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Analysis and Live Load Ratings of Illinois Bulletin Slab (IBS)

Bridges24th Annual Bridge Design Workshop

Kansas State University

Mike Briggs, PE, SEHNTB Corporation - Kansas City, MO

KDOT Concrete Ratings Project:

Objectives

To gain familiarity with:• IBS Bridges and Their Use in Kansas• Refined Analysis of IBS Bridges• Structural modeling• Implementation in Bentley STAAD.Pro

• Rating of IBS Bridges• Simplified analysis• Implementation in AASHTOWare BrR

Overview

• IBS Bridges• Description, development and history

• 1990s Field Investigations

• Current Study• Refined analysis• Simplified rating method

• Results and Conclusions

Example IBS

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Illinois Bulletin Slab (IBS) Bridges

Description:

• Reinforced Concrete Slab• Short Span Length• Integral Edge Curbs• Stiffen the slab• Reduce deck thickness• Economize reinforcement

• Limited Width (Two Lanes)Example Edge Curbs

Illinois Bulletin Slab (IBS) Bridges

Development:

• University of Illinois “Engineering Experiment Station”• Bulletin No. 315 (1939)• Bulletin No. 346 (1943)

• Semi-Empirical Design Method• Simple-Span Only

Bulletin 346 Cover

Illinois Bulletin Slab (IBS) Bridges

Use In Kansas:

• State Highway Commission Adapted to Make Continuous• “Go-To” Type in 1940s-1950s• 73 Bridges (43 Counties)• Continuous up to 8 spans• Only two are simple-span

Example IBS

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Illinois Bulletin Slab (IBS) Bridges

Use In Kansas:

• Span Length = 22-45 feet• Widths = 26-33 feet• 1960s widening up to 46 feet Example IBS - Elevation

Example IBS Widening - Section Example IBS Widening – T-Beam

1990s Field Investigations

Scope:

• Develop IBS Ratings – 113 in service• 6 Bridges Studied• Inspect, instrument, and load test (H/HS)

Field InspectionLoad Test Vehicles

1990s Field Investigations

Scope:

• 6 Bridges Studied• Refined structural analysis• Integrated shells, beams, support springs• Stiffness calibrated with measured results

Example Refined Analysis Model

Example Calibration Data

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Edge L-Beam - Section

1990s Field Investigations

Simplified Rating Method:

• Line Analysis• Edge “L” beams• Center slab

• Load Distribution from Refined Analyses• Spreadsheet Rating Tool• 1998 Review at KSU

Load Distribution Plots

Current StudyRationale:

• Specialized Hauling Vehicle (SHV) Ratings per FHWA

Not supported, but…• Model in AASHTOWare BrR• Add into statewide database• Integrate K-TRIPS permitting• Easily consider future vehicles

Example SHV

Current Study

Scope:

• Develop Methodology• Analyze/Load Rate• Deliver BrR Models

• 73 IBS Bridges• 29 “Others”

SHV Axle Configurations

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Current Study

Refined Analysis – Rationale:

• 1990s Study Limitations• SHVs not considered• Limited scope (6 bridges)• Calibrated results only

• Site-specific: soil, site, and structural condition• Vehicle-specific: H and HS

• Increase Confidence, Understanding, and Extend Applicability

Current Study

Refined Analysis – Scope:

• K42; Kingman Co.• 32’-40’-40’-40’-32’• “New-Build” Assumptions• Uncalibrated plan dimensions

Plan

Half Section

Current Study

Refined Analysis – Model Construction:

• Mixed-Modeling: Frames and“Thick” Shells• Replicate 1990s methodology• Section properties based on plans

• Discretization• Coarser mesh than 1990s model• Discretize L-beam/center slab boundary

Node Map - Plan

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Current StudyRefined Analysis – Model Construction:

• Boundary Conditions• Fixed supports at column bases• Abutment restraint per 1990s study

• Loads• Self-weight dead load• Five lane positions• H20, HS20, T170, SU4, SU7

Node Map - Section

Current StudyRefined Analysis – Model Construction:

• STAAD Live Load Positioning• BEAVA influence surface – not used• Shell element loading – not used• Load Generation – frame elements only

• “Dummy” Frame Elements• Placed along each line of nodes• Very low stiffness• Live load applied to frames

• Transfers into shared joints

Example Influence Surface

Node Map - Section

Current Study

Refined Analysis – Model Construction:

• Load Generation• One command per lane• 4-foot increments (0.1*L)• Vehicles completely enter and exit bridge• Each load case is one vehicle position• Coincident forces for

discrete members

SU7 – Lane 1

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Current Study

Refined Analysis – Output:

Model

Dead Load

Live Load

Current Study

Refined Analysis – Output:

• Critical Sections• (-) at face of integral pier capbeams• (+) at 0.4L (end spans) and 0.5L (all spans)

Elevation

(+) (+) (+)

(-)(-) (-)(-) Symmetric

(+)

Current Study

Refined Analysis – Output:

• Store Forces at Critical Sections• Frame ends and shell corners• Load case = coincident forces

• Integrate Elements Forces• L-beams and Center Slab• Translate to member centroid• Combine moments and eccentric axial

Force Integration – H20

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Current Study

Refined Analysis – Results:

• VBA Macro Automation• Update inputs• Recalculate• Stored outputs in summary

• Distribution Factors (DF)• Member/total at critical section• Valid at peak member moment

Partial Results Summary – H20

Current StudyRefined Analysis – Results:

• Distribution Factors (DFs)• Generally confirmed 1990s results

• Insensitive to vehicle configuration

• “New build” modeling estimates higher center slab moments

DLDF – Calibration vs. New Build

LLDF – Vehicle Configuration

DEAD 1990 s 1990 s STAADLOAD General 0 4 8 -0 4 4 0 4 8 -0 4 4

L-Beam+ 0 .4 2 0 .4 1 0 .35L-Beam- 0 .38 0 .39 0 .35

Ct Slab+ 0 .18 0 .18 0 .31Ct Slab- 0 .22 0 .22 0 .30

Current Study

Simplified Rating Method –Model Construction:

• AASHTOWare BrR• Line girder superstructure

• T-Beam Member Type• Inverted L-Beam• Wide “web” is unconservative for

shear, but does not controlT-Beam Input

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Current Study

Simplified Rating Method –Model Construction:

• Re-Define Self-Weight• Member alternative definition• Remove tributary self-weight• Apply as line load

Self-Weight Input

Current Study

Simplified Rating Method –Model Construction:

• Substructure Restraint• Add springs

• Control Options• Omit support point POIs

Control Options

Support Springs

Results and Conclusions

• Refined Analysis• Confirmed 1990s results• Expanded applicability of DFs• Bentley STAAD.Pro appropriate for advanced modeling

• Simplified Rating Method• Significant time-savings• DFs applicable within scope of refined analyses• Successful T-beam implementation in AASHTOWare BrR

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Questions?

Thanks to: