LRFD-Steel Design

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LRFD-Steel Design

• Dr.
• Ali I. Tayeh
• First Semester

2
Steel DesignDr. Ali I. Tayeh

• Chapter 2

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Design philosophies

• Structure member design means selection of a
  cross section that will be more safe and economic
  to resist the applied force
• Economic usually means minimum weight
• There are three different philosophies to design
  any structure
• Allowable stress design.
• Plastic design
• LRFD.
• Allowable stress design (elastic design
  working stress design)
• The member which selected should has cross
  sectional properties (area, moment of inertia)
  large enough to prevent the maximum stress from
  exceed the allowable

Ultimate stress Yield stress Allowable stress
Safety factor Safety factor Allowable stress
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Design philosophies

• Working stress
• Stress resulting from working load (service
  load).
• For any Designed member will stressed to no more
  than allowable stress when subjected to working
  load.
• Plastic Design
• Based on failure consideration conditions
• Member is selected by using the criterion that
  mean structure will fail at a load higher than
  the working load.
• The total load will be less than the failure
  loads by a factor of safety called load Factor.

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Design philosophies

• Design procedure
• Multiply the working load service load by the
  load factor to obtain the failure loads.
• Determine cross sectional proprieties to resist
  failure under these loads
• Select the lightest cross sectional properties
  that has the mentioned properties.

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Design Philosophies

• LRFD (Load and Resistance Factor Design)
• Failure condition is considered
• Load factor are applied to service load
• Resistance factor reduce the theoretical strength
• Factored load factored strength
• Load load Factor Resistance Resistance
  factor
• load Factor 1
• Resistance factor 1

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Load and Resistance Factors

• Factored load factored strength
• S ?i Qi Ø Rn
• Where
• Each effect have a different load factor
• Load factor Valve for a particular load effect
  will depend on combination of load under
  consideration.
• The load factors and load combinations are based
  on extensive statistical studies.

Load effect (force or moment) Q
Load factor ?
Resistance factor Ø
Nominal resistance or strength Rn
Design strength Ø Rn
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Load and Resistance Factors

• Load combinations
• 1.4(DF).
• 1.2(DFT)1.6(LH) 0.5(Lror S or R).
• 1.2D1.6(Lror S or R)(0.5L or 0.8W).
• 1.2D1.6W0.5L 0.5(Lror S or R).
• 1.2D1.0E0.5L0.25.
• 0.9D1.6W1.6H.
• 0.9D1.0E1.6H.

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Load and Resistance Factors

• Load combinations
• Where

Dead load D
Earthquake load E
Load due to fluid with well defined pressures and maximum heights. F
Load due to lateral earth pressure, ground water pressure or pressure due to bulk materials. H
Live load L
Roof live load Lr
Rain load R
Snow load S
Self strain force T
Wind load W
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Load and Resistance Factors

• Normally, fluid pressure F , earth pressure H,
  and self strain force T are not applicable to the
  design of structure steel members, so load
  combination become as
• 1.4(D).
• 1.2(D) 1.6(L) 0.5(Lror S or R).
• 1.2D 1.6(Lror S or R)(0.5L or 0.8W).
• 1.2D 1.6W0.5L 0.5(Lror S or R).
• 1.2D 1.0E0.5L0.25.
• 0.9D (1.6W or 1.0E).

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Load and Resistance Factors

• Example2.1
• Dead load ( D ) 109 kips compression.
• Floor live load ( L ) 46 kips compression.
• Roof live load ( Lr ) 19 kips compression.
• Snow ( S ) 20 kips compression.
• Find the controlling load combination and the
  corresponding factor load
• What is the required Nominal strength ? If the
  resistance factor Ø , equal 0.85

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Load and Resistance Factors

• Example2.1 (Cont.).
• the controlling load combination is that
  procedure largest factored load from the
  following load combination
• Combination 1
• 1.4(D).
• 1.4 109152.5 kips.
• Combination 2
• 1.2(D) 1.6(L) 0.5(Lror S or R).
• Because S larger than Lr we will use S instead of
  Lr
• 1.2(D) 1.6(L) 0.5(S)
• 1.2 109 1.6 20 0.5 46 214.4 kips

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Load and Resistance Factors

• Example2.1 (Cont.).
• Combination 3
• 1.2D 1.6(Lror S or R)(0.5L or 0.8W).
• 1.2D 1.6(S )(0.5L)
• 1.2 109 1.6 200.5 46 185.5 kips
• Combination 4
• 1.2D 1.6W0.5L 0.5(Lror S or R)
• 1.2D 0.5L 0.5( S )
• 1.2 109 0.5 46 0.5 20
• Smaller than combination 3

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Load and Resistance Factors

• Example2.1 (Cont.).
• Combination 5
• 1.2D 1.0E0.5L0.25.
• Smaller than combination 4
• Combination 6
• 0.9D (1.6W or 1.0E).
• Note that dead load was multiplied to 0.9 this
  was reduce it , so this case is samaller than any
  comination.
• Answer
• Combination 2 controls, and factored load 214.4
  kips

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Load and Resistance Factors

• Example2.1 (Cont.).
• S ? Q Ø Rn
• S ? Q214 kips , Ø 0.85 ?Rn (214 0.85
  252.2kips)
• Rn 252.2kips
• Answer.

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Manual of Steel Construction

• Part 1 , part 2, part 3 , part 4 , .. Part 17
• See Text Book.

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Introduction -Steel Design

• Home work
• 2.3-1.
• 2.3-2.
• Page 33 . Text book
• End