what is a curved girder · 2008. 12. 7. · effect of crossframe spacing compare lateral moments...

Development of an Appropriate Framing Plan

What is a Curved Girder ?

What is a Curved What is a Curved Girder ?Girder ?

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Not All Curved Bridges Require

Curved Steel Girders

Not All Curved Not All Curved Bridges Require Bridges Require

Curved Steel GirdersCurved Steel Girders

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Dev

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• Girder Spacing• Crossframe Spacing• Skew Considerations

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Development of an Appropriate Framing Plan –Girder Spacing

• Limits for Deck Design (i.e. Empirical Method)• Crossframe Member Length• Number of Girders (Redundancy)• Girder Capacity

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Effect of Crossframe SpacingCompare lateral moments & stresses using the lateral moment

approximation from the VLOAD method:Mw = Md2/12Rh, where

M = long. girder moment = 2018 kip-ftd = diaph. spacing

h = web depth R = girder radius

Top Flange

4.6 ksi20.9 kip-ft20’

3.0 ksi13.4 kip-ft16’

1.7 ksi7.5 kip-ft12’

Lateral Flg. StressLateral MomentDiapragm Spacing

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Effect of Crossframe Spacing

Now compare the effect of crossframe spacing on the bottom flange since all applied loads induce lateral bending in the flange

Bottom Flange Lateral Stresses

9.1 ksi82.233.98.420.920’

5.9 ksi52.721.75.413.416’

3.3 ksi29.812.43.07.512’

Lat. StressMfact5/3ML+IMDL2MDL1Diaph. Spa.

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46.8 ksi47.9 ksi48.8 ksiFbu

1.091.081.09ρw

6.7 ksi4.3 ksi2.4 ksifw

161.5 kip-ft103.4 kip-ft58.1 kip-ftMw

0.9880.9960.999ρb

46.8 ksi47.9 ksi48.8 ksiFbs

0.1470.1170.088λ

20’16’12’Diaph. Spa.

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• Uniform Spacing Within a Particular Span• Consider Tighter Crossframe Spaces in

Negative Moment Region• Per Section C6.7.4.2, the following equation

can be used to approximate crossframespacing for preliminary framing:

Lb = √ (5/3 rσRbf)

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Overview of the 2005 AASHTO LRFD Design Specifications for

Horizontally Curved I Girder Bridges

Developed By:Michael A. Grubb, P.E.

BSDI, Ltd.

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The Big Picture

Beginning of the “Big Picture”

• With the 2005 Interim Revisions, the AASHTO LRFD Steel Girder Design provisions will achieve “Unification”

• All steel girders, whether curved or straight, will be designed using the same code provisions

End of the “Big Picture”

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Recent Developments

• FHWA Curved Steel Bridge Research Project (CSBRP)• NCHRP Project 12-38 Leads to 2003 AASHTO LFD Guide

Specification• NCHRP Project 12-52 (Phase I)

– LRFD version of NCHRP Project 12-38• Complete re-write of Art. 6.10 for flexural design of straight I

girders and Art. 6.11 for flexural design of straight box girders in 2004 Third Edition LRFD Specification

Bottom Line: Third Edition revisions written to bet ter accommodate incorporation of curved-girder

provisions -- i.e. UNIFICATION

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Recent Developments (cont’d)

• NCHRP Project 12-52 (Phase II)– Mike Grubb of BSDI, Ltd. joins the NCHRP Project 12-

52 team to assist with the writing of the specification – Development of the LRFD spec based on White et al,

calibration by Nowak et al, and FHWA results -integrated into the Third Edition LRFD straight-girder spec

– Comparisons of 1993 LFD, 2003 LFD and LRFD specs– Updates to the curved I- and box-girder bridge design

examples from 2003 Guide Spec

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Recent Developments (cont’d)

• NCHRP Project 12-52 Specification accepted by AASHTO at the 2004 SCOBS meeting in Orlando, FL - appeared in the 2005 Interims to the Third Edition LRFD specification

