CE311 Lesson 7:Load Path & Trib Area III – ASD vs LRFD Fall 2018

Fact: the number one cause of structural collapses in the contiguous United States is $1 Bud Light Night1

LESSON 7: LOAD PATH & TRIBUTARY AREA III – ASD VS LRFDSeptember 10, 2018

LESSON OBJECTIVES1. Describe the differences between the ASD and LRFD methods (e.g., applied loads versus factored loads, allowable

strengths versus design strengths).2. Compute ASD applied forces and moments in a simple-braced building, using tributary areas and widths.3. Compute LRFD factored forces and moments in a simple-braced building, using tributary areas and widths.4. Assess the adequacy of members using the ASD or LRFD methods, given the member’s nominal resistance.

READING: Several pages on the website version of this handout Optional textbook (Segui) – Chapter 2

LAB THIS WEEK: Downstairs in the Structures Lab . Proper lab clothing is essential (long pants, shoes that fully cover all skin on the

feet, short-sleeve shirt, hair tied back, if applicable). Be prepared to get filthy, as you will learn to MIG weld. Wear clothing that you don’t mind ruining. Bring headphones or earbuds, as you will be following video instructions as you conduct computer models, when

not welding.LOAD COMBINATION:A load combination is a combination of loads that represents a plausible event. For example, the blizzard of the century is an event that is likely to control the roof beams of a building. Implausible means that it is unlikely that the biggest live load that can ever occur (probably a huge party) will occur on the same day as the biggest blizzard. Consequently, you must understand why the live load is not 100% when the snow is 100%, etc. Every member in the structure may be controlled by a different event. For example, braces are usually controlled by wind or earthquake events, floor beams are usually controlled by live load events, and roof beams are usually controlled by snow events.

IN-CLASS – EXAMPLE BUILDING:

1 This is utterly untrue. Starting a sentence with the word “Fact” does not make it factual.

ASD Load Combinations: LRFD Load Combinations:

20’, typ.

30’, typ.

Example Building

1

2

3

4

A B C D Given:6” thick slab (=150 lb/ft3)Fill beams: W14x30Girders: W16x50Columns: W12x40 (20’-long)

Design Live Load: 100psfLive Load Reduction Factor is applicable

Roof Live Load Lr = 20 psfRoof Snow Load S = 30 psf

W12x40 column buckling strength:Pn = 192 kips for a 20’-long column

LRFD: = 0.9 for column buckling (ductile)ASD = 1.67 for column buckling (ductile)

CE311 Lesson 7:Load Path & Trib Area III – ASD vs LRFD Fall 2018

Homework Assignment (Due Wednesday. Standard Homework Assignment – Presentation counts)0. Study Segui example 2.1. Do not hand in.THE FOLLOWING PROBLEMS REFER TO THE BUILDING ON THE NEXT PAGE.1. Compute the controlling ASD applied load P on the column at grid location B2 at the ground level, then determine if

the column is safe, per ASD.2. Compute the controlling LRFD factored load Pu on the column at grid location B2 at the ground level, then

determine if the column is safe, per LRFD.3. Compute the ASD controlling applied moment M on a typical interior 2ND FLOOR Long Girder. If the nominal

moment strength of a W21x93 is Mn = 920 kip-ft, determine if this 2nd Floor Girder is adequate, per ASD.4. Compute the ASD controlling applied moment M on a typical interior ROOF Long Girder. If the nominal moment

strength of a W21x93 is Mn = 920 kip-ft, determine if this ROOF Girder is adequate, per ASD.5. Compute the LRFD controlling Factored moment Mu on a typical interior ROOF Long Girder. If the nominal

moment strength of a W21x93 is Mn = 920 kip-ft, determine if this ROOF Girder is adequate, per LRFD.NOTE THE FACT THAT THE ROOF AND 2ND FLOOR BEAMS ARE CONTROLLED BY DIFFERENT LOAD EVENTS (LOAD COMBINATIONS). ALSO NOTE THAT LRFD AND ASD GIVE DIFFERENT CONCLUSIONS BECAUSE LRFD FEARS LIVE AND SNOW LOADS BUT ASD DOES NOT

20’

30’, typ.

1

2

3

4

A B CGiven:4” thick slab (=150 lb/ft3)Fill beams: W14x30Short Girders: W16x35Long Girders: W21x57All Columns: W12x40Each story height is 12’

40’

CE311 Lesson 7:Load Path & Trib Area III – ASD vs LRFD Fall 2018

30’

30’

30’ 20’

40’

20’

20’

Given:2nd Floor 4” thick slab (=150 lb/ft3)Roof does not have a slab. Instead, it has roofing material on corrugated metal deck, which have a dead load pressure of 20 psf. This 20 psf must be added to the dead loads of beams and girders, as necessary.

All Fill beams: W14x30All Short Girders: W16x36All Long Girders: W21x93All Columns: W12x40Each story height is 20’

Design Live Load for 2nd flr: 100 psfLive Load Reduction Factor is applicable

Roof Snow Load: 60 psfRoof Live Load Lr = 20 psf

W12x40 column buckling strength:Rn = 192 kips for a 20’-long column

LRFD: = 0.9 for column buckling (ductile)ASD: = 1.67 for column buckling (ductile)

Ground

2nd flr.

Roof

CE311 Lesson 7:Load Path & Trib Area III – ASD vs LRFD Fall 2018

CE311 Lesson 7:Load Path & Trib Area III – ASD vs LRFD Fall 2018

CE311 Lesson 7:Load Path & Trib Area III – ASD vs LRFD Fall 2018

CE311 Lesson 7:Load Path & Trib Area III – ASD vs LRFD Fall 2018

CE311 Lesson 7:Load Path & Trib Area III – ASD vs LRFD Fall 2018