NSBA TG10 PPT NCHRP 20-07-355 05-06-15 v5

OVERVIEW OF RESEARCH FINDINGS VIA CHALLENING CONSTRUCITON CASES

FROM NCHRP 20-07/TASK 355

(GUIDELINES FOR RELIABLE FIT-UP OF STEEL I-GIRDER BRIDGES)

Thanh Nguyen & Don White

Georgia Institute of Technology

AASHTO/NSBA TG 10 Meeting

Raleigh, NC

May 6 2015

NCHRP 20-07/TASK 355 PROBLEM STATEMENT

• Potential construction problems:

Difficult fit-up of girders and cross-frames

Out-of-plumb girder webs

Unaccounted locked-in stresses in the girders and cross-frames

Bearings rotated beyond design limits

Out-of-alignment of deck joints & barrier rails at abutments

• Overarching factors influencing the bridge responses:

Longer spans

Tighter curvature and/or

Sharper skew

• Practices that can be used to control/limit problems:

Framing arrangements

Cross-frame detailing

Erection sequencing & other techniques/practices 2

NCHRP 20-07/TASK 355 RESEARCH OBJECTIVES

• Improved design, detailing and erection guidelines to ensure reliable fit-up of skewed &/or curved steel I-girder bridges

• Clear understanding of the implications of

Framing arrangements,

Cross-frame detailing methods, &

Erection procedures

on

Ease of fit-up during erection of the steel,

Achievement of targeted constructed geometry, &

Limiting significant additive locked-in stresses & accounting for beneficial subtractive locked-in stresses in the cross-frames & girders

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RESEARCH TEAM

• Don White, Georgia Tech (PI)

• Thanh Nguyen, Georgia Tech

• Domenic Coletti, HDR Engineering

• Brandon Chavel, HDR Engineering

• Calvin Boring, Brayman Construction Corporation

• Mike Grubb, M.A. Grubb & Associates, LLC

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PRESENTATION OUTLINE

• Highlight several findings & illustrate with two extreme bridge cases

Curved radially-supported case: Ford City Veterans Bridge, PA

Large span length

Tight curvature

Erection constraints

Critical choice of cross-frame detailing method

Straight-skewed case: Parametric Bridge NISSS54 (NCHRP 12-79)

Large span length

Sharp skew

Critical choice of framing arrangement & erection scheme

• Simulation results based on

As-built scheme for Ford City Bridge (results match closely with field observations)

Alternative framing arrangement & erection scheme for NISSS54 that provide low fit-up forces 5

FORD CITY BRIDGE

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• Significant deflections in curved span

Second largest curved span length of cases studied in this research, Ls = 322’

Largest Ls/R = 0.78

Largest curved span Ls/w = 8.1

• Site constraints

Limits on location of shoring towers

• Curved span involved drop-in segments

• Detailing method

Essentially Steel Dead Load Fit (SDLF), by mistake

FORD CITY BRIDGE

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• Steel DL & Total DL cambers from 3D FEA, the bridge deflects as a system

• TDL camber determined neglecting staged deck placement effects

• SDL and Concrete DL defl. approx. equal; bridge detailed essentially for SDLF

• SDL & TDL cambers are central to:

Setting the cross-frame drops for fabrication

Calculation of SDLF & TDLF detailing effects

FORD CITY BRIDGE – COMPLETED STRUCTURE

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• SDLF & TDLF reduce vertical deflections of all the girders in curved span

SDLF & TDLF effects twist the girders opposite from their DL twist rotations

Twist rotations and vertical deflections are coupled

• SDLF & TDLF significantly influence the final elevations

SDLF: G1 max 2.5” higher than zero elevation line (matches field observations)

TDLF: G1 max 4.5” higher


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• TDLF enforces approximately plumb girder webs

• Highest layovers near mid-spans

6” max in curved span with NLF (girder webs 14 ft deep)

Layovers are not a structural concern

• Zero layovers at bearing locations


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• Negligible changes in fb due to SDLF & TDLF