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LRFD Curved Spec Overview

• Section 1 - Introduction– Elimination of curved steel girder exception

• Section 2 - General Features– More tracking of deformations for curved steel I-girders– More on span/depth ratios for curved steel systems– More constructibility - constructibility issues should

include, but not be limited to consideration of deflection, strength of steel and concrete, and stability during critical stages of construction

– Live-load deflections determined individually for each girder in curved steel systems

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LRFD Curved Spec Overview (cont’d)• Section 3 - Loads

– Clarification on load factors for construction – not curved steel specific

– New article on construction deflections

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• Section 3 – Loads (cont’d)– Prestressed concrete decks on steel girders– Centrifugal forces

• overturning effect on wheel loads • countereffect of superelevation may be considered• load for fatigue load combination less than for other load

combinations

LRFD Curved Spec Overview (cont’d)

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• Section 4 - Analysis– Analysis of Curved Structures - General

• Analysis advice• Exemptions from curved girder analysis for major-axis

bending, but not for lateral flange bending or torques– Analysis of Curved Structures - Approximate

• Discusses use of V-Load and M/R methods in Commentary

• Use straight girder distribution factors as starting point• Lateral flange bending:

(N = 10 or 12)NRD

MM lat

2l=


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• Section 4 - Analysis (cont’d)– Analysis of Curved Structures - Refined

• Generally grid, FEA or finite strip• Modeling advice

– Additional effective length factor K = 1.0 for single-angle struts


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• Section 6 - Steel Structures– No post-yield resistance used– Hybrid girders and tension-field action permitted– No curvature reduction in shear resistance or stiffener

spacing

– ρb and ρw equations removed– f

lincluded in base service and strength equations• Sometimes 2nd Order• Amplification factor provided to conservatively

guard against large unbraced lengths for which 2nd order lateral bending effects may be significant in compression flanges


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• Section 6 - Steel Structures (cont’d)– Additional shear stud requirements – NCHRP Project 12-

38– More on camber for intended geometry outcome– Continuous kinked (chorded) girders should be treated as

horizontally curved girders – Expanded specification and commentary language on

cross-frames, diaphragms and lateral bracing


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6.7.2 Dead Load Camber

• For straight skewed I-girder bridges and horizontally curved I-girder bridges with or without skewed supports, the contract documents should clearly state:– an intended erected position of the girders, and– the condition under which that position is to be

theoretically achieved.

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Commentary to 6.7.2

• The erection and cambering of straight skewed bridges and horizontally curved bridges with or without skewed supports is a more complex problem than generally considered.

• In some cases, failure to engineer the erection to achieve the intended final position of the girders, or to properly investigate potential outcomes when detailing to achieve an intended final position of the girders, has resulted in construction delays and claims.

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Commentary to 6.7.2 -- (cont’d)

• It is important that engineers and owners recognize the need for an engineered construction plan and the implied level of checking of shop drawings of girders and cross-frames or diaphragms, processing of RFI’s, and field inspection.

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Commentary to 6.7.2 -- (cont’d)

• Intended erected positions of I-girders in straight skewed and horizontally curved bridges are defined herein as either:– girder webs theoretically vertical or plumb, or– girder webs out-of-plumb.

• Three common conditions under which these intended erected positions can be theoretically achieved are defined herein as:– the no-load condition,– the steel dead load condition, or– the full dead load condition.

