ASD v LRFD

ASD v/s LRFD•Know the difference•What’s the underlying philosophy?•Where is it applied?•How to apply it?

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• ASDAllowable Stress DesignUsed to be called ‘Working Stress Design’The principle of ASD design format is to ensure that the equivalent stress induced due to combination of all contributing loads does not exceed the allowable stress for the material .

• LRFDLoad Resistance Factor DesignUsed to be called ‘Limit States Design’The principle of the LRFD design format is to ensure that the level of structural safety issuch that the design load on the pipeline does not exceed the design resistance of thepipeline except for a stated level of failure probability.

ASD• Uses service load stresses against an

Allowable Stress

• Provides a constant factor of safety for all designs regardless of load types

• Uses single safety factor independent of how and which load affects the failure the most

• Static analysis is acceptable for ASD

• Uses factored load forces against a maximum Strength

• Provides a higher factor of safety on the loads that are less well defined

• Allows proper, balanced statistical probabilities of failure

• Non-linear geometric analysis is required for LRFD

LRFD

ASD v/s LRFDMajor difference

ASDExample to understand the design methodology

Lets take the classic example of bar under tension in a universal testing machine (UTM)

Bar dimensions: Circular bar of φ 10mmC/s area, A=πφ2/4=78.54mm2 Material of bar:Steel X60 grade: SMYS=414 N/mm2 Load:Tension applied, F=2 Tons

F=2*9.81=19.62 kN Stress induced, σ=F/A=249 N/mm2 Allowable Stress, σall=(safety factor)*SMYS

σall=0.72*414=298.08 N/mm2

Since σ<σall the bar is safe

LRFDExample to understand the design methodology

Consider the same example of bar under tension in a universal testing machine (UTM) under same loading conditions

We know that SMYS of 414 N/mm2 may vary slightly with different bars (due to slight variations in metallurgy or imperfections), hence we apply a resistance factor of 0.9 just to be safe (this factor is also called ‘capacity reduction factor’)

Hence, Allowable resistance=0.9*414 = 372.6 N/mm2

Also a load factor is used to account for the uncertainty of exact load applied. For instance, lets apply a load factor of 1.2

Hence, Load applied, F=1.2*2 Tons = 2.4 TonsLimiting load is implied by introducing both resistance

factor and load factor into design

LRFD (contd…)

Example to understand the design methodology

Limiting load is implied by introducing both resistance factor and load factor into design

Hence the bar is safe if:1.2*F < (0.9*SMYS)*Area

Þ 1.2*F < 0.9*414*78.54Þ 1.2*F < 29.26 kN or 1.2*F < 2.98 Tons

Since, 1.2*F=2.4 Tons < 2.98 Tons

i.e., Applied load < Limiting load, the bar is safe

• In ASD you treat dead and live loads equally. • For example, if you have a case where your dead to live load is 1 to 1 (say 200 kips dead and

200 kips live) the safety factor would be exactly the same as if your dead to live ratio was 1 to 3 (say 100 kips dead and 300 kips live) given the same total load.

• LRFD recognizes the inherent unpredictability of loads and assigns a different factors for different loads. LRFD provides a higher factor of safety on the loads that are less well defined.

• Here, LRFD assigns much higher "factor of safety" to live loads (we increase them by 1.6), whereas it recognizes that dead loads are most likely much closer to what you calculate (we only multiply dead loads by 1.2).

At a LL/DL ratio < 3, ASD is more conservative, At a LL/DL ration = 3, it is a wash,

And at a LL/DL ratio > 3 LRFD is more conservativeYou cannot switch between the two philosophies in a given project!

More insight to ASD v/s LRFD Salient inclusions in LRFD

• Like discussed previously, LRFD assigns different load factors for different loads. For example, loads commonly acting on a member are:

• Dead Load, DL (such as self weight, constant reaction from supports)• Live Load, LL (such as environmental load)• Accidental Load, AL (such as pressure surge)• Earthquake Load, EL

• Hence each load carries a distinguished load factor• γDL

• γLL

• γAL

• γEL • Design load effect, Ld = DL*γDL + LL*γLL + AL*γAL + EL*γEL • For safe design, Design Load Effect < Design Resistance

Ld < Rd

More insight to ASD v/s LRFD (contd…)

Salient inclusions in LRFD

DNV-OS-F101, 2000 edition is based on LRFD methodConsequently the safe design follows the form:

Ld ≤ Rd

Where:Ld = LF ⋅ γ F ⋅ γC + LE ⋅ γE + LA ⋅ γA ⋅ γC

Rd=Rk(fk)/(γ SC ⋅ γm)

More specifically:Md = MF ⋅ γ F ⋅ γC + ME ⋅ γE + MA ⋅ γA ⋅ γC

ε d = ε F ⋅ γF ⋅ γC + εE ⋅ γE + εA ⋅ γA ⋅ γc

Δpd = γP (p⋅ ld – pe )

Sd = SF ⋅ γF ⋅ γC + SE ⋅ γE + SA ⋅ γA ⋅ γC

More insight to ASD v/s LRFD (contd…)

Salient inclusions in LRFD

Where suffix definitions are:F FunctionalE EnvironmentalA Accidental

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