Stress-strain relations obeyHooks law (linear)

working stress method 1.

Tensile stresses are taken by steel only.

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The Modular ratio is well defined value independent of time which is not true

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Results in larger compression steel percentage

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Working stress method does not give reasonable measure of strength, which is more fundamental measure of resistance than is allowable stress.

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Another drawback in working stress method is that safety is applied only to stress level. Loads are considered to be deterministic (without variation). For example, Permissible bending & direct compression are as fraction of crushing strength.

Yield Stress of a Material – Simple Explanation

Yielding of Steel Yield of a material is explained as the stress at which a material begins to deform irreversibly. Preceding the yield point, the material will deform elastically, meaning that it will return to its original shape when the applied stress is removed (i.e. no permanent, visible change in the shape of the material). Once the yield point is ed, however, some of the deformation will be permanent and non-reversible. Knowledge of the yield point is important when deg a component since it normally signifies an upper limit to the load that can be applied Not all materials have a well-defined yield region. In the absence of a distinct yield point, a .2% offset is used to obtain an approximate yield point. All deformation before the yield point is uniform throughout the narrow region of the material.

Yielding of Steel _ Stress Strain Curve