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6.10.2 & 6.11.2 Cross-Section Proportion Limits• Webs without longitudinal stiffeners

• Webs with longitudinal stiffeners

150≤wt

D

300≤wt

D

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6.10.2 & 6.11.2 Cross-Section Proportion Limits - (cont’d)• Compression flanges

• Tension flanges

0.122

≤fc

fc

t

b 6Db fc ≥ wfc tt 1.1≥ 101.0 ≤≤yt

yc

I

I

0.122

≤ft

ft

t

b 6Db ft ≥ wft tt 1.1≥

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6.10.3.2 & 6.11.3.2 Constructibility - Flexure• For I-sections:

– Discretely braced compression flanges

=>– Discretely braced tension flanges

– Continuously braced flanges

)max,(31

ychncfbu FRLTBorFLBFff =≤+ φl

yfhfbu FRf φ≤

ythfbu FRff φ≤+ l

ychfbu FRff φ≤+ l

crwfbu Ff φ≤2

9.0

=

w

crw

tD

EkF

( )29

DDk

c

=

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6.10.8 Flexural Resistance - Composite I Sections in Negative Flexure & Noncomposite I-Sections - (cont’d)

Bas ic Fo rm o f A ll F LB & LTB E q s

M m a x

M r

λ p λ r

co m p a ct n o n co m p a c t

n o n s le n d e r s le n d e r

(in e la s tic b u ck lin g )

(e la s tic b u ck lin g )

M m a x

M r

λ p λ r

co m p a ct n o n co m p a c t

n o n s le n d e r s le n d e r

(in e la s tic b u ck lin g )

(e la s tic b u ck lin g )

A n ch o r p o in t 1

A n ch o r p o in t 2

L b o r b fc /2 t fcL p o r λ p f L r o r λ r f

F n o r M n

F m ax o r M m ax

F r o r M r

ychbychbpr

pb

ych

yrbnc FRRFRR

LL

LL

FR

F11CF ≤

−−

−−=

ychbcrnc FRRFF ≤=

ychbnc FRRF =

ychbpfrf

pff

ych

yrnc FRR

FR

F11F

λ−λλ−λ

−−=

ychbnc FRRF =

2

t

b

2bb

r

L

ERC

π

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Amplification of First-Order Flange Lateral Bending Stress (Art. 6.10.1.6)

• If

Then second-order compression-flange lateral bending stresses may be approximated by amplifying first-order value as follows:

ycbu

bbpb Ff

RCLL 2.1>

111

85.0lll ff

Ff

f

cr

bu≥

−= 2

2

=

t

b

bbcr

rL

ERCF

π

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6.10.4.2 Service Limit State –Permanent Deformations

• Top steel flange of composite sections:

• Bottom steel flange of composite sections:

• Both steel flanges of noncomposite sections:

• Also:

yfhf FRf

f 95.02

≤+ l

crwc Ff ≤

yfhf FRf 95.0≤

yfhf FRf

f 80.02

≤+ l

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6.10.5 Fatigue and Fracture Limit State

• For horizontally curved I-girder bridges, fatigue stress ranges due to major-axis plus lateral bending shall be investigated.

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Article 6.6.2 Fracture - Addition

• The Engineer shall have the responsibility for determining which, if any, component is an FCM. Unless a rigorous analysis with assumed hypothetical cracked components confirms the strength and stability of the hypothetically damaged structure, the location of all FCMsshall be clearly delineated on the contract plans.

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Commentary C6.6.2

• The criteria for a refined analysis used to demonstrate that part of a structure is not fracture-critical has not yet been codified.

• Therefore, the loading cases to be studied, location of potential cracks, degree to which the dynamic effects associated with a fracture are included in the analysis, and fineness of models and choice of element type should all be agreed upon by the owner and the engineer.

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Commentary C6.6.2 - (cont’d)

• The ability of a particular software product to adequately capture the complexity of the problem should also be considered and the choice of software should be mutually agreed by the owner and the engineer.

• Relief from the full factored loads associated with the Strength I Load Combination of Table 3.4.1-1 should be considered, as should the number of loaded design lanes versus the number of striped traffic lanes.