• Significant changes in f due to SDLF & TDLF

Girders show overall lateral bending for NLF due to the twisting of the

narrow bridge cross-section

This effect is reduced by SDLF & TDLF

The f values are relatively small


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Total DL cross-frame stress contour (ksi), NLF detailing (all CF areas = 8.52 in2)

CFs with large stresses

in curved span


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Total DL cross-frame stress contour for the curved span (ksi), NLF detailing


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Detailing

Method

Average CF

Force

Percentage

Increase

Relative to

NLF

NLF 32 NA

SDLF 40 23 %

TDLF 46 44 %

Table 1: Average Total DL CF Forces in Curved Span(kips)

FORD CITY BRIDGE – STEEL ERECTION

14As-built erection stage involving drop-in segments


15As-built erection stage involving drop-in segments

G1G2G3G4


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• As-built erection simulation for Ford City Bridge

Including the effects of:

Holding crane

Lifting crane & spreader beam with optimum pick points

Shoring towers

Detailing methods

Cross-frames installed sequentially after girder segments are installed

Separate analyses for fit-up at top and bottom CF connections

For system stability & geometry control, all the CFs are installed & the connections are fully tightened at each stage of the erection

Crane and tower elevations:

No-load elevations

These elevations can be adjusted to achieve some reduction in the fit-up forces (Iteration on elevations in simulations is outside our scope)


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Table 2: Max fit-up forces (kips)

• Fit-up force: forces developed in the connection after the installation

• Typical come-along capacity = 20 kips

• Fit-up force larger than 40 kips is taken as difficult

DetailingMethod Fmax (kip) Level of Difficulty

NLF 38 Moderate

SDLF 86 Difficult

TDLF 130 Very difficult


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Table 3: Girder splice fit-up moments (kip*ft) and equivalent flange fit-up forces (kip)

DetailingMethod

M(kip*ft)Equivalent Flange

Force (kip)Level of Difficulty

NLF 315 22 Relatively easy

SDLF 7566 540 Very difficult

TDLF 11267 804 Extremely difficult

RESEARCH FINDINGS – CURVED RADIALLY-SUPPORTED BRIDGES

More difficult fit-up of girders and cross-frames in curved radially-supported bridges in cases involving:

Large span lengths

High Ls/R and/or Ls/w

Erection schemes with:

Drop-in segments

Site and/or job constraints that place limitations on the number and/or location of shoring towers and cranes

Erection from inside to the outside of the curve

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More difficult fit-up of girders and cross-frames in curved radially-supported bridges in cases involving:

Use of TDLF

Use of SDLF in some extreme cases

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Curved Bridges with Radial or Skewed Supports

Recommended Acceptable Avoid

Span lengths greater than 250’ +/-

and L/R > 0.1 +/-NLF SDLF TDLF

All other cases SDLF NLF TDLF

Table 4: NSBA Guide Document Recommendation on Fit Conditions for Curved Bridges


• Effects of SDLF & TDLF on the completed structure

Increase in elevation profiles

Approximately plumb webs under corresponding load condition

Negligible changes in fb; Significant changes in f Average CF forces are increased; Max CF forces are increased in

most cases (influence on distribution of CF forces will be illustrated for a range of bridges in our final reporting )

A simple “adjustment factor” can be used to account for the additive effects of SDLF and TDLF on CF forces and f values

For extreme cases, detailing effects should be considered in the analysis of the structure to predict CF forces & f values

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• In a large number of cases with proper erection schemes and use of NLF or SDLF, cross-frame and girder fit-up forces are very manageable

• In extreme cases, all the CFs should be installed in sequence & the connections should be fully tightened at each stage of the erection

• For cases with potential high fit-up difficulty, the erection engineer may consider analyzing the erection sequence to predict the fit-up forces including the effects of:

Placement of cranes and shoring towers, etc.

