CONTINUOUS SLAB BRIDGE CONTINUOUS SLAB BRIDGE COMPARITIVE STUDY - - PowerPoint PPT Presentation

CONTINUOUS SLAB BRIDGE CONTINUOUS SLAB BRIDGE COMPARITIVE STUDY COMPARITIVE STUDY

LRFD vs. Standard Specifications LRFD vs. Standard Specifications

By: Teddy Antonios & Matt Blythe By: Teddy Antonios & Matt Blythe Ohio Department of Transportation Ohio Department of Transportation

Overview Overview

LRFD Changes the following significantly in the LRFD Changes the following significantly in the design of continuous slab superstructures: design of continuous slab superstructures:

– – Load Factors, Modifiers & Combinations Load Factors, Modifiers & Combinations – – Live Load Cases Live Load Cases – – Live Load Extreme Effects Live Load Extreme Effects – – Equivalent Deck Strip Widths Equivalent Deck Strip Widths – – Impact Impact – – Shrink./Temp. Steel Shrink./Temp. Steel – – Max./Min. Reinforcing Limits Max./Min. Reinforcing Limits – – Crack Control Crack Control – – Fatigue Fatigue – – Edge Beam Design Edge Beam Design

Code Comparison Code Comparison

LRFD Basics LRFD Basics

η ηi

i = Load Modifier

= Load Modifier γ γi

i = Load Factor

= Load Factor Q Qi

i = Load Effect

= Load Effect φ φ = Resistance Factor = Resistance Factor R Rn

n = Nominal Resistance

= Nominal Resistance

• Std. Spec. Basics
• Std. Spec. Basics

γ γ = Load Factor for All Loads = Load Factor for All Loads β βDL

DL = Dead Load Combination Effect

= Dead Load Combination Effect β βLL

LL = Live Load Combination Effect

= Live Load Combination Effect DL = Dead Loads DL = Dead Loads (LL+I) = Live Loads + Impact (LL+I) = Live Loads + Impact φ φ = Reduction Factor = Reduction Factor R Ru

u = Ultimate Resistance or Strength

= Ultimate Resistance or Strength

n i i i

R Q φ γ η ≤ Σ ( ) ( )

u LL DL

R I LL DL φ β β γ ≤ + Σ + Σ . .S F R LL DL

u

≤ Σ + Σ

Live Load Live Load -

• Std. Spec. 3.7.1
• Std. Spec. 3.7.1

OR OR OR OR

Live Load Live Load -

• LRFD 3.6.1.2

LRFD 3.6.1.2

+ + OR OR + +

Live Live -

• Load LRFD 3.6.1.3

Load LRFD 3.6.1.3

HL HL-

• 93 is the design load.

93 is the design load. The The extreme force effect extreme force effect shall be taken as the following: shall be taken as the following:

– – Design Truck combined with the Design Lane Load Design Truck combined with the Design Lane Load

• r
• r

– – Design Tandem combined with the Design Lane Load Design Tandem combined with the Design Lane Load

• r
• r

– – For negative moment, 90% of 2 Design Trucks combined with For negative moment, 90% of 2 Design Trucks combined with 90% of the Design Lane Load. 90% of the Design Lane Load.

• r
• r

– – For negative moment, 2 Design Tandems combined with the For negative moment, 2 Design Tandems combined with the Design Lane Load. Design Lane Load.

At the Discretion of the District. At the Discretion of the District.

90% of 2 Design Trucks 90% of 2 Design Trucks

Double Tandem (C3.6.1.3.1) Double Tandem (C3.6.1.3.1)

Extreme Effects Extreme Effects

LRFD 3.6.1.3.1: LRFD 3.6.1.3.1:

“Axles that do not contribute to the extreme force “Axles that do not contribute to the extreme force effect under consideration shall be neglected.” effect under consideration shall be neglected.”

– – Remove design truck/tandem axles from analysis that Remove design truck/tandem axles from analysis that reduce response under consideration. reduce response under consideration.

MAXIMUM POSITIVE MOMENT @ SPAN #1 MAXIMUM NEGATIVE SHEAR @ SPAN #1 INFLUENCE LINE INFLUENCE LINE

Extreme Effects (cont’d) Extreme Effects (cont’d)

LRFD C3.6.1.3.1: LRFD C3.6.1.3.1:

“Only those areas or parts of areas that contribute to “Only those areas or parts of areas that contribute to the same extreme being sought should be loaded.” the same extreme being sought should be loaded.”

– – Remove sections of design lane from analysis that Remove sections of design lane from analysis that reduce response under consideration. reduce response under consideration.

MAXIMUM POSITIVE MOMENT @ SPAN #1 MAXIMUM NEGATIVE SHEAR @ SPAN #1 INFLUENCE LINE INFLUENCE LINE

Dynamic Allowance (Impact) Dynamic Allowance (Impact)

LRFD 3.6.2 LRFD 3.6.2 IM = 33% for design IM = 33% for design truck & tandem. truck & tandem.