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6.10.7 Flexural Resistance – Composite I-Sections in Positive Flexure

• 6.10.7.2 Noncompact Sections– Compact sections are not permitted for horizontally

curved girders– Compression flanges

– Tension flanges

1

3bu f h ytf f R F+ ≤ φl

bu f b h ycf R R F≤ φ

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6.10.8 Flexural Resistance – CompositeI-Sections in Negative Flexure and

Noncomposite I-Sections– For horizontally curved girders, use Article 6.10.8 only

-- the use of Appendices A and B is not permitted.

– Discretely braced compression flanges

– Discretely braced tension flanges

– Continuously braced flanges

)max,(31

ychbncfbu FRRLTBorFLBFff =≤+ φl

yfhfbu FRf φ≤

ythfbu FRff φ≤+ l31

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6.10.9 Shear Resistance

• The design of straight and horizontally curved girders for shear is now exactly the same -- use same shear resistance and maximum permitted stiffener spacings.

• The handling requirement for transversely stiffened webs has been eliminated since D/tw is now limited to 150.

• Tension-field action has been extended to horizontally curved girders and to interior panels of all hybrid girders.

• Moment-shear interaction has been eliminated.• Shear connector design has been moved to a separate Article

6.10.10.

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6.10.10 Shear Connectors (abridged)

• The pitch, p, of shear connectors shall satisfy:

• Vsr = horizontal fatigue shear range per unit length

• Vfat = longitudinal fatigue shear range per unit length

• Ffat = radial fatigue shear range per unit length taken as the larger of either:…

r

sr

nZp

V≤

2 2( ) ( )sr fat fatV V F= +

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6.13 Connections and Splices

• Where diaphragms, cross-frames, lateral bracing, stringers or floorbeams for straight or horizontally curved flexural members are included in the structural model used to determine force effects, or alternatively, are designed for explicitly calculated force effects from the results of a separate investigation, end connections for these members shall be designed for the calculated factored member force effects.

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6.13 Connections and Splices -- (cont’d)

• Unless expressly permitted otherwise by the contract documents, standard size holes shall be used in connections in horizontally curved bridges.

• Lateral flange bending and St. Venant torsion are to be considered, where applicable, in the design of bolted splices.

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LRFD Curved Spec Overview (cont’d)• Sec 14 – Joints and Bearings

– Curvature Forces and Displacements

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Summary

• With the 2005 Interim Revisions, the AASHTO LRFD Steel Girder Design provisions achieved “Unification”

• All steel girders, whether curved or straight, will be designed using the same code provisions

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� Minimum Flange Width > L/85� To facilitate handling during fabrication/erection� Wider flanges may be economical, even if they

require additional material

� Flange Width – 2” to 6” wider than typical flange for a straight girder of same span

� Provides larger lateral section to resist lateral bending stresses (fl)

� Use constant width flanges in field sections

Good Detailing PracticePD

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� Check mid-ordinate of field sections� To assure that girders can be shipped� Look at mid-ordinate of curve plus flange width

� Ratio of flange width to girder depth� 2006 LRFD code requires minimum flange

width of D/5� TxDOT uses minimum flange width of D/4, but

prefers designer to meet D/3

Good Detailing PracticePD

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� Generally optimum is 3” to 6” deeper than straight girder of similar span length and spacing

� Can vary girder depth across structure� Allows same plate across all girders - good� Requires more design time - bad� Increases crossframe cost since each bay is

unique - bad

Girder DepthPD

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In accordance with the provisions ofAASHTO 6.10.2:

Girder Proportions

150≤wt

D 122 ≤f

f

tb

6Db f ≥

wf tt 1.1≥

101.0 ≤≤yt

yc

I

I

Webs: Flanges:

300≤wt

D

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� Horizontal force is carried to the bearings by slab & end crossframes

� The CF acting 6’ above the deck causes an overturning moment.

