Structural Design Philosophies

Purpose of Structural Design is to design Structures which is economical, safer & serviceable for a specified lifetime.

The three(3) major Design Philosophies used while designing of RCC, Steel or any type of Civil Engineering Structures are discussed here:
  1. ASD (WSD or WSM)
  2. ULM (USD)
  3. LRFD (LSD or LSM)

(1) ASD (WSD)

                   Allowable Stress Design (ASD) also called Working Stress Design/method (WSD) is basically Allowable Strength Design. In this method we apply a Factor of Safety (FoS) on ultimate strength of the material as whole and compare the results with the actual load that actually to be applied during the life span of the structure, if actual load < allowable load then the design is OK. This method only satisfy the serviceability of the structure or Serviceability Limit State (SLS).

Drawbacks

  • ASD method uses only the elastic region of stress-strain curve of materials due to which most of the strength acts as a reserve.
  • ASD gives an uneconomical dimensions of the structural element.
  • ASD doesn’t show any sign about collapse of structure, it means doesn’t satisfy the Ultimate Limit State.
  • ASD uses only material strength reduction factors in the form of FoS: Which are Given below:
  1. Factor of Safety (FoS) for Concrete= 3
  2. Factor of Safety (FoS) for Steel= 1.78
Stress-Strain Curve for Concrete

(2) ULM (USD 0r Load factor)

Ultimate Load Method (ULM) or Ultimate Strength Design (USD) or Load Factor is the design philosophy of design structures in which load increasing factor is used while calculating different loads combinations and these factors generally (>1) to increase loading so is to adjust the uncertainties occurring in load applications. Some Load Combination According to ASCE and UBC-97 are Listed here:

ASCE Load Combinations
  • 1.4D
  • 1.2D + 1.6L + 0.5(Lr or S or R)
  • 1.2D + 1.6(Lr or S or R) + (L or 0.5W)
  • 1.2D + 1.0W + L+0.5(Lr or S or R)
  • 1.2D + 1.0E + L +0.2S
  • 0.9D + 1.0W
  • 0.9D + 1.0E
UBC-97 Load Combinations
  • 1.4D                                                       
  • 1.2D + 1.6L +0.5 (Lr or S)           
  • 1.2D +1.6 (Lr or S) + (f1L or 0.8W)       
  • 1.2 D + 1.3W + f1L + 0.5 (Lr or S)     
  • 1.2 D + 1.0E + (f1L + f2S)                   
  • 0.9D ± (1.0E or 1.3W) 

Load factor is the ratio of ultimate strength to the service loads

The ULM makes it possible to consider the effects of different loading acting simultaneously thus solving the shortcomings of WSM (WSD). As the ultimate strength of the material is considered we will get much slender/thinner sections for different elements of the structures e.g columns, beams etc. In this method we use the non-linear method of stress-strain curves for concrete and steel.

Stress-Stain Curve for Steel Rebar

Drawbacks

  • This method doesn’t satisfy the Serviceability Limit State (SLS).
  • This method gives very thin sections of members which is vulnerable to severe cracks and damaged.
  • This method only consider load factors while ignoring the strength of materials.

(3) LRFD (LSD or LSM)

                     Load and Resistance Factor Design (LRFD) or Limit State Method/Design (LSM or LSD) is the most advanced method while design any civil engineering structures. It considered both factored loads and material strength reduction factors partially.

There are Two Limit States which is satisfied in LRFD (LSM) Method:

  1. Ultimate Limit State: It considers strength, overturning, fatigue, sliding etc.
  2. Serviceability Limit State: It Considers Cracks width, deflection, vibration etc.

  This method use to multiply partial safety factors for required safety at ultimate load and serviceability at working load.

Partial Safety Factor for Materials:

The strength of concrete in actual structure is taken as (0.67*characteristic strength), i.e 0.67fck. The partial safety factor for (Ultimate limit state) for concrete is 1.5 and that for steel is 1.15. The value for concrete higher because it has more variability and uncertainties compared to steel. The partial safety factor ( for serviceability limit state) for concrete and that for steel is taken is 1. This is taken unity as we are interested in estimating the actual deflections and crack widths during service loads.

Partial Safety Factor for Loads:

Various load Combinations are used and it may be different in different design codes.

For Ultimate Limit States:

  • UL= 1.5(DL+LL)
  • UL=1.5(DL+QL) or 0.9DL+1.5QL (QL: Earthquake/Wind Load)
  • UL=1.2(DL+LL+QL) (1.2 because the probability of three loads reaching its peak together are less)

For Serviceability Limit States:

  • SL= 1.0 (DL+LL)
  • SL=1.0 (DL+QL)
  • SL=1.0DL+0.8LL+0.8QL

The load factor is taken as 0.8 in the third case as the probability of wind load or earthquake load acting with the peak of live load is less. For all other cases we consider the safety factor is 1.0 because we consider the serviceability of structure here.

So Why LRFD/LSM Design Philosophy is more reliable?
  •  LRFD/LSM is a more reliable and statistical based method for predicting both loads and material strengths.
  • Whereas the allowable stress safety factors were based on engineering judgement and past experiences.
  • This method gives you an economical, safer & serviceable structural elements/members.
  • The shortcomings in the previous methods were addressed and rectified here on a more rational basis.

Watch the video given below, this article has been discussed briefly:

========Thank You=======

Author: Engineer-Nisar

Civil Engineer

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