For All Limit States For All Limit States except Fatigue as per except Fatigue as per Table 3.6.2.1 Table 3.6.2.1-

• 1

1 IM IMFAT

FAT = 15%

= 15%

IM = 0% for lane IM = 0% for lane loading. loading.

For All Limit States as For All Limit States as per 3.6.1.2.4 per 3.6.1.2.4

Standard Spec. 3.8.2 Standard Spec. 3.8.2 IM = 50/(L+125) IM = 50/(L+125)

Maximum of 30% Maximum of 30% For All Limit States For All Limit States

Dead Load Dead Load

DL = DC & DW DL = DC & DW

– –DC = Dead load of component. DC = Dead load of component. – –DW = Dead load of wearing surface/utilities. DW = Dead load of wearing surface/utilities.

Structural Analyses & Evaluation Structural Analyses & Evaluation

AASHTO LRFD AASHTO LRFD AASHTO Std. Spec. AASHTO Std. Spec.

Computing # Of Design Lanes Computing # Of Design Lanes

LRFD 3.6.1.1.1 LRFD 3.6.1.1.1

– – Same as Std. Specs. 3.6.2 Same as Std. Specs. 3.6.2 – – N NL

L= Integer {(Bridge Width)/12}

= Integer {(Bridge Width)/12}

Bridges 20 to 24 feet wide shall be designed for 2 Bridges 20 to 24 feet wide shall be designed for 2 traffic lanes, each half the roadway width. traffic lanes, each half the roadway width. In cases where the traffic lanes are less than 12 In cases where the traffic lanes are less than 12 feet wide, N feet wide, NL

L is taken as the number of traffic lanes

is taken as the number of traffic lanes & the width of the design lane = the width of the & the width of the design lane = the width of the traffic lane traffic lane

Equivalent Strip Widths for Slab Equivalent Strip Widths for Slab-

• Type

Type Bridges (E) Bridges (E)

• Std. Spec. 3.24.3.2 (LFD)
• Std. Spec. 3.24.3.2 (LFD)

– – E = (4+0.06S) < 7.0 for Wheel loads E = (4+0.06S) < 7.0 for Wheel loads – – 2E 2E < 14 for Lane loads < 14 for Lane loads

Equivalent Strip Widths (cont’d) Equivalent Strip Widths (cont’d)

LRFD 4.6.2.3 LRFD 4.6.2.3

– – For Single Lane loaded: For Single Lane loaded: – – For more than one lane loaded For more than one lane loaded

1 1

. 5 . 10 W L E + =

L

N W W L E 12 44 . 1 . 84

1 1

≤ + =

Minimum Slab Depth for a Minimum Slab Depth for a Continuous Span Continuous Span

Same as Standard Specifications Same as Standard Specifications

– – LRFD Table 2.5.2.6.3 LRFD Table 2.5.2.6.3-

• 1

1 – – Std. Spec. Table 8.9.2

• Std. Spec. Table 8.9.2

' 54 . 30 ) 10 ( ≥ + S

Calculate Live Load Bending Calculate Live Load Bending Moment per Foot of Slab. Moment per Foot of Slab.

Design Truck/Tandem: Design Truck/Tandem: Design Lane: Design Lane:

( )

⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + = 100 1 IM E Moment Bending M IM

( )

E Moment Bending M IM =

Flexural Resistance LRFD 5.7.3.2 Flexural Resistance LRFD 5.7.3.2

Distribution Reinforcement Distribution Reinforcement

Same as Standard Specifications Same as Standard Specifications

– – LRFD 5.14.4.1 LRFD 5.14.4.1 – – Std. Spec. 3.24.10.2

• Std. Spec. 3.24.10.2

% 50 100 ≤ L

Shrinkage and Temperature Shrinkage and Temperature Reinforcement Reinforcement

LRFD 5.10.8 LRFD 5.10.8

• Std. Spec. 7.5.2.2
• Std. Spec. 7.5.2.2

⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ + ≥

y g s

F h b A A ) ( 2 3 . 1 ft in As / 125 .

2

= 60 . 11 . ≤ ≤

s

A

Maximum Reinforcement Maximum Reinforcement

LRFD 5.7.3.3.1 LRFD 5.7.3.3.1

– – 2006 Interim Specs 2006 Interim Specs eliminated this provision. eliminated this provision.

• Std. Specs. 8.16.3
• Std. Specs. 8.16.3

b

ρ ρ 4 3 ≤

Minimum Reinforcement Minimum Reinforcement

• Std. Specs. 8.17.1
• Std. Specs. 8.17.1

LRFD 5.7.3.3.2 LRFD 5.7.3.3.2

– – Same as Standard Same as Standard Specs with a Specs with a modification to the modification to the definition of y definition of yt

t

illustrated on next slide illustrated on next slide & f & fr

• r. (LRFD 5.4.2.6)

. (LRFD 5.4.2.6)

t g c r c cr n r cr

y I S f S M M M M = = = ≤ φ 2 . 1

c r

f f ' 37 . =

yt = Dist. from NA to the tensile face

– Composite Section

Minimum Reinforcement (cont’d.) Minimum Reinforcement (cont’d.)