� Part of overturning moment is balanced by SE

� AASHTO, consider girders to act as a pile group to account for CF

Centrifugal Force Effects

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30' - 0"

8"

8"

3.0' 3 @ 8.0' 3.0'

11½

"

SE = 6%6’

6.25’

0.96

13.21’

CF

G4 G1

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S = 45 mph R = 645’

C = f(V2/gR) = (4/3)(66)2/(32.2)(645) = 28.0%Balanceable by SE - = 6.0%Producing overturning moment = 22.0%

Ipile group = 2(12)2 + 2(4)2 = 320 ft2

CFG1 = (2x0.220)(13.21)(12)/320 = 0.218 lanesCFG2 = (0.440)(13.21)(4)/320 = 0.073 lanesCFG3 = CFG4 = 0 Conservatively assume 0 mph

Centrifugal Force EffectsPD

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LRFD Limit States:

• Four Limit States for Design:– Constructability (6.10.3)– Service (6.10.4)– Fatigue (6.10.5)– Strength (6.10.6)

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Lateral Bending StressThe position at

which the engineer requires the webs to be plumb may require computation of additional lateral bending stresses (fw) in the girder flanges.

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� If flanges are cut-curved� No change from normal computation methods

� If flanges are heat curved� AASHTO suggests modification to the cambers to

account for camber losses associated with the heat-curving process

� Some states do not require this additional camber� The additional often is within normal fabrication

tolerances and thus insignificant

CambersPD

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QU

ES

TIO

NS

??


Crossframe

Design


Why is Crossframe Design Important ?• Crossframes may represent around 5 to

10% of the structural steel, but a significantly larger percentage of the structural steel cost

• Crossframes can have a significant impact on the capacity and efficiency of the main load carrying members (girders)

• Crossframes can greatly impact the constructability and stability of the system –especially true for curved girder bridges

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Curved Steel Girder Design

L1 L2 OUTSIDEGIRDER

L ABUT

L PIER

L ABUT

C

CC

INSIDEGIRDER

CROSSFRAME

RADIUS

CURVED BRIDGE - PLAN VIEW

d

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Unrestrained by Cross Frame

Top Flange

Cross frame

CompressionCompression

TensionTension

Bottom Flange

Curved Steel Girder DesignPD

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CompressionCompression

TensionTension

Bottom Flange

Top Flange

Cross frame

Curved Steel Girder DesignPD

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Cur

ved

Ste

el G

irder

Des

ign


1993 AASHTO Guide Specifications for Horizontally Curved Highway Bridges -

Requirements

From Section 2.9:• Maximum crossframe spacing of 25'

per Standard Specs 10.20• Crossframes at each support &

intermediate locations• Single lines across the bridge• Full Depth• Transfer load to web and flanges• Connection plate copes 4t to 6t high

at flanges and longitudinal stiffeners

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AASHTO LRFD 3rd Edition (Incl. 2006 Interims) - Requirements

• Purpose of Crossframes• Spacing Limits• Depth of Crossframe• Configuration• Location Requirements• Connections

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AASHTO LRFD Unified Code Requirements

• Transfer lateral wind loads to deck and from deck to bearings

• Stability of bottom flange in compression• Stability of top flange in compression prior

to deck curing• Consideration of any flange lateral bending

effects• Distribution of vertical dead and live loads

Purpose of Crossframes – Section 6.7.4.1

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Depth for Plate

Girders

Depth for Rolled Beams

Maximum Spacing

Specification

Diaphragm / Crossframe Spacing and Depth

≥ 0.5D, prefer 0.75D

≥ 0.75D≥ 0.75D

≥ 0.33D, prefer 0.5D

≥ 0.5D≥ 0.5D

25'Eq. 6.10.8.2.3-5

or R/10 or 30'

Unlimited – By Design

1993 Curved2006 (Curved)

2006 (Straight)

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GEOMETRICS

Members Sloped

Dh

• Cross Frame Depth h > 0.75 D

W.P.