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STRAIGHT SKEWED BRIDGE: NISSS54

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Framing Plan 1 (Original Design)

• Parallel skew, large Ls = 300 ft. & high Is = 0.68

• Small stagger between CFs, large number of CFs

• Significant nuisance transverse stiffness

• Offsets (first intermediate CF to the bearing line) smaller than 1.5 x girder depth

STRAIGHT SKEWED BRIDGE: NISSS54

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Framing Plan 2 (Alternative Design)

• Offsets at least 1.5 x girder depth

• CFs are equally spaced except at the offsets adjacent to the bearing lines. Every other CF is left out within the interior of bridge plan. This reduced the number of CFs significantly & increased stagger distances

• Low nuisance transverse stiffness

NISSS54 – COMPLETED STRUCTURE

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Detailing

Method

Framing Plan 1

(Original)

Framing Plan 2

(Alternative)

NLF 354.0 58.5

SDLF 181.9 31.2

TDLF 18.1 8.8

Table 5: Comparison of Max Total DL Cross-Frame Forces (kips)

between Framing Plans 1 and 2*

*Results shown are based on line girder analysis cambers


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• G1 is a fascia girder. G5 is the innermost interior girder

• With the use of line girder analysis cambers, the desired elevations are obtained

only for the targeted DL condition


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• NLF and SDLF give high layovers at the skewed bearings, placing higher

rotation demand on the bearings

This effect can be offset by using beveled sole plates

• TDLF effectively enforces plumb webs


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• Measurable changes in fb due to SDLF/TDLF effects for this bridge

Due to changes in vertical forces transferred from CFs to the girders

These changes are not significant in the majority of cases

• TDLF gives close to zero f under TDL

NISSS54 – STEEL ERECTION

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Shoring Tower

Field Splice

Lifting Points

G1G2

• Large span length requires a field splice & shoring tower

• Shoring tower and lifting crane are set at SDL elevations

• Only a few CFs are installed for stability before the field splice is made

• The rest of the CFs are installed after the splice is made so that all the steel is

deflected close to the SDL elevation profile

NISSS54 – STEEL ERECTION

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Table 6: Max fit-up forces (kips)

• NLF is not considered

Fit-up difficulty (short of substantial shoring of the span)

High bearing rotation demands & girder layovers

• Typical come-along capacity = 20 kips

• Fit-up force larger than 40 kips is taken as difficult

DetailingMethod

Fmax (kip) Level of Difficulty

SDLF 8.4 Relatively easy

TDLF 47.9 Somewhat difficult

RESEARCH FINDINGS FOR STRAIGHT SKEWED BRIDGES

Difficult fit-up of cross-frames in straight skewed bridges when

• Large span length

• High skew index

• Use of TDLF

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Square Bridges and Skewed Bridges up to 20 deg +/- Skew


Any span length Any None

Skewed Bridges with Skew > 20 deg +/- and Is ≤ 0.30 +/-


Any span length TDLF or SDLF NLF

Skewed Bridges with Skew > 20 deg +/- and Is > 0.30 +/-


Span lengths up to 200’ +/- SDLF TDLF NLF

Span lengths greater than 200’ +/- SDLF TDLF & NLF

Table 7: NSBA Guide Recommendation on Fit Cond. for Straight Bridges

RESEARCH FINDINGS FOR STRAIGHT SKEWED BRIDGES

• Use of NLF is not recommended for bridges w/ sharp skew

• In almost all cases with proper erection schemes and use of SDLF, cross-frame fit-up forces are relatively low

• Alternative framing arrangement reduces the number of cross-frames as well as cross-frame forces

• Effects of SDLF & TDLF in the final structure

DLF detailing with line girder cambers gives correct elevations ONLY for the targeted steel or total DL condition

Effectively plumb webs under corresponding load condition

For extreme cases, measurable changes in fb . TDLF gives close to zero funder TDL

Total DL CF forces are nearly offset by TDLF

Engineers can use a reduction factor to account for the beneficial subtractive effects of SDLF and TDLF on CF forces and fvalues 32

Thank you!

Questions?

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NSBA TG10 PPT NCHRP 20-07-355 05-06-15 v5

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