• Std. Spec.
• Std. Spec.

LRFD LRFD yt = H/2

– Neglects the reinforcement

Service Limit State / Crack Control Service Limit State / Crack Control

LRFD 5.7.3.4 LRFD 5.7.3.4

• Std. Spec. 8.16.8.4
• Std. Spec. 8.16.8.4

– – f fact.

act.< f

< fs

s

– – Z = 130 kips severe Z = 130 kips severe exposure exposure

) ( 7 . 1

c c s

d h d − + = β

c act s e

d f s 2 700 − ≤ β γ

y c s

F A d Z f 6 .

3

≤ =

3

76 . A d f w

c s

β =

Fatigue Limit State Fatigue Limit State

LRFD 5.5.3 LRFD 5.5.3 γ γLL

LL = 0.75

= 0.75 –

– Table 3.4.1 Table 3.4.1-

• 1

1

IM 15% IM 15% –

– Table 3.6.2.1 Table 3.6.2.1-

• 1

1

Check fatigue in regions Check fatigue in regions

• f compressive stress if:
• f compressive stress if:
• Std. Spec. 8.16.8.3
• Std. Spec. 8.16.8.3

β β (L+I)

(L+I) = 1.0

= 1.0 –

– Table 3.22.1A Table 3.22.1A

Impact = 30% Impact = 30% –

– 3.8.2 3.8.2

W L ELane * 5 10 + =

( )

. 7 06 . 4 < + = S EWheel

( )

I LL DL

f f

+

< 2

Edge Beam Design Edge Beam Design

• Std. Spec. 3.24.8.3
• Std. Spec. 3.24.8.3

– – The edge beam of continuous The edge beam of continuous slabs should be designed to slabs should be designed to resist a live load moment of: resist a live load moment of:

P P20

20 = 16 kip

= 16 kip – – 3.24.3 3.24.3

( ) ( )

S P M

LL u 20

1 . 8 . =

Edge Beam Design Edge Beam Design

Bridge Standard Drawing CS Bridge Standard Drawing CS-

• 1

1-

• 03

03

– – Edge beam designed to resist required Std. Edge beam designed to resist required Std.

• Spec. moment.
• Spec. moment.

Edge Beam Design (cont’d) Edge Beam Design (cont’d)

LRFD 9.7.1.4 LRFD 9.7.1.4

– – Design an edge beam unless deck’s primary Design an edge beam unless deck’s primary direction is transverse or there is a direction is transverse or there is a continuous, composite barrier. continuous, composite barrier.

LRFD 4.6.2.1.4b LRFD 4.6.2.1.4b

– – Edge beams support 1 line of wheel loads Edge beams support 1 line of wheel loads and a portion of lane load if important. and a portion of lane load if important.

Edge Beam Design (cont’d.) Edge Beam Design (cont’d.)

Equivalent Edge Beam Strip Width Equivalent Edge Beam Strip Width

– – LRFD 4.6.2.1.4b LRFD 4.6.2.1.4b

" 72 , 2 4 " 12 ≤ ≤ + + =

• r

E E d E

EB EB

Shear Shear

LRFD 5.14.4.1 LRFD 5.14.4.1

– – Indicates that shear does Indicates that shear does not not need to be need to be checked in slab bridges. checked in slab bridges. – – Same as in Std. Specs. Same as in Std. Specs.

Conclusions Conclusions

LRFD Changes the following significantly in the LRFD Changes the following significantly in the design of continuous slab superstructures: design of continuous slab superstructures:

– – Load Factors, Modifiers & Combinations Load Factors, Modifiers & Combinations – – Live Load Cases Live Load Cases – – Live Load Extreme Effects Live Load Extreme Effects – – Equivalent Deck Strip Widths Equivalent Deck Strip Widths – – Impact Impact – – Shrink./Temp. Steel Shrink./Temp. Steel – – Max./Min. Reinforcing Limits Max./Min. Reinforcing Limits – – Crack Control Crack Control – – Fatigue Fatigue – – Edge Beam Design Edge Beam Design

CS CS-

• 1

1-

• 07

07

CS CS-

• 1

1-

• 07

07

CS CS-

• 1

1-

• 07

07

Why Standardize? Why Standardize?

References References

AASHTO LRFD Bridge Design Specifications, AASHTO LRFD Bridge Design Specifications, 2004 2004

– – Including up to the 2006 Interims Including up to the 2006 Interims

Standard Specifications for Highway Bridges, Standard Specifications for Highway Bridges, 2002 2002 Ohio Department of Transportation Bridge Ohio Department of Transportation Bridge Design Manual Design Manual

– – Including up to the 2006 Third Quarter Revisions Including up to the 2006 Third Quarter Revisions

Questions? Questions?

Thank you for your time. Thank you for your time.