• Working Point at Bolt Group Centroid

“X” Type Cross Frame

~45

• Diagonal Approximately 45

�

�

Cross Frame Design

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• X or K Configuration• Evaluate based on aspect ratio of the

crossframe bay• Crossframes should contain diagonals and

top and bottom chords

Crossframe Configuration

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• No Skew – Contiguous lines normal to the girders• Skew ≤ 20° - Contiguous skewed lines parallel to

the support• Skew > 20° - Normal to the girders in contiguous

lines or in staggered patterns• Staggering can reduce structure transverse

stiffness and lower crossframe forces – but this does increase girder lateral flange bending

• There must be a load path at supports to get loads into the bearings

Locations

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• Connection plates connected to both top and bottom flanges - Section 6.6.1.3.1

• At a minimum connection should be designed for a 20 kip lateral load (will likely be greater for curved bridges) – Section 6.6.1.3.1

• Where the diaphragm/crossframe members aren’t attached directly to flange, connection plates (and gusset plates) should be capable of transmitting th e required load – Section 6.7.4.1

• Per Section 6.7.4.2 end moments from the diaphragm / crossframe must be considered in the design of the connection

Connections

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DESIGN

Stiffener Plate Designed for Horizontal Flange

Force

HM

AASHTO Requires Stiffener Connection to

the Flange

Cross Frame Design

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• Diaphragms and crossframes not required for final condition could be “temporary” –may not be the best idea

• SIP forms should not be considered bracing• If included in model, must be designed –

duh!• In curved bridges, diaphragms and

crossframes shall be considered primary members

Miscellaneous Provisions

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• When modeling the bridge, it is important to enter reasonable crossframe properties, since their stiffness influences the distribution of forces

• Diaphragms with span-to-depth ratios greater than 4.0 may be designed as beams

• Crossframe / Diaphragm members must be designed to meet slenderness requirements per Section 6.8.4 and 6.9.3

Miscellaneous Provisions

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AASHTO LRFD Articles 6.8.4 & 6.9.3

Limiting Slenderness Ratio

Kl/r = 140l/r = 240Bracing Members

l/r = 200Not Subject

to Stress Reversals

Kl/r = 120

l/r = 140Subject to

Stress ReversalsMain

Members

Compression Members

Tension Members

Includes

Crossframemembers on

curved

bridges

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• K per Section 4.6.2.5 (0.75 for welded; 0.875 for pinned; 2003 Curved Girder Spec called for K = 1.0 for crossframes)

• l = unbraced length (use engineering judgement – WP to WP or Actual Length or Something?)

• r = minimum radius of gyration (could be r x, ry or r z for angles)

Limiting Slenderness Ratio

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Cross Frame Design

• In general, members (angles or WT’s) are eccentric

• Per Section 6.13.1, “Where eccentric connections cannot be avoided, members and connections shall be proportioned for the combined effects of shear and moment due to the eccentricity.”

• The provisions of the AISC “Load and Resistance Factor Design Specification for Single-Angle Members” should be applied per Section 6.12.2.2.4

• Single -Angles experience bi -axial bending

Member Design

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DESIGN

� Axial Load + Bending

� Fatigue

� Primary Members

Bottom Strut

hhGusset Plate

hCentroidal Axis

Sec. A

PM

� Explicitly Calculated Forces (Not 75% Capacity or Avg. of Applied Load & Capacity, Which Used to be the Case)

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� Critical for Grid Analysis

� Do not base on the sum of Ad2 for top and bottom struts

� Must base on how the frame functions in the bridge

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Crossframe Behavior

Behavior consistent with bending stiffness equal to ΣAd2 of struts

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Crossframe Behavior

Actual Behavior – develop bending stiffness based onlimiting relative deflection between girders

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Differences from Straight Girder Design

� Crossframes are primary members� Meet more stringent KL/r requirements

� Require CVN testing

� Check strength based on computed loads

� Check fatigue in crossframe members, connections and connection plates

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Lateral Bracing� Lateral bracing generally not necessary

� Provide if needed for lateral stability prior to deck placement

� Should not be required in the final condition

� Not necessary to place it full length

� Can be used to help shape the structure for long spans and deep girders

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Lateral Bracing

� Bracing should not participate in final structure

� Detail lateral bracing to carry wind only� Detail with oversized holes� Use top flange bracing

� Detail top flange bracing may require fill plates� Keep bracing below SIP form support

angles

� Connect bracing to flanges, not webs� Minimizes out-of-plane bending of webs� Connection details are more cost-

effective

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Cross Frame Design

• Use One-Piece Frames Instead of Knocked Down Frames – yields better geometry control

• Don’t Use Oversize Holes – See Section 6.13.1– “Unless expressly permitted by the contract

documents, standard-size bolt holes shall be used in connections in horizontally curved bridges.”

– Standard-size bolt holes in connections in horizontally curved bridges ensure that the steel fits together in the field.

• Read and Understand Section 6.7.2 and C6.7.2 regarding the detailing of girders and crossframes.

Some Closing Recommendations

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Plan Review

Session –Tips

for Plan

Checking


Tips For Plan Checking

Plan Notes & SpecificationsPlan Notes & Specifications

�� This structure is designed in accordance with the AASHTO LRFD Bridge Design Specifications, 4th Edition.

�� Charpy Impact Testing required for all flange plates in tension, web plates, field splice plates, cross frame members, and stiffener & gusset plates at cross frame connections.

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Tips For Plan CheckingCamber & DL DeflectionsCamber & DL Deflections

�� Separate camber and deflection results for each girder.

�� Differential deflections for a cross section are consistent with the framing layout.

�� Deflections greater for the outside girder (furthest from the center of curve).

�� Watch for large differential deflections for a cross section, say deflection difference of > 2” between inside and outside girders. Excessive rotation and fit-up problems may be encountered during construction.

�� Deck Pouring Sequence consistent with the framing layout.

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Tips For Plan CheckingFlangeFlangePlatesPlates

�� 12”x3/4” Minimum flange size.

�� Compression flange width to thickness ratio (B/t) <= 24 (Section 6.10.2.2)

�� Ratio of the larger flange area to the smaller flange area at a shop splice is <= 2.

�� Larger plates on the outside girder.

�� Larger bottom flanges than top flanges over the piers.

�� Extend the pier plates just past the 1st cross frame line.

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Web PlatesWeb Plates

�� D/t ≤ 150.

StiffenersStiffeners

�� Stiffener spacing ≤ 3 x(Dw).

�� 1st stiffener spacing at the end support ≤ 1.5 Dw

�Stiffeners at cross frames connected to the top and bottom flanges (including connections at bearing stiffeners).

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Cross framesCross frames

�� Cross frame lines continuous (no staggered cross frames).

�� Maximum spacing consistent with the Specifications. Typically use 20 ft maximum for radii around 1000 ft, and tighter spacing for smaller radius.)

�� Depth of cross frames at 3/4 or more times the depth of web.

�� Top strut member used.

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Cross frames (cont.)Cross frames (cont.)

�� The angle of the diagonals with respect to the top or bottom struts >= 30 degrees.

�� No oversized holes.

�� C.G. of member centroids coincides with the bolt group.

�� Typically gusset plate connections used.

�No all around welds (primary load carrying member).

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Tips For Plan CheckingBearingsBearings

�� Uplift potential at the obtuse corner of heavily skewed abutments.

�� Uplift potential at the abutment or piers for relatively short end spans or extreme span arrangements.

�� Higher reactions for the bearing of the inside girders (closer to the center of curve) at an interior bent.

�� Expansion bearing movements oriented towards the point of fixity.

�� Bearing type should allow for out of plane rotations.

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MiscellaneousMiscellaneous

�� Barrier rail cover plates at expansion joints (for large movement joints) should accommodate transverse thermal movements due to curvature.

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QUESTIO

NS??


what is a curved girder · 2008. 12. 7. · effect of crossframe spacing compare lateral moments...

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